Round robin testing of various back-sheets for PV-modules with different ultra-violet radiation sources and sample temperatures

Durability testing of materials exposed to natural weathering requires testing of the UV stability, especially for polymeric materials. The type approval testing of PV-modules according to the standards IEC 61215 and IEC 61646 includes a so-called UV-preconditioning test with a total UV dose of 15 kWh/m² not corresponding to the real loads during lifetime. Between 3- 10% of the UV radiation has to be in the spectral range between 280nm and 320nm in the recent editions of the standards. However, the spectral distribution of the radiation source is very important since the samples show a very individual spectral sensitivity for the radiation offered. Less than 6% of the intensity of solar radiation comes in the UV range. In case of an increase of the intensity of the light source for accelerating the UV-test the overheating of the samples would have to be prevented more strictly and the temperature of the samples have to be measured in order to avoid misinterpretation of the test results. Fraunhofer ISE organised an inter-laboratory comparison of testing different back-sheets with different UV-light sources in various test laboratories using different UV-sources. The interaction of the UV-radiation with the polymers used in PV-modules is main subject of this round robin. Laminates were produced by using solar glass (130mm * 200mm) and a respective EVA encapsulant combined with 7 different back-sheets. The sample was exposed in a sample holder equipped with a couple of different filters in order to investigate roughly the spectral sensitivity by means of 3 edge filters and the intensity impact by two grating filters. The sample temperature was measured by thermo-couples. The degradation was followed by spectral reflectance and transmittance measurements and calculation of the yellowness-index. Clear differences in the degradation behavior of the different products were found.