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

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.