Nondestructive Detection of Water Ingress in Solar Modules Using Near-Infrared Absorbance Spectroscopy

Abstract

Moisture ingress is a key factor in the degradation of photovoltaic module components. This study employs near-infrared absorption spectroscopy to nondestructively quantify water uptake in backsheets and encapsulants, using a water index derived from the 1910-1920 nm absorption band. Measurements covered short-term dynamics during rainfall, long-term outdoor monitoring, and spatial mapping. Short-term monitoring showed a 14% increase in the water index within 20 min of observations. Five months of rooftop measurements revealed strong sensitivity to humidity and temperature: the index rose by 75% as relative humidity increased from 20% to 50%, and fell by 50% as temperature rose from 0°C to 40°C. Comparative field campaigns in 2021 and 2023 showed material-specific trends: under identical conditions, polyamide and fluoropolymer-coated backsheets exhibited average water index increases of 32%, while polyvinylidene fluoride showed only a 17% increase. Changes in distribution shape indicated differing moisture resistance among materials. Gravimetric analysis confirmed material-dependent water retention. Spatial mapping and immersion tests revealed localized moisture accumulation and saturation-type sorption, with uptake rates—derived via kinetic fitting—ca. 27% higher in field-aged modules than in stored ones. These results establish near-infrared spectroscopy as a scalable and noninvasive tool for detecting moisture-related degradation in photovoltaic modules.