Papers - Araki Kenji
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Araki K., Ota Y., Nagaoka A., Nishioka K.
Energies 16 ( 11 ) 2023.6
Authorship：Lead author Publishing type：Research paper (scientific journal) Publisher：Energies
Building-integrated photovoltaics (BIPVs) and vehicle-integrated photovoltaics (VIPVs) receive solar irradiance through non-uniform shading objects. Standard scalar calculations cannot accurately determine the solar irradiance of BIPV and VIPV systems. This study proposes a matrix model using an aperture matrix to accurately calculate the horizontal and vertical planes affected by non-uniform shading objects. This can be extended to the solar irradiance on a VIPV by applying a local coordinate system. The 3D model is validated by a simultaneous measurement of five orientations (roof and four sides, front, left, tail, and right) of solar irradiance on a car body. An accumulated logistic function can approximate the shading probability. Furthermore, the combined use of the 3D solar irradiance model is effective in assessing the energy performance of solar electric vehicles in various zones, including buildings, residential areas, and open spaces. Unlike standard solar energy systems, the energy yield of a VIPV is affected by the shading environment. This, in turn, is affected mainly by the location of vehicle travel or parking in the city rather than by the climate zones of the city.
DOI： 10.3390/en16114414
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How did the knowledge of CPV contribute to the standardization activity of VIPV? Reviewed
Araki K., Ji L., Kelly G., van der Ham A., Agudo E., Antón I., Baudrit M., Carr A., Herrero R., Kurtz S., Liu Z., Pravettoni M., Ota Y., Tobita H., Yoon S., Yoshita M., Yamaguchi M., Nishioka K.
AIP Conference Proceedings 2298 2020.11
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2020 American Institute of Physics Inc.. All rights reserved. The standardization of VIPV (Vehicle-integrated photovoltaic) has been driven by the international discussion among scientists and engineers, from photovoltaic research, photovoltaic industries, automobile and other industries, and testing laboratories. Since the VIPV is not flat and fixed installation, as well as a wide variety of designs, the knowledge of CPV (Concentrator Photovoltaic) technology, has been useful for developing testing technologies. This paper took two examples in the performance testing and the test for the tolerance for the partial/dynamic shading, utilizing various techniques exclusively used in CPV technologies.
DOI： 10.1063/5.0032997
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Araki K., Ota Y., Saiki H., Tawa H., Nishioka K., Sato D., Yamaguchi M.
AIP Conference Proceedings 2298 2020.11
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2020 American Institute of Physics Inc.. All rights reserved. The performance of CPV is sensitive to the spectrum change. Several previous works have been published on the sensitivity of multi-junction cells to airmass variations as well as works on band gap tuning and optimization of multi-junction stacks under concentrated radiation in various locations. Such a variety of atmospheric conditions, airmass as well as climate (quality of direct sunlight) are dynamic and cannot be controlled by the product design. All we can do is to customize the design by locations (not capable of dynamic variation) or inventing a robust device configuration against a dynamic change of atmospheric conditions (also effective to regional variation). Enhancing a luminescent coupling is useful to suppress the spectrum mismatching loss inherent to outdoor CPV installation. The configuration of the solar cell with 100 % of the recycling of the surplus photon energy for compensating spectrum mismatching is called an SMJ solar cell (super-multi-junction solar cell). The advantage of the SMJ solar cells in non-concentrating fixed sloped angle installation was intensively analyzed in the previous works. However, that of CPV was only investigated under the combination of the worst-case atmospheric parameter distributions, and not under the realistic spectrum variations, so that the calculation result was too extreme. This paper intended to fill the missing stone, by updating a previous result on CPV modeled under the combination of the worst-case variations of atmospheric conditions in the worst locations for CPV, by the realistic fluctuation pattern of meteorological parameters and climate using the intensive analysis that was done to the non-concentration installation (fixed installation). The SMJ was also confirmed valid to CPV under the dynamic fluctuation of the direct solar resources. Different from the non-concentrating operation, the normal CPV was found raising annual average outdoor efficiency under realistic atmospheric conditions simulated by the regional states in Miyazaki, Japan, up to six-junctions. This analysis also implies the bandgap design guideline for the robustness of the spectrum variation, trying to place the bandgap energy of the some of the sub-cells close to the water-absorption band (1.3 eV, 1.1 eV, and 0.89 eV).
DOI： 10.1063/5.0032996
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The outdoor field test and energy yield model of the four-terminal on si tandem PV module Reviewed
Araki K., Tawa H., Saiki H., Ota Y., Nishioka K., Yamaguchi M.
Applied Sciences (Switzerland) 10 ( 7 ) 2020.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)
© 2020 by the authors. The outdoor field test of the 4-terminal on Si tandem photovoltaic module (specifically, InGaP/GaAs on Si) was investigated and a performance model, considering spectrum change affected by fluctuation of atmospheric parameters, was developed and validated. The 4-terminal on Si tandem photovoltaic module had about 40% advantage in seasonal performance loss compared with standard InGaP/GaAs/InGaAs 2-terminal tandem photovoltaic module. This advantage increases (subarctic zone < temperate zone < subtropical zone). The developed and validated model used an all-climate spectrum model and considered fluctuation of atmospheric parameters. It can be applied every type of on-Si tandem solar cells.
DOI： 10.3390/app10072529
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Measurement and modeling of 3D solar irradiance for vehicle-integrated photovoltaic Reviewed
Araki K., Ota Y., Yamaguchi M.
Applied Sciences (Switzerland) 10 ( 3 ) 872 2020.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)
© 2020 by the author. The energy yield of vehicle-integrated photovoltaics (VIPV) differs from that of standard photovoltaics (PV). It is mainly by the difference of the solar irradiance onto the car roof and car bodies as well as its curved shape. Both meaningful and practical modeling and measurement of solar irradiance for VIPV need to be established, rather than the extension of the current technologies. The solar irradiance is modeled by a random distribution of shading objects and car orientation with the correction of the curved surface of the PV modules. The measurement of the solar irradiance onto the car roof and car body is done using five pyranometers in five local axes on the car for one year. The measured dynamic solar irradiance onto the car body and car roof is used for validation of the solar irradiance model in the car.
DOI： 10.3390/app10030872
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Araki K., Ota Y., Saiki H., Tawa H., Nishioka K., Yamaguchi M.
Applied Sciences (Switzerland) 9 ( 21 ) 2019.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)
© 2019 by the authors. Licensee MDPI, Basel, Switzerland. The highest-efficiency solar cell in the efficiency race does not always give the best annual energy yield in real world solar conditions because the spectrum is always changing. The study of radiative coupling of concentrator solar cells implies that efficiency could increase by recycling the radiative recombination generated by the surplus current in the upper junction. Such a configuration is called a super-multi-junction cell. We expand the model in the concentrator solar cell to a non-concentrating installation. It is shown that this super-multi-junction cell configuration is robust and can keep maximum potential efficiency (50% in realistic spectrum fluctuation) for up to 10 junctions. The super-multi-junction cell is also robust in the bandgap engineering of each junction. Therefore, a future multi-junction may not be required for tuning the bandgap to match the standard solar spectrum, as well as relying upon artificial technologies such as epitaxial lift-off (ELO), wafer-bonding, mechanical-stacking, and reverse-growth, but merely uses upright and lattice-matching growth technologies. We present two challenging techniques; one is the optical cap layer that may be the directional photon coupling layer in the application of the photonics technologies, and another is the high-quality epitaxial growth with almost 100% radiative efficiency.
DOI： 10.3390/app9214598
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Araki K., Sato D., Masuda T., Lee K., Yamada N., Yamaguchi M.
AIP Conference Proceedings 2149 2019.8
Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2019 American Institute of Physics Inc.. All rights reserved. With this car-roof PV, 70 % of passenger's may be able to run by solar energy. The potential of the market size is 50 GW/year. However, it is not an easy task to meet the requirement of the main component of EV and the creation of the massive market. It is also apparent that the market will be small as far as we only try to apply the conventional crystalline Si cells. The CPV technology with high power conversion efficiency and capable of coloring and covering on the threedimensional curved surface is suitable for this market. The static CPV is useful for this application with a better collection of diffused sunlight. One of the problems is the spectrum mismatching. However, this issue is easily solved by adjustment of the bandgap of the bottom cell.
DOI： 10.1063/1.5124188
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Araki K., Ota Y., Lee K.H., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 2149 2019.8
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2019 American Institute of Physics Inc.. All rights reserved. It is well known that CPV is sensitive to the spectrum change, and its high-performance is often discouraged by the spectrum mismatching loss by the seasonal change of the atmospheric parameters. We found multiple methods could improve the robustness to the spectrum sensitivity, including enhancing luminescence coupling and fine-tuning to the bottom-bandgap matched to local atmospheric conditions (water absorption).
DOI： 10.1063/1.5124194
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Standardization of the CPV technology in 2019 - The path to new CPV technologies Reviewed
Araki K., Algora C., Siefer G., Nishioka K., Leutz R., Carter S., Wang S., Askins S., Ji L., Kelly G.
AIP Conference Proceedings 2149 2019.8
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2019 American Institute of Physics Inc.. All rights reserved. CPV incorporates a wide variety of technologies and options. The application of these technologies, however, is not limited to CPV only. The basic approach of the standardization for CPV is thus expanding the scope to a broader range of technologies. The knowledge gained through our activities is contributing to standardizations in PV technologies like, e.g. standards for trackers. In this regard, the standardization activities for CPV is also moving to new fields of applications.
DOI： 10.1063/1.5124220
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Araki K., Ota Y., Lee K.H., Nishioka K., Yamaguchi M.
2019 18th International Conference on Optical Communications and Networks, ICOCN 2019 2019.8
Language：English Publishing type：Research paper (scientific journal) Publisher：2019 18th International Conference on Optical Communications and Networks, ICOCN 2019
© 2019 IEEE. It is known that multi-junction solar cells are sensitive to the spectrum change [1]-[2], and its high-performance is often discouraged by the spectrum mismatching loss by the seasonal change of the atmospheric parameters [3]-[4]. It implies that the highest efficiency solar cell in the standard testing condition does not always promise the most excellent annual energy yield in the real world solar condition that the spectrum is ever-changing. We found the ultimate solution of the seasonal loss will be autonomous compensation of the carrier imbalance by the radiative coupling among junctions due to radiative recombination [5]. We call it a super-multi-junction solar cell. The annual operation of the super-multi-junction solar cell was simulated by the validated operation model of the photovoltaic module using multi-junction solar cells. It was shown that the super-multi-junction architecture was found robust up to 6 junctions. The robust high-efficiency solar cell can be the right candidate of the vehicle-integrated photovoltaic that run a majority of electric vehicles on solar energy.
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Alignment Tolerance Control of the Micro CPV Array Using Monte Carlo Methods
Araki K., Lee K.H., Hayashi N., Ichihashi K., Kanayama S., Inohara T., Morita Y., Takase M., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 202 - 209 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. Micro CPV is a promising technology for reducing high-precision assembly cost while making use of the high efficiency of CPV. A production method is often taken to manufacture small size modules with high accuracy, high homogeneity, low cost, and to compile them. There is a concern that the performance of the large-area panel may deteriorate due to accuracy in the assembly. We did the Monte Carlo analysis for simulate assemble error. It could successfully define inspection conditions and tolerance control thresholds with consideration of the worst-case of the tracking errors. With proper but reasonable inspections (component acceptance and alignment accuracy test), it is possible to collect micro-CPV units to a reasonable-size of the CPV panel keeping more than 90 % of the output even with the worst-case installation and tracking errors. This approach can be expanded to the analysis of impact by the partial shading of the lens that has a significant impact on the performance of CPV.
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Araki K., Ota Y., Lee K.H., Yamada N., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 1584 - 1591 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. Curved photovoltaic, typically using flexible one receives different irradiation from the sunlight. The energy yield is often overestimated by ignoring local cosine loss and other losses related to the curvature of the PV surface. Although the formula to some extent varies by the type of the curved surface, it is modeled by the curve-correction factor from the flat-plate conditions. The curve correction factor is calculated either geometrical calculation or the numerical ray-tracing simulation. In these proceedings, the one by Monte Carlo method (Ray-tracing simulation) is discussed. First, the incident angle the sunlight affected by the orientation (random orientation if necessary), shading by surrounding structures (randomly distributed), climate pattern, and both direct and diffused sunlight from a regional database, was calculated. Second, the absorbed flux onto the curved module surface was calculated using random rays by the above distribution of the sunlight. Third, the regional influences onto the energy generation of the curved PV panel were discussed. The energy yield of the curved PV drops significantly by the increase of the curvature and incident angle distribution affected by latitude, climate pattern, and shading environment.
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Araki K., Lee K.H., Masuda T., Hayakawa Y., Yamada N., Ota Y., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 1218 - 1225 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. Solar panels used for EV charging stations and car-roof PV are often partially shaded and lead to substantial energy loss. It is known that this mismatching loss can be reduced by increasing the number of parallel strings. First, a probability model was developed using a one-year solar irradiance monitoring around the car body in 5 directions. The developed model successfully matched to the measured solar resource in each direction and succeeded to quantify the probability of the partial shading on the car. Another related issue of the inherent mismatching loss of the car-roof PV is non-uniform illumination caused by the curved shape of the panel. This can also be modeled by ray-tracing simulation. Then, we calculated the PV output affected by mismatching due to various sunlight patterns and partial shade patterns by Monte Carlo method. It was found that the average efficiency asymptotically approached 1 - 1 / N (N is the number of strings). We also examined the relationship between partial shade quantity and power generation loss in a 30 kW solar system array and verified the above model.
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Araki K., Ji L., Kelly G., Agudo E., Anton I., Baudrit M., Carr A., Herrero R., Kurtz S., Liu Z., Limpinsel M., Lu Z., Pravettoni M., Robusto P., Tobita H., Wu Q., Wu Z., Yoon S., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2722 - 2729 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. A car-roof photovoltaic (PV) has significant potential. The activities of international standardization through a series of web meetings have been done by 51 volunteer scientists and engineers from 15 different countries with a good mixture of scientists, manufacturers, and testing laboratories. Specifically, it covered (1) Irradiance modeling onto the car-roof; (2) Curve correction of the PV output; (3) Quantification of partial shading loss; (4) Performance testing conditions; (5) Effective solar resource on the car-roof impacted by shadows of surrounding buildings; (6) Environmental testing conditions.
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Design of the Micro-Köhler Concentrator Optics for CPV Application
Araki K., Lee K.H., Hayashi N., Ichihashi K., Kanayama S., Inohara T., Morita Y., Takase M., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 196 - 201 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. Micro CPV is a promising technology for reducing high-precision assembly cost while making use of the high efficiency of CPV. However, the requirement of the installation and tracking accuracy remains the same. A production method is often taken to manufacture small size modules with high accuracy, high homogeneity, low cost, and to compile them. There is a concern that the performance of the large-area panel may deteriorate due to accuracy in the assembly. A micro-Köhler concentrator optics was designed suitable to the micro CPV modules, and the designed acceptance angle (90 % power point) reached 1.8 degrees with the 500 x axially symmetrical (truncated to square) primary plano-convex lens.
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Araki K., Ota Y., Nishioka K., Tobita H., Ji L., Kelly G., Yamaguchi M.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC 368 - 373 2018.11
Language：English Publishing type：Research paper (scientific journal) Publisher：2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC
© 2018 IEEE. With the progress of the automotive technology, it will be possible that the most of the personal cars will be able to run by the solar energy equipped with high-efficiency and 3-D curved solar panels. The issue is the conventional IEC60904 international standard is based on a 2-D plane, and it is needed to expand to 3-D. Our attempt is trying to define 3-D sunshine irradiation and rating to the 3-D curved solar panels, using a newly introduced curve correction factor. This approach can be applied to the rating of the flexible PV modules. Another issue is the environmental test needed as a car-component. The required tests were compared to the typical testing conditions of PV.
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Araki K., Ota Y., Lee K.H., Nishioka K., Yamaguchi M.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC 1661 - 1666 2018.11
Language：English Publishing type：Research paper (scientific journal) Publisher：2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC
© 2018 IEEE. Fluctuation of the atmospheric parameters substantially impact to the energy yield of multi-junction cells. The calculation considering the variation of atmospheric parameters showed a dramatic decline of the annual energy yield from the cells with more than five junctions. Partially Radiativecoupling multi-junction solar cells were proposed, and optimization work was done. The top and the bottom junctions have lower bandgap for buffering to the mismatching and collecting a full range of the sunlight by robust energy conversion, and the annual energy substantially increases even by five or six junctions.
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Analysis of fluctuation of atmospheric parameters and its impact on performance of CPV Reviewed
Araki K., Ota Y., Lee K.H., Sakai T., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 2012 2018.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2018 Author(s). The measurement of the atmospheric parameters by fitting to the measured spectrum was used to analyze the uncertainty of the energy generation of CPV. It was found the fluctuation of the atmospheric parameters substantially impact to the energy yield of multi-junction cells. The calculation considering the variation of atmospheric parameters showed a dramatic decline of the annual energy yield from the cells with more than five junctions. Optimized multijunction solar cells using luminescent coupling for an adjuster of the spectrum mismatching was proposed and analyzed. The top and the bottom junctions have lower bandgap for buffering to the mismatching and collecting a full range of the sunlight by robust energy conversion, and the annual energy substantially increases even by five or six junctions. The advanced modeling of the multi-junction cells considering the sensitivity of the atmospheric parameters and the degree of luminescent coupling is useful to the accurate prediction of the solar power in the smart control to the car-roof PV.
DOI： 10.1063/1.5053530
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Standardization of the CPV and car-roof PV technology in 2018 - Where are we going to go? Reviewed
Araki K., Algora C., Siefer G., Nishioka K., Leutz R., Carter S., Wang S., Askins S., Ji L., Kelly G.
AIP Conference Proceedings 2012 2018.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2018 Author(s). The work of IEC TC82 WG7 aims for the standardization of concentrator modules, and their optics, mechanics and other advanced photovoltaic systems. These standards will be in the general areas of safety, photoelectric performance and environmental reliability tests. The standards ultimately produced should be universal and non-restrictive in their application, taking into account different environments and manufacturing technologies. In addition to the essential electrical and mechanical characteristics, standards will be written for other important factors such as thermal performance, high voltage performance, angular performance, mechanical precision, specific testing fault resistance and fault-tolerant design. Recently an expanded group started to discuss standardization of the car-roof PV.
DOI： 10.1063/1.5053527
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The possibility of the static LCPV to mechanical-stack III-V//Si module Reviewed
Araki K., Lee K., Yamaguchi M.
AIP Conference Proceedings 2012 2018.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2018 Author(s). For further reduction of the cost of the III-V//Si cell, a 4-terminal III-V//Si partial concentrator cell was proposed. A Monte Carlo analysis showed that the 4-terminal configuration had an advantage under high aerosol density. The partial concentrator optics of 1.5x of concentration ratio was optimized, and the angular loss was 1% of the annual energy yield. The mono module efficiency was 27.08%. Moreover, corresponding cell efficiency was 31.03%. Although the area of the top cell in 2/3, the efficiency is the almost same as the non-concentration operation. The contribution from the bottom Si cell increases and reaches nearly 6 %, whereas regular Si cell contribution is around 4 %. The angular loss due to concentrator optics is almost ignored.
DOI： 10.1063/1.5053540
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Araki K., Lee K., Yamaguchi M.
2018 18th International Workshop on Junction Technology, IWJT 2018 2018-January 1 - 4 2018.4
Language：English Publishing type：Research paper (scientific journal) Publisher：2018 18th International Workshop on Junction Technology, IWJT 2018
© 2018 IEEE. This paper discusses the potential of the power conversion efficiency of solar cells from various materials considering external radiative efficiency, open-circuit voltage loss. The possibility of the power conversion efficiency is summarized in the 1st chapter. The examined cells are Silicon, III-V, CIS, CdTe, Organic, Dye-sensitized and Perovskite. The 2nd and successive chapters discuss the potential of the multijunction cells. The power conversion efficiency can be raised by using multi-junction structure. However, fluctuation of the atmospheric parameters substantially impacts the energy yield. The calculation considering the variation of atmospheric parameters showed a dramatic decline of the annual energy yield from the cells with more than five junctions. Super-multijunction solar cells were proposed, and optimization work was done. The top and the bottom junctions have lower bandgap for buffering to the mismatching and collecting a full range of the sunlight by robust energy conversion, and the annual energy substantially increases even by five or six junctions.
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Araki K., Ji L., Kelly G., Yamaguchi M.
Coatings 8 ( 7 ) 2018.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Coatings
© 2018 by the authors. A car-roof photovoltaic has enormous potential to change our society. With this technology, 70% of a car can run on the solar energy collected by the solar panel on its roof. Unfortunately, it is not a simple extension of conventional photovoltaic technology. This paper lists what we need to do to achieve the goal of running a majority of cars on renewable solar energy, after clarification of the difference to conventional photovoltaic technology. In addition to technological development, standardization will be important and this list was made highlighting standardization.
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Possibility of CPV for automobile energy source Reviewed
Araki K., Ota Y., Ikeda K., Lee K., Nishioka K., Yamaguchi M.
2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017 1 - 6 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017
© 2017 IEEE. Greenhouse gas emission from automobiles cannot be eliminated before having them run by the sun. Toyota Tec. Inst. is serious to develop CPV applications to automobile. One is an innovative static concentrator that selectively collects sunlight from shallow angle. Another is a solar powered EV charging station with practical size and capacity.
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Araki K., Herrero R., Anton I., Sala G., Nagai H., Lee K., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 549 - 553 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2016 IEEE. The acceptance angle of concentrator modules measured by short circuit current Isc is always larger than that by maximum power Pm even by use of advanced concentrator optics that keep uniform illumination onto the concentrator cells. The purpose of this study is to investigate the cause of that myth. An expanded Monte Carlo method that calculates acceptance angle of CPV modules was developed with considering 2-dimensional misalignment error vector. It was shown that the gap from Pm-based acceptance angle from that of Isc-based was a function of deviation of assembles misalignment in the module. The rough rule for acceptable assembles misalignment is that the standard deviation of assemble misalignment level will be less than 10 % of acceptance angle of the optics.
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Possibility of CPV for automobile energy source Reviewed
Araki K., Ota Y., Ikeda K., Lee K., Nishioka K., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 554 - 559 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2016 IEEE. Greenhouse gas emission from automobiles cannot be eliminated before having them run by the sun. Toyota Tec. Inst. is serious to develop CPV applications to automobile. One is an innovative static concentrator that selectively collects sunlight from shallow angle. Another is a solar powered EV charging station with practical size and capacity.
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Analysis of impact to optical environment of the land by CPV Reviewed
Araki K., Lee K., Yamaguchi M.
AIP Conference Proceedings 1766 2016.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2016, American Institute of Physics Inc. All rights reserved. With the increase of installations on the land, the impact to the optical environment result from shading by PV panels is needed to be concerned. This paper describes how to calculate illumination level on the land around the isolated CPV panel. The land under the flat-plate PV is often dark but the shading influence from the tracking CPV is not very significant in most of the cases. This is because the shadow by the tracking panels moves by the movement of the sun. The integrated illumination level is averaged after integration in a day or a year. This effect is enhanced in the high latitude area. The shading influence by CPV is further decreased by the design of a higher pedestal. With this advantage, CPV can be combined with agriculture as a double use of the land.
DOI： 10.1063/1.4962108
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Intermittent tracking (30 minutes interval) using a wide acceptance CPV module Reviewed
Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1766 2016.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2016, American Institute of Physics Inc. All rights reserved. For applications of stand-alone systems including an electric vehicle (EV) power station, a tracking system with low power consumption will be demanded. One possible solution to save idling power is intermittent tracking, for example, tracking in every 30 minutes. To achieve this, a new CPV module was designed. With the combination of an advanced feed-forward tracking control, the 30 minutes of intermittent tracking may be possible by 100 x of geometrical concentration (the projected lens area by the cell aperture area with alignment margin).
DOI： 10.1063/1.4962083
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Solving optimization problem of space factor of multiple CPV trackers using "butterfly approach" Reviewed
Araki K.
AIP Conference Proceedings 1616 224 - 227 2014
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
©2014 AIP Publishing LLC. Optimization of land use to multi-tracker CPV system is discussed by mathematical approach. Optimization problem using butterfly plot (Contour plot on the shading to adjacent tracker) is discussed to seek optimum allocation pattern. With initial solutions given by this optimum allocation pattern, numerical optimization calculation is done to obtain the optimum allocation including, skew angle to the North-South axis, aspect ratio between X and Y pitch and optimum panel aspect ratio. It is suggested that there are two candidate of optimum allocation pattern.
DOI： 10.1063/1.4897066
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Araki K., Nagai H., Tamura K.
AIP Conference Proceedings 1477 281 - 284 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
CPV cells are successively exposed by cycles of concentrated sunlight and huge cycling current flows out of contacts. The purpose of this research is to identify if the fatigue will be one of the life-end and wearing modes of CPV cells and how long the life-time will be. One of the best ways of giving cycling stress on cells and contacts is cycling forward bias injection. First, it is important to investigate if forward bias itself damages concentrator III-V cells. To confirm the forward bias current itself does not damage the CPV solar cells but cycling docs, we applied 4 times of Isc continuously in 500 hours, equivalently total injection of 90,000 cycles, but no damage or degradation was observed. What's more, we applied the cycling test after the stress of 500 hour continuous forward bias injection. We did not identify any changes between groups of with stress of continuous forward bias and without them. On-Off tests giving forward bias current as high as 4 times of Isc are conducted. The failure was analyzed by the Weible function. About 2,000 cells in total were examined. A small number of initial failure, that of failure number of cycles lied along exponential distribution and distinct portion of wear mode failure that of failure number of cycles lied along Gaussian distribution were identified. The EL measurement indicated that all the damaged cells in wear mode were local shunt under the top contacts suggesting stress under the top contacts would be possibly responsible to the fatigue failure. Advanced contact design that leads to reduce the stress on the top contact is expected to prolong the lifetime of the III-V concentrator cells. © 2012 American Institute of Physics.
DOI： 10.1063/1.4753886
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Araki K., Zamora P., Nagai H., Benítez P., Hobo K., Miñano J., Futo M., Sala G., Tamura K., Kumagai I.
Conference Record of the IEEE Photovoltaic Specialists Conference 1815 - 1819 2012.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
Modules are an important part of the CPV system. By pursing, in our objective of a 35% efficiency module, we need to look forward a significant improvement in the state of the art of CPV modules since no commercial module is capable of achieving that efficiency. Achieving this efficiency will require high efficiency cells, progress in the optics lenses that are implemented in these modules, and also integration into module. Basic design of 35 % CPV module is presented considering for practical and rapid industry application. The output is 385 W while its weight is only 18 kg. In spite of its high concentration ratio reaching 1, X, it acceptance angle is as high as 1.1 degree. © 2012 IEEE.
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Performance of the 30 KW CPV system installed in coastal area in Japan Reviewed
Araki K.
AIP Conference Proceedings 1277 239 - 242 2010.11
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
A 30 kW concentrator photovoltaic power plant was constructed and has started operation. The power consumption of the tracking motors was only 19.6 W, namely 0.07 % of the rated power. Improved optics that reduces the mismatching losses associated with variance of optical losses. 25.8 % (STC) of efficiency was achieved even in as large as 23.8 m2 of the array size. The annual energy generation was about 1,000 kWh/kW, corresponding almost equivalent value to the crystalline silicon system in spite of unwanted climate to CPV. © 2010 American Institute of Physics.
DOI： 10.1063/1.3509200
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30 kW Concentrator photovoltaic system using dome-shaped fresnel lenses Reviewed
Araki K., Yano T., Kuroda Y.
