Backsheet yellowing is defined as “The loss of maximum elongation of the polymer materials, which causes brittle panels, increasing the risk of their cracking under mechanical stress”.
Results of the Fraunhofer Study
The Fraunhofer Institute in Germany has conducted a study on round robin tests on solar panel back sheets. The aim was to analyze the effects of UV stress on the backsheets at different temperatures. The key findings in the Fraunhofer study regarding yellowing are as follows:
Yellowing starts at UV wavelengths of less than 360nm
Different backsheets degrade differently with UV exposure.
Fraunhofer observed that strong UV intensities and high temperatures trigger degradation in panels eventually leading to yellowing of back sheets.
Figure 1: Results from Fraunhover study (source: PVDiagnosis)
The results of Franhouver study are represented in the graph above. Backsheet yellowing of different types of materials exposed to a constant amount of UV is shown. Variation of yellowing is measured at two different temperatures (60C and 80C). The following observations was concluded from the study:
In DuPont’s field observations, the rate of backsheet degradation increased over time with all backsheet materials except for Tedlar polyvinyl fluoride (PVF).
The rate of degradation was very low in older photovoltaic systems in the DuPont field survey, because most of those featured Tedlar-based backsheets.
Tedlar PVF backsheets had a very low rate of degradation, much lower than all other backsheets.
Yellowing is attributed to physical and chemical mechanisms .It depends on the materials used, the features of the front glass and how well the material resists UV radiation. Sub-optimal components and bad manufacturing process combined with low resistance to UV radiation are the most likely causes of yellowing.
Influence on generation capacity and durability
Yellowing on the backsheet may lead to a loss of adhesion and can have a negative effect on the reliability of the PV module.
Defective modules and correction steps of Yellowing
It is important to communicate with the manufacturer after detecting the yellowing. But sometimes manufacturers accept only those warranty requests where the entire module performance is impacted. Regardless, the following steps should be carried out in the identification of modules with anomalies:
Measurement of yellowing breakthrough degree.
The electric parameters of yellowed and not yellowed modules should be recorded.
Three destructive tests should be done on affected modules, namely accelerated aging test, adherence measurement and mechanical properties measurements.