Degradation prediction of encapsulant-glass adhesion in the photovoltaic module under outdoor and accelerated exposures

Maintaining good encapsulant-glass adhesion is important to ensure the long-term durability of photovoltaic (PV) modules. An exposure dose based model was used in this study to predict the temporal and spatial degradation of encapsulant-glass adhesion inside the PV module under hygrothermal exposure. The exposure dose is a single parameter that combines the exposure temperature, humidity and time using an exponential relation that is governed by a degradation activation energy. Peel tests were used to measure degradation in ethylene-vinyl acetate (EVA)-glass adhesion in PV module laminates that are aged under various constant temperature and humidity environments. An optimization technique applied to fit an exponential relation between the degraded adhesion strength and exposure dose gave the degradation activation energy as 59.4 kJ/mol. The model was used to study EVA-glass adhesion degradation for glass-glass and glass-backsheet based PV modules exposed to 5 year outdoor (Delhi, India), damp heat, humidity freeze and thermal cycling environments. The spatial and temporal distribution of relative humidity at the EVA-glass interface inside the PV module was predicted considering Fickian diffusion in the backsheet and EVA. The predicted humidity is used to estimate the exposure dose, which is further used to predict adhesion degradation. The highest degradation in all exposures occurs at the edge for the glass-glass module, whereas near the gap between cells for the glass-backsheet module. The integrity of the interface near the exposed boundary can be assessed by the damp heat exposure, but the integrity away from the boundary cannot be assessed by the accelerated exposures.