Optics Express 18 ( 101 ) A53 - A63 2010.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Optics Express
A 30 kW concentrator photovoltaic power plant was constructed and has started operation with the following new technologies: A new Concentrating PhotoVoltaic (CPV) tracker developed for high wind area like Korea and Japan by Daido Steel. (The power consumption of the tracking motors was only 19.6 W, namely 0.07% of the rated power.) With improved optics that reduce the mismatch losses associated with optical aberrations, an efficiency of 25.8% was achieved under standard testing conditions (STC) even in a large 23.8 m2 array size. A rapid installation sequence was developed. It was designed for long-term power supply to a local sewage center. Peak power corresponds to 10% of the demand. As a result, the system performance ratio was 0.87, and the capacity factor was 11.7%. The energy generation per rated power was 1, 020 kWh/kWp. While it is true that CPV systems perform better in dry and high irradiance areas, our 30 kW system installed in a cloudy area like Japan, showed satisfactory performance. © 2010 Optical Society of America.
DOI： 10.1364/OE.18.000A53
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8-3-2 Questionnaire Survey on Renewable Energies to Agricultural High School in Aichi Prefecture Reviewed
AKISAWA Atsushi, Araki Kenji, Kumagai Isao
Proceedings of the Annual Conference of The Japan Institute of Energy 19 ( 0 ) 364 - 365 2010
Language：English Publishing type：Research paper (scientific journal) Publisher：The Japan Institute of Energy
Questionnaire survey was conducted to agricultural high schools in Aichi prefecture to survey activities and requirements for teaching renewable energy. Most schools already implement lectures for renewable energy. They are strongly interested in the economy of utilizing renewable energy in agriculture. The results suggest that practical knowledge is required for the students to manage the technologies.
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A simple rating method for CPV modules and systems Reviewed
Araki K., Kemmoku Y., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2008.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
Right now, there is no agreed definition and measurement method for rating concentrator photovoltaic. This paper discusses present situation of the ratings of CPV modules and systems. First, it was analyzed which factors have influence to the performance to CPV modules and systems. Next, the linear interpolation method that is known as a robust estimator especially to multi-junction thin film modules is discussed. The test data was provided from Toyohashi University of Technology where a 550 X CPV module (20 cells connected in series and the total area is 5,445 cm2) was tested independently. The relative standard deviation was 0.89 %, whereas that of the linear interpolation method was 1.2 %. Different from the linear interpolation method, this stratified averaging method is unbiased. © 2008 IEEE.
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Araki K., Kondo M., Uozumi H., Ekins-Daukes N.J., Kemmoku Y., Yamaguchi M.
Solar Energy Materials and Solar Cells 90 ( 18-19 ) 3312 - 3319 2006.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
A 400× and 7056 cm2 concentrator module was fabricated from 36 concentrator receivers, connected in series and with the same number of newly developed dome-shape Fresnel lenses. The averaged outdoor efficiency on a clear sky day was 26.8±1.5% (25C STC). This is the highest module efficiency achieved to date using a module of practical size and electrical rating. The heat was dissipated by the module wall and no heat sinks nor external cooling were used. A glass homogenizer was introduced to give uniform illumination to the square cell, and afforded a reasonable assembly tolerance, without the need for optical alignment. © 2006 Elsevier B.V. All rights reserved.
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Development of a new 550X concentrator module with 3J cells - Peformance and reliability Reviewed
Araki K., Uozumi H., Kondo M., Takamoto T., Agui T., Kaneiwa M., Egami T., Hiramatsu M., Miyazaki Y., Kemmoku Y., Akisawa A., Lee H.S., Ekins-Daukes N.J., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 631 - 634 2005.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. The peak uncorrected efficiency for a 7,056 cm2 400 X module with 36 solar cells connected in series was 26.6 % was measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1,176 cm2 area measured by Fraunhofer ISE and NREL were 27.4 % and 24.8 % respectively. The peak uncorrected efficiency for a 550X and 5,445 cm2 module with 20 solar cells connected in series was 28.9 %. The temperature corrected efficiency under the best sunshine condition in Japan for the 550X module was 31.5 ±2 %. For reliability, some new degradation modes inherent to high concentration III-V solar cell system are discussed and a 20 year lifetime under concentrated flux exposure proven. The fail-safe issues of the concentrated sunlight are also discussed. For cost, the overall scenario for the reduction of material cost is discussed. © 2005 IEEE.
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Araki K., Uozumi H., Egami T., Hiramatsu M., Miyazaki Y., Kemmoku Y., Akisawa A., Ekins-Daukes N.J., Lee H.S., Yamaguchi M.
Progress in Photovoltaics: Research and Applications 13 ( 6 ) 513 - 527 2005.9
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm2 400× module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm2 area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm2 module with 20 solar cells connected in series was 28·9% in house. The temperature-corrected efficiency of the 550× module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side-by-side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III-V solar cell system are discussed and a 20-year lifetime under concentrated flux exposure proven. The fail-safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.
DOI： 10.1002/pip.643
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Material study for the solar module under high concentration UV exposure Reviewed
Araki K., Kondo M., Uozumi H., Yamaguchi M.
Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion A 805 - 808 2003.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion
The use of high concentration constitutes an extremely harsh operating environment with high density and high energy photon flux. However, there is very few literatures on the material study for high concentration receiver. In this study, a super-high intensity UV weathering meter was developed. Initial examination of UV durability of materials used for concentrator receiver has been started.
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Influences of spectrum change to 3-junction concentrator cells Reviewed
Araki K., Yamaguchi M.
Solar Energy Materials and Solar Cells 75 ( 3-4 ) 707 - 714 2003.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
PV output of multi-junction cells is strongly influenced by spectrum change. The influence of Sun height was quantitatively analyzed, considering seasonally and daily changes of spectrum. The new model also considered the presence of clouds. The influence of daily random fluctuation of spectrum was shown averaged out in the integration of yearly PV output fluctuation. It was suggested that the overall mismatch loss by the change of Sun height and Sun orbital would be <4% for III-V multi-junction cells for concentration application, when the bandgap of each junction was well balanced. The sensitivity of spectrum fluctuation was shown enlarged with the discrepancy of current matching conditions. © 2002 Elsevier Science B.V. All rights reserved.
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Novel equivalent circuit model and statistical analysis in parameters identification Reviewed
Araki K., Yamaguchi M.
Solar Energy Materials and Solar Cells 75 ( 3-4 ) 457 - 466 2003.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
Parameter fitting based on the classical equivalent circuit model does not always reach reasonable solutions. It often gave negative series resistance and exaggerated diode ideality factor. Three problems were identified. The first problem was illuminated I-V is not a simple voltage shift by lumped series resistance. The second problem was rounded I-V curves in concentrator cells. The third problem was statistical instability in data fitting algorithm. A new model was proposed to describe the second problem. It was also effective to partly solve the first and the third problems. © 2002 Elsevier Science B.V. All rights reserved.
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A simple passive cooling structure and its heat analysis for 500 X concentrator PV module Reviewed
Araki K., Uozumi H., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 1568 - 1571 2002.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
A new and simple module structure for 500 X concentrator module made by printed epoxy and copper sheet on aluminum plate was pro-posed. The heat analysis was done and indicated that around 10 degree rises at the surface of the cell compared to normal irradiated flat-plane was expected without the help of classical heat sinks. Outdoor test was done and confirmed its cooling performance.
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Si concentrator cell by single photolithography process Reviewed
Araki K., Yamaguchi M.
Solar Energy Materials and Solar Cells 65 ( 1 ) 437 - 443 2001.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
A mid-range concentrator cell by low-cost process is investigated and a new design is proposed. We tried to develop a single lithography CZ-Si cell process. The current conversion efficiency (not optimized) is 16% under X20 concentration without AR coating. The present moderate efficiency is possibly due to low bulk lifetime (approx. 5 μs after process). Cell efficiency and open-circuit voltage (Voc) are expected to improve by using a better quality Si material and developing a low-stress process to maintain crystal quality. The equivalent circuit analysis reveals degradation of diode characteristics under concentration, which implies decline of fill-factor (FF) by use of low-quality concentrator optics.
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Outdoor evaluation of total cross-tied Si module under driving conditions Reviewed
太田 靖之, 西岡 賢祐
Solar Energy Materials and Solar Cells 274 112989 - 112989 2024.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier BV
This paper presents the fabrication and evaluation of a total cross-tied (TCT) Si module designed to mitigate the partial shading effect, which is known to reduce the output performance of vehicle-integrated photovoltaic (VIPV) modules. The TCT module, consisting of 27 Si solar cells interconnected both vertically and horizontally, demonstrated an 18.8 % decrease in output power when one cell was shaded. In contrast, the output of the series-parallel (SP) Si module decreased by 31.6 % under the same shading condition. Comparative analysis of the output performances of TCT, SP, and single-string commercial modules under three driving conditions revealed that increasing the number of strings helped alleviate the impact of partial shading. Notably, the TCT module configuration showed a positive impact on mitigating the partial shading effect.
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Consideration for importance of concentrating photovoltaics (PV) toward TW-scale PV production Reviewed
Yamaguchi M., Araki K., Zhang Y., Hallam B., Kojima N., Ohshita Y.
AIP Conference Proceedings 2841 ( 1 ) 2023.9
Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Photovoltaics will play a key role in a future net zero greenhouse gas emission energy systems. Especially, the concentrator photovoltaics (CPV) has great potential of higher efficiency, lower cost and potential overcoming sustainability of material resources compared to conventional crystalline Si PV and thin-film PV. This paper presents impact of material consumption upon annual production capacities of various solar cells by using CPV. Merits of CPV compared to conventional PV (1-sun use) are 190TW/1.6TW for Si (Si limit), 650GW/2.7GW for GaAs (Ga limit), 250GW/4.2GW for CdTe (Te limit) and 950GW/2GW for III-V 3-junction solar cells (Ga limit).
DOI： 10.1063/5.0146171
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Prediction of PPFD (photosynthetic photon flux density) under transparent CPV modules Reviewed
Toyoda T., Yajima D., Kirimura M., Araki K., Ota Y., Nagaoka A., Nishioka K.
AIP Conference Proceedings 2841 ( 1 ) 2023.9
Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Recently, the agro-photovoltaic (agri-photovoltaic) system is expected to penetrate the market due to its advantages in some crops and significant potential installation areas. In principle, the agro-photovoltaic system shares solar energy with PV and agriculture, and its appropriate and controlled distribution is the key for the economy. Accurate and reproducible optimization design is essential. Any PV modules can be used for the agro-photovoltaic system. The best may be the transparent CPV that uses direct sunlight to concentrate onto solar cells and diffused sunlight to illuminate the farming light through a transparent backplane. The diffused sunlight for typical CPV modules is not used for electricity generation, and the transparent CPV is utilized as the secondary light source to crops. The diffused sunlight does not create shadow and improves the inhomogeneity of PPFD (Photosynthetic Photon Flux Density) from the stripe of shadows by PV modules, thus improving the yield. This paper predicts PPFD by the transparent CPV modules and the effectiveness of its use for the agro-photovoltaic system.
DOI： 10.1063/5.0146145
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Shimizu R., Ota Y., Nagaoka A., Araki K., Nishioka K.
Applied Sciences (Switzerland) 13 ( 18 ) 2023.9
Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)
Featured Application: Solar panel fire accident diagnosis, analysis, and prediction. Fires in photovoltaic modules are caused by hot spots, which are typically monitored by thermal images. This method helps visualize the hot spot, but it is affected by the environment (solar irradiance, wind, ambient temperature) and is not reproducible. Assessing the heat dissipation of the hot cell can be used for alternative assessment of the fire risk. This method was validated by comparing the value measured by the surface potential meter and the module potential measured directly by adding a bypass measurement circuit. The substantial reverse-bias voltage caused by mismatching or partial shading (depending on the operating conditions) leads to local heat consumption of the partially shaded solar cells and potentially causes fire. The fire risk can be assessed in the worst-case conditions (ex. 1380 W/m2 solar irradiance) by non-contact measurement of the reverse-bias voltage and calculating the heat dissipation and temperature rise. This work suggested that −13 V is the criterion and was close to the known value of reverse voltage for Si cells. The current technology inspects solar cells before assembly to the module, and there is no way of inspecting in the product test or detecting after degradation that can be covered by the proposed method in this work.
DOI： 10.3390/app131810391
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Yamaguchi M., Masuda T., Nakado T., Yamada K., Okumura K., Satou A., Ota Y., Araki K., Nishioka K., Kojima N., Ohshita Y.
IEEE Journal of Photovoltaics 13 ( 3 ) 343 - 348 2023.5
Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
Development of photovoltaic (PV)-powered vehicles is very important to play a critical role in a future carbon neutrality society because it has been reported that the vehicle integrated PVs (VIPVs) have great ability to reduce CO2 emission from the transport sector. Usage of high-efficiency solar cell modules is essential due to the limited installable area of PV on vehicle exterior. This article presents test driving data of the Toyota Prius demonstration car installed with high-efficiency III-V compound triple-junction solar cell module with an efficiency of more than 30%. Average daily driving distance (DD) of 17 km/day under usage of air conditioning and 62% CO2 emission reduction are demonstrated by actual driving in Nagoya, Japan. In addition, analytical results for impact of high-efficiency VIPV modules of more than 35% on increases in DD of more than 30 km/day average and reducing CO2 emission of PV-powered vehicles of more than 70% reduction are also shown.
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Solar Electric Vehicles as Energy Sources in Disaster Zones: Physical and Social Factors Reviewed
Araki K., Ota Y., Maeda A., Kumano M., Nishioka K.
Energies 16 ( 8 ) 2023.4
Publishing type：Research paper (scientific journal) Publisher：Energies
Electric vehicles (EVs) have the advantage of being resilient to natural disasters. However, users hesitate to donate electricity when they lose the chance to recharge at the utility. Solar electric vehicles (SEVs) save energy through vehicle-integrated photovoltaics (VIPV) and make it possible to voluntarily donate excess energy, thus maintaining facility resilience. Given that the supply of solar energy to VIPV systems is not continuous and is difficult to forecast, the contribution of VIPV to the resilience of the larger energy system has been called into question. This is the first study in which the potential of VIPV to maintain utility resilience is investigated in the context of physical factors, such as irradiance, and social factors. The actual energy yield of a VIPV car was determined using an advanced 3D solar irradiation model under a nonuniform shading distribution, with validation from actual measures of solar irradiance on five orthogonal sides of the car body. The Monte Carlo method was used to model the complex factors in VIPV energy storage and energy donations under different scenarios. Depending on the climate, population density, and shading environment, the voluntary contribution of stored electricity in SEV is sufficient to provide disaster relief support.
DOI： 10.3390/en16083580
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Yajima D., Toyoda T., Kirimura M., Araki K., Ota Y., Nishioka K.
Energies 16 ( 7 ) 3261 2023.4
Publishing type：Research paper (scientific journal) Publisher：Energies
Climate change and increasing food demand are global issues that require immediate attention. The agrivoltaic system, which involves installing solar panels above farmland, can simultaneously solve climate and food issues. However, current systems tend to reduce agricultural production and delay the harvest period due to shading by the solar panels. A delayed harvest period impacts the income of farmers who wish to sell produce at specific times. Incorporating a model that calculates the amount of electricity generated by solar irradiation, this study establishes a model to estimate the correct start date of cultivation for solar panel covered crops to ensure the correct harvest date and determines the expected income of farmers by calculating agricultural production and power generation. Using taro cultivation in Miyazaki Prefecture as a case study, the model estimated that the start date of cultivation should be brought forward by 23 days to ensure the ideal harvest period and agricultural production. This would prevent an opportunity loss of USD 16,000 per year for a farm area of 10,000 m2. Furthermore, an additional income of USD 142,000 per year can be expected by adjusting shading rates for the cultivation and non-cultivation periods.
DOI： 10.3390/en16073261
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Analysis for the Potential of High-Efficiency and Low-Cost Vehicle-Integrated Photovoltaics Reviewed
Yamaguchi M., Nakamura K., Ozaki R., Kojima N., Ohshita Y., Masuda T., Okumura K., Satou A., Nakado T., Yamada K., Tanimoto T., Zushi Y., Takamoto T., Araki K., Ota Y., Nishioka K.
Solar RRL 7 ( 8 ) 2023.4
Publishing type：Research paper (scientific journal) Publisher：Solar RRL
The spread of the photovoltaic-powered electric vehicle (PV-EV) is desirable and is essential for reduction in CO2 emissions, increasing electric range, and creating a new clean energy infrastructure based on PVs. Development of highly efficient PV modules with reasonable cost is necessary to realize a longer PV-driving range of passenger cars. Herein, the potential of various solar cell modules for vehicle-integrated photovoltaic (VIPV) applications is analyzed. This article shows that the use of highly efficient solar cell modules with an efficiency of higher than 35% enables longer than 30 km day−1 PV driving under average irradiance of 4 kWh m−2day−1 without external charging. Cost reduction of VIPV modules is also very important for spreading the PV-powered vehicles. By considering electricity charging cost saving and reduction in CO2 emission of electric vehicles, the cost target of VIPV is discussed. The cost targets of the solar cell modules for the PV-EV by considering only electricity charging cost saving, $2.6/Wp for a module with conversion efficiency of 20% and $3.7/Wp for a module with that of 40%, are estimated in the case of electric mileage of 10 km kWh−1.
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Yajima D., Toyoda T., Kirimura M., Araki K., Ota Y., Nishioka K.
Cleaner Engineering and Technology 12 2023.2
Publishing type：Research paper (scientific journal) Publisher：Cleaner Engineering and Technology
Climate change and increasing food production due to population growth are global challenges that need immediate attention. The introduction of renewable energy to mitigate climate change and the requirement of adequate land to increase food production are generally mutually exclusive. However, an agrivoltaic system generates renewable electricity and produces agricultural products from a common piece of land, thus increasing the land productivity. In addition, this system contributes to local production, thus reducing the CO2 emissions from logistics. Photovoltaic arrays in previous studies were designed by calculating the irradiance in W/m2, even in recent studies. A careful design of the farmland's illumination must be developed for effective agriculture. The simulations must be scaled based on photosynthetic photon flux density rather than irradiance commonly applied in photovoltaic technology simulations. This study focused on the photosynthetic photon flux density and employed an all-climate solar spectrum model to calculate the photosynthetic photon flux density accurately on farmland partially shaded by solar panels and supporting tubes. This study described an algorithm for estimating the photosynthetic photon flux density values under solar panels. The calculated data were validated using the photosynthetic photon flux density sensors. To calculate the photosynthetic photon flux density under the solar panels, it is essential to weigh the direct and diffused components shaded by the solar panels separately because they have different spectrums. A method to quantify the shading was explored here by solar panels and their supporting tubes for the direct and diffused component as the sun moves. The calculation formula was established by defining the sun's moves and the positions of solar panels and their supporting tubes in terms of elevation and azimuth angles from the observation point. It was found that the waveform based on the calculation formula for the photosynthetic photon flux density under the solar panels reproduced the same tendency as the measured photosynthetic photon flux density. To evaluate this trend numerically, the measured and calculated photosynthetic photon flux densities were compared using the standard residuals. Generally, the similarity of the two values is confirmed by a standard residual value between −3 and 3. The result of this study showed that the standard residual values were negative in more frequencies except for the zero photosynthetic photon flux density at night. This indicates that the calculated photosynthetic photon flux density tends to be higher than the measured photosynthetic photon flux density. The peak frequency of the standard residuals was between −6 and −3. This difference probably occurred because the established calculation formula targets the shading provided by the solar panels and supporting tubes but does not cover the shading provided by the other system structures. The calculation formula enables farmers to evaluate the economic efficiency of the system before introducing it using measured solar irradiation data at the target farmlands by introducing published neighborhood solar irradiation data and considering, in advance, measures to avoid the effects of shading on agricultural production. The next study will be to improve the accuracy of the calculation formula by increasing the number of days and develop a method that leads to the best practices of agricultural production and solar power generation by introducing the system.
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CPV standardization 2021 - Maintenance and stability Reviewed
Araki K., Algora C., Kresse D., Siefer G., Timò G., Antón I., Nishioka K., Leutz R., Carter S., Wang S., Askins S., Iwasaki T., Ji L., Kelly G.
AIP Conference Proceedings 2550 2022.9
Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
The rapid and substantial decline of the cell-cost, CPV, relied on cell-saving and became less valuable. Many players and systems got off the stage as a result. Also, less and fewer new proposals in the CPV standardization. We decided to continue the standardization activity but separate the tracker's work by creating a new working group, WG9. The technical discussion on VIPV has also been discussed in WG7, and it is to be transferred to a new project team. Besides the existing standards' maintenance work, new proposals of the new CPV standards were posted and approved recently. One is the fire test of the CPV module, and another is a series of standards of the hybrid CPV/PV.
DOI： 10.1063/5.0099174
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The proposal of a new standard for the measurement procedures of CPV/PV hybrid modules Reviewed
Timò G., Minuto A., Araki K., Siefer G., Askins S., Antón I., Algora C.
AIP Conference Proceedings 2550 2022.9
Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Hybrid CPV/PV modules include CPV solar cells designed to collect concentrated light and PV solar cells designed to collect diffuse or global light. The power rating, as well as the module energy rating for such PV modules are complicated by the fact that, in some cases, the power is a strong and discontinuous function of angle of incidence (AOI). For such a reason, the IEC 61853 and IEC 62670 series do not sufficiently cover the specific requirements and procedures for evaluating the Hybrid CPV/PV performances. The standard conditions for assessing the power produced by the hybrid CPV/PV modules have also to be differentiated from those used to characterize flat PV or CPV modules. With this contribution, the motivations behind the new work item proposal on "Hybrid CPV/PV modules: General characteristics and measurement procedures"are therefore presented.
DOI： 10.1063/5.0099713
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Optimization of asymmetrical static-LCPV dependent on applications Reviewed
Araki K., Yamaguchi M., Ota Y., Nishioka K.
AIP Conference Proceedings 2550 2022.9
Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
The optimized design of the static-CPV was simplified by the reduction of variables considering geometrical symmetry. The previous works assumed 4-folded symmetry (grid arrangement) for horizontal installation (car-roof) where accumulated solar irradiance is axially symmetrical. Optimizing the lens shape under asymmetrical irradiance distribution may lead to a different story. Moreover, the lens profile optimization depends on the angular distribution of rays and varies by the mode of installations. In this study, the design method of the static LCPV was expanded to the arbitrary angular distribution of the solar ray so that the application of the static LCPV was searched for new applications. At first angular distributions of the annual solar beam were calculated using the METPV-11 database and its 840 sites, including VIPV application, BIPV (vertical) weighted average in 8 orientations, and horizontal single-axis west-east tracking. The lens profile in each condition was optimized, allowing geometrical asymmetry. Despite the optimization, the annual optical efficiency decreased by the concentration ratio. Its trend was almost the same among different modes of installations. The annual efficiency was further dropped by the more expansive and off-centered solar beam distribution. However, the difference was relatively small after optimization according to each distribution pattern of the solar beam, despite the substantial variation of the lens profiles. The relationship "the more concentration, the less energy yield"was unchanged among installations. The peak annual yield depending on the concentration ratios was almost the same among different modes of installations. Thus, the optimum concentration ratios were nearly the same among installations. More concentration saves the cell cost, but this advantage may be seen in the system using expensive solar cells (more than 70% of the system cost), regardless of the mode of installations.
DOI： 10.1063/5.0099173
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Uno T., Ota Y., Kanayama S., Araki K., Nishioka K.
AIP Conference Proceedings 2550 2022.9
Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
We developed a plastic-integrated concentrator photovoltaic (PIC) module and evaluated the long-term reliability of the PIC module with two concentrator solar cells; one is a conventional Ge-bottom subcell triple-junction solar cell, and the other is a 1-eV-bottom subcell triple-junction solar cell under outdoor operation. The daily energy yield of the PIC module with Ge-bottom subcell slightly decreased in the summer season. Moreover, the 1-eV-bottom subcell triple-junction solar cell is expected to be more affected by the solar spectrum change due to the increased precipitable water vapor in the summer season. The impact of spectral change on the output performance of the PIC module is continuously discussed by actual measurement data.
DOI： 10.1063/5.0099179
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Development of High-Efficiency Solar Cell Modules for Photovoltaic-Powered Vehicles Reviewed
Yamaguchi M., Ozaki R., Nakamura K., Lee K.H., Kojima N., Ohshita Y., Masuda T., Okumura K., Satou A., Nakado T., Yamada K., Araki K., Ota Y., Nishioka K., Takamoto T., Zushi Y., Tanimoto T., Thiel C., Tsakalidis A., Jäger-Waldau A.
Solar RRL 6 ( 5 ) 2022.5
Publishing type：Research paper (scientific journal) Publisher：Solar RRL
Photovoltaic (PV)-powered vehicles are expected to play a critical role in a future carbon neutral society because it has been reported that the onboard PVs have great ability to reduce CO2 emission from the transport sector. Although the demonstration car with a III−V-based solar cell module has shown the PV-powered driving range of 36.6 km day−1 at solar irradiance of 6.2 kWh m−2 day−1, practical driving ranges of PV-powered vehicles are shown to be lower than estimated values due to some losses such as nonradiative recombination and resistance losses of solar cell modules under sunshine condition. This article presents analytical results for the effects of illumination intensity properties of various solar cell modules on the PV-powered driving range to develop highly efficient solar cell modules for vehicle-integrated applications. The analysis shows that improvements in shunt resistance and saturation current density of solar cell modules are necessary to improve illumination intensity properties of solar cell modules under low intensity sunshine condition. The calculations show that the III−V-based 3-junction solar cell modules with an efficiency of more than 30% have a potential PV-powered driving range of 30 km/day average and more than 50 km day−1 on a clear day.
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Impact and recent approaches of high-efficiency solar cell modules for PV-powered vehicles Reviewed
Yamaguchi M., Masuda T., Araki K., Ota Y., Nishioka K.
Japanese Journal of Applied Physics 61 ( SC ) 2022.5
Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
Photovoltaic (PV)-powered vehicles are expected to play a critical role in a future carbon neutrality society because it has been reported that vehicle integrated PVs (VIPVs) have great ability to reduce CO2 emission from the transport sector. The development of high-efficiency solar cell modules is very important for this end. In this paper, analytical results for the impact of high-efficiency solar cell modules on increases in electric vehicle (EV) driving distance, reducing CO2 emission and saving charging cost of EV powered by PV are shown. The III-V triple-junction and Si tandem solar cell modules with an efficiency of more than 35% have the potential of the EV driving distance of more than 30 km d-1 on average and more than 50 km d-1 on a clear day. The effects of temperature, partial shading, curved surface, and color variation of solar cell modules upon output power of the VIPV are also briefly shown.
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Ota Y., Araki K., Nagaoka A., Nishioka K.
Cleaner Engineering and Technology 7 2022.4
Publishing type：Research paper (scientific journal) Publisher：Cleaner Engineering and Technology
By mounting photovoltaics (PV) on the roof of an electric vehicle (EV), solar energy can be used to supply a considerable portion of the energy demand of the EV. The roof is the best place for PV installation on the vehicle body because doors and engine hood have less yearly-average sunlight available and more stringent mechanical requirements. The roof of a modern passenger vehicle such as a sedan is not flat, and parts with high curvature or a significant slope do not need to be covered. However, there are no design rules available for mounting PV on a vehicle roof, such as determining the coverage ratio. In this study, the distributions of roof shapes and sizes were obtained from trace drawings of various commercially available passenger vehicles. This was then used to calculate the distributions of the mechanical (i.e., local curvature) and optical (i.e., local solar utilization rate) properties. Based on the results, general guidelines were developed, including a potential coverage ratio of 96% for a hemispherical roof with a radius of curvature of 1 m. These guidelines have already been successfully demonstrated and prototyped with a small module.
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Impact of climatic conditions on prospects for integrated photovoltaics in electric vehicles Reviewed
Thiel C., Gracia Amillo A., Tansini A., Tsakalidis A., Fontaras G., Dunlop E., Taylor N., Jäger-Waldau A., Araki K., Nishioka K., Ota Y., Yamaguchi M.
