Degradation in photovoltaic encapsulant transmittance: Results of the second PVQAT TG5 artificial weathering study. (5th April 2022)

• Record Type: Journal Article
• Authors: Morse, Joshua; Thuis, Michael; Holsapple, Derek; Willis, Ryan; Kempe, Michael D.; Miller, David C.
• Is Part Of: Progress in photovoltaics. Volume 30:Number 7(2022)
• Page Start: 763
• Page End: 783
• Publication Date: 2022-04-05
• Subjects: durability -- encapsulant -- EVA -- optical scattering -- optical transmittance -- POE -- PVB -- PVQAT -- reliability -- thermal activation -- TPO; Solar cells -- Periodicals; Photovoltaic cells -- Periodicals; Solar power plants -- Periodicals; 621.31245
• DOI: 10.1002/pip.3551
• Languages: English
• ISSNs: 1062-7995

Abstract

Abstract: The optical degradation of encapsulants from ultraviolet (UV) radiation has historically resulted in a significant loss in performance throughout the life of a photovoltaic (PV) module. International Electrotechnical Commission (IEC) test methods have recently been developed to screen for PV encapsulants prone to loss in optical performance. The present study was performed to benchmark polymeric packaging materials relative to IEC 62788‐1‐4 (covering the measurement of optical transmittance) and IEC 62788‐1‐7 (on the durability of transmittance), provide feedback toward improvement of the methods, and develop insight regarding optical degradation. Contemporary materials were examined, including poly(ethylene‐co‐vinyl acetate) (EVA), thermoplastic polyolefin (TPO), polyolefin elastomer (POE), and polyvinyl butyral (PVB) encapsulants; a poly(ethene‐co‐tetrafluoroethene)/poly(ethylene terephthalate) (ETFE/PET) transparent backsheet; and a polystyrene (PS) working reference material. The use of silica‐, specialty‐, and rolled‐glass was also compared in laminated coupons. Specimen size was separately examined from 2.5 to 12.5 cm. Weathering was performed with a xenon source, using IEC TS 62788‐7‐2 methods A2, A3, A4, and A5 (chamber temperature of 55°C, 65°C, 75°C, or 85°C), respectively. Characterizations were made using a UV–visible–near‐infrared (UV–VIS–NIR) spectrophotometer (transmittance and reflectance, with and without an integrating sphere), a UV–VIS fluorescence spectrophotometer, a camera, and an optical microscope. Performance was analyzed, including solar weighted transmittance, yellowness index, UV cut‐off wavelength, and haze (scattering). Separate Arrhenius analyses were performed to assess retention of transmittance and changes in yellowness index. The activation energy for both characteristics was found to range from 15–80 kJ·mol −1, with an average of 48 kJ·mol −1, similar to the average of 45 kJ·mol −1 identified in the previous international PV Quality Assurance Task Force (PVQAT) Task Group 5 (TG5) study of more traditional encapsulants. The separate degradation modes of discoloration and scattering were distinguished in the encapsulants using a comprehensive spectral characterization. Based on these results, the IEC 62788‐1‐7 pass/fail criteria of 5% change in transmittance was confirmed to identify a known bad encapsulant. Abstract : Contemporary encapsulants were compared through the recent IEC TS 62788‐7‐2 (A2–A5) UV weathering methods. The pass/fail criteria of 5% transmittance loss was demonstrated to screen for inappropriate encapsulants through early PV module life. Optical haze gave a similar magnitude of optical loss and occurred as well as discoloration in polyolefin encapsulants. A specimen size effect was identified for coupons smaller than 7.5 cm. The activation energy for UV degradation ranged in the order of 15 kJ·mol −1 to 80 kJ·mol −1, with an average of 48 kJ·mol −1 .