Influence of soiling and moisture ingress on long term PID susceptibility of photovoltaic modules

Abstract

The impact of soiling and moisture on long term PID susceptibility of PV modules is investigated based on the voltage divider model. In the first instance, the electric conductance of the glass surface is measured before and after a well-defined soiling experiment using Arizona Test Dust. Secondly, the change of the electric conductivity of the encapsulating polymer foils is addressed by a combined study of water uptake behavior, electrical resistivity and PID susceptibility of mini modules with three different common, commercially available EVA materials. The ingress of moisture is triggered under controlled, realistic conditions by pre-soaking of the mini modules under damp heat conditions (85 °C, 85 % RH). Samples of the EVA are then characterized with respect to their water content and electrical resistivity. The measured electrical material parameters for glass soiling and damp heat soaked polymers are implemented in a simplified electrical model of the module encapsulation. The resulting voltage across the anti-reflective coating of the solar cell is indicative for the degree and speed of PID. After damp heat soaking the mini modules develop a PID sensitivity that is correlated to the moisture ingress and the electrical resistivity. Furthermore, soiling of the glass surface has a promotive impact on the strength of PID due to decrease of potential drop across the glass surface.