Renewable and Sustainable Energy Reviews 158 2022.4
Publishing type：Research paper (scientific journal) Publisher：Renewable and Sustainable Energy Reviews
Integrated photovoltaics are an emerging technology which can extend the range of electric vehicles. However, up to now there is a lack of a consensus method that would provide consumers with an estimate of the fraction of annual driving which could be covered by solar power generated onboard in different usage scenarios and locations. To address this, we assess the energy implications of vehicle integrated photovoltaics for a commuter car and light delivery van for six climatic regions and for typical daily usage profiles over a ten-year period. The analysis captures the energy needs for driving and cabin temperature control to an unprecedented level of detail. Our results reveal that the grid power needed to drive such vehicles on identical routes can vary by more than 44% between climate regions. In the best case the solar power generated can cover up to 35% of the driving range per year. This contribution can vary by a factor of 2.5 between different climates, from 1800 to 5100 km annually, considerably mitigating the effect of ambient conditions on electric range. We propose developing consumer labels for solar electric vehicles that would consider this variation according to climatic conditions. The method described in this paper could help to frame initial discussions for such labels. We identify further requirements for research and development, standardisation, and policy needs.
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Curve correction of vehicle-integrated photovoltaics using statistics on commercial car bodies Reviewed
Ota Y., Araki K., Nagaoka A., Nishioka K.
Progress in Photovoltaics: Research and Applications 30 ( 2 ) 152 - 163 2022.2
Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
Typically, car roofs are curved, which means that vehicle-integrated photovoltaics (VIPVs) are also curved along the roof surface. The performance of the PV module is influenced by the local cosine loss and self-shadowing loss due to the curved surface. The ratio between the solar irradiances of curved and flat surfaces is defined as the curve correction factor, and it widely varies with the shape of the curved surface. When the curve correction factor is less than unity, the PV on a curved car roof performs worse than the PV on a flat plate. Understanding the typical range of the curve correction factor is important for the estimation of the energy generated by VIPV. We investigated the curved shapes of 100 lines of cars and 200 cases. The curved shapes were then used to extract eight nondimensional geometric parameters, and the distributions and correlations among the parameters were investigated. The parent population of the curved surfaces was estimated via a Monte Carlo simulation based on an analysis of the statistical characteristics. The distribution of curve correction factors for the car population was calculated via differential geometry weighted by the distribution of incident angles of sunlight (direct and diffused) affected by shading along the streets, which was obtained from 1 year of driving data for Miyazaki City, Japan. The curve correction factors were highly skewed, but the average value was 0.92. This means that VIPV requires a 10% boost in performance to compensate for the inherent loss due to the geometry of curved roofs.
DOI： 10.1002/pip.3473
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Maeda M., Nagaoka A., Araki K., Nishioka K.
Solar Energy 231 243 - 251 2022.1
Publishing type：Research paper (scientific journal) Publisher：Solar Energy
A precise and practical power generation forecasting method for outdoor photovoltaic (PV) arrays and its performance evaluation are essential for advancements in solar power generation. Specifically, accurate temperature measurements of each of the PV modules in an array, the entire outdoor PV array, and the power generation system are crucial regardless of climatic and irradiance conditions. However, outdoor PV temperatures are often affected by environmental factors, such as wind direction, wind speed, solar irradiance, and outdoor air temperature fluctuations. Hence, the temperature data vary greatly depending on the measurement point of the module, which is affected by the above environmental factors. The variations of and uncertainties in temperature often extend over long arrays of PV modules common to large-scale PV systems. Therefore, it is necessary to establish a straightforward and reproducible inspection method to determine a single and robust representative PV module temperature for an array. In this study, we have established one such inspection method for outdoor PV arrays. It was observed that the temperature at the center of the array was closest to the average temperature regardless of the measurement period and climatic conditions; thus, it can be considered as the unbiased and robust representative temperature for the PV array. Moreover, lower irradiance conditions are better for accurate and repeatable PV module temperature measurements. The mean temperature differences of the PV array from its average measured at the center of the array were 0.2 °C for low irradiance and 0.5 °C for high irradiance. The temperature difference observed will have 0.1% to 0.2% impact on the power estimation for monitoring purposes.
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Yamaguchi M., Masuda T., Araki K., Ota Y., Nishioka K., Takamoto T., Thiel C., Tsakalidis A., Jaeger-Waldau A., Okumura K., Satou A., Nakado T., Yamada K., Zushi Y., Tanimoto T., Nakamura K., Ozaki R., Kojima N., Ohshita Y.
Journal of Physics D: Applied Physics 54 ( 50 ) 2021.12
Publishing type：Research paper (scientific journal) Publisher：Journal of Physics D: Applied Physics
Development of vehicles that are powered by photovoltaics (PV) is desirable, and is crucial for reduction in CO2 emissions from the transport sector to realize a decarbonized society. Our investigations show that the majority of the passenger cars that cruise only with solar energy can be realized by installing a high-efficiency PV module. Although the Toyota Prius demonstration car, which is equipped with a 860 W rated-output power PV module, has shown a 36.6 km d-1 PV-powered driving range at solar irradiance of 6.2 kWh m-2 d-1, practical driving ranges of PV-powered vehicles are shown to be shorter than the estimated values due to some losses of solar cell modules, such as temperature rise under sunny conditions. In this paper, we conduct a systematic analysis of the effects of these losses on the PV-powered driving range in order to obtain guidelines for the development of highly efficient solar cell modules for vehicle integrated applications. The analytical results show that the III-V compound solar cell modules have more suitable properties compared to other cells because of their higher potential conversion efficiencies of 37% with a smaller temperature coefficient of -0.19% C-1 compared to -0.29% C-1 for Si back contact solar cell modules and -0.26% C-1 for Si heterojunction solar cell modules. Our theoretical calculations that take these losses into account suggest that installing the III-V-based triple-junction solar cell modules provides a potential PV-powered driving range of 30 km d-1 on average, and more than 50 km d-1 on a sunny day under the irradiation conditions in Japan.
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Sato D., Masuda T., Araki K., Yamaguchi M., Okumura K., Sato A., Tomizawa R., Yamada N.
Communications Materials 2 ( 1 ) 2021.12
Publishing type：Research paper (scientific journal) Publisher：Communications Materials
Stretchable photovoltaics are emerging power sources for collapsible electronics, biomedical devices, and buildings and vehicles with curved surfaces. Development of stretchable photovoltaics are crucial to achieve rapid growth of the future photovoltaic market. However, owing to their rigidity, existing thin-film solar cells based predominantly on silicon, compound semiconductors, and perovskites are difficult to apply to 3D curved surfaces, which are potential real-world candidates. Herein, we present a stretchable micro-scale concentrator photovoltaic module with a geometrical concentration ratio of 3.5×. When perfectly fitted on a 3D curved surface with a sharp curvature, the prototype module achieves an outdoor power conversion efficiency of 15.4% and the daily generated electricity yield improves to a maximum of 190% relative to a non-concentration stretchable photovoltaic module. Thus, this module design enables high areal coverage on 3D curved surfaces, while generating a higher electricity yield in a limited installation area.
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Fabrication of 3D surface PV module with 1m radius of curvature using monocrystalline Si cells Reviewed
Aoki Yuma, Araki Kenji, Tanaka Makoto, Yamada Noboru
Proceedings of the Annual Meeting of the Japan Photovoltaic Society 1 ( 0 ) 115 - 115 2021.10
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Photovoltaic Society
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Evaluating the Output of a Car-Mounted Photovoltaic Module under Driving Conditions Reviewed
Ota Y., Araki K., Nagaoka A., Nishioka K.
IEEE Journal of Photovoltaics 11 ( 5 ) 1299 - 1304 2021.9
Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
In this article, we measured and estimated the outputs of car-mounted PV panels under actual driving conditions and different effective shading angles. On a sunny day, the performance ratio (PR) decreased to 0.99, despite the decrease in module temperature caused by the increased wind speed when driving. The PR decreased because of the effect of partial shading over the PV module. Public infrastructure could have led to dynamic partial shading passing across the PV module at high speeds. Furthermore, the effective shading angle being close to the sun's altitude could also have led to static partial shading. In contrast, on an overcast day the PR value was almost the same ($>$1) under both driving and parking conditions; this was because of the low module temperature and the spectrum effect.
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Nagaoka A., Ota Y., Sakai K., Araki K., Matsuo H., Kadota N., Maeda K., Nakajima A., Nishioka K.
Renewable Energy 174 147 - 156 2021.8
Publishing type：Research paper (scientific journal) Publisher：Renewable Energy
The concept of zero-energy building (ZEB) has attracted global attention in recent years as it involves offsetting the primary energy usage of a building on an annual base by the energy generated from renewable resources. The performance of wall-mounted photovoltaics (PV) for building applications has great potential for ZEB. In this study, we compared wall-mounted and common roof-mounted PV systems using a combination of experimental and theoretical studies. The wall-mounted PV system exhibited an unusual performance behavior depending on the season. Winter is a suitable season for energy production from the wall-mounted PV system because of the low solar altitude and resulting lower incident angle of the sun. However, the performance ratio (PR) had twin peaks throughout the day that could not be explained by the classical model that uses irradiance and temperature variation. In summer, there was less irradiance on the vertical wall and the PR had triple peaks that were also hard to explain using the conventional model. We established a detailed output power model of the wall-mounted PV using spectrum variation on a vertical plane. Our approach is the development of spectrum model that can be applied to all-climate and computed the performance variation according to the sun's orientation and incident angle. Our model quantitatively explains unique seasonal and daily efficiency variations for the wall-mounted PV system (twin peaks in winter and triple peaks in summer). Our validated model can be utilized to map a suitable location for a wall-mounted PV system.
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Potential of Si Tandem Solar Cell Modules for PV-Powered Vehicles Reviewed
Yamaguchi M., Araki K., Ota Y., Nishioka K., Takamoto T., Masuda T., Okumura K., Satou A., Nakado T., Yamada K., Zushi Y., Tanimoto T., Nakamura K., Ozaki R., Kojima N., Ohshita Y.
Conference Record of the IEEE Photovoltaic Specialists Conference 120 - 122 2021.6
Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
Development of high-efficiency solar cell modules and new application fields such as PV (Photovoltaics)-powered vehicles are significant for the further development of PV and the creation of new clean energy infrastructure based on PV. In this paper, analytical results for impact of high-efficiency solar cell modules on increasing driving distance, reducing CO2 emission and saving charging cost of electric vehicles by PV-powered vehicles. Because the Si tandem solar cells are expected to have significant potential for PV-powered vehicle applications, potentials of high-efficiency and driving distance are also analyzed. The III-V/Si 3-junction solar cell modules have potential of driving distance of 30 km/day average and more than 50 km/day on a clear day.
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Yamaguchi M., Tampo H., Shibata H., Lee K.H., Araki K., Kojima N., Ohshita Y.
Japanese Journal of Applied Physics 60 ( SB ) 2021.5
Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
The efficiency potential of chalcopyrite and kesterite solar cells including CIGSe (CuInGaSe2), CIGS (CuInGaS2), CZTS (Cu2ZnSnS4) and CZTSSe [Cu2ZnSn(S,Se)4] solar cells is discussed based on external radiative efficiency (ERE), open-circuit voltage loss, fill factor loss, non-radiative recombination and resistance loss. CIGSe cells achieve efficiency potential of 26.8% and 27.5% by improving the ERE from around 1% to 10% and 20%, respectively. CIGS and CZTS(Se) cells achieve the efficiency potential of 25% and 22%, respectively, by improvement in ERE from around 1 × 10−4% to 3%–5%. The effects of non-radiative recombination and resistance loss upon the properties of wide-bandgap CIGSe, CIGS and CZTS(Se) cells are discussed. In the case of wide-bandgap CIGSe cells, lattice mismatching between the buffer layer and CIGSe active layer and deep-level defects are thought to originate from non-radiative recombination loss. CIGS and CZTS(Se) cells are shown to have lower ERE and higher resistance loss compared to that of CIGSe cells.
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Overview of Concentrator Solar cells and Analysis for their Non-radiative Recombination Reviewed
Yamaguchi M., Lee K.H., Sato D., Araki K., Kojima N., Ohshita Y.
Proceedings - ISES Solar World Congress 2021 92 - 100 2021
Publishing type：Research paper (scientific journal) Publisher：Proceedings - ISES Solar World Congress 2021
Concentration photovoltaics have great potential of higher efficiency and lower cost compared to conventional crystalline Si and thin-film solar cells. Although excellent results for concentrator solar cells such as 27.6%, 30.5%, 44.4% and 47.1% with Si, GaAs, InGaP/GaAs/InGaAs 3-junction and AlGaInP/AlGaAs/GaAs/GaInAs/GaInAs/GaInAs 6-junction concentrator cells have been demonstrated, there are still problems to be solved. This paper overviews concentrator solar cells. In addition, this paper presents analytical results for efficiency potential of concentrator solar cells based on analysis of non-radiative recombination loss in concentrator solar cells. Concentrator Si, GaAs, CIGS and InGaAs/InGaAs 3-junction solar cells have efficiency potential of more than 34%, 36% 31% and 50%, respectively, by realizing external radiative efficiency of 20% and reducing series resistance. This paper also presents our recent approaches for photovoltaic-powered vehicle applications and static low concentrator InGaP/GaAs/InGaAs 3-junction solar cell module with efficiency of 32.84%, and so forth.
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Impact of Photovoltaics-powered Vehicles Reviewed
Yamaguchi M., Masuda T., Araki K.
Proceedings - ISES Solar World Congress 2021 176 - 182 2021
Publishing type：Research paper (scientific journal) Publisher：Proceedings - ISES Solar World Congress 2021
Development of high-efficiency solar cell modules and new application fields such as PV (Photovoltaics)powered vehicles are significant for the further development of PV and the creation of new clean energy infrastructure based on PV. In this paper, analytical results for impact of high-efficiency solar cell modules on increasing driving distance, reducing CO2 emission and saving charging cost of electric vehicles by PV-powered vehicles are presented. Because the Si tandem solar cells are expected to have significant potential for PV-powered vehicle applications, potentials of high-efficiency of Si tandem solar cells and driving distance of vehicles installed with Si tandem solar cells are also analyzed. The III-V/Si 3-junction solar cell modules with more than 35% have potential of driving distance of more than 30 km/day average and more than 50 km/day on a clear day.
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Sato D., Araki K., Takamoto T., Juso H., Yamada N., Masuda T., Yamaguchi M.
Applied Physics Express 13 ( 7 ) 2020.7
Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics Express
© 2020 The Japan Society of Applied Physics. Two-types of nearly 30%-efficient (27.4%-32.8%) low-concentration static photovoltaic modules with inverted metamorphic triple-junction (IMM-3J) solar cells were designed. The designed concentrator optics with a geometrical concentration ratio of 1.76× have different optical characteristics. The outdoor testing results confirm that daily conversion efficiency of the modules is almost stable between clear-sky and partly cloudy days, which suggests that the modules can utilize both direct and diffuse components of sunlight. Furthermore, the modules generate 30%-40% larger daily electricity compared to a flat Si module and maintain 70%-80% of the electricity of a flat module with the IMM-3J cells, while reducing cell usage.
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Overview and Loss Analysis of High-Efficincy III-V Compound Single-Junction Solar Cells
Yamaguchi M., Araki K., Kojima N., Ohshita Y.
Conference Record of the IEEE Photovoltaic Specialists Conference 2020-June 0149 - 0151 2020.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
The III-V compound solar cells have contributed as space and concentrator solar cells and are important as sub-cells for multi-junction solar cells. This paper reviews progress in III-V compound single-junction solar cells such as GaAs, InP, AlGaAs and InGaP cells. In addition, analytical results for non-radiative recombination and resistance losses in III-V compound solar cells are shown for further understanding and reducing major losses in III-V compound materials and solar cells.
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Importance of Developing High-Efficiency Solar Cells for PV-Powered Vehicles
Yamaguchi M., Araki K., Kojima N., Takamoto T., Masuda T., Satou A., Yamada K., Yamazaki M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2020-June 0221 - 0223 2020.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
Development of PV-powered vehicle applications is desirable and very important for creation of clean energy society. This paper presents PV efficiency impact on reduction in CO2 emission and increase in driving distance. This paper shows analytical results for efficiency potential of various solar cells for choosing candidates of high-efficiency solar cell modules for automobile applications. As a result of analysis, static low concentrator PV and III-V/Si tandem solar cells are thought to be some of their candidates.
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Tawa H., Saiki H., Ota Y., Araki K., Takamoto T., Nishioka K.
Applied Sciences (Switzerland) 10 ( 2 ) 703 2020.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)
© 2020 by the authors. Because semiconductors absorb wavelengths dependent on the light absorption coefficient, photovoltaic (PV) energy output is affected by the solar spectrum. Therefore, it is necessary to consider the solar spectrum for highly accurate PV output estimation. Bird's model has been used as a general spectral model. However, atmospheric parameters such as aerosol optical depth and precipitable water have a constant value in the model that only applies to clear days. In this study, atmospheric parameters were extracted using the Bird's spectrum model from the measured global spectrum and the seasonal fluctuation of atmospheric parameters was examined. We propose an overcast spectrum model and calculate the all-weather solar spectrum from clear to overcast sky through linear combination. Three types of PV modules (fixed Si, two-axis tracking Si, and fixed InGaP/GaAs/InGaAs triple-junction solar cells) were installed at the University of Miyazaki. The estimated performance ratio (PR), which takes into account incident angle and spectral variations, was consistent with the measured PR. Finally, the energy yield of various PVs installed across Japan was successfully estimated.
DOI： 10.3390/app10020703
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Hara T., Liang J., Araki K., Kamioka T., Sodabanlu H., Watanabe K., Sugiyama M., Shigekawa N.
ECS Transactions 98 ( 4 ) 125 - 133 2020
Language：English Publishing type：Research paper (scientific journal) Publisher：ECS Transactions
© The Electrochemical Society We investigate nanostructural properties of GaAs//indium tin oxide (ITO)/Si junctions fabricated by surface-activated bonding with emphasis on impacts of thermal process. Both of the Ga 2p3/2 and As 2p3/2 core-level spectra obtained by hard X-ray photoemission spectroscopy show that the GaAs layers are oxidized by annealing at 400 ℃. This finding is consistent with the formation of amorphous-like layers at 400 ℃ annealed GaAs//ITO interfaces. Concentration depth profiles of O, Ga, and As suggest that the oxidation markedly occurs at GaAs//ITO interfaces annealed at temperatures higher than 200 ℃, which is consistent with the dependence of resistance in GaAs//ITO/Si junctions on annealing temperature. These results suggest that annealing brings about the reaction between GaAs and ITO layers and causes the degradation of the electrical properties of GaAs//ITO interfaces. Low-temperature process technologies are essential so as to make a full use of ITO as intermediate layers in III-V-on-Si multijunction cells.
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Analysis for nonradiative recombination loss and radiation degradation of Si space solar cells Reviewed
Yamaguchi M., Lee K.H., Araki K., Kojima N., Okuno Y., Imaizumi M.
Progress in Photovoltaics: Research and Applications 29 ( 1 ) 98 - 108 2020
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
© 2020 John Wiley & Sons, Ltd. Silicon space solar cells are currently attracting attention again for their relatively low-cost feature with sufficient performance, and they are expected to resume into the space market especially by short-term mission spacecraft designers. In this paper, efficiency potential of crystalline Si space solar cells is analyzed by considering external radiative efficiency (ERE), voltage and fill factor losses. Crystalline Si space solar cells have efficiency potential of more than 26% by realizing ERE of 20% from about 0.2% and normalized resistance of less than 0.05 from around 0.15. Nonradiative recombination and resistance losses in Si space solar cells are also discussed. Radiation degradation of Si space solar cells is also analyzed. Advanced Si solar cells such as passivated emitter, hetero-junction, and back contact solar cells are expected to use as space solar cells. Potential of advanced Si solar cells for space applications is discussed from point view of radiation degradation.
DOI： 10.1002/pip.3346
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Development of high-efficiency and low-cost solar cells for PV-powered vehicles application Reviewed
Yamaguchi M., Masuda T., Araki K., Sato D., Lee K.H., Kojima N., Takamoto T., Okumura K., Satou A., Yamada K., Nakado T., Zushi Y., Ohshita Y., Yamazaki M.
Progress in Photovoltaics: Research and Applications 29 ( 7 ) 684 - 693 2020
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
© 2020 John Wiley & Sons, Ltd. Development of high-efficiency solar cell modules and new application fields are significant for the further development of photovoltaics (PVs) and the creation of new clean energy infrastructure based on PV. Notably, the development of PV-powered vehicle applications is desirable and very important for this end. According to the NEDO's Interim Report “PV-Powered Vehicle Strategy Committee,” a new broader PV markets with more than 10 GW and 50 GW in 2030 and 2040, respectively, are expected to be established when PV-powered vehicles are developed. Cumulative PV capacity for PV-powered vehicles will be 50 GW and 0.4 TW in 2030 and 2040, respectively. This paper presents impacts on efficiency and cost for PV-powered vehicles. According to our survey, the use of more than 30% of high-efficiency PV enables 30 km per day driving without external charging and the society that the majority of the family cars run by the sunlight and without supplying gas. Thus, we are developing high-efficiency and low-cost solar cells and modules for automobile applications. In this paper, our analytical results for the efficiency potential of various solar cells for choosing candidates of high-efficiency solar cell modules for automobile applications. This paper also presents our recent approaches: demonstration car (Toyota Prius PHV) by using Sharp's high-efficiency III-V triple-junction solar cell modules with an output power of 860 W, static low concentrator InGaP/GaAs/InGaAs triple-junction solar cell module with efficiency of 32.84%, and so forth.
DOI： 10.1002/pip.3343
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Overview of Si Tandem Solar Cells and Approaches to PV-Powered Vehicle Applications Reviewed
Yamaguchi M., Lee K.H., Sato D., Araki K., Kojima N., Takamoto T., Masuda T., Satou A.
MRS Advances 5 ( 8-9 ) 441 - 450 2020
Language：English Publishing type：Research paper (scientific journal) Publisher：MRS Advances
Development of high-efficiency solar cell modules and new application fields are significant for the further development of photovoltaics (PV) and creation of new clean energy infrastructure based on PV. Especially, development of PV-powered EV applications is desirable and very important for this end. This paper shows analytical results for efficiency potential of various solar cells for choosing candidates of high-efficiency solar cell modules for automobile applications. As a result of analysis, Si tandem solar cells are thought to be some of their candidates. This paper also overviews efficiency potential and recent activities of various Si tandem solar cells such as III-V/Si, II-VI/Si, chalcopyrite/Si, perovskite/Si and nanowire/Si tandem solar cells. The III-V/Si tandem solar cells are expected to have a high potential for various applications because of high efficiency with efficiencies of more than 36% for 2-junction and 42 % for 3-junction tandem solar cells under 1-sun AM1.5 G, lightweight and low-cost potential. Recent results for our 28.2 % efficiency and Sharp's 33% mechanically stacked InGaP/GaAs/Si 3-junction solar cell are also presented. Approaches to automobile application by using III-V/Si tandem solar cells and static low concentration are presented.
DOI： 10.1557/adv.2020.66
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Analysis of nonradiative recombination in quantum dot solar cells and materials Reviewed
Zhu L., Lee K.H., Yamaguchi M., Akiyama H., Kanemitsu Y., Araki K., Kojima N.
Progress in Photovoltaics: Research and Applications 27 ( 11 ) 971 - 977 2019.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
© 2019 John Wiley & Sons, Ltd. Quantum dot (QD) solar cells have drawn much attention in research because of their tunable band gap and potential to realize many novel concepts, such as intermediate transitions. However, high nonradiative recombination rates in the QD layer stand in the way of realizing high-efficiency QD solar cells. In this paper, the efficiency potential of QD solar cells is discussed based on external radiative efficiency (ERE), open-circuit voltage loss, fill factor loss, and nonradiative recombination loss via current-voltage characteristics in a detailed balance model. The intrinsic loss of QD solar cells substantially increases with increasing binding energy and volume density of the QDs. The ERE of QD solar cells decreases with increasing binding energy and volume density of the QDs due to nonradiative recombination.
DOI： 10.1002/pip.3110
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Impact of Nonplanar Panels on Photovoltaic Power Generation in the Case of Vehicles Reviewed
Tayagaki T., Araki K., Yamaguchi M., Sugaya T.
IEEE Journal of Photovoltaics 9 ( 6 ) 1721 - 1726 2019.11
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2011-2012 IEEE. We investigate the impact of nonplanar panels on power generation in the case of photovoltaic panels mounted on vehicles. The power generation of nonplanar photovoltaic panels on vehicles is analyzed using the numerical geometric model, comprising the incident angle distribution of irradiation. We calculated power generated by the photovoltaic panels equipped on the roof, side, and tilted surfaces of vehicles, showing that even the panels on the side, which are fitted vertically to the ground, generate more than a quarter of the total power as that generated by the panel horizontal to the ground. Furthermore, we evaluated the curve correction factor that reflects the relative power generation of nonplanar panels with respect to the reference flat panels on vehicles. The geometrical model reproduces a simple relationship between the curve correction factor and effective surface ratio between the nonplanar and reference flat panels. Our findings indicate that the curve correction factor is useful to estimate power generation for nonplanar photovoltaic devices on vehicles; this provides a guiding principle for designing nonplanar photovoltaic devices for vehicles.
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Demonstration of the performance static low-concentration module using hybrid lens arrays Reviewed
Lee K.H., Daisuke S., Araki K., Yamada N., Yamaguchi M.
AIP Conference Proceedings 2149 2019.8
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2019 American Institute of Physics Inc.. All rights reserved. We demonstrated a static (non-tracking) low concentration photovoltaic module that employs a new design paradigm that mixes two different types of the lens arrays. This design approach brings the annual optical collection efficiency closerto the theoretical upper hmitbutwith high manufacturabilityand scalabilhy, because this design paradigm gives more degree of freedom to optimize the overall efficiencies. We demonstrated that the module with the hybnd lens array is 2-%(absolute) more efficient than its single lens array counterparts at on-axis illumination. Also, the good match of the modeled and experimental incident-angle dependent module efficiency confirms the robustness of our fabrication process, showing the promise of achieving high annual collection efficiency of this module.
DOI： 10.1063/1.5124208
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Nuñez N., Vazquez M., Schilling R., Araki K., Algora C.
AIP Conference Proceedings 2149 2019.8
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2019 American Institute of Physics Inc.. All rights reserved. Qualification standards are one the driving forces for the commercialization and production of the Concentrator Photovoltaic (CPV) technology. After several years of preparation the Committee Draft of IEC 62787 has been finally submitted. The new standard IEC 62787 (Concentrator photovoltaic (CPV) solar cells and cell-on-carrier (CoC) assemblies - Qualification) fills the gap between the IEC TS 62789:2014 (Photovoltaic concentrator cell documentation) and the IEC 62108 Ed 2 (Concentrator photovoltaic (CPV) modules and assemblies - Design qualification and type approval). In the present article, apart from analyzing the objective, main guidelines, the innovative characteristics, and the life estimation from several tests of the qualification standard is also explained.
DOI： 10.1063/1.5124221
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Analysis for Radiation Degradation of Advanced Si Space Solar Cells
Yamaguchi M., Lee K.H., Araki K., Kojima N., Okuno Y., Imaizumi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2377 - 2380 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. Advanced Si solar cells such as passivated emitter, hetero-junction and back contact solar cells are expected to use as space solar cells. In this paper, efficiency potential of crystalline Si space solar cells is analyzed by considering external radiative efficiency (ERE), voltage and fill factor losses. Crystalline Si space solar cells have efficiency potential of more than 26% at AM0 by realizing ERE of 20% from about 0.2% and normalized resistance of less than 0.05 from around 0.15. Non-radiative recombination and resistance losses in Si space solar cells are also discussed. Radiation degradation of Si space solar cells is also analyzed. Potential of advanced Si solar cells such as passivated emitter, hetero-junction and back contact solar cells for space applications is discussed from point view of radiation degradation.
