Scientists in Italy investigated the formation of cracks in silicon solar cells, and the subsequent performance loss this can cause. Their work finds that current models don’t fully take into account the influence of encapsulants and other module components outside of the cell, and that smaller stresses such as vibrations during transport or caused by wind in the field could have a role to play. And all of this could have implications for testing and module quality standards.
Solar cell cracking has been identified as one of the major causes of module degradation and performance loss in the field. Cracks are known to be caused by mechanical stress placed on the module, which can occur in manufacturing, handling during transport & installation, and in the field once modules are already operational. And once initial cracks are formed, performance loss can spread very slowly over years in the field, making it difficult to detect until its too late.
Awareness of cell cracking has improved in recent years, and the industry is hard at work looking to minimize the risk of cracks forming in cells. Advancements in technology – such as the switch to monocrystalline silicon and to multibusbar cell interconnection – have been shown to lessen the likelihood of performance loss due to cracking, there is still more work to be done to fully understand and mitigate the risk here.
A group of scientists led by the IMT Institute for Advanced Studies in Lucca, Italy investigated the formation of cracks in silicon modules, and found that many models used to predict formation of cracks and associated module performance loss might not be looking at the full picture. “Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios,” the group explained. “Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon.”
Fatigue degradation
The experimental study, Fatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modules, is published in the journal Scientific Reports. The group took small modules made up of two rows of five 156mm monocrystalline cells, and made ‘moderate impacts’ on these to induce cracks, and then subjected the modules to a series of bending, loading and humidity freeze tests to observe how the cracks spread.
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The work reveals a need to give more consideration of module components outside of the cell, and for further research combining the fields of electronics, materials science and computational mechanics. A key finding, which the group says could have impact on module testing and qualification standards, is the discovery of ‘fatigue degradation’, showing that over time smaller stresses than previously thought may be contributing to performance loss.
“The present results imply that the quasi-static application of very high distributed pressures as requested by qualification standards is not enough if we are interested in evaluating the actual degradation rate and possibly infer about the lifetime of produced PV modules,” they conclude. “…other unexpected forms of damage, like fatigue degradation, are indeed possible due to the composite structure of the module and can be induced by very common sub-critical loads like vibrations due to transportation or wind gusts, phenomena not yet characterized so far.”
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Source: pv magazine
