Backsheet Durability Sequence

PVEL’s Backsheet Durability Sequence (BDS) assesses vital reliability risks for backsheets including yellowing and polymer cracking. While backsheet cracking is certainly more concerning, yellowing can be a sign of material degradation that may ultimately lead to backsheet failure in extreme cases. UV exposure is the primary driver of polymer degradation, so backsheet durability is most important in high irradiance and high albedo conditions.

Backsheet cracks that allow moisture to enter a PV module can severely impact field performance and safety. Other signs of backsheet aging, such as yellowing and chalking (powder accumulation on the backsheet surface), are potential indications of material degradation. BDS utilizes high temperature and high humidity to stress the backsheet. UV light can break polymer chains in backsheets, resulting in degraded mechanical properties. Cyclic thermal stress allows for the full level of material degradation to be realized.

Why BDS Testing Matters

Extel Energy, a member of the Kiwa Group, has analyzed multiple sites with backsheet issues. In 2017, they conducted a study on a 10 MW ground mount project in Thailand that was commissioned in 2013 and had portions of the site that were underperforming. Numerous backsheet cracks were noticed around the cell perimeters allowing moisture into the laminate. This further led to corrosion and discoloration. In extreme cases the issue caused ground and arc faults from the cells’ electrical circuit to the racking via a path through the backsheet cracks, resulting in burn marks and broken glass.

Extel’s further investigations discovered that one 2 MW section of the array had 5% lower power than the module’s power guarantee. Based on these findings, the module manufacturer supplied replacement modules, but the site owner still had to pay for the labor to replace the modules.

Requiring PVEL PQP Backsheet Durability Sequence testing helps to prevent unreliable backsheets from entering the market, thus avoiding backsheet-related failures.

Backsheet

Materials Assessed

The backsheet durability sequence is solely focused on the backsheet’s susceptibility to polymer degradation. This test is therefore not required for glass//glass modules or when other module materials are changed.

Backsheet
Backsheet Color

Explore PVEL’s Test Methodology

Key Takeaways

Click through the key takeaways.

No Backsheet Cracking Detected
Percentage of Available Backsheets
BDS Failures
Field Failures

PVEL has not identified any catastrophic failures during BDS testing.

Over the past three years, PVEL’s BDS testing for PQPs has not revealed any backsheets that have suffered from backsheet cracks. This covers backsheets with a range of outer layer materials including coatings, PVF, PVDF and PET films.

PVEL’s BDS test has qualified ~40% of the available backsheets.

The BDS results represent less than half of the 130 backsheets available on the market according to ENF’s solar backsheet directory. It is possible that the untested backsheets, including those from new market entrants, are susceptible to cracking.

Electrical insulation and power label failures exist.

While no cracking has been observed, PVEL has detected wet leakage/electrical insulation failures during BDS that were traced to the junction boxes. Additionally, UV exposure made some power labels illegible, a major defect according to IEC 61215.

Historical modules still suffer from backsheet failures.

PVEL is frequently contacted by site owners experiencing insulation failures, delamination, and corrosion issues due to modules with backsheet cracks. Repairs are often the suggested remedy, the reliability of which remains unclear.

Test Procedure

The BDS begins with exposing glass//backsheet modules to 1000 hours of damp heat, which weakens bonds in susceptible polymer backsheet materials. This is followed by rear-side UV exposure, 50 thermal cycles (TC), and 10 humidity freeze (HF) cycles. This sequence of UV, TC and HF testing is repeated twice more, followed by a short UV exposure for photobleaching, which removes a specific form of discoloration that occasionally happens in laboratory environments, but not in the field. Throughout BDS, PVEL performs visual inspection, electrical insulation resistance testing, and colorimeter measurements at ten different backsheet locations for two identical samples per BOM.

These colorimeter measurements use the Commission Internationale de l’Eclairage (CIE) L*a*b* coordinate system, with b* representing the yellow/blue coordinate. As material yellowness increases, so does the b* value. Calculating the difference between b* values (“delta b star”) at the beginning and end of BDS testing provides a marker for backsheet discoloration and potential material degradation.

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