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Lee K.H., Sato D., Araki K., Yamada N., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2519 - 2523 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. We demonstrated a static (non-tracking) low concentration photovoltaic module that employs a new design paradigm that mixes two different types of the lens arrays. This design approach brings the annual optical collection efficiency closer to the theoretical upper limit but with high manufac-turability and scalability, because this design paradigm give more degree of freedom to optimize the overall efficiencies. We demonstrated that the module with the hybrid lens array is 2-%(absolute) more efficient than its single lens array counterparts at on-axis illumination. Also, the good match of the modeled and experimental incident-angle dependent module efficiency confirms the robustness of our fabrication process, showing the promise of achieving high annual collection efficiency of this module.
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Yamaguchi M., Lee K.H., Araki K., Kojima N., Masuda T., Kimura K., Satou A.
Conference Record of the IEEE Photovoltaic Specialists Conference 1724 - 1728 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. Although photovoltaics (PV) are expected to contribute as major electricity and energy source, there are some important issues such as continuous development of PV RD and creation of new markets to be developed. This paper emphasizes acceleration of PV system installations in Japan in order to overcome difficulty of increasing nuclear energy. This paper also presents importance of high-efficiency, low cost and highly reliable solar cells and modules by overviewing efficiency potential of various solar cells including Si tandem solar cells. Our approaches to automobile application by using high-efficiency and low-cost solar cell modules are presented.
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Sato D., Lee K., Araki K., Masuda T., Yamaguchi M., Yamada N.
Progress in Photovoltaics: Research and Applications 27 ( 6 ) 501 - 510 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
© 2019 John Wiley & Sons, Ltd. A low-concentration static III-V/Si partial concentrator photovoltaic (CPV) module for area-limited car roofs was designed and experimentally demonstrated. The concentrator lens geometry was optimized for an incidence-angle dependency of annual global irradiance on a car-roof (horizontal-flat) surface by considering arbitrary car orientations. The designed lens with a geometrical concentration ratio of 3.5× for III-V triple-junction (3-junction) cells can collect 46.6% and 36.4% of the annual irradiation onto a III-V 3-juction and Si cells, respectively, which are located under the III-V cell. The module design was validated by indoor and outdoor tests using a prototype submodule. The results show that the present module can potentially achieve up to 27.3% annual module efficiency, as projected from the actual daily submodule efficiency of 20.8% assuming state-of-the-art record cell efficiency. Furthermore, the outdoor test on a real curved car roof demonstrated that the effect of the curved surface on the power generation performance is comparable with that of a conventional PV module.
DOI： 10.1002/pip.3124
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Lee K.H., Sato D., Araki K., Yamada N., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 255 - 259 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2019 IEEE. We demonstrated a static (non-tracking) low concentration photovoltaic module that employs a new design paradigm that mixes two different types of the lens arrays. This design approach brings the annual optical collection efficiency closer to the theoretical upper limit but with high manufacturability and scalability, because this design paradigm give more degree of freedom to optimize the overall efficiencies. We demonstrated that the module with the hybrid lens array is 2%(absolute) more efficient than its single lens array counterparts at on-axis illumination. Also, the good match of the modeled and experimental incident-angle dependent module efficiency confirms the robustness of our fabrication process, showing the promise of achieving high annual collection efficiency of this module.
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Ota Y., Masuda T., Araki K., Yamaguchi M.
Solar Energy 184 84 - 90 2019.5
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
© 2019 International Solar Energy Society Photovoltaic (PV) panels mounted on vehicle surfaces could directly power automobiles in the future. However, accurate predictions of energy yield from PV systems will be required to quantify vehicle mileage and additional power requirements. Therefore, to model the energy generation of car roof PV systems, it is essential to define a meaningful and scientifically accurate method for measuring solar irradiance. We measure irradiance with five pyranometers fixed to a car body using a mobile multipyranometer array (MMPA). This paper discusses a new approach that focuses on the incidence angle distribution model of sunlight to account for direct and diffuse components, and reports on our irradiance measurements on a stationary car using the proposed system. We define the incidence angle regarding the vector components of sunlight and analyze a model for sunlight angle distribution. Our data show that the peak incidence angle as predicted by MMPA is larger than the peak incidence angle predicted based on the conventional method. Consequently, we conclude that an irradiance distribution model based on MMPA can be applied to effectively design automotive static concentrator PV systems.
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The potential for concentrator photovoltaics: A feasibility study in India Reviewed
Kamath H., Ekins-Daukes N., Araki K., Ramasesha S.
Progress in Photovoltaics: Research and Applications 27 ( 4 ) 316 - 327 2019.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
© 2018 John Wiley & Sons, Ltd. India has aggressive plans for scaling up photovoltaic installations in the coming decades. Currently fixed tilt, flat plate crystalline silicon (c-Si) technology sets the standard for cost and performance and is both robust and relatively easy to deploy. Concentrator photovoltaics (CPV) systems have a different cost structure; using solar cells with the highest efficiencies, system efficiencies greater than 30% are possible, but the system is also more sensitive to meteorological conditions. India has a complex and varied atmosphere that prevents a straightforward comparison of technologies, and hence, in this paper, we use a computer model to simulate the power output from CPV systems located in locations in India where the Aerosol Robotic Network (AERONET) stations are based and additionally, in Bangalore where we have a CPV test station. We quantify the increased intermittency suffered by CPV systems that arises from the larger dynamic range in direct beam irradiance over global irradiance. Nevertheless, by calculating the target system costs required to attain a competitive levelized cost of electricity (LCOE), we find that CPV systems in some, but not all locations have the opportunity to compete against dual-axis tracked and inclined c-Si based PV in India.
DOI： 10.1002/pip.3099
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Performance Analysis and Fault Diagnosis Method for Concentrator Photovoltaic Modules Reviewed
Kamath H., Ekins-Daukes N., Araki K., Ramasesha S.
IEEE Journal of Photovoltaics 9 ( 2 ) 424 - 430 2019.3
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2011-2012 IEEE. Concentrator photovoltaic (CPV) systems use high efficiency multi-junction solar cells with efficiencies >40%, but the module efficiency is often much lower. The increased complexity of a CPV module, with optics, receiver, and the tracker give an increased probability that faults will arise during the operational lifetime. In addition, a location like India has varied atmospheric conditions that further complicate the diagnosis of faults. It is therefore important to decouple effects due to the external environment (such as the atmosphere) from effects due to the degradation of the module. By applying a computer model to outdoor CPV test data in Bangalore, India, we have established a method to assess the performance of the CPV module and finally we present a method to diagnose faults in the module.
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Sato D., Lee K., Araki K., Yamaguchi M., Yamada N.
IEEE Journal of Photovoltaics 9 ( 1 ) 147 - 153 2019.1
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2011-2012 IEEE. This paper describes the design and evaluation of a III-V/Si partial concentrator photovoltaic (CPV) module with a stacked structure comprising a highly transparent CPV module and a Si cell, which aims to maximize the power generation from global normal irradiation (GNI) by harvesting not only direct, but diffuse sunlight as well. The module structure was designed by considering the optical and heat transfer characteristics, and its performance was evaluated by outdoor and indoor tests using a prototype submodule with a geometrical concentration ratio of 100×. The submodule achieves diffuse sunlight transmission of over 80%, thus generating more power from diffuse sunlight. Under clear-sky condition, the submodule with a single-sided Si cell exhibits the maximum GNI-based module efficiency of 30.7%. The submodule with a bifacial Si cell further improves the power generation and tolerance to tracking error angle for various sunlight conditions.
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Lee K., Araki K., Yamaguchi M.
IEEE Access 7 97208 - 97215 2019
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Access
© 2013 IEEE. Equivalent circuit network simulation is widely used in modeling solar cells in three dimensions. However, the computational time and numerical instability increases dramatically when the number of circuit element increases. This problem is exacerbated by increasing the number of junctions in the solar cells. We propose a downsampling algorithm to reduce the time complexity but retain reasonable accuracy within the appropriate parameter space of multi-junction solar cells. We also publish a full-featured software that implements this algorithm and the full circuit network simulation along with this paper.
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Review of recent progress in car-roof PVs for applications as an automobile energy source Reviewed
ARAKI Kenji, YAMAGUCHI Masafumi
Oyo Buturi 88 ( 2 ) 84 - 90 2019
Language：English Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Applied Physics
A car-roof photovoltaic has significant potential to change our society. With this technology, 70% of a car will be able to run on the solar energy collected by the solar panel on its roof. However, it is not a simple extension of conventional photovoltaic technology. This paper discusses what we need to do to achieve the goal of running a majority of cars on solar energy, after clarification of the differences from conventional photovoltaic technology.
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Ota Y., Masuda T., Araki K., Yamaguchi M.
Coatings 8 ( 12 ) 2018.12
Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：Coatings
© 2018 by the authors. For modeling the energy generation of three-dimensional car roof photovoltaic (PV) panels, it is essential to define a scientifically accurate method to model the amount of solar irradiance received by the panel. Additionally, the average annual irradiance incident on car roofs must be evaluated, because the PV module is often shaded during driving and when parked. The curve-correction factor, which is a unique value depending on the three-dimensional curved shape of the PV module, is defined in this paper. The curve-correction factor was calculated using a ray-trace simulator. It was found that the shape of the curved surface affected the curve-correction factor. The ratio of the projection area to the curved surface area of most car roofs is 0.85-0.95, and the annual curve-correction factor lies between 0.70 and 0.90. The annual irradiance incident on car roofs was evaluated using a mobile multipyranometer array system for one year (September 2017-August 2018). It is estimated that the effective annual solar radiation for curved PV modules is 2.53-3.52 kWh m-2/day.
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Verification of uncertainty in CPV's outdoor performance Reviewed
Saiki H., Sakai T., Araki K., Ota Y., Lee K.H., Yamaguchi M., Nishioka K.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC 949 - 953 2018.11
Language：English Publishing type：Research paper (scientific journal) Publisher：2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC
© 2018 IEEE. Multi-junction cells have high efficiency but their performance ratio was less than the conventional single junction cell. One of the issues is lower outdoor performance in the outdoor operation conditions due to spectrum mismatch influenced by fluctuation of sun-height and atmospheric parameters. In this study, to predict the energy yield at an arbitrary point, firstly, we estimated the range of atmospheric parameter fluctuation that determines the spectrum from Bird's spectrum model using the DNI spectrum measured at University of Miyazaki. Furthermore, we examined the uncertainty of the outdoor performance of CPV system using the DNI spectrum resynthesized from the Monte Carlo method. As a result, when considering the atmospheric parameters, calculated values approached to the measured values and the uncertainty of outdoor performance was confirmed.
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Achieving High Efficiency Static Low-Concentration Photovoltaic Module Using Hybrid Lens Arrays
Lee K.H., Araki K., Yamaguchi M.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC 905 - 908 2018.11
Language：English Publishing type：Research paper (scientific journal) Publisher：2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC
© 2018 IEEE. We propose a new design paradigm of static low concentration photovoltaics for III-V or silicon solar cells. Although static/non-tracking low concentrator optics has been studied extensively, most of these designs are either difficult to implement or low optical efficiency. In this work, we present a design that uses lens array that mixes two different optics, giving more degrees of freedom to optimize the optics and the size of solar cells. This design approach brings the optical efficiency closer to the theoretical upper limit while maintaining manufacturability and scalability. Although the design example presented in this work is aimed to mount on sloped surface, this can be also applied to the building integrated photovoltaics or on car-roof.
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Sato D., Lee K., Araki K., Masuda T., Yamaguchi M., Yamada N.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC 954 - 957 2018.11
Language：English Publishing type：Research paper (scientific journal) Publisher：2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC
© 2018 IEEE. This paper describes the design and evaluation of a low-concentration static III-V/Si partial CPV module that aims to maximize the power generation in the limited car-rooftop area by harvesting full global sunlight. The concentrator lens was designed considering the incident angle dependence of annual global irradiance on the car-rooftop (horizontal flat) surface. The fundamental characteristics were investigated by optical simulation and indoor test using a prototype submodule with a geometrical concentration ratio of 3.5×. The designed lens can collect 46.6% and 36.4% of the annual solar irradiation onto triple-junction and Si cells, respectively, achieving the annual electricity yield which is equivalent to 30% efficiency flat PV module.
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Analysis for efficiency potential of crystalline Si solar cells Reviewed
Yamaguchi M., Lee K., Araki K., Kojima N., Ohshita Y.
Journal of Materials Research 33 ( 17 ) 2621 - 2626 2018.9
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Materials Research
© 2018 Materials Research Society. Efficiency potential of crystalline Si solar cells is analyzed by considering external radiative efficiency (ERE), voltage, and fill factor losses. Crystalline Si solar cells have an efficiency potential of more than 28.5% by realizing ERE of 20% from about 5% and normalized resistance of less than 0.05 from around 0.1. Nonradiative recombination losses in single-crystalline and multicrystalline Si solar cells are also discussed. Especially, nonrecombination and resistance losses in multicrystalline Si solar cells are shown to be higher than those of single-crystalline cells. Importance of further improvement of minority-carrier lifetime in crystalline Si solar cells is suggested for further improvement of crystalline Si solar cells. High efficiency of more than 28.5% will be realized by realizing high minority-carrier lifetime of more than 30 ms. Key issues for those ends are reduction in carbon concentration of less than 1 × 1014 cm-3, oxygen precipitated and dislocations even in single-crystalline Si solar cells, and reduction in dislocation density of less than 3 × 103 cm-2 in multicrystalline Si solar cells.
DOI： 10.1557/jmr.2018.262
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Lee K., Araki K., Kojima N., Yamaguchi M.
AIP Conference Proceedings 2012 2018.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2018 Author(s). We proposed a low-profile static low-concentration optics design paradigm for building-integrated photovoltaics or as alternative of flat PV panels. Based on the measurement results of our prototype, this design could reach more than 80% on-axis optical efficiency and more than 60% collection efficiency over an annual solar insulation, with a concentration of around 3x. The result is approaching the analytical maximum of non-tracking concentrator derived in previous study.
DOI： 10.1063/1.5053514
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Analysis for efficiency potential of high-efficiency and next-generation solar cells Reviewed
Yamaguchi M., Lee K., Araki K., Kojima N., Yamada H., Katsumata Y.
Progress in Photovoltaics: Research and Applications 26 ( 8 ) 543 - 552 2018.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
Copyright © 2017 John Wiley & Sons, Ltd. This paper overviews photovoltaic R&D projects in Japan. Recently, world-record and second highest efficiencies of various types of solar cells have been demonstrated under the New Energy and Industrial Technology Development Organization Project: 44.4% (under concentration) and 37.9% (under 1 sun) InGaP/GaAs/InGaAs inverted metamorphic 3-junction solar cells by Sharp, 26.7% single crystalline Si heterojunction back-contact solar cell by Kaneka, 22.3% copper indium gallium selenide solar cell by Solar Frontier, a-Si/μc-Si/μc-Si thin-film triple-junction solar cell with stabilized efficiency of 14.0% by AIST, 11.9% dye-sensitized solar cell by Sharp, and 11.2% organic solar cell by Toshiba. This paper also presents efficiency potential of high-efficiency and next-generation solar cells analyzed by considering external radiative efficiency, open-circuit voltage loss, and fill factor loss. Efficiency potential of crystalline Si, GaAs, III-V compound 3-junction and 5-junction, CIGSe, CdTe, CZTS(Se), multiquantum well, and quantum dot and perovskite solar cells is shown to be 28.5%, 29.7%, 40%, 43%, 26.5%, 26.5%, 20%, 25.8%, and 24.9% under 1 sun, respectively.
DOI： 10.1002/pip.2955
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Hara T., Ogawa T., Liang J., Araki K., Kamioka T., Shigekawa N.
Japanese Journal of Applied Physics 57 ( 8 ) 2018.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. The electrical properties of GaAs//indium tin oxide (ITO)/Si junctions fabricated by surface-activated bonding (SAB) are investigated with emphasis on their dependence on the temperature of postbonding annealing. The current–voltage (I–V) characteristics of n+-GaAs//ITO/p+-Si and n+-GaAs//ITO/n+-Si junctions without annealing are linear. Those of p+-GaAs//ITO/p+-Si and p+-GaAs//ITO/n+-Si junctions without annealing are nonlinear. Although the interface resistance of all the junctions increases with increasing annealing temperature, the resistances of the respective junctions after the annealing at 400 °C are still smaller than the series resistance of the actual SAB-based InGaP/GaAs//Si hybrid triple-junction cells (>4 Ω cm2). The n+-GaAs//ITO/n+-Si junction reveals the lowest resistance among the investigated junctions after annealing. These results demonstrate that GaAs//ITO/Si junctions with an ITO intermediate layer could be effective for reducing series resistance in hybrid multijunction cells.
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Evaluation and optimization of coating for wide acceptance angle concentrator photovoltaic module Reviewed
Ahmad N., Ota Y., Araki K., Lee K.H., Yamaguchi M., Nishioka K.
Japanese Journal of Applied Physics 57 ( 8 ) 2018.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. Because of the reflection on the module surface, the irradiance received by the solar cells constituting the PV module is less than that received by the module surface. In this situation, an antireflection coating is crucial in improving the performance of the PV module. In this study, we experimented with and simulated a middle concentrator photovoltaic (MCPV) module with a large-acceptance-angle lens. The module was coated with a silica-based coating, which has an antisoiling and an antireflection function. Using the silica-based coating, the optical efficiency of the optics of the MCPV module increased. As a result, under outdoor conditions, the short circuit current and the conversion efficiency of the MCPV module with the silica-based coating was increased by 3 and 2.9%, respectively, compared with the module without the coating. The short circuit current of the MCPV module with a 100-nm-thick coating achieved a 5.6% increase in the optical simulation.
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Shigekawa N., Hara T., Ogawa T., Liang J., Kamioka T., Araki K., Yamaguchi M.
IEEE Journal of Photovoltaics 8 ( 3 ) 879 - 886 2018.5
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2011-2012 IEEE. Effects of GaAs/indium tin oxide (ITO)/Si junctions on III-V-on-Si multijunction solar cells are examined by fabricating and characterizing InGaP/GaAs/ITO/Si triple-junction (3J) solar cells. The 3J cells are fabricated by evaporating ≈100-nm-thick ITO films on the surfaces of Si bottom cells and bonding the InGaP/GaAs double-junction (2J) subcells and the ITO films using surface-activated bonding technologies at room temperature. The current-voltage characteristics of 3J cells with p+-GaAs/ITO/ n+-Si and n+-GaAs/ITO/n+-Si junctions are compared with those of an InGaP/GaAs/Si 3J cell. The parasitic resistance of the respective 3J cells is estimated by analyzing their characteristics in the dark. We find that the 3J cell with an n+-GaAs/ITO/ n+-Si junction shows the lowest parasitic resistance, which is the origin of its lowest differential resistance at the open-circuit voltage and highest fill factor. This means that n+-GaAs/ITO/ n+-Si junctions are promising for improving the performances of III-V-on-Si hybrid multijunction cells. The spectral response characteristics of these cells indicate that the thickness of the ITO films must be optimized.
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Analysis of future generation solar cells and materials Reviewed
Yamaguchi M., Zhu L., Akiyama H., Kanemitsu Y., Tampo H., Shibata H., Lee K.H., Araki K., Kojima N.
Japanese Journal of Applied Physics 57 ( 4 ) 2018.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. The efficiency potentials of future generation solar cells such as wide bandgap chalcopyrite, Cu2ZnSnS4 (CZTS), Cu2ZnSn(S,Se)4 (CZTSSe), multi quantum well (MQW) and quantum dot (QD) solar cells are discussed on the basis of external radiative efficiency (ERE), open-circuit voltage loss, fill factor loss, and nonradiative recombination losses. CZTS and CZTSSe solar cells have efficiency potentials of more than 20% owing to the improvement in ERE from about 0.001 to 1%. MQW and QD cells have efficiency potentials of 24.8%, and 25.8% owing to the improvement in ERE from around 0.01 to 0.1%, and 1%, respectively. In this paper, the effects of nonradiative recombination on the properties of future generation solar cells are discussed.
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A review of recent progress in heterogeneous silicon tandem solar cells Reviewed
Yamaguchi M., Lee K., Araki K., Kojima N.
Journal of Physics D: Applied Physics 51 ( 13 ) 2018.3
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Physics D: Applied Physics
© 2018 IOP Publishing Ltd. Silicon solar cells are the most established solar cell technology and are expected to dominate the market in the near future. As state-of-the-art silicon solar cells are approaching the Shockley-Queisser limit, stacking silicon solar cells with other photovoltaic materials to form multi-junction devices is an obvious pathway to further raise the efficiency. However, many challenges stand in the way of fully realizing the potential of silicon tandem solar cells because heterogeneously integrating silicon with other materials often degrades their qualities. Recently, above or near 30% silicon tandem solar cell has been demonstrated, showing the promise of achieving high-efficiency and low-cost solar cells via silicon tandem. This paper reviews the recent progress of integrating solar cell with other mainstream solar cell materials. The first part of this review focuses on the integration of silicon with III-V semiconductor solar cells, which is a long-researched topic since the emergence of III-V semiconductors. We will describe the main approaches - heteroepitaxy, wafer bonding and mechanical stacking - as well as other novel approaches. The second part introduces the integration of silicon with polycrystalline thin-film solar cells, mainly perovskites on silicon solar cells because of its rapid progress recently. We will also use an analytical model to compare the material qualities of different types of silicon tandem solar cells and project their practical efficiency limits.
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Electrical conduction of Si/indium tin oxide/Si junctions fabricated by surface activated bonding Reviewed
Liang J., Ogawa T., Hara T., Araki K., Kamioka T., Shigekawa N.
Japanese Journal of Applied Physics 57 ( 2 ) 2018.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2018 The Japan Society of Applied Physics. The electrical properties of n+-Si//indium tin oxide (ITO)/n+-Si, n+-Si//ITO/p+-Si, and p+-Si//ITO/n+-Si junctions fabricated by surface activated bonding (SAB) were investigated. The current-voltage (I-V) characteristics of n+-Si//ITO/n+-Si, n+-Si//ITO/p+-Si, and p+-Si//ITO/n+-Si junctions showed excellent linear properties. The interface resistances of n+-Si//ITO/n+-Si, n+-Si//ITO/p+-Si, and p+-Si//ITO/n+-Si junctions were found to be 0.030, 0.025, and 0.029 Ω•cm2, respectively, which are lower than required for concentrator photovoltaics. The interface resistances of all the junctions increased with increasing annealing temperature. The degradation of the interface resistance is lower in n+-Si//ITO/n+-Si junctions than in n+-Si//ITO/p+-Si and p+-Si//ITO/n+-Si junctions, when the annealing temperature is higher than 100 °C. These results demonstrate that the ITO thin film as an intermediate layer has high potential application for the connection of subcells in the fabrication of tandem solar cells.
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HOW DOES R&D ON THE CAR-ROOF PV IMPACT TO THE ENTIRE PV TECHNOLOGIES? Reviewed
Araki Kenji, Lee Kan-Hua, Kojima Nobuaki, Yamaguchi Masafumi
グランド再生可能エネルギー国際会議論文集 1 ( 0 ) 2018
Language：English Publishing type：Research paper (scientific journal) Publisher：特定非営利活動法人 再生可能エネルギー協議会
With the advancement of both vehicle and PV technologies, it will be possible that 70 % of the cars will run by the solar energy, 8 % of the Greenhouse gas emission will be cut, and 50 GW/year of the new PV market will be created. The R&D of the car-roof PV impacts to entire PV technologies. The progress of R&D of the car-roof PV impacts to various aspects of PV technologies including 3-D nowcasting leading to the establishment of the real-time market and smoothing PV output to the grid.
DOI： 10.24752/gre.1.0_39
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TOWARD STANDARDIZATION OF SOLAR TRACKERS, CONCENTRATOR PV, AND CAR-ROOF PV Reviewed
Araki Kenji, Ji Liang, Kelly George, Algora Carlos, Siefer Gerald, NIshioka Kensuke, Muller Mathew, Leutz Ralf, Carter Sam, Wang Shitao, Askins Steve
グランド再生可能エネルギー国際会議論文集 1 ( 0 ) 2018
Language：English Publishing type：Research paper (scientific journal) Publisher：特定非営利活動法人 再生可能エネルギー協議会
IEC TC82 WG7 works for standardization of concentrator modules, and their optics, mechanics and other advanced photovoltaic systems. These standards will be in the general areas of safety, photoelectric performance and environmental reliability tests. The standards ultimately produced should be universal and non-restrictive in their application, taking into account different environments and manufacturing technologies. In addition to the essential electrical and mechanical characteristics, standards will be written for other relevant factors such as thermal performance, high voltage performance, angular performance, mechanical precision, specific testing fault resistance and fault-tolerant design. Recently an expanded group started to discuss standardization of the car-roof PV.
DOI： 10.24752/gre.1.0_37
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SUPER-MULTIJUNCTION CELL, A NEW SOLAR CELL OVERCOMING THE SPECTRUM LOSS OF MULTIJUNCTION CELLS Reviewed
Araki Kenji, Lee Kan-Hua, Kojima Nobuaki, Yamaguchi Masafumi
グランド再生可能エネルギー国際会議論文集 1 ( 0 ) 2018
Language：English Publishing type：Research paper (scientific journal) Publisher：特定非営利活動法人 再生可能エネルギー協議会
Fluctuation of the atmospheric parameters substantially impacts the energy yield of multi-junction cells. The calculation considering the variation of atmospheric parameters showed a dramatic decline of the annual energy yield from the cells with more than five junctions. Super-multijunction solar cells were proposed, and optimization work was done. The top and the bottom junctions have lower bandgap for buffering to the mismatching and collecting a full range of the sunlight by robust energy conversion, and the annual energy substantially increases even by five or six junctions.
DOI： 10.24752/gre.1.0_45
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Herrero R., Antón I., Victoria M., Domínguez C., Askins S., Sala G., De Nardis D., Araki K.
Energy Science and Engineering 5 ( 5 ) 257 - 269 2017.10
Language：English Publishing type：Research paper (scientific journal) Publisher：Energy Science and Engineering
© 2017 The Authors. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd. An inherent characteristic of high concentrator photovoltaics (HCPV) modules is a tight mechanical tolerance caused by the narrow angular transmission of the optical system, typically below or close to 1°. Misalignments in the modules caused during the assembly process in the production line will degrade not only the electrical but also the angular performance of the module. Moreover, dispersion in the electrical characteristics of the elementary units comprising a module would lead also to power loss. Quality control and data analysis on the production line is of great significance for adjusting the production line and preserving the angular tolerance and the electrical performance. This is particularly critical during the set-up and tuning of an automated production line. This paper presents the results of a pilot production line for HCPV modules carried out within the European funded ECOSOLE project. Several quality controls were established, which are the binning of the photovoltaic receivers, the measurement of misalignments among the elementary units within every module, and the indoor electrical characterization of the modules. Collected experimental data during the tuning phase of the pilot line were used to validate a module performance model based on production parameters. Monte Carlo method is lately applied to the model to assess the influence of production defects of diverse nature and the adequacy of quality controls, in several manufacturing scenarios beyond the specific constrains of the ECOSOLE experience.
DOI： 10.1002/ese3.178
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Efficiency potential and recent activities of high-efficiency solar cells Reviewed
Yamaguchi M., Yamada H., Katsumata Y., Lee K., Araki K., Kojima N.
