Cell gap change in crystalline silicon PV modules with aluminum rear covers: temperature-dependent cell gap measurements and analytical modeling

PV-activated aluminum façade elements offer a straight forward replacement of common façade elements, but have a major issue with module reliability due to interconnector fatigue under cyclic thermal stress. For better understanding of the degradation mechanism and to find adequate measures against the interconnector fatigue, we perform temperature-dependent cell gap measurements on PV modules with aluminum rear covers and develop an analytical model for estimating cell gap changes. We measure identical relative cell gap changes for conventionally wire-interconnected solar cells and interconnected solar cells with strain relief. Furthermore, we measure 1.76 times greater relative cell gap changes in a test module containing M6 full cells instead of M6 half cells. Reducing the length of the cell strings from ten to four M6 half cells lowers the cell gap changes by about 35%. We create a model with linear elastic springs representing the encapsulant between the silicon solar cells and the aluminum rear cover to estimate cell gap changes analytically. The calculated relative cell gap changes for the investigated module configurations with varying module rear cover material, varying cell width and varying string length agree well with the measured values. Both measured and modelled relative cell gap changes correlate with the improved reliability of aluminum modules with narrower cells and shorter cell strings in thermal cycling experiments. It confirms that smaller cell gap changes correspond to less thermal stress and longer module lifetime.