Widespread reports of backsheet failures in fielded modules prompted PVEL to introduce the Backsheet Durability Sequence (BDS) to the PQP in 2019.
PVEL is pleased to share that no catastrophic backsheet cracking failures have occurred in PQP testing to date.
However, it is important to note that PQP participants choose which of their BOMs are factory witnessed and submitted for PQP testing. It is also important to note that less than 10% of commercially available backsheet models have completed BDS testing.
After module certification testing, manufacturers are free to use any of the 100+ backsheet models on the market in commercially available products. Some will suffer failures, and it is well-documented that certification testing does not identify failure-prone backsheets. Specifying PQP-tested BOMs with strong BDS results will help ensure that backsheets and modules perform as anticipated.
Backsheet Yellowing in the Field
Although increased yellowness does not always result in backsheet failure in the field, in some cases yellowing is an indicator of mechanical breakdown that leads to embrittlement and cracking in the field.
This occurred in a <5MW site in the southern U.S. located in a high desert climate. The yellowed backsheets began cracking after 10 years in the field and a full module replacement was required.
PVEL's BDS characterizations include visual inspections and colorimeter measurements to quantify yellowing
During BDS PVEL performs colorimeter measurements at ten different backsheet locations for two identical samples per BOM. The measurements use the Commission Internationale de l’Eclairage (CIE) L*a*b* coordinate system, with b* representing the yellow/blue coordinate.
As yellowness increases so does the b* value. Calculating the delta b* from the average initial measurement throughout the course of BDS helps quantify the change in yellowness.
In this example, there were no signs of material cracking in the yellowed samples, but it may still be cause for concern. Yellowing can be indicative of material degradation, but not always.
The graph shows examples of two backsheet types. One sample has minimal change in the b* coordinate while the other backsheet type yellows significantly.
When BDS reports expose cases of yellowness PVEL recommends that module purchasers perform additional diligence.
In these cases the module manufacturer should provide evidence that despite their backsheet’s discoloration it will perform reliably for the expected module lifetime.