Analysis of Non-Linear Long-Term Degradation of PV Systems

The current work presents the degradation evaluation of different PV systems under the weather conditions prevailed in the Swiss midlands, Jura and Alps. The purpose of this paper is to analyze degradation rates and change of degradation rates over time. The analysis is done for the degradation rates of three 25-30 years old PV systems in Switzerland, one of which is the oldest grid connected PV system in Europe of its size (30-year-old 555 kW PV system Mont-Soleil). The two other PV systems are located on Jungfraujoch and in Burgdorf. The examined degradation metric in this study is the performance ratio (PR) which is normalized energy yield with received insolation. The focus of this study is to examine the linear degradation rate of the PV plants and find the best non-linear fitting functions to the degradation. It is found that the annual degradation differs between the systems although they have identical PV modules. The highest linear degradation was found for Tiergarten East system with 0.6 %, 0.5 % for Tiergarten West, 0.3 % for Mont-Soleil and 0.02 %, for the Junfraujoch system. For non-linear degradation, 2nd order polynomial and breakpoint functions were used. The performance of both functions varies depending on to the PV system, and it is found that breakpoint function provided the best results and fit better than polynomial function.