Degradation in PV Encapsulant Transmittance: Results of the First PVQAT TG5 Study

Reduced optical transmittance of encapsulants resulting from ultraviolet (UV) degradation has frequently been identified as a cause of decreased PV module performance through the life of service in the field. The present module safety and qualification standards, however, apply short UV doses only capable of examining design robustness or 'infant mortality' failures. The latest versions of the safety and qualification apply ~40 - ~160 days cumulative dose of the AM1.5G total UV radiation. Essential information that might be used to screen encapsulation through product lifetime also remains unknown. For example, the relative efficacy of xenon-arc and UVA-340 fluorescent sources or the typical range of activation energy for degradation is not quantified. We have conducted an interlaboratory experiment to provide the understanding that will be used towards developing a higher fidelity, more rigorous UV weathering test. Five representative, known formulations of EVA were studied in addition to one TPU material. Replicate laminated silica/polymer/silica specimens were examined at 10 institutions using a variety of indoor chambers (including Xenon, UVA-340, and high pressure mercury light sources). Specimens were artificially weathered for 180 cumulative days at pre-designated accelerated test conditions. The solar-weighted transmittance, yellowness index, and the UV cut-off wavelength, determined from the measured hemispherical transmittance, are examined to provide understanding and guidance for the UV light source (lamp type) and temperature used in accelerated UV aging tests.