Multilayer Backsheet Characterization Using Diffusion Experiments and Optimization Method for Water Diffusion Simulation Inside the Photovoltaic Module

Photovoltaic (PV) modules are expected to be durable against exposure to outdoor environments for prolonged time durations. One of the primary causes for the loss of durability is water exposure. Water concentration prediction within the PV module is challenging under the varying environmental conditions as well as the multilayer nature of the backsheet. Therefore, there is a need for systematic multilayer water diffusion characterization of backsheet as well as to incorporate the varying environments in the water diffusion simulations of the PV module. In this article, backsheet is considered as a two-layer laminate whose water diffusion parameters were determined using both water diffusion experiments (without separation of each layer) and an optimization technique. The gravimetric technique was used to measure experimental water mass uptake of the backsheet, whereas Fick's second law based diffusion model was used to predict numerical water mass uptake. An optimization technique was used to determine the water diffusion and solubility coefficients that minimized the difference between the numerical and experimental water mass uptake. The optimized water diffusion and solubility coefficients in the test range of 24-50 °C reasonably fitted the Arrhenius rate equations. Backsheet was also considered as a homogeneous configuration that was used to determine the effective water diffusion parameters. These effective parameters were found to be significantly different from the optimized parameters of the two-layer backsheet model. Water diffusion simulations with a commercial PV module exposed to outdoor environment showed significant differences in the water concentration between the two-layer and homogeneous backsheet models.