Fig. 2

Why Do Some Solar Panels Fail Prematurely?

March 24, 2020
NIST suggest plastics used in them are not compatible with the task.

Unlike diamonds, solar panels are not forever. Ultraviolet rays, gusts of wind and heavy rain wear away at them over their lifetime.

Manufacturers typically guarantee panels will withstand the elements for at least 25 years before experiencing significant drop-offs in power generation. Recent reports, however, call out a trend of panels failing decades before expected. For some models, there has been a spike in the number of cracked backsheets, the layers of plastic that electrically insulate and physically shield the back of solar panels.

The premature cracking has largely been attributed to the widespread use of certain plastics, such as polyamide, but the reason for their rapid degradation has been unclear. By closely examining cracked polyamide-based backsheets, researchers at the National Institute of Standards and Technology (NIST) discovered how interactions between these plastics, environmental factors and solar panel architecture may be speeding up the degradation. This could help researchers develop better, more accurate durability tests and lead to longer-lived solar panels.

Cracks in backsheets often show up first near certain features such as the grid-shaped space in between the blue or black solar cells. The cracks can eventually make their way all the way through the sheet. These defects let in oxygen and moisture to damage the interior where the cells lie and let electrical current escape, increasing the risk of electrocution.

If left outside for long enough, any plastic-based backsheet will start to fall apart, but some plastics deteriorate more rapidly than others.

“In the 2010 to 2012 timeframe, many modules were deployed containing polyamide-based backsheets, which had dramatic cracking failures in as little as four years, despite meeting standard requirements,” says Xiaohong Gu, a NIST materials engineer.

To get to the root of polyamide’s degradation problem, Gu and her team acquired backsheet samples from solar panels deployed in regions around the globe, including the U.S., China, Thailand and Italy. Most panels, which were used from three to six years, showed clear signs of premature cracking.

The researchers conducted a range of chemical and mechanical tests to examine the patterns and severity of degradation throughout the depth of the sheets they had collected. The areas of the sheets that had undergone the worst cracking were those that had become the most rigid. And the most brittle areas were on the inner side of the sheets.

How could the walled-off interior degraded more quickly than the exposed outer layer? Gu and her team speculated that the sunlight-induced degradation of the top side of the encapsulant, a film that surrounds the solar cells, created damaging chemicals that went down toward the backsheets, speeding up their decay. The proposed mechanism would explain why cracks form in between solar cells, as chemicals could find passage to the back through these regions.

Researchers identified acetic acid as a prime suspect, as it is known to be harmful to polyamide when the polymer commonly used as an encapsulant (ethylene vinyl acetate) degrades. To test their theory, the researchers put several polyamide strips in vials of acetic acid and after five months, then analyzed how they decayed compared with strips placed in either air or water.

Under the microscope, the strips exposed to acetic acid had cracks that mirrored those from the weathered backsheets, and they looked much worse than those on strips that had been in air or water. Chemical analysis showed higher concentrations of polyamide’s degradation products of polyamide in the acetic-acid-exposed strips, providing further evidence the acid accelerates deterioration of the backsheet material.

The study indicates the interplay between solar panel components (the EVA encapsulant and polyamide backsheet in this case) are potentially critical factors to consider when designing solar panels.

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