Improving the spectral correction function

Plane of array irradiance derived from common measurement sources is not usually directly applicable in modelling the energy yield of photovoltaic systems. Corrections to an effective irradiance value are required due to the differences in the spectral and angular response of irradiance measurement instruments (typically pyronometers or reference cells) when compared to the installed system. Empirical methods to provide this correction based on easily measured parameters are attractive both because of the simplicity of their implementation and the ability to tailor the solution to fit the system under study. Unfortunately the most common methods to correct for spectrum and angle of incidence do not adequately capture performance under non-ideal conditions. Here we demonstrate that for silicon modules an improved surface function combining air mass and clearness index is capable of being used as a site independent measure of spectral mismatch. Following a correction for precipitable water the same result is obtained from the procedure applied to cadmium telluride modules.