FEM stress analysis of various solar module concepts under temperature cycling load

The reliability of photovoltaic modules is essential for high electrical performance and a long operational lifetime. Both issues increase the profitability of photovoltaic electricity because these systems require only an initial installation invest. There are several aspects in a PV module which are able to reduce its profitability. An important aspect is the thermo-mechanical stress which is induced by day to night shifts at every day of operation. Since this stress obviously cannot be omitted the PV module set-up should reduce the resulting internal loads to a minimum. This paper analyzes the effects of the thermal induced stresses in three different module constructions. The reliability of photo voltaic modules under thermomechanical loads is tested by the TEC standard 61215. This test method is reproduced in FEM simulations which are able to directly analyze the internal stresses of different PV modules. The investigation presented here applies a classic module assembly for H-patterned cells with a single front glass and a plastic back sheet which is the reference type. Another packaging type for Hpatterned cells is the glass-glass module which replaces the back sheet by a second glass board. Finally there is a novel module type applying back contact solar cells. Tn this module type all electrical interconnections are supported by a substrate which is situated below the solar cells. This assembly is enclosed by a front glass and a plastic back sheet. The different module assemblies are transferred to 3D FE-models and subjected to temperature cycles. The mechanical analyses show that the solar cells are moved towards each other when temperatures decline and vice versa during temperature increase. This forced movement causes stresses and strains in the interconnection structures of the modules. The analyses reveal that those structures which are subjected to high mechanical loads are not supposed to cut the electrical interconnection because a failure may appear only in a small section of the according interconnection structure. Only in case of the glass-glass module the copper ribbons are subjected to high mechanical loads which may result in a complete cut of the series-connected solar cells.