Lifetime of terrestrial PV modules is determined not by limits of the photovoltaic process but by ingression of moisture into the module laminate. To avoid (or at least to limit) moisture ingression an adequate sealing at the module edges is very helpful, but also a good adhesion of the laminate layers is necessary. This paper addresses the adhesion quality. Adhesion of a standard PV module lamination is consisting of the sequence glass-EVA-cell-EVA-glass or glass-EVA-cell-EVA-backsheet. The adhesion quality depends mainly on: 1.1. the cleanliness of the glass sheets 1.2. the condition of the EVA (prior to lamination) 1.3. the lamination process (process temperature, profile and duration, pressure, homogeneity) To test lamination quality, three different tests (on the module center as well as close the module edges) have been applied: 2.1. Chemical analysis of samples to determinate the state of “curing” or cross-linking of the co-polymer EVA after lamination 2.2. Peel tests to determine the force to peel-off the layer of the laminate 2.3. Damp-heat treatment (1000 h at a temperature of 85°C and 85% of relative humidity) It was found out that the chemical analysis of the EVA curing-state (2.1) is good to find out about the accuracy of the lamination process, following the recommended temperature profile and homogeneity (as 1.3), and the initial condition of the EVA used (1.2.). However, this test method may be misleading to allow a statement on the overall quality of the lamination: the surface glass sheet may be treated with oil (e.g., to prevent adhesion of the individual glass sheets during storage) which has not been removed properly before lamination, thus causing low laminate adhesion (especially after the damp heat treatment 2.3), early moisture ingression and a reduced module lifetime. Therefore peel tests (2.2) are essential to determine the overall quality of the lamination. Several results of measurements are presented in the paper.
The hot spot stability is part of the product qualification test sequence of pv modules. Therein the major challenge is the selection of the cell with the highest hot spot risk. Currently, the electrical properties of the cells are used as selection criteria, e.g., the highest threshold currents at the point where the bypass diode turns “on” and/or cells with maximum and minimum values for the parallel resistance. The weak point of this procedure is, that the cells with a hot spot risk are not ranked by temperature, which may cause a hot spot event. The paper presents the results of a comparison between the electrical and thermal properties of shadowed solar cells in pv modules. Finally, an improved hot spot test sequence is presented that shows a better prediction probability for this specific module failure.