Journal of Materials Research 32 ( 18 ) 3445 - 3457 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Materials Research
Copyright © Materials Research Society 2017 Â. The present status of R&D for various types of solar cells is presented by overviewing research and development projects for solar cells in Japan as the PV R&D Project Leader of the New Energy and Industrial Technology Development Organization (NEDO) and the Japan Science and Technology Agency (JST). Developments of high-efficiency solar cells such as 44.4% (under concentration) and 37.9% (under 1-sun) InGaP/GaAs/InGaAs 3-junction solar cells by Sharp, 26.6% crystalline Si heterojunction back-contact (HBC) solar cells by Kaneka, 22.3% CIGS solar cells by Solar Frontier have been demonstrated under the NEDO PV R&D Project. 15.0% efficiency has also been attained with 1 cm2 perovskite solar cell by NIMS under the JST Project. This article also presents analytical results for efficiency potential of high-efficiency solar cells based on external radiative efficiency (ERE), open-circuit voltage loss and fill factor loss. Crystalline Si solar cells, GaAs, III-V compound 3-junction and 5-junction, CIGSe, and CdTe solar cells have efficiency potential of 28.5%, 29.7%, 42%, 43%, 26.5%, and 26.5% under 1-sun condition, respectively, by improvements in ERE.
DOI： 10.1557/jmr.2017.335
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Towards creation of mobility society using solar energy Reviewed
Yamaguchi M., Araki K., Lee K., Kojima N., Masuda T., Kimura K., Satou A., Yamada H.
2017 5th IEEE International Conference on Smart Energy Grid Engineering, SEGE 2017 394 - 399 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 5th IEEE International Conference on Smart Energy Grid Engineering, SEGE 2017
© 2017 IEEE. This paper presents current status and future direction of Photovoltaic (PV) electric power system installations in the world. This article also overviews PV R&D activities in Japan as the PV R&D Project Leader of NEDO and JST. Present status of various solar cells efficiencies under NEDO and JST PV R&D projects are presented: 44.4% for concentrator III-V compound 3-junction solar cell, 37.9% for 1-sun III-V compound 3-junctiion cell, 26.7% for single-crystal Si cell, 22.3% for CIGS cell, and 14.0% for a-Si based 3-junction cell. Efficiency potential of various solar cells is also discussed. Prospects of PV and our recent approaches towards the creation of 'Mobility Society by using Solar Energy' are discussed.
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Design of the partial concentrator lens for III-V on Si static concentration Reviewed
Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). To compete with the flat-plate crystalline Silicon cell module, III-V on Si structure is developed. However, it is clear that the situation of the higher cost of III-V cell relative to the Silicon cell will be unchanged. Then, it is preferred concentrating III-V cell for further savings. The partial concentrator is expanding the acceptance angle despite the higher concentration ratio. It is achieved by better performance balance of on-axis and high incidence angle. The new and generalized design method of the partial concentrator was developed. The profile function was constructed by selected Zernike's polynomial considering rotational symmetry. The full conditions of the calculation including the initial value and the radial and azimuthal degree of the function were examined. It was found that the recommended radial and azimuthal degree were 12 and 12.
DOI： 10.1063/1.5001412
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Quantifying the potential of III-V/Si partial concentrator by a statistical approach Reviewed
Lee K., Araki K., Ota Y., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). We propose a theoretical framework for analyzing the energy yields of partial concentrators. A partial concentrator uses a concentrator cell to absorb the principal defracted or reflected light rays from its concentrator optics and a backplane cell to absorbs the diffused or defocused light. This concept can be applied to the concentrator system when accurate sun-tracking is not available, such as on a vehicle. This analysis framework provides a simplified way to describe the uncertainties of solar incidences dealt by partial concentrator. This help identified a clearer design criteria of partial concentrator in order to outperform the flat-panel PV or conventional CPV.
DOI： 10.1063/1.5001445
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Outdoor validation of the 30 minutes intermittent tracking of 100 x CPV Reviewed
Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). HCPV uses trackers, but their cost and reliability have raised problems. These problems may be solved by substantially increasing acceptance angle and mechanical tolerance. For a demonstration of the possibility of such substantial improvement, a 30 minutes intermittent tracking in high concentration applications was examined. To allow for 30 minutes intermittent tracking, both advanced optics and an advanced tracking control were investigated. For advanced optics, a 100 x refractive and dielectric concentrator was designed. It had about plus or minus 5°of acceptance angle. For advanced tracking control, a feed-forward control with a linear error correction was investigated. With combining both optimizations, it was shown that 30 minutes intermittent tracking was validated in outdoor experiments even for an as high as 100 x concentration.
DOI： 10.1063/1.5001401
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Optical and thermal simulation for wide acceptance angle CPV module Reviewed
Ahmad N., Ota Y., Araki K., Lee K., Yamaguchi M., Nishioka K.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). Concentrator photovoltaic (CPV) technology has the potential to decrease the cost of systems in the near future by using less expensive optical elements in the system which replace the receiving surface aperture and concentrate the sunlight onto small solar cells. One of the main concerns of CPV is the need for high precision tracking system and the relation to the acceptance angle. In this paper, we proposed a CPV module with concentration ratio larger than 100 times and wide acceptance angle. An optical simulation for the module with S-TIM2 glass as a lens material was conducted to estimate the optical performance of the module. Thermal and electrical simulation was also conducted using COMSOL Multiphysics and SPICE respectively to evaluate the working temperature and electrical characteristics of the multijunction solar cell under concentration conditions.
DOI： 10.1063/1.5001400
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Monte Carlo simulation to analyze the performance of CPV modules Reviewed
Herrero R., Antón I., Sala G., De Nardis D., Araki K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). A model to evaluate the performance of high concentrator photovoltaics (HCPV) modules (that generates current-voltage curves) has been applied together with a Monte Carlo approach to obtain a distribution of modules with a given set of characteristics (e.g., receivers electrical properties and misalignments within elementary units in modules) related to a manufacturing scenario. In this paper, the performance of CPV systems (tracker and inverter) that contain the set of simulated modules is evaluated depending on different system characteristics: inverter configuration, sorting of modules and bending of the tracker frame. Thus, the study of the HCPV technology regarding its angular constrains is fully covered by analyzing all the possible elements affecting the generated electrical power.
DOI： 10.1063/1.5001450
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Is it CPV? Yes, but it is a partial CPV Reviewed
Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). III-V on Si cell is one of the most promising candidates of the low-cost and high-efficiency solar cell beyond the limit of Si cells. However, it is likely that the cost of III-V cell will keep greater than that of Si cells. The partial concentrator technology was invented to concentrate the sunlight onto smaller III-V cells on the Si cell with the increased acceptance angle for static concentrator operation. The partial concentrator allows on-axis optical loss but expands the apparent acceptance angle often beyond the thermodynamic limit. The spilled light will be saved and collected by the bottom Si cell. It is not just placing the Si cell around the concentrated III-V cell, but its design philosophy is "partially" collecting the sunlight to the III-V cell to expand the acceptance. Depending on the values of cost parameters, the cost of the partial CPV module becomes minimum at 4 x - 10 x of concentration onto the top III-V cell, meaning that the area of expensive III-V cell can be saved to 1/4 - 1/10. In the partial concentrator. There may be a better optics than the ideal concentrator. The partial concentrator can be applied not only to III-V on Si cells but also III-V "by" Si cells. This lateral configuration does not have as excellent efficiency as the III-V on Si cell, but the usage of III-V cell is only several percents of the Si cell and has good similarity on the conventional Si flat-plate module technology.
DOI： 10.1063/1.5001439
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Araki K., Lee K., Yamaguchi M.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). The spectrum influence on the CPV multi-junction cell is not only responsible for the air mass but significantly influenced by the fluctuation of the atmospheric parameters. The Monte Carlo simulation assuming the best and worst conditions of the distribution of parameters was examined to the CPV multi-junction cells. It was shown both aerosol density and water precipitation affects to the optimum bandgap design. The optimum bandgap set in the area of the worst aerosol density was significantly different from those of the best area. Even though the bandgap was designed maximizing the annual energy yield in a set of annual data, the fluctuation of the atmospheric parameters affects the annually mean efficiency to ±5 % (best area) and ± 9 % (worst area). Considering this considerable influence on the annual energy yield of the CPV multi-junction cell, with junctions connected in series, the best number of the junctions was 4 and more than five junctions were not better for the annual energy yield.
DOI： 10.1063/1.5001435
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Estimation of conversion efficiency for partially static concentrator with III-V on Si solar cell Reviewed
Ota Y., Araki K., Lee K., Yamaguchi M., Nishioka K.
AIP Conference Proceedings 1881 2017.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2017 Author(s). We estimated the annual mean conversion efficiency of partially static concentrator (PSC) module. The PSC module consistes of static concentrator, small III-V top solar cell and large Si bottom solar cell. The large Si bottom solar cell absorbes the solar power that is not collected by the III-V top soler cell. The mean optical efficiency decreased with the increase of concentration ratio. The decreasing ratio of mean optical efficiency between 1X to 4X was 49.9%. By using PSC module structure, it was possible to boost the annually mean conversion efficiency. The decreasing ratio of annually mean conversion efficiency between 1X to 4X was 21.5%. The acceptance angle of conversion efficiency was larger than that of optical efficiency. The PSC module structure also improved acceptance angle of the conversion efficiency.
DOI： 10.1063/1.5001409
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Electrical conduction of Si/ITO/Si junctions fabricated by surface activated bonding Reviewed
Liang J., Ogawa T., Araki K., Kamioka T., Shigekawa N.
Proceedings of 2017 5th International Workshop on Low Temperature Bonding for 3D Integration, LTB-3D 2017 2017.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of 2017 5th International Workshop on Low Temperature Bonding for 3D Integration, LTB-3D 2017
© 2017 JSPS 191st Committee on Innovative Interface Bonding Technology. The electrical properties of n-Si/ITO/n-Si, n-Si/ITO/p-Si, and p-Si/ITO/n-Si junctions fabricated by surface activated bonding (SAB) were investigated. The current-voltage (I-V) characteristics of n-Si/ITO/n-Si, n-Si/ITO/p-Si, and p-Si/ITO/n-Si junctions showed excellent linearity. The interface resistance of n-Si/ITO/p-Si junctions was found to be 0.0249 O·cm2, which is the smallest value observed in all the samples.
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Orlando V., Gabás M., Galiana B., Espinet-González P., Palanco S., Nuñez N., Vázquez M., Araki K., Algora C.
Progress in Photovoltaics: Research and Applications 25 ( 1 ) 97 - 112 2017.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
Copyright © 2016 John Wiley & Sons, Ltd. Accelerated life tests are frequently used to provide reliability information in a moderate period of time (weeks or months), and after that, a failure analysis is compulsory to detect the failure origins. In this paper, a failure analysis has been carried out after a temperature accelerated life test on lattice matched GaInP/Ga(In)As/Ge triple junction commercial solar cells. Solar cells were forward biased in darkness inside three climatic chambers in order to emulate the photo-generated current under nominal working conditions (a concentration level of 820 suns). After the accelerated aging test, a characterization of the resulting cells by means of quantum efficiency, dark and illumination I–V curves, electroluminescence, scanning electron microscope, energy dispersive X-ray, scanning transmission electron microscope and X-ray photoelectron spectroscopy has been carried out. Current is identified as the cause of degradation while temperature just dominates the accelerating factor of the aging test. Current promotes the front metal damage produced by the chemical evolution of the electroplating impurities together with those of the tab soldering process. Semiconductor structure does not seem to be responsible of any failure. Therefore, this kind of lattice matched GaInP/Ga(In)As/Ge triple junction solar cells, that as of 2016, are the workhorse of CPV technology, exhibits as a very robust device if the front metal connection is properly accomplished. Copyright © 2016 John Wiley & Sons, Ltd.
DOI： 10.1002/pip.2818
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Analysis of impact to optical environment of the land by flat-plate and array of tracking PV panels Reviewed
Araki K., Nagai H., Lee K., Yamaguchi M.
Solar Energy 144 278 - 285 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
© 2017 Elsevier Ltd The impact to the optical environment result from shading of PV panels becomes concerned. This paper describes how to calculate illumination level on the land around tracking PV (both isolated and arrayed) panel. The land under the flat-plate PV is dark by shading, but the influence from the tracking PV is not very significant, typically half in average. This is because shadow by tracking panel moves. The shading influence is further decreased with the increase of pedestal height. With this advantage, tracking PV can be combined with agriculture as a double use of the land.
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Ikeda K., Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017 1 - 4 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017
© 2017 IEEE. For the purpose of designing the high efficient static solar concentrator on automobile, we discuss the way to increase the aperture area. The 2-dimensional (2D) single and double refraction models based on the Snell's law are prepared under the conditions of random azimuth and large off-angle of the incident ray. We find that the center height of the lens is a key parameter to increase the aperture size. For lens array systems, the optimum center height is determined by the upper limit which is required to suppress the shadowing loss at the adjacent lenses. We demonstrate the ray tracing of the 2D lens model.
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Araki K., Nagai H., Herrero R., Antón I., Sala G., Lee K., Yamaguchi M.
Solar Energy 147 448 - 454 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
© 2017 Elsevier Ltd CPV (Concentrator photovoltaic) requires accuracy in optical alignment. Optical misalignment in the module was measured and analyzed. Such information was useful to identify assembly problems and improved module performance. 1-D and 2-D Monte Carlo simulations were developed to analyze the impact of alignment errors by assembly errors. The 1-D simulation was useful to anticipate the power output of the module, whereas 2-D was for acceptance angle. The 2-D model showed that the gap from Pmp-based acceptance angle (Pmp: Power at the maximum power point) from that of Isc-based (Isc: Short-circuit current) was a function of deviation of assembles misalignment in the module. The rough rule for the acceptable assembles misalignment is that the standard deviation of the assemble misalignment will be less than 10% of acceptance angle of the optics, whereas that of power output of the single module is 20%. This fact implies that the inspection of the acceptance angle is highly recommended in the CPV module production.
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Araki K., Herrero R., Anton I., Sala G., Nagai H., Lee K., Yamaguchi M.
2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017 1 - 5 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017
© 2017 IEEE. The acceptance angle of concentrator modules measured by short circuit current I sc is always larger than that by maximum power Pm even by use of advanced concentrator optics that keep uniform illumination onto the concentrator cells. The purpose of this study is to investigate the cause of that myth. An expanded Monte Carlo method that calculates acceptance angle of CPV modules was developed with considering 2-dimensional misalignment error vector. It was shown that the gap from Pm-based acceptance angle from that of I sc -based was a function of deviation of assembles misalignment in the module. The rough rule for acceptable assembles misalignment is that the standard deviation of assemble misalignment level will be less than 10 % of acceptance angle of the optics.
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Ahmad N., Ota Y., Araki K., Lee K., Yamaguchi M., Nishioka K.
Japanese Journal of Applied Physics 56 ( 9 ) 2017
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2017 The Japan Society of Applied Physics. In this study, we propose a new concentrator photovoltaic (CPV) system design with a large-acceptance-angle lens, which tracks the sun on the basis of a new 30-min intermittent tracking method that does not require a special high-precision CPV tracking system. This will reduce costs, because a large percentage of the cost of a typical CPV system comes from the expensive accurate tracking system. The present system had a concentration ratio of 111 and an acceptance angle of 4.5°. We conducted an experiment to evaluate the thermal and electrical performances of the system in an outdoor test site in Miyazaki Japan. The experimental results were compared with optical thermal and electrical simulation results. The simulated results showed good agreement with the experimental ones.
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Araki K., Lee K., Yamaguchi M.
Solar Energy 153 445 - 453 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
© 2017 Elsevier Ltd Bandgap optimization of the tandem cells has been calculated by many scientists under the standard air-mass 1.5 spectrum or the site-specific conditions using statistics of meteorological observations and spectrum data. Calculation of the optimized bandgap combination at the specific site required many annually-observed atmospheric parameters. It is not realistic, because most of the installation site does not have detailed annual spectrum information. This paper has two purposes. One is to quantitatively understand the impact of the variance of the atmospheric conditions to the bandgap optimization design. Another is to propose the practical method to take the spectrum information of the specific site by even in a single day's measurement. For evaluating the impact of the atmospheric conditions, both the best and the worst distributions were taken from the measurement data. With including other meteorological and geographical data variance, the dataset of the annual spectrum with different sun-height was generated and distribution of the calculated optimized bandgap combination was examined. The optimum bandgaps varied by aerosol depth, water precipitation, latitude, and seasonal sunshine duration change as it was expected. It was also found that the optimization of the bandgaps combination at the specific site, considering ever-changing spectrum, may be determined by the matching condition to the sun height at the culmination on the winter solstice, because the distributions of optimum bandgap combinations were much closed to those of annual integration, in spite of the fact that they were calculated by a single time (single date) of the measurement. Different from AM1.5 the sun height can be taken two times a day throughout a year.
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Static concentrator photovoltaics for automotive applications Reviewed
Masuda T., Araki K., Okumura K., Urabe S., Kudo Y., Kimura K., Nakado T., Sato A., Yamaguchi M.
Solar Energy 146 523 - 531 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
© 2017 Elsevier Ltd This research investigated the benefits of utilizing solar power as an energy source for future passenger vehicles and an innovative static concentrator photovoltaic module for practical automotive applications. Due to strict emissions standards, alternative energy sources must be found for vehicles in the future. It was estimated that replacing all passenger vehicles with hybrid vehicles (HVs) equipped with an 800 W rated-power solar module that generates an average output of 1.8 kW h/day would reduce CO2 emissions by 63% in Japan. To confirm the validity of this estimation, a test vehicle was created by installing a 6.8 m2 solar module onto a commercially available plug-in HV. An average power generation of 2.1 kW h/day was obtained by this solar module over 100 days under real-world conditions, which was larger than the power required to achieve the estimated CO2 emissions reduction. In addition, an innovative static low-concentrator with III–V cells was studied to help reduce the installation area of the solar module on the vehicles, which is essential for practical passenger vehicles. A new concentrator design method was proposed that can be easily integrated into a standard vehicle design procedure by utilizing numerical optimization in a CAD-friendly environment. Both design equations and a design example are discussed in this paper. The proposed lens design (asymmetric-aspheric type) can expand the acceptance incident angle of solar light and increase the annual energy yield of a solar panel, while maintaining the essential thin structure for automotive applications. In spite of the wide range of sun positions, this asymmetric-aspherical lens can maintaining stable illumination on the cell while suppressing the maximum spot intensity to 20×.
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Optimization of static concentrator photovoltaics with aspherical lens for automobile Reviewed
Ota Y., Nishioka K., Araki K., Ikeda K., Lee K., Yamaguchi M.
2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017 1 - 4 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017
© 2017 IEEE. In this paper, we optimized the basic aspherical lens for automobile using optical method. Since the panel is installed in horizontal, the distribution of incident angle that is important to concentrator optics design needs to be newly defined. By optimization of lens structure, the optical efficiency increased to 46.7%.
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Next environment-friendly cars: Application of solar power as automobile energy source Reviewed
Masuda T., Araki K., Okumura K., Urabe S., Kudo Y., Kimura K., Nakado T., Sato A., Yamaguchi M.
2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017 1 - 5 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017
© 2017 IEEE. We investigate the benefits of utilizing solar power as an energy source for future automobiles. Since there are strict emission standards developed by governments, we must find an alternative energy source for the future automobiles. We show that replacing all passenger cars with hybrid vehicles equipped with a 800 W rated-power solar module would reduce greenhouse gas (GHG) emission by 63% in Japan. We obtained an average power generation of 2.1 kWh/day over 100 days by solar modules installed on a Prius under the real environment, which was larger than the required power to achieve the GHG emission reduction. We also investigate an innovative statistic low-concentrator with III-V cells in order to reduce installation area for the automobiles. We show that our proposed design (asymmetric-aspheric design) can expand the acceptance incident angle and increase the annual energy yield, while keeping thin structure, which is essential to automobile application.
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Fabrication and performance analysis of a mechanical stack InGaP/GaAs//Si solar cell Reviewed
Lee K., Nakamura K., Kamioka T., Kojima N., Lee H., Wang L., Araki K., Ohshita Y., Ogura A., Yamaguchi M.
2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017 1 - 3 2017
Language：English Publishing type：Research paper (scientific journal) Publisher：2017 IEEE 44th Photovoltaic Specialist Conference, PVSC 2017
© 2017 IEEE. We present the measurement results and analyses of a InGaP/GaAs//Si mechanical stack four-terminal solar cell. From these results, we performed detailed discussions of the loss mechanisms and efficiency limits of InGaP/GaAs//Si mechanical stack solar cells.
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Ikeda K., Araki K., Ota Y., Lee K., Nishioka K., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 566 - 569 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2016 IEEE. For the purpose of designing the high efficient static solar concentrator on automobile, we discuss the way to increase the aperture area. The 2-dimensional (2D) single and double refraction models based on the Snell's law are prepared under the conditions of random azimuth and large off-angle of the incident ray. We find that the center height of the lens is a key parameter to increase the aperture size. For lens array systems, the optimum center height is determined by the upper limit which is required to suppress the shadowing loss at the adjacent lenses. We demonstrate the ray tracing of the 2D lens model.
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Lee K., Araki K., Wang L., Kojima N., Ohshita Y., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 1906 - 1908 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2016 IEEE. We present a quantitative approach to investigate and compare the material qualities of III-V on silicon (III-V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality for achieving high efficiency III-V/Si solar cells. This result yields several implications of designing high efficiency III-V/Si solar cells.
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Fabrication and performance analysis of a mechanical stack InGaP/GaAs//Si solar cell Reviewed
Lee K., Nakamura K., Kamioka T., Kojima N., Lee H., Wang L., Araki K., Ohshita Y., Ogura A., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 1957 - 1959 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2016 IEEE. We present the measurement results and analyses of a InGaP/GaAs//Si mechanical stack four-terminal solar cell. From these results, we performed detailed discussions of the loss mechanisms and efficiency limits of InGaP/GaAs//Si mechanical stack solar cells.
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Optimization of static concentrator photovoltaics with aspherical lens for automobile Reviewed
Ota Y., Nishioka K., Araki K., Ikeda K., Lee K., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 570 - 573 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2016 IEEE. In this paper, we optimized the basic aspherical lens for automobile using optical method. Since the panel is installed in horizontal, the distribution of incident angle that is important to concentrator optics design needs to be newly defined. By optimization of lens structure, the optical efficiency increased to 46.7%.
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Next environment-friendly cars: Application of solar power as automobile energy source Reviewed
Masuda T., Araki K., Okumura K., Urabe S., Kudo Y., Kimura K., Nakado T., Sato A., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November 580 - 584 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2016 IEEE. We investigate the benefits of utilizing solar power as an energy source for future automobiles. Since there are strict emission standards developed by governments, we must find an alternative energy source for the future automobiles. We show that replacing all passenger cars with hybrid vehicles equipped with a 800 W rated-power solar module would reduce greenhouse gas (GHG) emission by 63% in Japan. We obtained an average power generation of 2.1 kWh/day over 100 days by solar modules installed on a Prius under the real environment, which was larger than the required power to achieve the GHG emission reduction. We also investigate an innovative statistic low-concentrator with III-V cells in order to reduce installation area for the automobiles. We show that our proposed design (asymmetric-aspheric design) can expand the acceptance incident angle and increase the annual energy yield, while keeping thin structure, which is essential to automobile application.
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Durability of polymeric encapsulation materials in a PMMA/glass concentrator photovoltaic system Reviewed
Miller D.C., Kempe M.D., Muller M.T., Gray M.H., Araki K., Kurtz S.R.
Progress in Photovoltaics: Research and Applications 24 ( 11 ) 1385 - 1409 2016.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
Copyright © 2016 John Wiley & Sons, Ltd. The durability of polymeric encapsulation materials was examined using outdoor exposure at the nominal geometric concentration of 500 suns. The results for 36-month cumulative field deployment are presented for materials including: poly(ethylene-co-vinyl acetate), (EVA); polyvinyl butyral (PVB); ionomer; polyethylene/polyoctene copolymer (PO); thermoplastic polyurethane (TPU); poly(dimethylsiloxane) (PDMS); poly(diphenyl dimethyl siloxane) (PDPDMS); and poly(phenyl-methyl siloxane) (PPMS). Measurements of the field conditions including ambient temperature and ultraviolet (UV) dose were recorded at the test site during the experiment. Measurements for the experiment included optical transmittance (with subsequent analysis of solar-weighted transmittance, UV cut-off wavelength, and yellowness index), mass, visual photography, photoelastic imaging, and fluorescence spectroscopy. While the results to date for EVA are presented and discussed, examination here focuses more on the siloxane materials. A specimen recently observed to fail by thermal decomposition is discussed in terms of the implementation of the experiment as well as its fluorescence signature, which was observed to become more pronounced with age. Modulated thermogravimetry (allowing determination of the activation energy of thermal decomposition) was performed on a subset of the siloxanes to quantify the propensity for decomposition at elevated temperatures. Supplemental, Pt-catalyst- and primer-solutions as well as peroxide-cured PDMS specimens were examined to assess the source of the luminescence. The results of the study including the change in optical transmittance, observed failure modes, and subsequent analyses of the failure modes are described in the conclusions. Copyright © 2016 John Wiley & Sons, Ltd.
DOI： 10.1002/pip.2796
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Lee K., Araki K., Wang L., Kojima N., Ohshita Y., Yamaguchi M.
Progress in Photovoltaics: Research and Applications 24 ( 10 ) 1310 - 1318 2016.10
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
Copyright © 2016 John Wiley & Sons, Ltd. The paper presents a quantitative approach to the investigation and comparison of the material qualities of III–V on silicon (III–V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality in order to achieve high-efficiency III–V/Si solar cells. This result yields several implications for the design of high-efficiency III–V/Si solar cells. Copyright © 2016 John Wiley & Sons, Ltd.
DOI： 10.1002/pip.2787
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Which is optimum tracker allocation, checkerboard or rectangular grid? Reviewed
Araki K., Lee K., Yamaguchi M.
AIP Conference Proceedings 1766 2016.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2016, American Institute of Physics Inc. All rights reserved. The optimum allocations of multiple trackers for an infinite land will be either rectangular allocation parallel to N-S and E-W axis, or checkerboard allocation. We solved an optimization problem. Type of the optimum allocation pattern and its aspect ratio depend on latitude, climate pattern, aspect ratio of panels and space factor. Roughly speaking, the checkerboard allocation will be desired to higher latitude, sunshine in summer, higher space factor, and highly landscaped rectangular solar panels. A decision flow chart was proposed by the calculation result.
DOI： 10.1063/1.4962097
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Possibility of solar station to EV Reviewed
Araki K., Nagai H., Yamaguchi M.
AIP Conference Proceedings 1766 2016.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2016, American Institute of Physics Inc. All rights reserved. For reduction of the green-house gas emission in the transportation sector, it is important to look for the way to supply renewable energies to automobiles. The car running 30 km a day may be powered by solar panel on the car-roof, but the rest of the cars may be charged by the charging station from solar energy. In this study, feasibility of the charging station powered by the practical size of CPV was investigated using the charging and discharging simulation based on the meteorological database in Japan by three criteria. It was concluded that 6 m x 6 m panel would be sufficient to charge maximum 5 electric vehicles in a day.
DOI： 10.1063/1.4962099
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Study on tolerance control for optical alignment of CPV modules using a Monte Carlo simulation Reviewed
Araki K., Nagai H., Lee K., Yamaguchi M.
AIP Conference Proceedings 1766 2016.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2016, American Institute of Physics Inc. All rights reserved. CPV requires accuracy in optical alignment. It is required to control the assembly tolerance quantitatively. It is convenient to control the tolerance by statistical parameters. A Monte Carlo method was developed to relate how statistical parameters in component and assemble variance affects to the module performance. The rough rule will be the standard deviation of misalignment should be controlled less than 20 % of designed acceptance angle, unless the number of the cells inside the module was not very small (Typically N= 4). Namely, if the acceptance angle of the optics is plus or minus 1 degree, then the standard deviation of assembly error should be less than 0.2 deg.
DOI： 10.1063/1.4962109
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Possibility of static low concentrator PV optimized for vehicle installation Reviewed
Araki K., Ota Y., Ikeda K., Lee K., Nishioka K., Yamaguchi M.
