Experimental and numerical investigation on the stress field induced by impact loadings in semi-flexible photovoltaic modules

In the present work, the effects of impacts on silicon (Si) cells embedded in semi-flexible photovoltaic (PV) modules are investigated from both the experimental and the numerical point of views. Semi-flexible PV modules, frequently installed on curved surfaces, such as on sailing boat decks, mountain refuge roofs, motorhomes and innovative electrical cars, are in fact particularly sensitive to the impact of hailstones, due to the fact that they are entirely made of soft polymeric layers. In order to simulate the hailstone impact, a polyamide sphere with a radius of 20 mm has been shot with a compressed-air apparatus against the PV module at different velocities, up to a maximum value of 10m/s. The effects of the impact load on the Si cells, invisible by the naked eye, have been analyzed through pictures taken with the electroluminescence (EL) technique [1]. Different typologies of substrates on which the panels are laid on have been considered, in order to identify the configuration that minimizes the extension of the zone around the impact point characterized by a high stress level. In the case of a rigid substrate, the damage is localized in a narrow circular area, where the Si cell is completely destroyed, whereas a completely different damage pattern is occurring in the case of a soft substrate, showing several concentric cracks developed around the point of impact. As regards the numerical modelling, an axisymmetric finite element model of the laminate has been proposed using the finite element analysis program FEAP. Linear elastic constitutive laws have been adopted for the materials composing the layers, except for the epoxy material encapsulating the Si cells for which a neo-Hookean constitutive behavior has been considered. An implicit integration scheme has been used to solve the contact problem, using the node-to-segment contact strategy [2] and the penalty method to model the contacting interfaces. Different values of the penalty parameter have been used along the contact interface between the PV module and the substrate, to simulate different substrate stiffnesses. Numerical predictions regarding the extension of the damaged areas are in good agreement with our own experimental results obtained in the laboratory.