Long-term reliability of photovoltaic c-Si modules - A detailed assessment based on the first Italian BIPV project

Assessing the long-term reliability of PV systems is important for understanding their energy and cost efficiency. Typically, estimates and predictions are based on indoor tests and accelerated ageing. However, fluctuating and differently interacting outdoor factors such as solar radiation, dust, and shadowing in real environment can impact the actual performance. This paper examines alterations related to ageing of c-Si PV modules, firstly by classifying the main factors that affect aged c-Si PV modules and then assessing the impact on their performance degradation by analysing a pilot BIPV system at Politecnico di Milano after 20 years of actual operation. Such system, which is highly representative since is the first public BIPV plant funded in Italy, was carefully and continuously monitored during its operating life. In particular, according to the visual/IR inspection carried out after the 20th year of operation, the main observed alteration in the modules were discoloration of the encapsulant, delamination, and chalking of the backsheet. The I-V characterization shown that all sampled modules had an overall degradation rate of less than 20 %, which is within the warranty limit, although in many cases the degradation rate over time shows a non-linear trend. Only one module experienced a severe fault that caused the complete loss of functionality. Obtained results confirm the reliability of c-Si technology, stressing the importance of a careful monitoring especially after the 15th year, when an increase of the degradation rate might occur.