Assessment of long term reliability of photovoltaic glass–glass modules vs. glass-back sheet modules subjected to temperature cycles by FE-analysis

Quantifying the reliability of photovoltaic (PV) modules is essential for consistent electrical performance and achieving long operational lifetimes. Optimisation of these parameters increases the profitability of photovoltaic electricity because such systems should only require an initial capital investment. There are several aspects in a PV module which compromise its profitability. One such important aspect is the thermo-mechanical stress that is induced by day to night temperature cycles during 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 analyses the effects of the thermally induced stresses in two different module constructions. The thermo-mechanical reliability of photovoltaic modules is tested by the IEC standard 61,215 which accelerates the day to night cycles. Detailed analysis of this experimental test method is done by FEM simulations. Results of those numerical analyses are able to directly analyse the internal stresses in a PV module. 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. The second packaging type for H-patterned PV cells is the glass–glass module which replaces the back sheet by a second glass sheet. Both module types have the same base area including 60 solar cells and the same total thickness. Each of the module assemblies are transferred to 3-D FE-models and subjected to temperature cycles. The simulation results show no module deformation for the symmetrical glass–glass module while the glass-back sheet assembly deforms by several mm. The mechanical results show that the solar cells are displaced 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 inside the glass–glass module the copper ribbons and solder layers are subjected to higher mechanical loads compared to the reference type. In case of the glass–glass module the copper ribbons may fail which can result in a complete cut of the series-connected solar cell strings.