Uniqueness verification of solar spectrum obtained from three sites in Japan based on similar index of average photon energy

The output energy yield from the photovoltaic (PV) module is influenced mainly by the spectral irradiance under outdoor conditions. To evaluate the effects upon PV modules, a quantitative index is essential for appropriate statistical analysis, because spectral irradiance distribution is a function of wavelength. Average photon energy (APE) is an index which represents a quantitative value of spectral irradiance distribution, and an APE value calculated from 350 to 1050 nm wavelength (APE350-1050) uniquely describes the shape of the solar spectrum at Kusatsu-city. Meanwhile, our previous study showed that APE has effects on PV performance, especially on short circuit current (ISC) of PV modules. Therefore, APE is a useful index of evaluation of PV performance under actual outdoor conditions. In Japan, the uniqueness of APE have been demonstrated at Ritsumeikan University in Kusatsu-city, Shiga prefecture as an index of spectral irradiance distribution and its effect on PV performance. However, the spectral characteristics in other regions of Japan with respect to Ritsumeikan University in Kusatsu-city are not yet clearly understood. This study aims to describe and discuss the uniqueness of solar spectrum obtained from three sites: Ritsumeikan University in Kusatsu-city, National Institute of Advanced Industrial Science and Technology in Tsukuba-city, and the University of Miyazaki in Miyazaki-city, Japan based on similar APE for standardization with respect to Kusatsu-city. In regard to this view, the coefficient of determination, denoted R2, was used to quantify the degree of coincidence for the shape of the solar spectrum. The results obtained from the R2 method were compared for evaluating consistency among three sites on the basis of Ritsumeikan University. It was found that the high R2 values of 0.988-0.998 correspond to small APE errors of 0.006-0.028 eV. These errors are raised from the different sites due to the slight variations in the shapes of the solar spectrum. For further evaluation of the influence of ISC of different PV modules, spectral mismatch correction factor (MM) based on the irradiance obtained from a pyranometer as a function of APE is examined and compared with three sites using similar APE. Results showed that the MM of the PV modules which have spectral sensitivities in the long wavelength band is approximately 1 regardless of the APE, except for cadmium telluride (CdTe) PV module. We thus conclude that the ISC of different PV modules except the CdTe PV module have uniqueness, if the irradiance and APE350-1050 measured from all three sites are similar.