AIP Conference Proceedings 1766 2016.9
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2016, American Institute of Physics Inc. All rights reserved. One of the most direct and efficient ways in eliminating greenhouse gas emission from cars is to install a PV panel on the roof. Since the area of the roof of the car is limited, high-efficiency III-V cells will be useful. Because of the space limitation, it should be high-efficiency panel. III-V cells may be a candidate, but it should be CPV for saving cost of the cell. Considering its quick movement and appearance, trackers were thought difficult to implement. Our choice was a static concentrator customized to automobile. Special static concentrator that collects sunlight from shallow incident angle was successfully developed.
DOI： 10.1063/1.4962069
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Design and Development of Dome-Shaped Fresnel Lens Reviewed
Araki K., Nagai H., Lee K., Ikeda K., Yamaguchi M.
IEEE Journal of Photovoltaics 6 ( 5 ) 1339 - 1344 2016.9
Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2011-2012 IEEE. Dome-shaped Fresnel lenses were developed with a new design algorithm. The lens was defined as the integration of pentagon prisms. This method is flexible and generates a dome-shaped Fresnel lens suitable for high concentration by keeping the thickness approximately equal in the entire region of the lens so that the lens becomes mechanically robust with the minimum amount of materials.
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Araki K., Nagai H., Herrero R., Anton I., Sala G., Yamaguchi M.
IEEE Journal of Photovoltaics 6 ( 5 ) 1353 - 1359 2016.9
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2011-2012 IEEE. Concentrator photovoltaics (CPV) requires accuracy in optical alignment. Optical misalignment in the module was measured and analyzed. Such information was useful to identify assembly problems and improved module performance. A Monte Carlo simulation was developed to analyze the impact of alignment errors by assembly errors. The simulation result implied that the key statistical parameters for module performance were not averaged errors but the standard deviation and the worst value. 'Cutting above certain level' was effective, including the introduction of jigs that keep misalignment at a certain level. The standard deviation of misalignment should be controlled to less than 20% of the designed acceptance half angle.
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Optimization of Land Use for a Multitracker System Using a Given Geometrical Site Condition Reviewed
Araki K., Nagai H., Ikeda K., Lee K., Yamaguchi M.
IEEE Journal of Photovoltaics 6 ( 4 ) 960 - 966 2016.7
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Journal of Photovoltaics
© 2016 IEEE. Shading loss of a multitracker concentrator photovoltaic (CPV) system can be predicted, and for doing this, geometrical optimization will be useful. The optimum allocation for infinite land will be either rectangular allocation parallel to the N-S and E-W axis or checkerboard allocation. We solve an optimization problem with the introduction of nondimensional parameters in geometry, namely, optimum conditions as a function of space factor and panel aspect ratio. The type of optimum allocation pattern and its aspect ratio depend on latitude, climate pattern, aspect ratio of panels, and space factor. Our calculation results show that, in general, checkerboard allocation is preferred for locations with higher latitude, higher sun irradiation in summer, higher space factor, and horizontally longer rectangular solar panels.
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Recent results for concentrator photovoltaics in Japan Reviewed
Yamaguchi M., Takamoto T., Araki K., Kojima N.
Japanese Journal of Applied Physics 55 ( 4 ) 2016.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
© 2016 The Japan Society of Applied Physics. We summarize the Europe-Japan Collaborative Research Project on Concentrator Photovoltaics (CPV); NGCPV Project (a New Generation of Concentrator PhotoVoltaic cells, modules and systems). The aim of this project was to accelerate the move to very high efficiency and lower cost CPV technologies and to enhance the widespread deployment of CPV systems. Seven European partners and nine Japanese partners have contributed to this international collaboration. The following objectives have been reached: 1) a CPV cell with InGaP/GaAs/InGaAs three-junction world-record efficiency of 44.4% has been developed by Sharp, 2) 50kW and 15kW CPV plant operations with an average DC efficiency of 27.8% have been demonstrated in Spain (since mid-2012), 3) a new "Intrepid" CPV module with 31.3% efficiency has been developed by Daido Steel, 4) standard measurement of CPV cells has been established by FhG-ISE and AIST, and 46.0% efficiency has been confirmed for the direct-bonded GaInP/GaAs/GaInAsP/GaInAs four-junction solar cell under this project, 5) the fundamental research on novel materials and structures for CPV has also been conducted.
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Potential and activities of III-V/Si tandem solar cells Reviewed
Yamaguchi M., Lee K., Araki K., Kojima N., Ohshita Y.
ECS Journal of Solid State Science and Technology 5 ( 2 ) Q68 - Q73 2016
Language：English Publishing type：Research paper (scientific journal) Publisher：ECS Journal of Solid State Science and Technology
© 2015 The Electrochemical Society. All rights reserved. The paper presents efficiency potential of III-V/Si tandem solar cells with efficiencies of more than 35% under 1-sun AM1.5G and effects of dislocation upon solar cell properties of III-V-on-Si single-junction solar cells and III-V/Si tandem solar cells. Because III-V/Si system has large lattice mismatching and difference in thermal expansion coefficient, generation of misfit and thermal stress induced dislocations affect on solar cell properties. Effects of dislocations upon minority-carrier lifetime are derived from considering one dimensional transport of minority carriers to dislocations. In order to realize high efficiency solar cells with similar efficiency by homo-epitaxially grown solar cells, low density dislocation of less than 3 × 105 cm-2 is necessary. This paper also reviews approaches on reduction in dislocation density in III-V compound films on Si and improvements in efficiencies of III-V compound single-junction solar cells on Si substrates and III-V/Si tandem solar cells, along with reviewing the efficiency potentials of using other efficiency potential of other materials/Si tandem solar cells such as II-V compound/Si. Chalcopyrite/Si and Perovskite/Si tandem solar cells.
DOI： 10.1149/2.0311602jss
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Espinet-González P., Algora C., Núñez N., Orlando V., Vázquez M., Bautista J., Araki K.
Progress in Photovoltaics: Research and Applications 23 ( 5 ) 559 - 569 2015.5
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
Copyright © 2014 John Wiley & Sons, Ltd. A temperature accelerated life test on commercial concentrator lattice-matched GaInP/GaInAs/Ge triple-junction solar cells has been carried out. The acceleration of the aging has been accomplished by subjecting the solar cells at temperatures markedly higher than the nominal working temperature inside a concentrator, and the nominal photo-current condition (820 X) has been emulated by injecting current in darkness. Three tests at different temperatures have been carried out. The failure distributions across the three test temperatures have been fitted to an Arrhenius-Weibull model. An Arrhenius activation energy of 1.59-eV was determined from the fit. The reliability functions and parameters of these solar cells at two nominal working conditions (80-and-100-°C) have been obtained. In both cases, the instantaneous failure rate function monotonically increases, that is, the failures are of the wear-out kind. We have also observed that the reliability data are very sensitive to the nominal temperature condition. In fact, at a nominal working condition of 820 X and 80-°C, assuming that the concentration module works 5-h per day, the warranty time obtained for a failure population of 5% has been 113-years. However, for a nominal working condition of 820 X and 100-°C, the warranty time obtained for a failure population of 5% has been 7-years. Therefore, in order to offer a long-term warranty, the working temperature could be a key factor in the design of the concentration photovoltaic systems.
DOI： 10.1002/pip.2461
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Potential and activities of III-V/si tandem solar cells Reviewed
Yamaguchi M., Lee K., Araki K., Kojima N., Ohshita Y.
ECS Transactions 69 ( 4 ) 11 - 20 2015
Language：English Publishing type：Research paper (scientific journal) Publisher：ECS Transactions
© The Electrochemical Society. The paper presents efficiency potential of III-V/Si tandem solar cells with efficiencies of more than 35% under 1-sun AM1.5G and effects of dislocation upon solar cell properties of III-V-on-Si single-junction solar cells and III-V/Si tandem solar cells. Because III-V/Si system has large lattice mismatching and difference in thermal expansion coefficient, generation of misfit and thermal stress induced dislocations affect on solar cell properties. Effects of dislocations upon minority-carrier lifetime are derived from considering one dimensional transport of minority carriers to dislocations. In order to realize high efficiency solar cells with similar efficiency by homo-epitaxially grown solar cells, low density dislocation of less than 3×105 cm-2 is necessary. This paper also reviews approaches on reduction in dislocation density in III-V compound films on Si and improvements in efficiencies of III-V compound single-junction solar cells on Si substrates and III-V/Si tandem solar cells.
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Characterization of CPV arrays based on differences on their thermal resistances Reviewed
Núñez R., Antón I., Askins S., Sala G., Araki K.
AIP Conference Proceedings 1616 144 - 148 2014
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
©2014 AIP Publishing LLC. Thermal characterization of Concentrating Photovoltaics (CPV) modules and arrays is needed to determine their performance and modelling of energy forecast. Module-ambient thermal resistance is easily obtained from its definition but the cell-module thermal resistant needs to be estimated from indirect procedures, two of them are presented in this paper. In addition, an equivalent parameter is defined, the Concentrator Nominal Operating Module/Cell Temperature (CNOMT/CNOCT), the temperature at Concentrator Standard Operating Conditions (CSOC). Definitions and expression to relate (CNOMT/CNOCT) to thermal resistances are presented, plus several examples of estimations from real operating arrays.
DOI： 10.1063/1.4897048
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Module optical analyzer: Identification of defects on the production line Reviewed
Herrero R., Askins S., Antón I., Sala G., Araki K., Nagai H.
AIP Conference Proceedings 1616 119 - 123 2014
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
©2014 AIP Publishing LLC. The usefulness of the module optical analyzer when identifying module defects on production line is presented in this paper. Two different case studies performed with two different kind of CPV modules are presented to show the use of MOA both in IES-UPM and Daido Steel facilities.
DOI： 10.1063/1.4897042
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Oke S., Saeki N., Motoda H., Kemmoku Y., Araki K.
IEEJ Transactions on Power and Energy 134 ( 5 ) 436 - 442 2014
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEJ Transactions on Power and Energy
A concentrator photovoltaic (CPV) system which can reduce the power generation cost of PV system has been developed and assessed for commercialization. An early-type CPV module consists of 20 pairs of dome Fresnel lenses the geometrical concentration ratio of which is X550 and three junction solar cells (InGaP/InGaAs/Ge). We had carried out a field test of the early-type CPV module to evaluate the power generation, the efficiency, and other characteristics from 2004. The Fresnel lenses had never been washed until 2012 were washed and some characteristics of the CPV module were compared in before and after cleaning the lenses. Before the cleaning, many micro particulate dust included 46wt.% of C was on the lenses. Its number density decreased from 600mm-2 to 230mm-2 with cleaning the lenses. However, some characteristics of the CPV module such as I-V curve, FF and electricity efficiency were not changed by cleaning the lenses. The influences of yellow sand (Asian dust) which decreased the direct irradiance and covered on the lens surface from some deserts in interior of East Asia did not continue persistently. This is because, any dirt would not cover easily on the lenses surface, since the CPV module was moved to track to the Sun, and it had the dome-shaped Fresnel lenses which had smooth surface. © 2014 The Institute of Electrical Engineers of Japan.
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Validation of energy prediction method for a concentrator photovoltaic module in Toyohashi Japan Reviewed
Chan N.L.A., Young T.B., Brindley H.E., Ekins-Daukes N.J., Araki K., Kemmoku Y., Yamaguchi M.
Progress in Photovoltaics: Research and Applications 21 ( 8 ) 1598 - 1610 2013.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
III-V concentrator photovoltaic systems attain high efficiency through the use of series connected multi-junction solar cells. As these solar cells absorb over distinct bands over the solar spectrum, they have a more complex response to real illumination conditions than conventional silicon solar cells. Estimates for annual energy yield made assuming fixed reference spectra can vary by up to 15% depending on the assumptions made. Using a detailed computer simulation, the behaviour of a 20-cell InGaP/In0.01GaAs/Ge multi-junction concentrator system was simulated in 5-min intervals over an entire year, accounting for changes in direct normal irradiance, humidity, temperature and aerosol optical depth. The simulation was compared with concentrator system monitoring data taken over the same period and excellent agreement (within 2%) in the annual energy yield was obtained. Air mass, aerosol optical depth and precipitable water have been identified as atmospheric parameters with the largest impact on system efficiency. Copyright © 2012 John Wiley & Sons, Ltd. The behaviour of a 20-cell InGaP/In0.01GaAs/Ge multi-junction concentrator system was simulated in 5-min intervals over an entire year with agreement within 2% between measured and modelled annual energy yields. Air mass, aerosol optical depth and precipitable water have been identified as the atmospheric parameters with the largest impact on system efficiency. Copyright © 2012 John Wiley & Sons, Ltd.
DOI： 10.1002/pip.2241
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Effect of anti-soiling photocatalytic coating for concentrator photovoltaic module Reviewed
Ota Y., Sueto T., Yokota M., Araki K., Nishioka K.
Proceedings of the 20th International Workshop on Active-Matrix Flatpanel Displays and Devices: TFT Technologies and FPD Materials, AM-FPD 2013 193 - 194 2013.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the 20th International Workshop on Active-Matrix Flatpanel Displays and Devices: TFT Technologies and FPD Materials, AM-FPD 2013
An anti-soiling layer was coated on the surface of polymethylmethacrylate (PMMA), which is the primary material of Fresnel lenses for concentrator photovoltaics. The layer was prepared by coating the photocatalytic surface layer with modified WO3 and partially hydrolyzed tetraethyl orthosilicate. When sand was discharged onto the PMMA samples, the mass of adherent sand was more than 0.010 g for the sample without the coating and approximately 0.005 g for the sample with the coating. The electrostatic potential of the sample without the coating increased with increasing amount of incident sand, reaching a maximum value of 0.25 k V. On the other hand, the electrostatic potential of the sample with the coating was suppressed to 0.1 kV. The presence of electrostatic charges on the surface of the samples was a main factor for the adhesion of sand, and it could be suppressed by the anti-soiling photocatalytic layer.. © 2013 JSAP.
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Durability of polymeric encapsulation materials for concentrating photovoltaic systems Reviewed
Miller D., Muller M., Kempe M., Araki K., Kennedy C., Kurtz S.
Progress in Photovoltaics: Research and Applications 21 ( 4 ) 631 - 651 2013.6
Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Photovoltaics: Research and Applications
The durability of polymeric encapsulation materials was examined using outdoor exposure at the nominal optical concentration of 500 suns. The results for 12 months' cumulative field deployment are presented for materials including ethylene-co-vinyl acetate, polyvinyl butyral (PVB), ionomer, polyethylene/polyoctene copolymer, thermoplastic polyurethane, poly(dimethylsiloxane), and poly(phenyl-methyl siloxane). Measurements during the experiment included optical transmittance (direct and hemispherical), mass, visual appearance, and fluorescence spectroscopy in addition to the initial thermogravimetry of the materials. Measurements of the field conditions and ultraviolet dose at the test site were facilitated by numerous laboratory instruments; characterization of the specimen temperature was performed using thermography. Discovery experiments identified the importance of a secondary homogenizer optic and the importance of contamination control. To date, the formal experiment verified a thermal-runaway-motivated combustion failure mechanism for one of the PVB formulations and identified densification, cracking, and haze-formation behaviors in some of the silicone specimens. The behaviors observed for the silicone specimens may be facilitated by their greater thickness. Copyright © 2012 John Wiley & Sons, Ltd.
DOI： 10.1002/pip.1241
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Reduction in operating temperature of 25 series-connected 820X concentrator photovoltaic module Reviewed
Ota Y., Sueto T., Nagai H., Araki K., Nishioka K.
Japanese Journal of Applied Physics 52 ( 4 PART 2 ) 2013.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
Under concentration conditions, it is important to manage the operating temperature of a concentrator photovoltaic (CPV) module, because a high-density solar energy enters into the solar cell. We measured the receiver temperature of the CPV module and evaluated the relationship between the temperature and output of the CPV module. 25 series-connected 820X CPV modules with 2- and 4-mm-thick back chassises were fabricated. In the case of a CPV module with a 4-mm-thick back chassis, the receiver temperature was markedly reduced owing to the effective thermal diffusion of the thicker chassis. The absolute conversion efficiency of the CPV module with the 4-mm-thick back chassis was 1.5% higher than that of the module with the 2-mm-thick back chassis. Moreover, we developed a thermal transfer model of the CPV module and calculated the thermal distribution in the CPV module using a thermal transfer simulator. © 2013 The Japan Society of Applied Physics.
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Heat reduction of concentrator photovoltaic module using high radiation coating Reviewed
Nishioka K., Ota Y., Tamura K., Araki K.
Surface and Coatings Technology 215 472 - 475 2013.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Surface and Coatings Technology
A thermal radiation layer was coated on the aluminum chassis of a concentrator photovoltaic (CPV) module. The temperature of the solar cell in the CPV module with the thermal radiation coating was approximately 10 °C lower than that of the module without the thermal radiation coating. The uniformity of the temperature distribution in the CPV module was considerably improved. The thermal radiation coating acted not only as a thermal radiation layer but also as a thermal conduction layer. The open-circuit voltage of the CPV module with thermal radiation coating was 0.5. V higher than that of the module without the coating during the period evaluated. The conversion efficiency of the CPV module with thermal radiation coating was 0.5% higher than that of the module without the coating. © 2012 Elsevier B.V.
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Sueto Tsuyoshi, Ota Yasuyuki, Araki Kenji, Kumagai Isao, Nishioka Kensuke
The Journal of the Institute of Electrical Installation Engineers of Japan 33 ( 7 ) 547 - 552 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Installation Engineers of Japan
We propose a simple guidepost for the installation of photovoltaic systems using the field test data in Miyazaki,Japan. Concentrator photovoltaic (CPV) system generates electricity only from direct solar irradiance. Therefore,the output of CPV system is drastically influenced by scattering ratio due to humidity,aerosols,and yellow sand,The scattering ratio varies with the seasons. We proposed an insta11ation guidepost of CPV systems using the scattering ratio and gradient global irradiance. We measured the output of CPV and flat-p1ate multi-crystalline Si PV systems. We have measurement systems for direct and global solar irradiance. Therefor,we can obtain the scattering ratio. Gradient global irradiance was also measured. These field test results were analyzed and a guidepost was derived. Based on the guidepost,we can decide which systems of CPV or Si PV systems is suitable at a candidate site for system installation
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Espinet-Gonzalez P., Romero R., Orlando V., Gabas M., Nunez N., Vazquez M., Palanco S., Bijani S., Contreras Y., Galiana B., Algora C., Araki K.
Conference Record of the IEEE Photovoltaic Specialists Conference 1666 - 1671 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
In this work the failure analysis carried out in III-V concentrator multijunction solar cells after a temperature accelerated life test is presented. All the failures appeared have been catastrophic since all the solar cells turned into low shunt resistances. A case study in failure analysis based on characterization by optical microscope, SEM, EDX, EQE and XPS is presented in this paper, revealing metal deterioration in the bus bar and fingers as well as cracks in the semiconductor structure beneath or next to the bus bar. In fact, in regions far from the bus bar the semiconductor structure seems not to be damaged. SEM images have dismissed the presence of metal spikes inside the solar cell structure. Therefore, we think that for these particular solar cells, failures appear mainly as a consequence of a deficient electrolytic growth of the front metallization which also results in failures in the semiconductor structure close to the bus bars. © 2013 IEEE.
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Thermal transfer simulation for concentrator photovoltaic receiver under concentration condition Reviewed
Ota Y., Nagai H., Araki K., Nishioka K.
AIP Conference Proceedings 1556 18 - 21 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2013 AIP Publishing. Under concentration conditions, it is important to manage the operating temperature of a concentrator photovoltaic (CPV) module, because a high-density solar energy enters into the solar cell. We measured the temperature of the CPV module. In the outdoor operation, the module back surface temperature of the CPV module was 3.3 K lower than the aluminum stage temperature. Moreover, we developed a thermal transfer model for the CPV module and calculated the temperature in it using a thermal transfer simulator. The temperatures in CPV module were calculated as a function of the thermal resistance between aluminum stage and back chassis. The temperature coefficient of Voc using calculated solar cell temperature and measured Voc was approximately -0.22%, which was in good agreement with previous report. The simulation results showed that the thicker back chassis and higher heat emissivity were effective for the reduction of operating temperature of CPV module.
DOI： 10.1063/1.4822189
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Thermal transfer simulating for concentrator photovoltaic module under concentration condition Reviewed
Ota Y., Nagai H., Araki K., Nishioka K.
Conference Record of the IEEE Photovoltaic Specialists Conference 470 - 473 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
Under concentration conditions, it is important to manage the operating temperature of a concentrator photovoltaic (CPV) module, because a high-density solar energy enters into the solar cell. We measured the temperature of the CPV module. In outdoor operation, the back surface temperature of the CPV module was 6.2 K lower than aluminum stage temperature. Moreover, we developed a thermal transfer model for the CPV module and calculated the temperature in CPV module as functions of the thermal resistance and the heat transfer coefficient using a thermal transfer simulator. The simulation results showed that the lower thermal resistance between the solar cell and the back surface and higher heat transfer coefficient of back surface were effective for the reduction of operating temperature. The estimated heat transfer coefficient was comparatively near with the theoretical value. By using the developed model, we can estimate the operating temperature in concentrating operation. © 2013 IEEE.
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Al Husna H., Shibata N., Sawano N., Ueno S., Ota Y., Minemoto T., Araki K., Nishioka K.
AIP Conference Proceedings 1556 252 - 255 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2013 AIP Publishing. Multi-junction solar cell is designed to have considerable effect towards the solar spectrum distribution so that the maximum solar radiation could be absorbed hence, enhancing the energy conversion efficiency of the cell. Due to its application in CPV system, the system's characteristics are more sensitive to environmental factor in comparison to flat-plate PV system which commonly equipped with Si-based solar cell. In this paper, the impact of environmental factors i.e. average photon energy (APE) and temperature of solar cell (Tcell) towards the performance of the tracking type CPV system were discussed. A year data period of direct spectral irradiance, cell temperature, and power output which recorded from November 2010 to October 2011 at a CPV system power generator plant located at Miyazaki, Japan was used in this study. The result showed that most frequent condition during operation was at APE = 1.87±0.005eV, Tcell = 65±2.5°C with performance ratio of 83.9%. Furthermore, an equivalent circuit simulation of a CPV subsystem in module unit was conducted in order to investigate the influence of environmental factors towards the performance of the module.
DOI： 10.1063/1.4822243
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Espinet-González P., Algora C., Núñez N., Orlando V., Vázquez M., Bautista J., Araki K.
AIP Conference Proceedings 1556 222 - 225 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2013 AIP Publishing. A temperature accelerated life test on commercial concentrator lattice-matched GaInP/GaInAs/Ge triplejunction solar cells has been carried out. The solar cells have been tested at three different temperatures: 119, 126 and 164 °C and the nominal photo-current condition (820 X) has been emulated by injecting current in darkness. All the solar cells have presented catastrophic failures. The failure distributions at the three tested temperatures have been fitted to an Arrhenius-Weibull model. An Arrhenius activation energy of 1.58 eV was determined from the fit. The main reliability functions and parameters (reliability function, instantaneous failure rate, mean time to failure, warranty time) of these solar cells at the nominal working temperature (80 °C) have been obtained. The warranty time obtained for a failure population of 5 % has been 69 years. Thus, a long-term warranty could be offered for these particular solar cells working at 820 X, 8 hours per day at 80 °C.
DOI： 10.1063/1.4822236
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Durability of polymeric encapsulation materials in a PMMA/glass concentrating photovoltaic system Reviewed
Miller D.C., Kempe M.D., Muller M.T., Kennedy C.E., Araki K., Kurtz S.R.
Optics InfoBase Conference Papers 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：Optics InfoBase Conference Papers
Many CPV cells are coupled to an optical component/coverglass with a polymer. We describe a screen test to identify field-induced failure modes in popular encapsulation materials. Results are presented for 36 months cumulative exposure. Renewable Energy and the Environment Congress. © 2013.
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Detailed effects of wind on the field performance of a 50 kW CPV demonstration plant Reviewed
Castro M., Domínguez C., Núñez R., Antón I., Sala G., Araki K.
AIP Conference Proceedings 1556 256 - 260 2013
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
© 2013 AIP Publishing. The effect of the wind speed and direction on the arrays in a 50 kW plant was analyzed along several days. The wind is measured in different positions of the plant: the general plant meteorological station, the module's boundary layer and the close surroundings of the array. The averaged thermal resistance between the back of the module and the ambient is calculated and its dependence with the wind direction analyzed. Early results suggest that Northern wind has a better cooling effect than Western and Eastern winds.
DOI： 10.1063/1.4822244
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Two interactive and practical methods for optimization of tracker allocation in a given land Reviewed
Araki K.
AIP Conference Proceedings 1477 244 - 247 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
The purpose of this paper is proposals of two interactive methods of tracker allocation effective to relatively smaller installation. Interaction helps designers to consider every detail of the limitation and possibly more accurate approach to the project of narrow land space rather than blindly relying mathematical optimum calculation. One is Overlay of skewed rectangular mesh method. This is a simple implementation of classical optimization calculation to interactive environment. Another is Butterfly method. The idea of using butterfly pattern was first invented by Spanish engineer, Dobon, and often called Dobon's butterfly in European regions. The original definition is the contour plot on the influence shading from a single tracker. We have improved this method for interactive design tool. © 2012 American Institute of Physics.
DOI： 10.1063/1.4753878
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Impact of volcanic ash on CPV system in Miyazaki Japan Reviewed
Ota Y., Araki K., Nishioka K.
AIP Conference Proceedings 1477 340 - 343 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Recently, large-scale photovoltaic (PV) systems have attracted attention. In the field-operation, the electric power generations of PV systems, including CPV system, are affected by the natural environment such as ambient temperature, snowfall, soiling, etc. In this study, we assessed the impact of the volcanic ash on CPV system, and the output characteristics of the CPV and CIS systems were compared. The output of CIS PV system was drastically decreased by the volcanic ash. On the other hand, in the case of the CPV system, the output was not decreased so much. © 2012 American Institute of Physics.
DOI： 10.1063/1.4753900
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The dome-shaped Fresnel-Köhler concentrator Reviewed
Zamora P., Benitez P., Li Y., Miñano J., Mendes-Lopes J., Araki K.
AIP Conference Proceedings 1477 69 - 72 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Manufacturing tolerances, along with a high concentration ratio, are key issues in order to obtain cheap CPV systems for mass production. Consequently, this manuscript presents a novel tolerant and cost effective concentrator optic: the domed-shaped Fresnel-Köhler, presenting a curved Fresnel lens as Primary Optical Element (POE). This concentrator is based on two previous successful CPV designs: the FK concentrator, based on a flat Fresnel lens, and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. The manuscript shows outstanding simulation results for geometrical concentration factor of Cg=1,230x: high tolerance and high optical efficiency, achieving acceptance angles of 1.18° (dealing to a CAP*=0.72) and efficiencies over 85% (without any anti-reflective coating). Moreover, Köhler integration provides good irradiance uniformity on the cell surface without increasing system complexity by means of any extra element. Daido Steel advanced technique for demolding injected plastic pieces will allow for easy manufacture of the dome-shaped POE of DFK concentrator. © 2012 American Institute of Physics.
DOI： 10.1063/1.4753836
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Temperature distribution in 820X CPV module during outdoor operation Reviewed
Ota Y., Nagai H., Araki K., Nishioka K.
AIP Conference Proceedings 1477 364 - 367 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Under the concentration conditions, it is important to manage the operating temperature of CPV module, because the high density solar energy is emitted to the solar cell. The operating temperature is a dominant factor for CPV module efficiency. We analyzed the temperature distribution in a 820X module modified to measure the temperature and output of each receiver. The relationship between the operating temperature and the module efficiency was clarified. © 2012 American Institute of Physics.
DOI： 10.1063/1.4753906
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Proposal of an energy rating method fair to countries of lower irradiance resources Reviewed
Araki K.
