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The performance of photovoltaic modules is influenced by solar spectrum even under the same solar irradiance conditions. Spectral factor (SF) is a useful index indicating the ratio of available solar irradiance between actual solar spectrums and the standard AM1·5‐G spectrum. In this study, the influence of solar spectrum on photovoltaic performance in cloudy weather as well as in fine weather is quantitatively evaluated as the reciprocal of SF (SF−1). In the cases of fine weather, the SF−1 suggests that solar spectrum has little influence (within a few %) on the performance of pc‐Si, a‐Si:H/sc‐Si, and copper indium gallium (di)selenide modules, because of the “offset effect”. The performance of a‐Si:H modules and the top layers of a‐Si:H/µc‐Si:H modules can vary by more than ± 10% under the extreme conditions in Japan. The seasonal and locational variations in the SF−1 of the bottom layers are about ± several %. A negative correlation is shown between the top and bottom layers, indicating that the performance of a‐Si:H/µc‐Si:H modules does not exceed the performance, at which the currents of the top and bottom layers are balanced, by the influence of solar spectrum. In the cases of cloudy weather, the SF−1 of the pc‐Si, a‐Si:H/sc‐Si, and copper indium gallium (di)selenide modules is generally higher, suggesting favorable for performance than that in fine weather. Much higher SF−1 than that in fine weather is shown by the a‐Si:H module and the top layer of the a‐Si:H/µc‐Si:H module. The SF−1 of the bottom layer neither simply depend on season nor on location. Copyright © 2011 John Wiley & Sons, Ltd.

Various kinds of photovoltaic (PV) modules have been developed and practically operated as PV systems up to present. Investigation of the long‐term reliability of PV modules is indispensable for the use of PV systems as reliable energy sources. In this study, we show the results of outdoor exposure test in which the performance of 14 PV modules composed of five different kinds made by six different PV manufacturers have been measured since July 2004. The average performance is calculated in each year from 2005 to 2008, and the performance degradation is quantitatively evaluated. The results are that the magnitude of the performance degradation can be clearly classified by the kinds of the PV modules. The performance difference of the single‐crystalline silicon (sc‐Si) modules between 2005 and 2008 is from 1.9% to 2.8%. Polycrystalline silicon (pc‐Si) modules show performance degradation from 0.7% to 1.4%. The performance of an amorphous silicon/crystalline silicon (a‐Si:H/c‐Si) decreased by 0.7%. Although a pair of a‐Si modules had been already exposed to sunlight for about 6 months, the pair of modules show 4.4% of performance degradation. More than half of the performance degradation happened during the initial period from 2005 to 2006. This indicates that it takes about 2 years until the performance of a‐Si modules is stable. The performance is quite stable after 2006. Interestingly, the performance of the cupper indium gallium diselenide modules in 2008 is about 0.8% higher than that in 2005. Copyright © 2010 John Wiley & Sons, Ltd.

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The performance of six photovoltaic (PV) modules composed of polycrystalline silicon (pc-Si), amorphous silicon (a-Si), and hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) modules was investigated at eight locations in Japan from August 2007 to December 2008. In addition, solar irradiance, solar spectrum, and module temperature were simultaneously measured in these round-robin measurements. In this study, we evaluate quantitatively the effects of module temperature and solar spectrum on the performance of the PV modules as thermal factor (TF) and spectral factor (SF), respectively. Furthermore, we investigate the variation in module performance, which is converted into module performance under standard test conditions (STC) using the TF and SF. In the case of the pc-Si modules, the variations in performance ratio under STC (PR STC ) for these modules range from 0.056 to 0.074 through the round-robin measurements. The TF indicates that the contribution of module temperature to the variation in performance is large, between about 15 and 20%. However, the SF suggests that the contribution of solar spectrum is quite small, less than 3%. In the case of the a-Si modules, the contribution of module temperature is about 8%. The performance is largely influenced by solar spectrum, more than 12% at its maximum. Consequently, the variations in the corrected PR STC of the a-Si modules are between 0.117 and 0.141. These large variations may result from the effects of thermal annealing and light soaking. The variation in PR STC of the a-Si:H/c-Si module is similar to that of the pc-Si modules.

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