Papers - Araki Kenji
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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.