Modellng Outdoor Service Lifetime Prediction of PV modules: Effect of Combined Climatic Stressors on PV modules Power Degradation
I Kaaya1,2, M Koehl1, A.P Mehilli1, M Sidrach2, K.A Weiss1
1Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany
/2University of Malaga, Malaga, Spain

Photovoltaic modules are exposed to a variety of climatic loads during outdoor operation. Over time, these loads trigger a number of degradation modes within the modules leading to performance loss. This paper quantifies the impact of combined climatic loads on the module’s maximum power output using a mathematical approach. Three degradation precursor reactions namely: hydrolysis, photodegradation, and thermomechanical degradation, are assumed to be necessary for service lifetime prediction. For each reaction an empirical kinetics model is proposed and validated with indoor test measurements. A generalized model to quantify the effects of combined climatic loads is proposed. The generalized model is calibrated and validated using outdoor test measurements. The model is then applied to predict the annual degradation rates and a 20% performance loss of three identical monocrystalline modules installed in three benchmarking climates: maritime (Gran Canaria, Spain), arid (Negev, Israel) and alpine (Zugspitze, Germany) using real monitored meteorological data. A degradation of 0.74%/year corresponding to 21.4 years operation time was predicted as the highest for an arid environment; compared to 0.50%/year and 0.3%/year degradation for maritime and alpine environments, respectively. This paper also discusses the pitfalls of using a constant degradation rate in service lifetime prediction modeling. These pitfalls include: over estimation of warranty time, especially when small rate constants are predicted, and over estimation of energy yield, in this study a 5% difference was evaluated for Negev.  The proposed model will not only in outdoor prediction but also in the currently discussed combined stresses accelerated tests to develop test designs.