A Dual-Transport Model of Moisture Diffusion in PV Encapsulants for Finite-Element Simulations

A better understanding of moisture ingress in photovoltaic modules is crucial for better predictions of their longterm behavior in the field. Current calculations and simulations of moisture uptake in photovoltaic modules are based on the Fickian diffusion model in a homogeneous material. In this article, in situ humidity measurements in four different encapsulants exposed to transient humidity conditions are compared with Fickian simulations. It is found that the model cannot accurately describe the measured moisture ingress and egress curves. Thus, a new model for finite-element simulations based on two transport mechanisms is applied. The mesh is split into two regions, where channels with a high diffusion coefficient lead through a bulk with a low diffusion coefficient. The diffusion in both regions as well as the flow between them is simulated as Fickian. The new model is able to predict the measured ingress and egress curves more accurately than the Fickian model in all four encapsulants. The same simulation parameters can accurately describe ingress for various relative humidity values between 20% and 80%, as well as egress from 40% to 20%. This allows further prediction of moisture ingress after the measurement of a single ingress curve and a corresponding parameter optimization. However, a nonlinearity of the diffusion during egress at higher moisture values inthe encapsulants is found.