Degradation in PV Power Plants: Theory and Practice

Our contribution is based exclusively on data from PV power plants in Germany that have been measured in detail. The comprehensive evaluations and statements are based on 44 plants, each with a rated power between 500 kWp and 1500 kWp and an operating life of almost 10 years. The main conclusion of this study is that almost all observed yield degradation effects are repairable or reversible, like inverter failures or soiling of the PV modules. But these effects lead to a negative trend in the Performance Ratio. PV Modules are taken from the field after 10 years of operation. The measurements show very small power deviations from the initial values. Furthermore, due to a positive trend of irradiance in plane of array no decreasing trend in annual energy yields is observed. The most frequent causes of larger fluctuations are inverter failures and inverter malfunctions. The sensors for measuring solar irradiation are very stable throughout the entire measurement period and the soiling of the sensors is very low. The results further support the thesis that beside climatological and site specific influences also operation and maintenance strategies have a significant influence on observed performance loss rates of commercially operated PV systems.