Thermal Stress and Strain of Solar Cells in Photovoltaic Modules

The long-term stability of photovoltaic (PV) modules is largely influenced by the module’s ability to withstand thermal cycling between −40°C and 85°C. Due to different coefficients of thermal expansion (CTE) of the different module materials the change in temperature creates stresses. We quantify these thermomechanical stresses by performing a Finite-Element-analysis of a 60 cell module during thermal cycling. We therefore start by the experimental characterization of each material layer. In particular, the polymeric encapsulant is characterized by three alternative models in order to stepwise consider the time- and temperature-dependence in the simulation. Experiments performed with laminated samples are used to validate the computational model. We find that taking into account the viscoelasticity of the encapsulation layers gives the best agreement with experiments. The Finite-Element-analysis of the complete module shows that the solar cells are under high compressive stress of up to 76 MPa as they are sandwiched between the stiff front glass and the strongly contracting plastic back sheet. The non-symmetrical structure of the 5.55 mm thick module with glass being the thickest component (4 mm) leads to bending during the thermal cycle.