Investigation of UV and hygrothermal stress on back side of rack-mounted photovoltaic modules

Abstract

When discussing the reliability of backsheet of PV modules, it is important to accurately determine the amount of stress it receives. In this study, the UV irradiation dosage on the back side of a rack-mounted module, which has been estimated from multiplication of the UV irradiation dosage on the front side and the UV albedo, was actually measured. It was found that the measured value was 1.4 kW h/m2/y, which was almost 1/3 of the estimated value of 4.2 kW h/m2/y. The environmental stress on the back side of the PV module was also investigated by comparing changes in the molecular weight distribution of transparent poly(ethylene terephthalate) films subjected to outdoor exposure or indoor acceleration test. It is shown that UV and hygrothermal stress can be separated by analysis of the molecular weight distribution peak height and shift so that the extent of UV dose in fielded modules can be estimated. It was found that the stress on the back side of the PV module, exposed for 5.5 years using asphalt with 6% UV albedo as the flooring material, was close to that of 8 kW h/m2 after a UV irradiation test, a subsequent 6 h pressure cooker test, or a subsequent 200 h damp heat test. Acceleration tests that reproduce stress itself rather than deterioration phenomena should be developed, and for that, it is important to measure the amount of stress in various cases with reference to this measurement and separation method of stress.