Yield predictions for photovoltaic power plants: Empirical validation, recent advances and remaining uncertainties

Yield predictions are performed to predict the solar resource, the performance and the energy production over the expected life time of a Photovoltaic (PV) system. In this study we compare yield predictions and monitored data for 26 PV power plants. The monitoring data includes in-plane irradiance for comparison with the estimated solar resource and energy yield for comparison with predicted performance. The results show that, due to increased irradiance in recent years (""global brightening"") the yield predictions systematically underestimate the energy yield of PV systems by about 5%. As common irradiance databases and averaging times were used for the yield predictions analyzed in this paper, it is concluded, that yield predictions for areas where the global brightening effect occurred in general underestimated the energy yield by the same magnitude. Using recent satellite derived irradiance time series avoids this underestimation. The observed Performance Ratio of the analyzed systems decreases by 0.5 %/year in average with a relatively high spread between individual systems. This decrease is a main factor for the combined uncertainty of yield predictions. It is attributed to non-reversible degradation of PV cells or modules and to reversible effects, like soiling. Based on the results of the validation the combined uncertainty of state of the art yield predictions using recent solar irradiance data is estimated to about 8%.