Why do solar PV module backsheets fail and what can be done about it?
The backsheet is one of the most likely components of the solar module to fail, which impacts about 1 percent of all modules, and according to A. Bradley et al., 9 percent of all module failures are related to the backsheet component. With premature degradation affecting all kinds of modules in all climates, costs are climbing for the solar industry. Of the 113 GW PV estimated capacity installed in 2020, 1.1 GW risks backsheet failure — with a potential repair bill of $500 million. Why is this happening? And how can solar owners and operators avert disaster?
This column originally appeared in the Spring 2020 issue of Solar Builder magazine. Get your free subscription (print or digital) right here.
Why backsheets fail
As the final layer on the back of a PV module, the backsheet is the first line of defense against air and moisture which can corrode electrical components. Cracking, delamination (peeling), and abrasion are all symptoms of backsheet failure which impact the mechanical properties of the solar module.
The demand for low-cost modules in recent years has put every component under extreme price pressure, including backsheets. To cut costs, inferior materials are used in solar modules at the expense of module lifespan and performance.
A weak core: Traditional backsheets use low-stabilized PET as a core layer material because it’s cheap and provides good electrical insulation. However, the PET polymer is very sensitive to moisture and it ages quickly in sunlight. Because stabilizing the full PET core layer would make the backsheet too expensive, thin and durable outer layers of fluoropolymers like PVF, PVDF or highly stabilized PET are added to protect the core layer. Once these outer layers fail, get damaged or delaminate, the weak core is not able to stop the rapid degradation of the backsheet.
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Zero-margin backsheet designs: Adding to the problem, the protective fluoropolymer outer film meant to protect the module has halved in thickness from more than 40 to less than 20 microns for most fluoro-based backsheets on the market today. In some cases, the outer protective layer is now little more than a polymer coating just a few microns thick. In most cases, the fluoropolymer inner layer has entirely disappeared and been replaced by a non-fluoropolymer film or coating. The predictable result is backsheet failure.
Inconsistent and ambiguous manufacturing practices: Due to cost pressure, parts of the bill of materials are exchanged for lower cost or more readily available materials — with backsheet manufacturers sourcing base films from multiple suppliers and laminating them with various adhesives. The use of inferior, untraceable and uncertified materials and adhesives in the lamination process leads to an inferior backsheet that is more likely than ever to fail. This practice also makes it virtually impossible for solar park owners and operators to pinpoint how and why a fault occurred.
The case for quality backsheets
The cost savings of solar modules made with inferior backsheet materials just don’t add up over the lifetime of the project. Quality backsheets use co-extrusion in place of traditional lamination and polyolefins as the core instead of PET (which has a molecular structure that’s intrinsically sensitive to moisture).
Using a quality backsheet will protect your investment by avoiding poor power output or outright module replacement due to backsheet failure. In addition, high-quality backsheets can extend the useful life of solar projects. Assuming an average useful life of 30 years for projects built today and a prevalence of backsheet issues at 9 percent, a project with no backsheet issues is expected to last approximately three years longer.
Combined with other considerations (O&M reserves and appropriate scope, module manufacturing inspections, quality and design life of other components, etc.), it may be reasonable to assume a 35-year useful life when high-quality backsheets are utilized. The advantage of additional years of power output far outweigh any materials cost increase, if any for a high-quality backsheet.
Vivek Chaturvedi, Business Leader Endurance Backsteets, DSM Advanced Solar
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