Evaluation of different indicators for representing solar spectral variation

Abstract

In studies analyzing the performance of photovoltaic (PV) modules, average photon energy (APE) is often used as an indicator for the effect of solar spectral variation on PV module performance, as it is said to accurately distinguish individual spectra. Especially for a-Si devices, there is a strong correlation between APE and performance. However, there can be significant variation in spectra measured at specific APE values. In this study we analyze the variation of spectra at a range of APE values, and also compare APE as an indicator of spectra to other spectral indicators, namely Blue Fraction (BF), Useful Fraction (UF), Airmass (AM) and Spectral Mismatch Factor (MMF). We compare the indicators by binning spectra at different values of each parameter, and calculating the Root-Mean-Square-Deviation (RMSD) of all spectra in the bin to the mean spectrum in the bin. Subsequently, we compare these calculated results between the different parameters Our results indicate that APE was found the best indicator of spectral variation, with the lowest mean RMSD over the whole range of measured data. However, BF is an almost equally good indicator, and UF and MMF also show a low mean RMSD. Airmass was found to be a quite poor indicator of spectral variation.