AIP Conference Proceedings 1477 344 - 347 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
International standard must be fair among nations. Fairness means fair among manufactures, testers and users. Present typical approaches, based on linear approximation and sampling only "good climate", may be improved by assuring fairness among spectrum and irradiance conditions among nations. In unwanted area, low efficiency days are usually humid and white-sky days. The energy anticipation will be underestimated based on the measurement by manufacturers in unwanted countries. This is not fair among manufactures. If we only filter "good climate data", it may be helpful to users in good irradiance area. This filtering may be blind to the realistic conditions. This is not fair to users. The robust approach will be parabolic representation of "daily energy" based on more than 10 days data. © 2012 American Institute of Physics.
DOI： 10.1063/1.4753901
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Photovoltaic performance of the dome-shaped fresnel-köhler concentrator Reviewed
Zamora P., Benítez P., Li Y., Miñano J., Mendes-Lopes J., Araki K.
Proceedings of SPIE - The International Society for Optical Engineering 8468 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of SPIE - The International Society for Optical Engineering
In order to have a cost-effective CPV system, two key issues must be ensured: high concentration factor and high tolerance. The novel concentrator we are presenting, the dome-shaped Fresnel-Köhler, can widely fulfill these two and other essential issues in a CPV module. This concentrator is based on two previous successful CPV designs: the FK concentrator with a flat Fresnel lens and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. The concentrator has shown outstanding simulation results, achieving an effective concentrationacceptance product (CAP) value of 0.72, and an optical efficiency of 85% on-axis (no anti-reflective coating has been used). Moreover, Köhler integration provides good irradiance uniformity on the cell surface and low spectral aberration of this irradiance. This ensures an optimal performance of the solar cell, maximizing its efficiency. Besides, the domeshaped FK shows optimal results for very compact designs, especially in the f/0.7-1.0 range. The dome-shaped Fresnel- Köhler concentrator, natural and enhanced evolution of the flat FK concentrator, is a cost-effective CPV optical design, mainly due to its high tolerances. Daido Steel advanced technique for demolding injected plastic pieces will allow for easy manufacture of the dome-shaped POE of DFK concentrator. © 2012 SPIE.
DOI： 10.1117/12.929698
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Optical design of the dome-shaped fresnel-köhler concentrator Reviewed
Zamora P., Benítez P., Li Y., Miñano J., Mendes-Lopes J., Araki K.
Proceedings of SPIE - The International Society for Optical Engineering 8485 2012.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of SPIE - The International Society for Optical Engineering
The dome-shaped Fresnel-Köhler concentrator is a novel nonimaging optical design for photovoltaic applications. It is based on two previous successful CPV optical designs: the FK concentrator with a flat Fresnel lens and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. This optical concentrator will be able to achieve large concentration factor, high tolerance (i.e. acceptance angle) and high optical efficiency, three key issues when dealing with photovoltaic applications. Besides, its irradiance is distributed on the cell surface in a very even way. The concentrator has shown outstanding simulation results, achieving an effective concentration-acceptance product (CAP*) value of 0.72, an on-axis optical efficiency over 85% and good irradiance uniformity on the cell provided by Köhler integration. Furthermore, due to its high tolerances, we will present the dome-shaped Fresnel-Köhler concentrator as a cost-effective CPV optical design. This all makes this concentrator superior to other conventional competitors in the current market. © 2012 SPIE.
DOI： 10.1117/12.929723
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Ota Y., Sueto T., Nagai H., Araki K., Nishioka K.
Conference Record of the IEEE Photovoltaic Specialists Conference 961 - 964 2012.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
Under the concentration conditions, it is important to manage the operating temperature of CPV module, because the high density solar energy is emitted to the solar cell. The operating temperature is a dominant factor for CPV module efficiency. We analyzed the temperature distribution in a 820X module modified in order to measure the temperature and output of each receiver. As the results, the relationship between the operating temperature and the module efficiency was clarified. © 2012 IEEE.
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Nabemoto K., Sakurada Y., Ota Y., Takami K., Nagai H., Tamura K., Araki K., Nishioka K.
Japanese Journal of Applied Physics 51 ( 10 PART 2 ) 2012.10
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
An anti-soiling layer was coated on a poly(methyl methacrylate) (PMMA) substrate. The anti-soiling layer was prepared by coating the acrylic urethane capping layer, the inorganic/organic nano-graded intermediate layer, and the photocatalytic surface layer including modified WO 3 and partial hydrolyzed tetraethyl orthosilicate. The layers were coated by spin-coat method. The photocurrent from each subcell of the InGaP/InGaAs/ Ge triple-junction solar cell can be calculated by multiplying the solar spectrum of AM1.5D (850W/m 2), the transmittance of the PMMA substrate, and the spectral response of the triple-junction solar cell. After 8 months exposure, the reduction rate of photocurrent of the sample without antisoiling coat was 9.6%. On the other hand, that with anti-soiling coat could be suppressed to 3.3%. © 2012 The Japan Society of Applied Physics.
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Sandblasting durability of acrylic and glass Fresnel lenses for concentrator photovoltaic modules Reviewed
Nishioka K., Ikematsu K., Ota Y., Araki K.
Solar Energy 86 ( 10 ) 3021 - 3025 2012.10
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
The impact of sandblasting on Fresnel lenses for concentrator photovoltaic (CPV) modules must be carefully considered when CPV systems are installed in desert areas. In this study, the transmittance after sandblasting of acrylic polymethylmethacrylate (PMMA) and glass substrates was evaluated, and their durability was assessed. The transmittance decreased as the momentum of the blown sand increased. The conversion efficiency of the CPV modules was estimated using equivalent circuit calculations, and the influence of sandblasting on the output of the CPV modules was quantitatively predicted. The coefficients of degradation for the PMMA and glass CPV modules were 0.17 and 0.08 points per unit of momentum, respectively. The durability of the CPV module with the glass Fresnel lens is approximately twice that with the PMMA lens. © 2012 Elsevier Ltd.
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Two-dimensional mapping of localized characteristics of concentrator photovoltaic module Reviewed
Yano H., Nagai H., Tamura K., Araki K., Nishioka K.
Materials Science Forum 725 187 - 190 2012
Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Science Forum
For the development of concentrator photovoltaic (CPV) module realizing high efficiency, it is necessary to achieve the high efficiency optical system including Fresnel lens and homogenizer. For the improvement of optical systems, it is very important to understand the contribution of the light irradiated to a localized position on the Fresnel lens. The light beam induced current (LBIC) system was constructed to evaluate the focusing characteristic of the CPV module. We locally irradiated a light from solar simulator to the CPV mini-module and measured the generation current, and the localized characteristics were represented by two-dimensional mappings. Moreover, we evaluated the influence of tracking error by changing the irradiation angle. In the case of tracking errors, the peak of generated current was shifted and the current was decreased. In the case of tracking error of 1.0°, the total generated current was reduced 12%, in comparison with the normal irradiance. © (2012) Trans Tech Publications, Switzerland.
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Espinet-González P., Algora C., Orlando V., Núñez N., Vázquez M., Bautista J., Xiugang H., Barrutia L., Rey-Stolle I., Araki K.
Conference Record of the IEEE Photovoltaic Specialists Conference 2 2012
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
A quantitative temperature accelerated life test on sixty GaInP/GaInAs/Ge triple-junction commercial concentrator solar cells is being carried out. The final objective of this experiment is to evaluate the reliability, warranty period, and failure mechanism of high concentration solar cells in a moderate period of time. The acceleration of the degradation is realized by subjecting the solar cells at temperatures markedly higher than the nominal working temperature under a concentrator, while the photo-current nominal conditions are emulated by injecting current in darkness. Three experiments at three different temperatures are necessary in order to obtain the acceleration factor which relates the time at the stress level with the time at nominal working conditions. However, up to now only the test at the highest temperature has finished. Therefore, we can not provide complete reliability information but we have analyzed the life data and the failure mode of the solar cells inside the climatic chamber at the highest temperature. The failures have been all of them catastrophic. In fact, the solar cells have turned into short circuits. We have fitted the failure distribution to a two parameters Weibull function. The failures are wear-out type. We have observed that the busbar and the surrounding fingers are completely deteriorated.
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Araki K., Al Taher O., Nagai H., Hebert P., Valles J.
AIP Conference Proceedings 1407 303 - 306 2011.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Two types of failure of III-V cells in CPV system by Daido Steel have been observed. One is thermal runaway and another is what we call a electrical shock. This paper will discuss on the frequency of the cell failure seen in a field and on experiments to determine the root cause of thermal runaway. Failures by the electrical shock were not related to thermal runaway, and a packaging solution to the failure by the electrical shock was found which will be published at another time. A detailed investigation of 30 kW field was undertaken to identify failed cells. After the other failure mechanism has been removed, experiments can be conducted on thermal runaway. Thermal runaway can occur due to loss of thermal conduction, such as voids or discontinuities in the thermal interchange material bonding cell to heat sink. It has been hypothesized that thermal runaway can also occur at location of cell defects as identified by electroluminescence. So far it we have not been able to induce thermal runaway at locations of electroluminescence defects. © 2011 American Institute of Physics.
DOI： 10.1063/1.3658349
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Output comparison of CPV and flat-plate systems in Japanese meteorological condition Reviewed
Shibata N., Ota Y., Sakurada Y., Takahashi Y., Kumagai I., Araki K., Nishioka K.
AIP Conference Proceedings 1407 341 - 344 2011.12
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
The output characteristics of tracking type concentrator photovoltaic (CPV), tracking type silicon solar cell, and fixed silicon solar cell were compared in Japanese meteorological condition. At first, the module characteristics were evaluated. In the noon, output power of the CPV module was twice the crystalline silicon modules. In the morning and evening, tracked modules generated higher power due to the tracking. Next, the characteristics of 14 kW CPV system and 50 kW multi-crystalline silicon PV system were evaluated and compared. The output of CPV system was high but unstable because the CPV system was susceptible to meteorological conditions. The CPV system runs ahead of the crystalline silicon PV system under the conditions that the global irradiance per a day is more than 5kWh/m2 and the direct normal irradiance per a day is more than 4kWh/m2. © 2011 American Institute of Physics.
DOI： 10.1063/1.3658357
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Anti-soiling layer coarted on PMMA Fresnel lens for concentrator photovoltaic modules Reviewed
Nishioka K., Nabemoto K., Sakurada Y., Ota Y., Takami K., Nagai H., Araki K.
Conference Record of the IEEE Photovoltaic Specialists Conference 003332 - 003334 2011.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
An anti-soiling layer was coated on the PMMA substrates. The anti-soiling layer was prepared by coating the inorganic/organic nano-graded intermediate layer (Ube-Nitto Kasei Co., Ltd.) and photocatalytic surface layer including modified WO 3 (ILUMIO®, Sumitomo Chemical Co., Ltd.) and partial hydrolyzed tetraethylorthosilicate (Wako Pure Chemical Industries, Ltd.). After 6 months, the reduction rate of photocurrent without anti-soiling coat was 7.5%. On the other hand, that with anti-soiling coat could be suppressed to 3%. The experimental environment of the exposure site is clean, and there is a lot of rainfall. It is considered that the effect of the anti-soiling coat must be more significant if CPV systems are installed in dusty areas. © 2011 IEEE.
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Lightning test for concentrator photovoltaic system Reviewed
Tamura K., Araki K., Kumagai I., Nagai H.
Conference Record of the IEEE Photovoltaic Specialists Conference 000996 - 000998 2011.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
This paper describes the results of the lightning test for CPV system. The CPV system is provided with a tracking device. For 14kW system, the entire aperture area is 75 m 2 and its height is approximately 10 m. This is relatively tall structure and subject to receive lightning damage. The IEC62305 standard describes the durability of CPV modules against lightning strike effect. Our test revealed that lightning current passed through the module's aluminum chassis and flowed directly into the ground. Module IV measurement was performed, and no degradation or change was observer before and after the lightning test. © 2011 IEEE.
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Present and future of high efficiency multi-junction solar cells Reviewed
Yamaguchi M., Takamoto T., Araki K., Imaizumi M., Kojima N., Ohshita Y.
Optics InfoBase Conference Papers 2011.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Optics InfoBase Conference Papers
This paper reviews our research activities for III-V compound multi-junction (MJ) solar cells and recent R&D topics on concentrator MJ solar cells under the NEDO's R&D Project on Innovative Solar Cells initiated since 2008. The conversion efficiency of InGaP/(In)GaAs/InGaAs triple-junction solar cells has been improved to 35.8% (1-sun, AM1.5G) and 42.1% (230-suns) as a result of proposing double-hetero wide-band-gap tunnel junctions, understanding dislocation behavior in the lattice mismatched system and inverted epitaxial layer structure. We have contributed to industrialization of InGaP-based multi-junction solar cell for space use in Japan. The authors have also shown great potential of concentrator PV for terrestrial applications by developing concentrator 3-junction solar cell modules (area of 5,500cm2 for 550suns use) with an efficiency of 31.5%. Now, we are challenging higher efficiency toward 50% under the NEDO's Innovative R&D Project. © OSA/ CLEO 2011.
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Long-term performance estimation of a 500X concentrator photovoltaic system Reviewed
Kemmoku Y., Oke S., Araki K.
30th ISES Biennial Solar World Congress 2011, SWC 2011 1 710 - 715 2011.12
Language：English Publishing type：Research paper (scientific journal) Publisher：30th ISES Biennial Solar World Congress 2011, SWC 2011
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Present and future of high efficiency multi-junction solar cells Reviewed
Yamaguchi M., Takamoto T., Araki K., Imaizumi M., Kojima N., Ohshita Y.
2011 Conference on Lasers and Electro-Optics: Laser Science to Photonic Applications, CLEO 2011 2011.9
Language：English Publishing type：Research paper (scientific journal) Publisher：2011 Conference on Lasers and Electro-Optics: Laser Science to Photonic Applications, CLEO 2011
This paper reviews our research activities for III-V compound multi-junction (MJ) solar cells and recent R&D topics on concentrator MJ solar cells under the NEDO's R&D Project on Innovative Solar Cells initiated since 2008. The conversion efficiency of InGaP/(In)GaAs/InGaAs triple-junction solar cells has been improved to 35.8% (1-sun, AM1.5G) and 42.1% (230-suns) as a result of proposing double-hetero wide-band-gap tunnel junctions, understanding dislocation behavior in the lattice mismatched system and inverted epitaxial layer structure. We have contributed to industrialization of InGaP-based multi-junction solar cell for space use in Japan. The authors have also shown great potential of concentrator PV for terrestrial applications by developing concentrator 3-junction solar cell modules (area of 5,500cm2 for 550suns use) with an efficiency of 31.5%. Now, we are challenging higher efficiency toward 50% under the NEDO's Innovative R&D Project. © 2011 OSA.
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Analysis of shadow by HCPV panels for agricilture applications Reviewed
Araki K., Akisawa A., Kumagai I., Nagai H.
Conference Record of the IEEE Photovoltaic Specialists Conference 2994 - 2997 2010.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
This paper discusses optical impact to the cultivation area with comparison of flat-plate PV and tracking PV - typically HCPV. Sometimes the PV area is covered by concrete, asphalt or graves for suppression of weeds. In this case, the atmosphere may be better but the PV owner will have to pay additional land cost preparation. Another aspect is the land utilization. This complies that massive unused land space is required. In this situation, it is reasonable to think of utilizing PV-uncovered area for agriculture. This paper discusses the space factor of the PV field by comparison of measured and calculated results as well as possibility of utilizing the land underneath PV array by agriculture. The farmland equipped with high-pole CPV has about 2 times more cultivated land, and the minimum illumination level of the cultivated land is around 2 times, the total sunlight energy to the farming land is almost 3 times. The PV output per lost cultivated land is about 2 times. © 2010 IEEE.
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What is the most appropriate and practical index to represent spectrum sensitivity of CPV? Reviewed
Araki K.
AIP Conference Proceedings 1277 205 - 208 2010.11
Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Conference Proceedings
Different from crystalline silicon solar cells, CPV using III-V multi-junction cells does not receive great influences from temperature, rather receives from solar spectrum. Some rating values related to spectrum sensitivity are needed both for the label of modules and the energy rating. Generally, an accurate model needs measurement of many parameters, which is not always accurate and requires complicated calculations. First, input parameter should be common and easy to measure. This simplicity is important for repeatability among manufactures and testing labs. Second, input parameter is not vector but scalar value. In this regards, data set from direct solar spectrum is not appropriate. Third, output parameter that describes spectrum sensitivity is a single value. The method proposed by this paper is energy or capacity factor is expressed by quadric function of DNI. © 2010 American Institute of Physics.
DOI： 10.1063/1.3509190
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A new simple model of direct spectral irradiance with easily observable atmospheric parameters Reviewed
Oke S., Fukushige N., Kemmoku Y., Takikawa H., Sakakibara T., Araki K.
IEEJ Transactions on Electrical and Electronic Engineering 5 ( 5 ) 548 - 552 2010.9
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEJ Transactions on Electrical and Electronic Engineering
Direct irradiance and direct spectral irradiance were measured for 16 months along with other basic atmospheric parameters. The direct irradiance changed smoothly on a clear day and fluctuated on a fair day. Then, we considered the possibility of a new, simple model of direct spectral irradiance expressed with fewer atmospheric parameters and applicable for even fair days. This new model, called DISPEC, can calculate the direct spectral irradiance from basic atmospheric parameters for wavelengths >400 nm. In DISPEC, the ozone absorption was ignored and the thin cloud effect was taken into account as a new parameter, which was considered to be proportional to the direct irradiance, instead of aerosol parameters. The performance of DISPEC was then compared with that of a well-known model of SPCTRAL2. Although the calculation results by SPCTRAL2 exhibited considerable error, especially in visible wavelength range on a fair day, those calculated using DISPEC showed good agreement with observed data on both clear and fair days. © 2010 Institute of Electrical Engineers of Japan.
DOI： 10.1002/tee.20571
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30 kW concentrator photovoltaic system using dome-shaped Fresnel lenses. Reviewed
Araki K, Yano T, Kuroda Y
Optics express 18 ( 9 ) A53 - 63 2010.4
Language：English Publishing type：Research paper (scientific journal)
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Nishimura A., Hayashi Y., Tanaka K., Hirota M., Kato S., Ito M., Araki K., Hu E.
Applied Energy 87 ( 9 ) 2797 - 2807 2010
Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Energy
In this study, the environmental load of photovoltaic power generation system (PV) during its life cycle and energy payback time (EPT) are evaluated by LCA scheme. Two hypothetical case studies in Toyohashi, Japan and Gobi dessert in China have been carried out to investigate the influence of installation location and PV type on environmental load and EPT. The environmental load and EPT of a high-concentration photovoltaic power generation system (hcpV) and a multi-crystalline silicon photovoltaic power generation system (mc-Si PV) are studied. The study shows for a PV of 100MW size, the total impacts of the hcpV installed in Toyohashi is larger than that of the hcpV installed in Gobi desert by 5% without consideration of recycling stage. The EPT of the hcpV assumed to be installed in Gobi desert is shorter than EPT of the hcpV assumed to be installed in Toyohashi by 0.64 year. From these results, the superiority to install PV in Gobi desert is certificated. Comparing with hcpV and mc-Si PV, the ratio of the total impacts of mc-Si PV to that of hcpV is 0.34 without consideration of recycling stage. The EPT of hcpV is longer than EPT of mc-Si PV by 0.27 year. The amount of global solar radiation contributing to the amount of power generation of mc-Si PV is larger than the amount of direct solar radiation contributing to the amount of power generation of hcpV by about 188 kW h/(m2 year) in Gobi desert. Consequently, it appears that using mc-Si PV in Gobi desert is the best option. © 2009 Elsevier Ltd. All rights reserved.
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Low-Concentration Linear-Array Photovoltaic System with Two-axis Sun Tracking Reviewed
OKE Shinichiro, ICHIKAWA Naoki, TAKIKAWA Hirofumi, SAKAKIBARA Tateki, ARAKI Kenji
IEEJ Transactions on Power and Energy 129 ( 9 ) 1154 - 1155 2009.10
Language：English Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan
In a concentrator photovoltaic (PV) system, high concentration causes a decrease of electrical output, since the temperature of the PV cell increases. A linear-array photovoltaic system (LAPS) with a relatively low concentration using a cylindrical mirror was developed. In this system, the sunlight was concentrated on a linearly-arrayed PV system with 7 mono-crystalline silicon PV modules. The LAPS was mounted on a two-axis sun-tracker system. The electrical efficiency was approximately 11% under typical operating conditions.
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Spectrometric outdoor characterization of CPV modules using isotype monitor cells Reviewed
Peharz G., Siefer G., Araki K., Bett A.
Conference Record of the IEEE Photovoltaic Specialists Conference 2008.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
In this paper we present a method to investigate the impact of the solar spectrum on the output parameters of a CPV module equipped with triple-junction solar cells. The changes of the sun's spectrum were monitored by using calibrated isotype solar cells. A correlation of the CPV module outdoor performance to changes in the sun spectrum has been determined. Experimental results of the spectral impact on the current of a Daido Steel CPV module are presented. Finally, a correction method is applied in order to derive the module's current under reference spectral conditions. © 2008 IEEE.
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Present and future of super high efficiency multi-junction solar cells Reviewed
Yamaguchi M., Takamoto T., Araki K.
Proceedings of SPIE - The International Society for Optical Engineering 6889 2008.3
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of SPIE - The International Society for Optical Engineering
While single-junction solar cells may be capable of attaining AM1.5 efficiencies of up to 29%, multi-junction (MJ, Tandem) III-V compound solar cells appear capable of realistic efficiencies of up to 50% and are promising for space and terrestrial applications. In fact, the InGaP/GaAs/Ge triple-junction solar cells have been widely used for space since 1997. In addition, industrialization of concentrator solar cell modules using III-V compound MJ solar cells have been announced by some companies. This paper presents principles and key issues for realizing high-efficiency MJ solar cells, issues relating to development and manufacturing, and applications for space and terrestrial uses.
DOI： 10.1117/12.784117
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Novel materials for high-efficiency III-V multi-junction solar cells Reviewed
Yamaguchi M., Nishimura K.I., Sasaki T., Suzuki H., Arafune K., Kojima N., Ohsita Y., Okada Y., Yamamoto A., Takamoto T., Araki K.
Solar Energy 82 ( 2 ) 173 - 180 2008.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm2. In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells. © 2007 Elsevier Ltd. All rights reserved.
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Evaluation of the Output Fluctuation of a Grid-connected Concentrator Photovoltaic System Reviewed
KEMMOKU Yoshishge, ARAKI Kenji, HIRAMATSU Masao, MIYAZAKI Yoshinori
2007 361 - 364 2007.10
Language：English Publishing type：Research paper (scientific journal)
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Influence of Yellow Sand to Generated Electricity of Concentrator PV System Reviewed
OKE Shinichiro, TAKIKAWA Hirofumi, KEMMOKU Yoshishige, ARAKI Kenji
2007 365 - 368 2007.10
Language：English Publishing type：Research paper (scientific journal)
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Photoluminescence analysis of intragrain defects in multicrystalline silicon wafers for solar cells Reviewed
Sugimoto H., Araki K., Tajima M., Eguchi T., Yamaga I., Dhamrin M., Kamisako K., Saitoh T.
Journal of Applied Physics 102 ( 5 ) 2007.9
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Applied Physics
Structures of intragrain defects were investigated by photoluminescence (PL) mapping tomography in multicrystalline silicon wafers for solar cells. PL dark patterns were observed in short minority carrier diffusion length regions, and we confirmed that the patterns came from the intragrain defects. The tomography revealed that the defects have planelike structures extended to the crystal growth direction. We also found that the growth conditions affect the structures of the defects: slower solidification leads to larger defects with lower density. Origins of the defects were analyzed by low-temperature PL spectroscopy, electron backscatter diffraction pattern measurement and etch-pit observation. We concluded that the defects are metal contaminated dislocation clusters which originate from small-angle grain boundaries. © 2007 American Institute of Physics.
DOI： 10.1063/1.2776003
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Concentrator solar cell modules and systems developed in Japan Reviewed
Yamaguchi M., Araki K., Takamoto T.
Springer Series in Optical Sciences 130 321 - 340 2007.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Series in Optical Sciences
Dissemination of photovoltaic (PV) systems has advanced, and solar cell module production has also significantly increased in Japan as a result of R&D programs such as the New Sunshine Project under the Ministry of Economy, Trade and Industry (METI), Residential PV System Dissemination Program, as well as others. Figure 15.1 shows Japanese cumulative installed capacity of PV systems. The total installed capacity of PV systems in 2004 reached 268.8MW and the cumulative installed capacity recorded 1128.2MW (over 1-GW level). The PV market in Japan is working toward a target of 4.82GW by 2010. As a result of discussion at the Committee for the 2030 PV Road Map in Japan organized by the New Energy and Industrial Technology Development Organization (NEDO) and METI, we also expect about 100-GW cumulative installed capacity [1], about 10% of Japanese electricity consumption, by 2030. The rapid growth in the PV market in recent years has been driven by the rooftop installations in the range 3-5 kWp; however, a market for larger PV systems is beginning to emerge, and in 2003 more than 20 systems of over 450kWp in size were commissioned in Europe and the U.S., with an installed capacity of 26MWp [2]. In addition, large-scale PV systems are necessary to develop in order to realize installation targets of PV systems shown in PV road maps. These facts suggest that concentrating PV systems can play a very important role for a growing PV market and a largescale productions of PV systems when cost-effectiveness against flat-plate arrays is demonstrated. © 2007 Springer-Verlag Berlin Heidelberg.
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Recent and future trends in PV technology Reviewed
Araki K., Beneking C., Faiman D., Gigliucci G., Paletta F.
Energy from the Desert: Very Large Scale Photovoltaic Systems: Socio-economic, Financial, Technical and Environmental Aspects 63 - 77 2007.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Energy from the Desert: Very Large Scale Photovoltaic Systems: Socio-economic, Financial, Technical and Environmental Aspects
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Araki K., Kondo M., Uozumi H., Ekins-Daukes N., Egami T., Hiramatsu M., Miyazaki Y., Yamaguchi M.
Solar Energy Materials and Solar Cells 90 ( 18-19 ) 3320 - 3326 2006.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
A new concentrator receiver containing a 7 mm×7 mm 3J concentrator solar cell with a 37.4% peak efficiency was developed. The receiver design includes a homogenizer, heat-handling (epoxy lamination) technologies and a low-resistance soldered connection and can be applied to various concentrator optics, including dish systems. The outdoor efficiency with a combination of a plastic Fresnel lens, made by low-cost injection molding, reached 27% on a hot summer day under 35.0 °C ambient temperature without additional cooling. With this newly developed receiver, mechanical engineers will be able to design their own concentrator module suitable for their environment, using their mechanical knowledge and local industrial resources. A 400X and 7056 cm2 concentrator module was fabricated with 36 concentrator receivers connected in series and the same number of newly developed dome-shaped, non-imaging Fresnel lenses. The power rating was 200 Wp. The peak outdoor efficiency on a clear sky day was 26.8±1.5%. The integrated efficiency over the course of the day was 25.3±1.4%. This is the highest module efficiency that has been achieved using a practical module size and electrical rating. © 2006 Elsevier B.V. All rights reserved.
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Super high-efficiency multi-junction and concentrator solar cells Reviewed
Yamaguchi M., Takamoto T., Araki K.
Solar Energy Materials and Solar Cells 90 ( 18-19 ) 3068 - 3077 2006.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
III-V compound multi-junction (MJ) (tandem) solar cells have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications. We have proposed AlInP-InGaP double hetero (DH) structure top cell, wide-band gap InGaP DH structure tunnel junction for sub cell interconnection, and lattice-matched InGaAs middle cell. In 2004, we have successfully fabricated world-record efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cells with an efficiency of 37.4% at 200-suns AM1.5 as a result of widening top cell band gap, current matching of sub cells, precise lattice matching of sub cell materials, proposal of InGaP-Ge heteroface bottom cell, and introduction of DH-structure tunnel junction. In addition, we have realized high-efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cell modules (with area of 7000 cm2) with an out-door efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules. Future prospects are also presented. We have proposed concentrator III-V compound MJ solar cells as the 3rd-generation solar cells in addition to 1st-generation crystalline Si solar cells and 2nd-generation thin-film solar cells. We are now challenging to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m2 for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications. © 2006 Elsevier B.V. All rights reserved.
