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Austrian researchers have analyzed the cracking propensity of polypropylene (PP) backsheets and have concluded that they could be used in the manufacture of solar modules, unlike co-extruded polyamide backsheets, which caused many headaches for the photovoltaic industry in the past.
A group of researchers led by Austrias Polymer Competence Center Leoben has investigated the cracking propensity of co-extruded backsheets based on three layers of polypropylene (PP) in an effort to evaluate their reintroduction into the solar module manufacturing industry.
“For several years, we have been testing polypropylene-based coextruded backsheets with regard to their properties, performance and reliability,” lead research author Gernot Oreski told pv magazine . “The biggest risk with these new types of backsheets is cracking after a few years of operation in the field.”
In the article “ Investigation of the crack propensity of co-extruded polypropylene backsheet films for photovoltaic modules ”, published in Solar Energy Materials and Solar Cells , the scientists point out that these backsheets have already been used in industry, giving rise to the so-called “sheet disaster”. polyamide backs”, which according to them refers to several defective installations developed between 2009 and 2015 in various countries with this technology.
“In my opinion, it was a really innovative idea to use co-extruded backsheets, where each layer has different requirements and therefore different fillers and additives,” says Oreski. “However, the selection of materials and, above all, their qualification was not done thoroughly. The test protocols in use at the time focused on exposure to point stresses, so the problematic long-term behavior of polyamide was not uncovered, and therefore the backsheets would crack after a few years.”
“In the meantime, new test protocols have been developed specifically to cause such backsheet failures, and the weld nub coupon test is one of them,” he continued. "Unfortunately, this has made the photovoltaic industry reluctant not only to use polyamides, which is understandable, but also to use coextruded backsheets in general."
The academics explained in the study that coextruded PP backsheets have lower stiffness and higher flexibility than laminated ones. In addition, they found that PP backsheets have low water vapor transmission rates, comparable to their PET counterparts, but have high oxygen and acetic acid permeability rates.
Using a standard encapsulant, the scientists fabricated test samples, which they called weld coupons, and subjected them to accelerated aging tests alongside reference laminated backsheets.
They tested five different samples: a PP-based coextruded backsheet; a coextruded backsheet based on polyamide (PA); a laminated backsheet based on polyethylene (PE), polyethylene terephthalate (PET) and polyvinylidene difluoride (PVDF); a laminated backsheet based on polyvinyl fluoride (PVF) and PET; and a laminated backsheet based on fluorinated coating (fc), PET and PVDF.
Next, the Austrian group exposed the samples, once laminated, to 5 cycles consisting of 500 hours of xenon and 100 thermal cycles, with temperatures between -40ºC and 85ºC. “Two samples of each material combination were subjected to the accelerated aging tests: For one sample the back sheet was directly irradiated, for the second sample the front glass was irradiated,” he explained.
The tests showed that only the second and third types of backsheet had severe cracks. The other three types, on the contrary, did not show cracks or embrittlement of the outer layer.
“The results of this study confirm previously published results on the long-term stability of PP-based coextruded backsheets, indicating excellent stability against temperature, humidity and UV rays,” the group said referring to the remarkable group of the first type of dorsal lamina. “Overall, coextruded PP backsheets show great potential to be a valid replacement for standard PET-based backsheets in PV modules.”
The research team also includes scientists from the Austrian Research Institute for Chemistry and Technology, Silicon Austria Labs, Kyoto Solar and Borealis Polyolefine GmbH.
“Co-extrusion makes it possible to tailor the backsheet exactly to the requirements needed for specific conditions, as the process itself is much more flexible than the lamination process,” Oreski said, referring to the real possibilities for co-extruded backsheets to re-enter the PV industry through the adoption of PP material. |
