Comparison of accelerated UV test methods with Florida exposure for photovoltaic backsheet materials

Photovoltaic (PV) modules currently have lifetime guarantees of 25 years or more. Ideally, all components of the modules would be tested in field conditions for at least 25 years, however this is impractical, so accelerated, laboratory-based testing is required to subject components to an approximation of field conditions. When using accelerated testing it is important to ensure the test does not result in false negatives, hence ruling out good materials, and more importantly, does not give rise to false positives, which could lead to material failures in the field. Current PV qualification standards can only test for “premature” failures and hence are not end-of-life tests. This has resulted in some products passing the qualification tests and subsequently failing in the field within the product lifetime guarantee. This paper compares the mechanical properties of PV backsheet materials weathered outdoors in Florida against those weathered using both ISO 4892-2 (Plastics – Methods of exposure to laboratory light sources – Part 2: Xenon-arc lamps) and the newly proposed IEC 62788-7-2 (Measurement procedures for materials used in photovoltaic modules – Part 2: Environmental exposure - Accelerated weathering tests of polymeric materials).