Combined experimental and simulatory evaluation of thermal and mechanical loads on pv modules

A key factor for the long-term performance degradation of PV modules is an in-use damage of the actual solar cells by loads causing (micro-)cracks. Besides external mechanical forces, the main source of these is forces created by different thermal expansions of the module components, inducing mechanical loads as soon as the module temperature changes. These thermo-mechanical effects are complex and highly interdependent, also since they are not only affected by the momentary temperature situation but likewise by the module’s life history. This becomes an issue since manufacturing and use of PV modules necessarily comprise temperature changes; residual stresses thus become an inherent part of each module. To achieve a better understanding of these effects, resistant strain gauge rosettes and temperature sensors have been included in dedicated test mini-modules comprising only one solar cell each. Using these, the principal strains occurring during lamination and later in thermal cycles simulating use have been monitored and analysed. It can be shown that the thermal loads of the lamination process impose the highest stresses on the solar cells, and that this stress magnitude is not correlated to the lamination time. The experi- ments also confirm the protective effect of the ethylene/vinyl acetate (EVA) encapsulant for preventing cell fracture. Related simulations taking into account the highly temperature-dependent material properties of the EVA can qual- itatively reproduce the experimental findings, although a detailed reproduction will requiring future efforts taking even more effects into account.