DuPont calls for extended testing to prevent premature module failure caused by defective backsheets.
The guarantee throughout the industry is an installed solar system should survive past its 25th birthday before a drop-off in production is noticed. There are tests and standards that confirm this and warranties that back it up. Survivability is often measured by the solar cells and the frontside of the panel. Unfortunately, the same attention has not been paid to what’s behind the module, and failing backsheets are threatening solar system longevity.
Backsheets are the final piece to the solar module puzzle, sealing a module from outside forces like moisture and UV light. Usually made from a type of polymer, backsheets adhere to the backside of modules to provide electrical insulation. Tedlar, a brand of polyvinyl fluoride (PVF) film produced by DuPont, is a popular choice for backsheet manufacturers. In the field for more than 30 years in all types of climates, DuPont’s backsheet material is a trusted industry staple.
Still, premature degradation of backsheets (well before year 25) has recently shown up in waves and might be closely associated to the quick ramp-up of solar production and panel manufacturers turning to untested, cheap backsheets.
“I think there was a period in the industry about three to five years ago where there was very rapid growth and putting out an awful lot of material into the field,” said John Trout, research and development manager for DuPont Photovoltaic Solutions. “Those panels are starting to see the aging and the degradation that’s associated with some of those [unproven material] choices.”
As polymers are exposed to sunlight, they age and lose their mechanical properties. They can turn brittle and, most notably, a yellowish color. A yellowing backsheet is more than just an unattractive color change; it’s usually the first sign of a mechanical breakdown and a warning that more drastic consequences aren’t far off.
“You can walk by the front of the modules and see yellow stripes between the cells, where the sun-facing portion of the backsheet has turned bright yellow. It’s not a good thing,” Trout said. “Around the back of the module, we often see the backsheet is no longer white, it’s turning various shades of yellow and, in bad cases, almost brown.”
Yellowing leads to cracking and sometimes delamination. The backsheet’s purpose is to protect the interior of the module, but cracking and delamination allow the environment in, and things like moisture can interrupt consistent solar production. Then there are risks of shock hazards and electricity leakage.
Unfortunately, this type of polymer breakdown started showing up on panels barely three or five years old. Seeing this unfortunate trend and knowing how backsheets affect a module, DuPont set out to research the problem and find a solution. Through its long involvement with the solar industry, DuPont established working relationships with third parties and created field programs to measure the lifetime results of nearly two million solar modules.
The company ultimately found that testing full modules (especially the backsheets) in sequential patterns, which mirror nature, is the best way to determine if a module will hit its warrantied lifetime. With this knowledge, DuPont developed the MAST (module accelerated sequential test) process, which is an accelerated 25-year test that runs modules through damp-heat cycles, UV cycles and temperature cycling.
“These are the most important stresses that happen in the field, and they happen sequentially,” Trout said. “This test best predicts and matches what we’ve seen in all our fielded work. Almost everyone does the single stress [tests]. Some people do them at very low levels and some people do them at such high levels, we think it has no meaning. People are now starting to pick up on the sequential testing.”
Trout said there are various requirements and standards for testing, but they’re not designed to predict lifetime expectations, yet the industry seems to use the standards just for that. In the last few years, other organizations like NREL have also come out with testing suggestions (stating that “the present qualifications and safety standards are inadequate for UV testing of the backsheet material”), but no specific standards have been established.
To ensure solar panels have long term durability, DuPont execs believe more attention needs to be paid past Day 1 functionality.
“[We need] to move on beyond Day 1 testing and appearance and look at better aged testing, testing that more represents a life in the field,” said DuPont’s global marketing director Bob Olsen. “More companies that own, insure and finance the solar systems are now more concerned about this, getting beyond that Day 1 because they know this will ultimately affect their returns and safety.”
The end-game, as DuPont sees it, is to develop industry-wide standards to ensure the backsheet is paid the same attention during testing as the module’s frontside.
“[We want] sequential testing and adoption on an industry-basis,” Trout said. “We want to educate the market on using better testing and standards and codes that would be industry-wide to continue to raise that bar.”
Hopefully certification and testing organizations take note, and soon.