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Development of Low-Concentration Linear-Array Photovoltaic System with Two-Axes Tracking Reviewed
ICHIKAWA Naoki, OKE Shinichiro, KEMMOKU Yoshishige, TAKIKAWA Hirofumi, SAKAKIBARA Tateki, ARAKI Kenji
2006 257 - 260 2006.10
Language：English Publishing type：Research paper (scientific journal)
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A small sun in an ETUI -Possibilities in HCPV- Reviewed
Araki K., Hiramatsu M., Kemmoku Y., Akisawa A., Yamaguchi M.
Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4 1 603 - 608 2006
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4
This paper discusses on possibilities of HCPV, including (1) cost (2) reliability (3) performance and (4) applications. The reduction of the cost is achieved by the reduction of the material itself. The material is not special. HCPV does not covered by special materials including pure Si, TCO, Ru-based dye, or Pt electrode or other materials using CVD or other vacuum process. HCPV modules are made without special tools. Local assembly is also possible. The weight of the 300 W HCPV module Is 22 kg corresponding 0.07 kg/W. It is about 70 % of the MC-Si flatplate modules and less than half of the high efficiency tandem thin film modules. The recent 1-year field test done in Japan showed that the HCPV generated 1.6 times energy per area than that of the crystalline Si module (fixed installation). This implies that the HCPV has an advantage not only desert area but wet and cloudy area. Intensive reliability tests were done to the HCPV module. One of the keys are lifetime of the receiver against 20 years of accumulated CONCENTRATED UV flux in wet environment and other accelerated environment. Finally, the new applications of HCPV including rooftop installation and see-through design are discussed. © 2006 IEEE.
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High efficiency concentrator multi-junction solar cells and modules Reviewed
Yamaguchi M., Takamoto T., Araki K.
Proceedings of the Solar World Congress 2005: Bringing Water to the World, Including Proceedings of 34th ASES Annual Conference and Proceedings of 30th National Passive Solar Conference 2 998 - 1003 2005.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the Solar World Congress 2005: Bringing Water to the World, Including Proceedings of 34th ASES Annual Conference and Proceedings of 30th National Passive Solar Conference
This paper reviews Japanese research and development (R&D) activities of III-V compound multi-junction (MJ) solar cells, concentrator MJ solar cells and modules. Potential of high-efficiency and low-cost using concentrator MJ solar cell modules is shown. 39.2% efficiency at 200- suns AM1.5G has been realized with InGaP/InGaAs/Ge 3- junction solar cells at Sharp Co. The peak efficiency for a 7,056 cm2 400 X module with 36 solar cells connected in series was 26.6 % (measured in house). The peak efficiencies (around 27.5%) of the same type of the modules with 6 solar cells connected in series and 1,176 cm2 area were confirmed by measurements at the Frunhofer ISE, NREL and Toyohashi University of Technology. Future prospects will also be presented.
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Comparison of efficiency measurements for a HCPV module with 3J cells in 3 sites Reviewed
Araki K., Emery K., Siefer G., Bett A.W., Sakakibara T., Kemmoku Y., Ekins-Daukes N.J., Lee H.S., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 846 - 849 2005.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
A new HCPV with 3J solar cells and new concentrator optics consisting in a dome-shaped Fresnel lens and a kaleidoscope homogenizer was presented in the last SCC2003. This paper summarizes the evaluation done by three independent organizations and a manufacturer. The peak uncorrected efficiency for a 7,056 cm2 400 X (Geometrical concentration ratio) module with 36 solar cells connected in series was 26.6 % was measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1,176 cm2 area measured by Fraunhofer ISE, NREL and Toyohashi University of Technology were 27.4 %, 24.8 % and 25.9 % respectively. The peak uncorrected efficiency for a 550X and 5,445 cm2 module with 20 solar cells connected in series was 28.9 % in house. The peak uncorrected efficiency of the 550X (Geometrical concentration ratio) module evaluated by Toyohashi University of Technology was 27.0 %. The temperature corrected efficiency under the best sunshine condition in Japan for the 550X module was 31.5 ± 1.7 %. © 2005 IEEE.
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Syracuse - A multi-junction concentrator system computer model Reviewed
Ekins-Daukes N.J., Betts T.R., Kemmoku Y., Araki K., Lee H.S., Gottschalg R., Boreland M.B., Infield D.G., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 651 - 654 2005.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
A multi-junction concentrator system computer model is presented, that aims to accurately model the power generated based on environmental data input, such as the irradiance, air temperature, humidity and pressure. The device model is discussed, illustrating the importance of statistical variation of module components when simulating a multi-cell module. This comprehensive concentrator computer model will be released for general use, aiming to become a useful resource for those designing and testing multi-junction based concentrator systems. ©2005 IEEE.
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Lee H.S., Ekins-Daukes N.J., Araki K., Kemmoku Y., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 754 - 757 2005.11
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
The behavior of triple junction concentrator cell (InGaP/(In)GaAs/Ge) under the control of cell temperature is investigated and evaluated at Toyota Technological Institute of Japan. The aim of this study is to investigate an accurate temperature coefficient as comparing the outdoor test with the theoretical calculation as a function of a sunlight concentration. Recently, the field test consisted of a total of 6 modules at two sites (Inuyama, Toyohashi) of Japan shows the peak efficiency of 26.8%±1.5%, and the triple junction cell shows the efficiency of 38.9% at 489suns. In this study, a new dome-typed Fresnel lens (550X) that has the optical efficiency of 85.8% was used. The homogenizer is placed on the holder designed to control the cell temperature, and the outdoor test using the solar tracker was carried out under the 400X, 550X lenses. The conversion efficiency increases with the direct solar radiation, and decreases as increasing the receiver temperature. However, no change in Isc was observed, and Voc decreased slightly as a function of the receiver temperatures. The temperature coefficient obtained from the receiver at TTI shows the similar trends with them from the bare cell at Nara-AIST. © 2005 IEEE.
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FUKUSHIGE Naoyuki, MATSUZAKI Yozo, OKE Shinichiro, KEMMOKU Yoshishige, TAKIKAWA Hirofumi, SAKAKIBARA Tateki, ARAKI Kenji
2005 291 - 294 2005.10
Language：English Publishing type：Research paper (scientific journal)
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Influence of Spectrum Intensity of Each Wavelength Range on Output of Si Photodiode Pyrheliometer Reviewed
MATSUZAKI Yozo, FUKUSHIGE Naoyuki, OKE Shinichiro, KEMMOKU Yoshishige, TAKIKAWA Hirofumi, SAKAKIBARA Tateki, ARAKI Kenji
2005 185 - 188 2005.10
Language：English Publishing type：Research paper (scientific journal)
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Multi-junction III-V solar cells: Current status and future potential Reviewed
Yamaguchi M., Takamoto T., Araki K., Ekins-Daukes N.
Solar Energy 79 ( 1 ) 78 - 85 2005.7
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy
Our recent R&D activities of III-V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31-32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP-Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm2) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules. Future prospects are also presented. We have proposed concentrator III-V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m2 for terrestrial applications. © 2004 Elsevier Ltd. All rights reserved.
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Generated Energy of a 500X Concentrator PV System with Dome Fresnel Lens in Summer Season Reviewed
KEMMOKU Yoshishige, EGAMI Toshio, MIYAZAKI Yoshinori, HIRAMATSU Masao, ARAKI Kenji, SAKAKIBARA Tateki
Proceedings of JSES/JWEA Joint Conference 2004 173 - 176 2004.11
Language：English Publishing type：Research paper (scientific journal)
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Measurement of Dynamic Spectrum Distribution of Direct Insolation Reviewed
FUKUSHIGE Naoyuki, MATSUZAKI Yozo, OKE Shinichiro, KEMMOKU Yoshishige, TAKIKAWA Hirofumi, SAKAKIBARA Tateki, ARAKI Kenji
Proceedings of JSES/JWEA Joint Conference 2004 25 - 28 2004.11
Language：English Publishing type：Research paper (scientific journal)
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Japanese programs on novel concepts in PV Reviewed
Yamaguchi M., Takamoto T., Araki K., Imaizumi M.
Semiconductors 38 ( 8 ) 956 - 961 2004.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Semiconductors
Japanese R&D activities in photovoltaics (PV) and our R&D activities with III-V compound multijunctipn (MJ) solar cells are presented. We have realized high-efficiency InGaP/InGaAs triple-junction solar cells with an efficiency of 36.5-37% (AM1.5G, 200 suns) and concentrator triple-junction solar cell modules with an outdoor efficiency of 27% as a result of designing a grid structure, developing low optical loss Fresnel lens and homgenizers, and designing low thermal conductivity modules. Our challenge now is to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m2 for terrestrial applications. © 2004 MAIK "Nauka/Interperiodica".
DOI： 10.1134/1.1787119
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Analysis on Efficiency Limit of Terrestrial Photovoltaic Reviewed
Araki Kenji, N. D. Ekins-Daukes, Yamaguchi Masafumi
DENKI-SEIKO 75 ( 3 ) 155 - 163 2004
Language：English Publishing type：Research paper (scientific journal) Publisher：Daido Steel Co., Ltd.
Monolithic multi-junction solar cells are constrained in that the current output of the stack is limited to the current being generated in the worst performing cell. This becomes a problem when the spectral content of the radiation incident on the solar cell is different to the 'designed' spectrum. The terrestrial spectrum always changes by different sun height or air mass as well as fluctuation of aerosol density. The annual power generation is expected to be smaller than the designed one, due to the spectrum mismatching. This influence may be emphasized by the increase of the number of junctions, while the efficiency under the designed standard spectrum will be increased with the number of junctions. The annual efficiency of monolithic multi-junction solar cells of more than three junctions were calculated and compared. No distinct advantage with more than 5 junctions in view of annual and daily power generation. The practical efficiency limit in real terrestrial sunshine is a little above 50 %.
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Development of 28 percent Efficient Solar Electric Concentrators Reviewed
Araki Kenji, Kondo Michio, Uozumi Hisafumi, Takamoto Tatsuya, Egami Toshio, Hiramatsu Masao, Miyazaki Yoshinori, Kemmoku Yoshishige, Yamaguchi Masafumi
DENKI-SEIKO 75 ( 3 ) 165 - 172 2004
Language：English Publishing type：Research paper (scientific journal) Publisher：Daido Steel Co., Ltd.
A 400 X and 7056 cm<sup>2</sup> concentrator module was fabricated from 36 concentrator receivers, connected in series and with the same number of newly developed dome-shape Fresnel lenses. The averaged outdoor efficiency on a clear sky day was 26.8 ±1.5 % (25C STC). This is the highest module efficiency achieved to date using a module of practical size and electrical rating. The heat was dissipated by the module wall and neither heat sinks nor external cooling were used. A glass homogenizer was introduced to give uniform illumination to the square cell, and afforded a reasonable assembly tolerance, without the need for optical alignment.
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Which is the best number of junctions for solar cells under ever-changing terrestrial spectrum? Reviewed
Araki K., Yamaguchi M., Kondo M., Uozumi H.
Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion A 307 - 312 2003.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion
Monolithic multi-junction solar cells are constrained in that the current output of the stack is limited to the current being generated in the worst performing cell. This becomes a problem when the spectral content of the radiation incident on the solar cell is different to the 'designed' spectrum, The terrestrial spectrum always changes by different sun height or air mass as well as fluctuation of aerosol density. The annual power generation is expected to be smaller than the designed one, due to the spectrum mismatching. This influence may be emphasized by the increase of the number of junctions, while the efficiency under the designed standard spectrum will be increased with the number of junctions. The annual efficiency of monolithic multi-junction solar cells of more than three junctions were calculated and compared. No distinct advantage with more than 5 junctions in view of annual and daily power generation. The practical efficiency limit in real terrestrial sunshine is a little above 50%.
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Development of a robust and high efficiency concentrator receiver Reviewed
Araki K., Kondo M., Uozumi H., Yamaguchi M.
Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion A 630 - 633 2003.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion
One of the difficulties in development of concentrator receiver using III-V solar cells is realization of robust structure with capable of high current density. Most of the cases, the top electrodes were cabled by wire bonding, conductive epoxy and silver ribbon blazing, which gradually increases series resistance under severe environment and loses reliability. Extending the MJ cell performance envelope to the economically required higher current levels and higher current density levels has involved serious and unexpected challenges. A new and robust concentrator receiver was developed with capable of more than 500 X of concentration. A 6300 cm2 and 400X concentrator module were fabricated with the new receivers. The average efficiency in clear day was 21.7 %.
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Study of the space factor for spectrum-sensitive MJ solar cell arrays Reviewed
Araki K., Kemmoku Y., Yamaguchi M.
Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion C 2350 - 2353 2003.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion
This is the first study of the influence of the annual power generation of PV array considering the optimum space factor. Supposing DNI (Direct Normal Irradiance) spectrum is calculated by the most recent reduced aerosol optical density model, and 3J concentrator solar cell is completely matched to air mass 1.5 calculated from that reduced-AOD or other models, the power loss factor in arbitrary months and hour angle would be calculated. The model was proved by actual observation of PV output under both concentration and non-concentration operation. It was thought that the optimum space factor in 3J concentrator solar cell was more packed than single junction cells such as crystalline silicon solar cells. It was also thought that the area efficiency in 3J solar cells was greater than the single junction cells. These were because that shadow loss in 3J solar cells appeared in low sun height was relatively less impact to the annual power generation.
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Araki K., Kondo M., Uozumi H., Yamaguchi M.
Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion A 853 - 856 2003.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion
This study presents the success of proving significant improvement of PV output by kaleidoscope homogenizers and their robust operation to various kinds of concentrator optics. It was also supported by theoretical calculation.
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KEMMOKU Yoshishige, MIYAKAWA Masayuki, EGAMI Toshio, ARAKI Kenji, SAKAKIBARA Tateki
2003 201 - 204 2003.11
Language：English Publishing type：Research paper (scientific journal)
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Research and Development on Concentrator Photovoltaic in the World Reviewed
ARAKI Kenji, YAMAGUCHI Masahumi
Journal of Japan Solar Energy Society 29 ( 5 ) 31 - 39 2003.9
Language：English Publishing type：Research paper (scientific journal)
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Solar Collector and Module for Cocentration Photo-voltaic Systems Reviewed
AKISAWA Atsushi, EGAMI Toshio, ARAKI Kenji
Journal of Japan Solar Energy Society 29 ( 5 ) 13 - 19 2003.9
Language：English Publishing type：Research paper (scientific journal)
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Extended distributed model for analysis of non-ideal concentration operation Reviewed
Araki K., Yamaguchi M.
Solar Energy Materials and Solar Cells 75 ( 3-4 ) 467 - 473 2003.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
Non-ideal illumination is often observed in practical concentrator systems. The simulation to that problem was usually done by a time-consuming 2D numerical calculation or a linear approximation of algebraic calculation. This paper proposes a simple method to anticipate non-linear response to non-ideal illumination operation for concentrator cells. © 2002 Elsevier Science B.V. All rights reserved.
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Sunshine environment and spectrum analysis for concentrator PV systems in Japan Reviewed
Araki K., Yamaguchi M.
Solar Energy Materials and Solar Cells 75 ( 3-4 ) 715 - 721 2003.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
Normal direct irradiance levels for concentrator photovoltaic were investigated and anticipated in 26 sunshine regions in Japan. The spectrum and photon numbers were also examined. Normal direct photon number was assumed to be 92-60% of global the sunshine on sloped surfaces in all the districts in Japan. © 2002 Elsevier Science B.V. All rights reserved.
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Strategies for improving radiation tolerance of Si space solar cells Reviewed
Khan A., Yamaguchi M., Ohshita Y., Dharmaraso N., Araki K., Khanh V., Itoh H., Ohshima T., Imaizumi M., Matsuda S.
Solar Energy Materials and Solar Cells 75 ( 1-2 ) 271 - 276 2003.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
The present study explored first time the better radiation tolerance of gallium-doped silicon solar cells as compared to conventional boron-doped silicon solar cells after heavy fluence of 1MeV electron irradiation. One of the approaches to improve the end of life of silicon solar cells is by increasing the effective base carrier concentrations. Analysis of the carrier removal rate RC in boron, gallium and aluminum-doped Si solar cells showed that carrier removal effects can be partially offset by using gallium as dopant instead of boron. © 2002 Elsevier Science B.V. All rights reserved.
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Development of a metal homogenizer for concentrator monolithic multi-junction-cells Reviewed
Araki K., Leutz R., Kondo M., Akisawa A., Kashiwagi T., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 1572 - 1575 2002.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
A high reflectance, low cost and robust metal homogenizer suitable for monolithic multi-junction concentrator cells was developed. Mismatching loss caused by chromatic aberration and FF loss caused by flux intensity inhomogenity are shown to have significantly improved.
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Japanese R&D activities of multi-junction and concentrator solar cells Reviewed
Yamaguchi M., Araki K.
Conference Record of the IEEE Photovoltaic Specialists Conference 820 - 823 2002.12
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
Japanese R&D activities of concentrator solar cells and systems are overviewed. Japan Energy developed high efficiency (31.7% at 1-sun AM1.5G) InGaP/InGaAs/Ge 3-junction cell under support by NEDO (New En-ergy and Industrial Technology Development Organization) in addition to the world-record efficiency InGaP/GaAs//InGaAs 3-junction cell (33.3% at 1-sun AM1.5G) by Japan Energy, Sumitomo and TTI. This project is now taken over by Sharp and targeted to concentration application also supported by NEDO. The new target is 40% efficiency under 500-sun concentration by March 2006. The SPFG (Single and non-alignment Photolithography Fine Grid) silicon cell is developed by TTI. The first prototype of non-imaging Fresnel lens, which expands acceptance half angle to 2°/12° and enabled low cost 1-axis tracker was fabricated and tested by TUAT. The module with this nonimaging lens and the SPFG cell is now assembled and subjected to be tested.
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Vu T., Ohshita Y., Araki K., Yamaguchi M.
Journal of Applied Physics 91 ( 8 ) 4853 - 4856 2002.4
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Applied Physics
Defects that reduce the minority-carrier lifetime in silicon crystal are produced by minority-carrier injection (forward bias or light illumination) when the boron-doped Czochralski-grown silicon (Cz-Si) is used as a solar cell material. The number of induced defects is determined from changes in open-circuit voltage (V OC) of the cells. It increases with the carrier injection time, and then becomes saturated. The saturated value increases as the ambient temperature increases, during the carrier injection. These defects are observed to be vanished by thermal annealing at 200°C for 20 min, indicating that they are in an unstable state and that some of them are annihilated even during the carrier injection. Therefore, the total number of induced defects to be determined by the difference between the generation and the annihilation rates. The activation energies for the generation process and annihilation process are evaluated to be 0.77 eV and 0.32 eV, respectively. © 2002 American Institute of Physics.
DOI： 10.1063/1.1459609
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26pWE-3 Optical spin injection into Si substrate across the GaAs/GaAsP/Si interfaces Reviewed
Torikai E., Wakahara K., Ikedo Y., Shimomura K., Nagamine K., Saka T., Kato T., Araki K., Mizuno Y.
Meeting Abstracts of the Physical Society of Japan 57 ( 0 ) 2002
Language：English Publishing type：Research paper (scientific journal) Publisher：The Physical Society of Japan
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Khan A., Yamaguchi M., Ohshita Y., Dharmarasu N., Araki K., Abe T., Itoh H., Ohshima T., Imaizumi M., Matsuda S.
Journal of Applied Physics 90 ( 3 ) 1170 - 1178 2001.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Applied Physics
The present extensive systematic study of defect introduction rates as a function of boron, gallium, oxygen, and carbon concentrations by means of deep level transient spectroscopy has drawn a quite complete picture towards the identification of the dominant radiation-induced defects in Si. The radiation-induced defect EV+ 0.36 eV has been identified as Ci-Oi complexes. The absence of an EC - 0.18 eV complex center in gallium-doped samples and the linear dependence of its introduction rates on both the boron and oxygen content fixed its identification as the Bi- Oi complex in boron-doped Si. One of the technologically important results of present study is that the gallium appears to strongly suppress the radiation induced defects, especially hole level EV+0.36eV (Ci-Oi), which is thought to act as a recombination center as well as the dominant compensating center at EC-0.18eV (Bi-Oi). As a result, the effects of lifetime degradation and carrier removal could be partially offset to higher radiation fluences by using Ga as a dopant instead of boron in Si space solar cells. The anneal out of the new hole level EV+0.18eV in gallium-doped samples at around 350°C, together with recovery of free carrier concentration, suggests that this level may act as a donor-like center which compensates free carrier concentration in gallium-doped Si. © 2001 American Institute of Physics.
DOI： 10.1063/1.1384855
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Characteristics of GaAs-based concentrator cells Reviewed
Araki K., Yamaguchi M., Takamoto T., Ikeda E., Agui T., Kurita H., Takahashi K., Unno T.
Solar Energy Materials and Solar Cells 66 ( 1-4 ) 559 - 565 2001.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Solar Energy Materials and Solar Cells
GaAs-based cells, including GaAs single-junction cells, AlGaAs/GaAs two-junction cells, and InGaP/GaAs two-junction cells grown on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) are examined in various levels of concentration and backside cooling temperature. All types of cells have shown boost of efficiency in low and medium ranges of concentration. The cell efficiencies obtained are 31.5% at 20-suns of AM1.5 for InGaP/GaAs tandem cell, and 29.2% at 7-suns of AM1.5 for AlGaAs/GaAs tandem cell, respectively. The GaAs single-junction cell is also examined as the reference. A new equivalent circuit model reveals that increase of apparent leakage current is responsible for a rapid efficiency drop in the high-concentration region. It is possible to improve it by reducing contact resistance and using uniform concentrated illumination.
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Impact of boron and gallium on defects production in silicon Reviewed
Khan A., Dharmarasu N., Yamaguchi M., Araki K., Vu T., Saga T., Abe T., Annzawa O., Imaizumi M., Matsuda S.
Materials Research Society Symposium-Proceedings 650 2001
Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Research Society Symposium-Proceedings
We report the results of comparison of radiation-induced defects (1 MeV electrons) in n+-p-p+ Si diodes doped with gallium or boron ranging in concentration from 8 × 1014 to 5 × 1016 cm-3, together with the impact of oxygen on radiation-induced defects. Present results provide evidence for new defects states in addition to those previously reported in gallium- and boron-doped Si. The combined boron and gallium data provide enough information to gain valuable insight into the role of the dopants on radiation-induced defects in Si. The interesting new future of our results is that the gallium appears to strongly suppress the radiation induced defect, especially hole level EV+0.36 eV, which is thought to act as a recombination center. Similarly the dominant electron level at EC-0.18 eV in B-doped Si (which act as a donor) has not been observed in Ga-doped CZ-grown Si. © 2001 Materials Research Society.
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Magnetic Materials and Semiconductors. A Concentrator Si Solar Cell Made by Low-cost Process. Reviewed
Araki Kenji, Yamaguchi Masafumi
DENKI-SEIKO 71 ( 4 ) 303 - 309 2000
Language：English Publishing type：Research paper (scientific journal) Publisher：Daido Steel Co., Ltd.
Obviously, concentrator solar cells reduce the total area of cells required, which provides lower system costs. For non-concentrator solar panels, the solar cells represent 80% of the system cost. The use of concentrator cells provides a cost reduction for cells inversely proportional to the concentration ratio. However conventional Si concentrator cells normally require more complex photolithography which increases the cost of manufacture. In this novel approach a single step photolithography process is described which includes superfine random pyramid texture, with the electrode directly defined onto the textured surface. Additionally, improved electrode design was applied, allowing for operation under non-uniform surface voltage and current density, experienced during high concentrator conditions. This has lead to energy conversion efficiencies in excess of 19%. Experimental results on the conversion efficiency are in good agreement with the results calculated by the proposed equations taking into account of the distributed diode effect at high concentration ratio.
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AM0 concentration operation of III-V compounds solar cells
Araki K., Yamaguchi M., Imaizumi M., Matsuda S., Takamoto T., Kurita H.
Conference Record of the IEEE Photovoltaic Specialists Conference 2000-January 968 - 971 2000
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2000 IEEE. III-V compounds solar cells were examined by concentration illumination of AM0 sunlight. High concentration or non-uniform illumination was found to bring significance degradation of FF due to apparent degradation of a diode ideality factor or "rounded" I-V curve. The output current at the maximum power point dropped as a result. This paper covers an experimental evaluation, analysis and solutions to that problem. The quantified approach was highly generalized and could be applied every type of high efficiency solar cells. It was found that a distributed diode model [1] explained a behavior of the I-V curve under high concentration or non-uniform concentration operation. The output current drop was responsible from modulation of surface voltage of the emitter. Raised voltage at inter-grid region ignited parasitic diodes and had it leak a part of the output current. For the purpose of quantified analysis and solutions, the distributed diode model [1] was expanded. Behaviors of l-V curve or FF were described by simple arithmetic functions with two basic new parameters, a non-dimensional finger pitch and a normalized resistance, which were independent to the type of solar cells, accompanied with the combination of cell-dependent basic parameters, such as Voc or Isc. The impact to FF degradation could be summarized into counter plots. Basic design parameters such as finger pitch and series resistance could be designed with avoiding FF degradation by high concentration illumination. It did not require complicated numerical calculations. It was found that reduction of series resistance was not very important for multi-junction space concentrator cells. The key was reduction of non-dimensional finger pitch and maintenance of uniform surface voltage. For single junction concentrator cells such as a silicon cell, still, series resistance was essential.
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Silicon concentrator cells by low cost process Reviewed
Araki K., Yamaguchi M.
Conference Record of the IEEE Photovoltaic Specialists Conference 2000-January 164 - 167 2000
Language：English Publishing type：Research paper (scientific journal) Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference
© 2000 IEEE. A new Si concentrator cell was proposed and developed. In this novel approach a single step photolithography process is described which includes superfine random pyramid texture, with the electrode directly defined onto the textured surface. Additionally, improved electrode design was applied, allowing for operation under non-uniform surface voltage and current density, experienced during high concentrator conditions. This has lead to energy conversion efficiencies in excess of 19%.
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Planar, contact, yoke GMR head vs. conventional, flying, shielded GMR head: A comparative study Reviewed
Yoshida Y., Araki K., Sugano S., Kaneta Y.
IEEE Transactions on Magnetics 34 ( 4 PART 1 ) 1507 - 1509 1998.12
Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Transactions on Magnetics
A comparative study was conducted between planar, contact, yoke GMR (Giant Magneto-resistive) heads and conventional, flying, shielded GMR heads by modeling. Magnetic flux efficiencies to the GMR element of the various planar yoke GMR heads were estimated using the reluctance network method and FEM (Finite Element Method). Results showed that it would be necessary to separate the reading magnetic circuit from the writing magnetic circuit in order to obtain higher efficiencies which could be up to 37%. S and 10 Gbits/in2 recording simulation shows that the shielded GMR are expected to be superior to the yoke GMR even at 10 Gbite/in2 but yoke GMR will start to show its advantages in the higher capability of writing and ease of processing at higher densities. © 1998 IEEE.
DOI： 10.1109/20.706598
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Shiosaki T., Adachi M., Kobayashi H., Araki K., Kawabata A.
Japanese Journal of Applied Physics 24 25 - 27 1985.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics
Transparent, core-free and good quality single crystals of Li2B4O7 have been successfully grown by the Czochralski method. All dielectric, elastic and piezoelectric constants, and their first- and second-order temperature coefficients have been determined by measuring resonant and antiresonant frequencies of bars and plates with various orientations. The theoretical and experimental results show the potential usefulness of Li2B4O7 for bulk wave or SAW applications. Further, measurements on optical, acousto-optic and electro-optic properties of Li2B4O7 have also been carried out. © 1985 IOP Publishing Ltd.