PV module degradation caused by thermomechanical stress: real impacts of outdoor weathering versus accelerated testing in the laboratory

Temperature cycling tests are part of the IEC 61215 qualification testing of crystalline silicon (c-Si) PV modules for evaluating PV module degradation caused by the impact of thermo-mechanically induced stresses. The defined temperature gradient and the cycle time by far exceed the actual impact of natural weathering, however. As a contribution to comparisons between laboratory testing and natural weathering our work provides data from standard temperature cycling tests as defined in IEC 61215 and extended from 200 (standard) to 800 cycles. The results of these tests for seven commercial c-Si PV modules from various manufacturers are compared with results from identical module types exposed outdoors in different climates for a period of 3 years. Degradation effects are evaluated with respect to changes in output power, changes in insulation properties and with respect to interruptions in the electrical interconnection circuits such as cell interconnects. Temperature gradients obtained at the different exposure locations are used to model the thermo-mechanical stress arising from the mismatches of the thermal expansion coefficients of the employed materials.