Over the past decade, degradation and power loss have been observed in PV modules resulting from the stress exerted by system voltage bias. This is due in part to qualification tests and standards that do not adequately evaluate for the durability of modules to the long-term effects of high voltage bias experienced in fielded arrays. High voltage can lead to module degradation by multiple mechanisms. The extent of the voltage bias degradation is linked to the leakage current or coulombs passed from the silicon active layer through the encapsulant and glass to the grounded module frame, which can be experimentally determined; however, competing processes make the effect non-linear and history-dependent.Appropriate testing methods and stress levels are described that demonstrate module durability to system voltage potentialinduced degradation (PID) mechanisms. This information, along with outdoor testing that is in progress, is used to estimate the acceleration factors needed to evaluate the durability of modules to system voltage stress. Na-rich precipitates are observed on the cell surface after stressing the module to induce PID in damp heat with negative bias applied to the active layer.
Accelerated lifetime testing of five crystalline silicon module designs was carried out according to the Terrestrial Photovoltaic Module Accelerated Test-to-Failure Protocol. This protocol compares the reliability of various module constructions on a quantitative basis. The modules under test are subdivided into three accelerated lifetime testing paths: 85°C/85% relative humidity with system bias, thermal cycling between -40°C and 85°C, and a path that alternates between damp heat and thermal cycling. The most severe stressor is damp heat with system bias applied to simulate the voltages that modules experience when connected in an array. Positive 600 V applied to the active layer with respect to the grounded module frame accelerates corrosion of the silver grid fingers and degrades the silicon nitride antireflective coating on the cells. Dark I-V curve fitting indicates increased series resistance and saturation current around the maximum power point; however, an improvement in junction recombination characteristics is obtained. Severe shunt paths and cell-metallization interface failures are seen developing in the silicon cells as determined by electroluminescence, thermal imaging, and I-V curves in the case of negative 600 V bias applied to the active layer. Ability to withstand electrolytic corrosion, moisture ingress, and ion drift under system voltage bias are differentiated according to module design. The results are discussed in light of relevance to field failures.
Interest in renewable energy has motivated the implementation of new polymeric materials in photovoltaic modules. Some of these are non-cross-linked thermoplastics, in which there is a potential for new behaviors to occur, including phase transformation and visco-elastic flow. Differential scanning calorimetry and rheometry data were obtained and then combined with existing site-specific time-temperature information in a theoretical analysis to estimate the displacement expected to occur during module service life. The analysis identified that, depending on the installation location, module configuration and/or mounting configuration, some of the thermoplastics are expected to undergo unacceptable physical displacement. While the examples here focus on encapsulation materials, the concerns apply equally to the frame, junction-box, and mounting-adhesive technologies.
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