Simplifying Measurements of Spectral Irradiance

Abstract

Solar spectral irradiance variations and related PV energy gains or losses are one of the last great unknowns in PV yield prediction. Energy gains or losses due to ‘spectral effects’ are reflected by the so-called spectral mismatch factor, which is calculated from prevailing solar spectral irradiance distributions, the reference spectral distribution and the actual spectral response of solar cells concerned. The main reason for the current lack of knowledge on potential energy gains or losses is the scarcity of solar spectral irradiance data. To this end, we present a new way to gain information on solar spectral irradiance by using measurement devices that are sensitive in differing wavelength regimes. Here, the best correlation to determine mismatch factors using our new approach is found to be the ratio of illuminance to irradiance. Using illuminance-to-irradiance ratios, we show that it is principally possible to reproduce measured spectra. A comparisons of mismatch factors for amorphous silicon cells calculated with our procedure shows that our approach is a promising one. However, future research is needed for testing solar cell types other than amorphous silicon, model optimization and a closer analysis of measurement uncertainties.