Accelerated laboratory testing has been recognized as a common means to simulate in-field exposure in a time-saving way, however, caution is needed as highly accelerated stresses can cause degradation which is never observed in the field. In this work, the effects of the environmental factors on backsheet degradation have been investigated using a commercial PPE backsheet (polyethylene terephthalate (PET/PET/ethylene-vinyl acetate (EVA)). The PPE films were exposed to NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) under different UV intensities, wavelengths, temperatures and humidities. The chemical and optical degradation of the backsheets were examined by FTIR and UV-vis spectroscopy. The reciprocity law and action spectrum were studied and the activation energy for PPE degradation was calculated. A preliminary statistical model for predicting the service life of the PPE backsheet has been established based on the SPHERE exposure data and further validated by the initial degradation of the fielded PPE in Florida, Arizona and Maryland.
Polyamide (PA)-based backsheet as an emerging product has been developed in recent years to partially substitute fluoropolymer materials under the intensified cost-reduction pressure. However, a number of reports indicate that backsheet cracks have been observed in fielded modules in a few locations after only a few years of exposure. An in-depth analysis on degradation and crack formation and their dependence on climatic conditions is needed. In this work, the field PV modules with PA-based backsheet under five different climatic conditions up to six years were retrieved and analyzed, including humid subtropical climate (Changshu, China), dry-summer subtropical climate (Rome, Italy), marine west coast climate (Bergamo, Italy), desert climate (Arizona, United States) and tropical climate (Thailand). Macroscopic cracks in backsheet were observed for modules aged in Italy and Thailand, while only hairline cracks showed up in backsheet from Changshu, and no cracks could be seen in Arizona. Backsheet in Changshu also experienced much higher yellowing than other sites, while the gloss loss of the backsheet in Italy is the highest. Spectroscopic analyses were also performed to identify various degradation products and to understand the possible changes in degradation mechanism of backsheets under different climates. The intercorrelations between various degradation modes of PA-based backsheet and weathering factors will be further established, providing a valuable information on the material selection and lifetime prediction for the backsheet.
Performance of a photovoltaic (PV) module is related to the micro-environment around the module. The position of photovoltaic modules in an array row have a large effect on the yellowing and gloss of PV module backsheet exposed in Dfa climatic zone (Gaithersburg, MD) with a polyethylene naphthalate (PEN) outer layer. <Stress/ Response< models of yellowing and gloss-losing as function of location parameters of module, including the shed, row, measurement position in a same module and the distance of module location to the row center, are under development. The module installation height had the greatest influence on degradation of PEN PV backsheet in the Dfa climatic zone. The module backsheets at the end of an array have higher degradation rate (edge effect). The edge effect decreases with increasing of module installation heights.
The selection of polymeric materials utilized in photovoltaic (PV) modules has changed relatively little since the inception of the PV industry, with ethylene-vinyl acetate (EVA), polyethylene terephthalate (PET), and fluoropolymer-based laminates being the most widely adopted primary components of the encapsulant and backsheet materials. The backsheet must serve to electrically insulate the solar cells and protect them from the effects of weathering. Due to continued downward pressure on cost, other polymeric materials are being formulated to withstand outdoor exposure for use in backsheets to replace either the PET film, the fluoropoymer film, or both. Because of their relatively recent deployment, less is known about their reliability and if they are durable enough to fulfill the ≥25 year warranties of current PV modules. This work presents a degradation analysis of field-exposed modules with polyamide- and polyester-based backsheets. Modules were exposed for up to five years in different geographic locations: USA (Maryland, Ohio), China, and Italy. Surface and cross-sectional analysis included visual inspection, colorimetry, glossimetry, and Fourier-transform infrared spectroscopy. Each module experienced different types of degradation depending on the exposure site, even for the same material and module brand. For instance, the polyamide-based backsheet experienced hairline cracking and greater yellowing and chemical changes in China (Changsu, humid subtropical climate), while in Italy (Rome, hot-summer Mediterranean climate) it underwent macroscopic cracking and greater losses in gloss. Spectroscopic studies have permitted identification of degradation products and changes in polymer structure over time. Comparisons are made to fielded modules with fluoropolymer-based backsheets, as well as backsheet materials in accelerated laboratory exposures. Implications for qualification testing and service life prediction of the non-fluoropolymer-based backsheets are discussed.