24 Mar 2020
Interactions between backside plastics, weather, environment and design thought to be speeding up degradation.National Institute of Standards and Technology (NIST).
The causes are a combination of ultraviolet rays, gusts of wind and heavy rain, which wear away at the panels – often shortening the advertised lifetime. 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 backsides of solar panels.
NIST says that 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.Progress in Photovoltaics.
Cracks in backsheets often show up first near certain features — such as the grid-shaped space in between the blue or black electricity-producing solar cells — and can eventually propagate through the entire thickness of a sheet. These defects make way for oxygen and moisture to infiltrate and damage the interior where the cells lie and also allow electrical current to escape, increasing risks of electrocution.
“In the 2010 to 2012 timeframe, many modules were deployed containing polyamide-based backsheets, which presented dramatic cracking failure in as little as four years despite meeting standard requirements,” said Xiaohong Gu, NIST materials engineer and co-author of the study.
To get to the root of polyamide’s degradation problem, Gu and her team acquired backsheet samples from solar panels deployed in the U.S., China, Thailand and Italy. Most of the panels, in use from three to six years, showed clear signs of premature cracking.
The NIST group analyzed the weathered backsheets and realized that the areas that showed the worst cracking were those that had become the most rigid. Furthermore, the most brittle areas were on the inner side of the sheets, Gu said. She speculated that the sunlight-induced degradation of the top side of the encapsulant produced damaging chemicals that descended toward the backsheets, speeding up their decay.
The researchers identified acetic acid as a prime suspect, as it is known to be harmful to polyamide and is produced during the degradation of a polymer commonly used as an encapsulant, called ethylene vinyl acetate (EVA). To test their hypothesis, the researchers stowed several polyamide strips away in vials of acetic acid and then, after five months, analyzed how they decayed compared with strips placed in either air or water.
Chemical analysis showed that degradation products of polyamide were higher in the acetic-acid-exposed strips, providing further evidence that the acid accelerates the deterioration of the backsheet material.
The study highlights the interplay between solar panel components (the EVA encapsulant and polyamide backsheet in this case) as a potentially critical factor to consider when designing solar panels that are built to last. The NIST statement concluded, “These new insights into premature failures could also become valuable to NIST researchers and others who seek to replicate the degradation process in the lab as a way to test and predict the longevity of solar panel components.”