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Bulged Backsheet! Is it detrimental? Why is it important to rectify it? Let’s find that out…

Updated: Jul 29



While inspecting a 7-year-old utility-scale PV asset, we found that the back-sheet of a module bulged. Let’s find out what caused this.


While conducting thermal inspection at this site we found hotspots at this module ranging from 90-100 degrees Celsius above operational temperature. To find out what caused it, we conducted a visual inspection.


What did we find during visual inspection?

We found browning on a few cells of some of the PV modules at the site, which was receiving lesser solar radiation than a normal operating cell.


So why were the solar cells with lesser solar radiation, warmer?

When a part of the module receives lesser solar radiation it resists the normal current flow which results in release of heat energy, as high as 90 to 100 degrees Celsius higher than normal operating temperature.


You may be wondering why we are talking about browning when we started with bulging of the back-sheet. Let’s find out, why...

It so happens that when there is browning in the modules, the affected cells at high temperature (above 80 degrees Celsius), melt the back-sheet material causing it to bulge.


Is bulging of back-sheet detrimental?

Yes, it is. If the bulged back-sheet is left unnoticed, with time this bulge might lead to back-sheet puncture, exposing the modules to the outside air which may, in turn, lead to safety hazards.


Dangerous, right! So What caused the browning in the first place? And how can it be prevented?

There is a phenomenon called the back-sheet breathing, wherein due to poor back-sheet material quality, there is moisture ingress that is seen inside the module. This moisture causes degradation of EVA encapsulation and leads to bus bar corrosion. This corrosion spreads to the entire solar cell over time.


The only way to prevent moisture entrance is by using a true hermetic seal or by using a low-diffusivity edge seal containing a large amount of desiccant. Therefore, if a PV device is very sensitive to moisture, it may be more reasonable to figure out ways to reduce the corrosion that are enhanced by moisture entrance.

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