Laboratory study of field-aged PV module degradation in Thailand’s tropical climate using EL imaging

This study evaluates long-term degradation in crystalline silicon photovoltaic (PV) modules operating at two utility-scale PV power plants located in central and northern Thailand. Electrical performance measurements and electroluminescence (EL) diagnostics were conducted at years 5 and 8 for the central site and compared with field data from the north site. The central PV plant shows stable behavior with minimal changes in maximum power (Pmax) and series resistance (Rs), whereas the north plant exhibits substantially higher degradation, with Pmax losses approximately three times greater (8.6–11.8%) than those at the central site (3.2%). Correspondingly, Rs at the north site nearly doubled, reflecting stronger environmental stressors typical of hot–humid tropical climates. Quantitative EL imaging reveals that the peak-pixel index is the most sensitive indicator of degradation, demonstrating strong correlation with both Pmax reduction and Rs. In contrast, low-threshold and black–white pixel metrics show limited diagnostic effectiveness in later years. These results highlight the value of integrated electrical and EL-based diagnostics and emphasize the need for tailored monitoring strategies to ensure reliable PV module aging assessment in tropical environments.