Relative efficiency revealed: Equations for k1–k6 of the PVGIS model

Abstract

The European PV Geographical Information System (PVGIS) describes module performance in terms of the relative efficiency with respect to Standard Testing Conditions (STC). The efficiency's dependence on irradiance and operating temperature is modeled with a bi-quadratic polynomial with respect to the relative temperature and the logarithm of relative irradiance. In earlier works, the present author derived relations between two model coefficients describing the irradiance dependence at 25°C, k1 and k2, and I-V curve model parameters such as the series resistance RS and the ideality factor n. There was good agreement between the theoretical and fitted values of k1, but the fitted values of k2 were overestimated for most of the studied crystalline-silicon (c-Si) modules. The present paper derives a correction factor for k2 and equations for k3 - k6. The results are limited to the case of PV modules behaving well according to the one-exponential I-V curve model, with negligible current leakage via cell shunts. The effects of several I-V curve non-idealities on performance are discussed.