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.
Channel cracking fragmentation testing and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were utilized to study mechanical and chemical degradation of a multilayered backsheet after outdoor and accelerated laboratory aging. A model sample of commercial PPE backsheet, namely polyethylene terephthalate/polyethylene terephthalate/ethylene vinyl acetate (PET/PET/EVA) was investigated. Outdoor aging was performed in Gaithersburg, Maryland, USA for up to 510 days, and complementary accelerated laboratory aging was conducted on the NIST (National Institute of Standards and Technology) SPHERE (Simulated Photodegradation via High Energy Radiant Exposure). Fracture energy, mode I stress intensity factor and film strength were analyzed using an analytical model based on channel cracking fragmentation testing results. The correlation between mechanical and chemical degradation was discussed for both outdoor and accelerated laboratory aging. The results of this work provide preliminary understanding on failure mechanism of backsheets after weathering, laying the groundwork for linking outdoor and indoor accelerated laboratory testing for multilayer photovoltaic backsheets.
The channel crack and delamination phenomena that occurred during tensile tests were utilized to study surface cracking
and delamination properties of a multilayered backsheet. A model sample of commercial PPE (polyethylene
terephthalate (PET)/PET/ethylene vinyl acetate (EVA)) backsheet was studied. Fragmentation testing was performed
after accelerated aging with and without ultraviolet (UV) irradiation in two relative humidity (RH) levels (5 % RH and
60 % RH) at elevated temperature (85 °C) conditions for 11 days and 22 days. Results suggest that the embrittled surface
layer resulting from the UV photo-degradation is responsible for surface cracking when the strain applied on the sample
is far below the yielding strain (2.2 %) of the PPE sample. There was no surface cracking observed on the un-aged
sample and samples aged without UV irradiation. According to the fragmentation testing results, the calculated fracture
toughness (KIC) values of the embrittled surface layer are as low as 0.027 MPa·m1/2 to 0.104 MPa·m1/2, depending on the
humidity levels and aging times. Surface analysis using attenuated total reflectance Fourier transform infrared and
atomic force microscopy shows the degradation mechanism of the embrittled surface layer is a combination of the photodegradation
within a certain degradation depth and the moisture erosion effect depending on the moisture levels.
Specifically, UV irradiation provides a chemical degradation effect while moisture plays a synergistic effect on surface
erosion, which influences surface roughness after aging. Finally, there was no delamination observed during tensile
testing in this study, suggesting the surface cracking problem is more significant than the delamination for the PPE
backsheet material and conditions tested here.
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