Abstract
Soiling of solar module cover glass is a serious problem for solar asset managers. It causes a reduction in power output due to attenuation of the incident light, and reduces the return on investment. Regular cleaning is required to mitigate the effect but this is a costly procedure. The application of transparent hydrophobic, anti-soiling coatings to the cover glass is a promising solution. These coatings have low surface energy and contaminants do not adhere well. Even if soiling does remain on the coated surface, it is much more easily removed during cleaning. The performance of the coatings is determined using the water contact angle and roll-off angle measurements. However, although hydrophobic coatings hold out great promise, outdoor testing revealed degradation that occurs surprisingly quickly. In this study, we report on results using laboratory-based damp heat and UV exposure environmental tests. We used SEM surface imaging and XPS surface chemical analysis to study the mechanisms that lead to coating degradation. Loss of surface fluorine from the coatings was observed and this appeared to be a major issue. Loss of nanoparticles was also observed. Blistering of surfaces also occurs, leading to loss of coating material. This was probably due to the movement of retained solvents and was caused by insufficient curing. This mechanism is avoidable if care is taken for providing and carrying out carefully specified curing conditions. All these symptoms correlate well with observations taken from parallel outdoor testing. Identification of the mechanisms involved will inform the development of more durable anti-soiling, hydrophobic coatings for solar application.
Highlights
The deployment of photovoltaics (PV) has grown at an extraordinary rate over the last decade.The manufacturing output of solar modules was reported to be 102 GW in 2018 and there is over500 GW of cumulative installed capacity worldwide [1]
Hydrophilic coatings typically used to coat windows and mirrors are designed to allow water drops to merge and form a film of water. These coatings were applied on module cover glass so that water will ‘sheet’ across the glass surface and as it drops to the bottom of the module, photo-catalytically remove contamination [11,12,13]
The hydrophobicity of a coating is determined by measuring the water contact angle (WCA) of a water droplet resting on the surface
Summary
The deployment of photovoltaics (PV) has grown at an extraordinary rate over the last decade.The manufacturing output of solar modules was reported to be 102 GW in 2018 and there is over500 GW of cumulative installed capacity worldwide [1]. These coatings were applied on module cover glass so that water will ‘sheet’ across the glass surface and as it drops to the bottom of the module, photo-catalytically remove contamination [11,12,13] These coatings are high surface energy and can lead to increased soiling in the absence of water. Hydrophobic coatings have low surface energy and reduce the adhesion of dust particles to the glass surface, thereby, reducing the level of soiling and enhancing the cleaning effect of rain and wind [5]. Such highly-repellent surface properties, which are inherent to many natural surfaces, derive from either specific surface texture or chemical composition. The most popular example is the lotus leaf naturally exhibiting WCA’s of ~ 160◦ and roll off angle (RoA) below
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