Fluoropolymer coatings for solar cover glass: Anti-soiling mechanisms in the presence of dew

Soiling of solar cover glass can cause a significant loss of electrical output from PV panels. Dew condensed on the surface exacerbates soiling rates and promotes reactions between dust and glass leading to stronger adhesive forces that make cleaning more expensive. To reduce soiling rates and lower cleaning costs, anti-soiling coatings are being developed. Although the interactions between dust particles and uncoated low-iron glass surfaces are known, the mechanisms by which anti-soiling coatings reduce soiling rates are less well understood. In this paper we report on the effect of wetting properties on both the soiling rate as well as the cleaning efficiency of polymer coated solar cover glass substrates in the presence of condensed water in a controlled environment. Coatings were fabricated with water contact angles (CM) ranging from 149 degrees to 51 degrees and compared to bare glass. Arizona Test Dust was applied onto substrates on which water condensate (i.e. artificial dew) had formed. The surfaces were baked to enable cementation to occur. Such dew-dust-dry cycles were repeated to simulate naturally occurring, cumulative soiling processes. Changes in dust accumulation rates were assessed as a function of coating properties. Hydrophobic coated glass exhibited 42% lower soiling rates compared to hydrophilic glass. A dust "herding" mechanism was identified to account for these reduced soiling rates, by which dust is concentrated into discrete piles during the lateral shrinkage of droplets on low-energy surfaces. These hydrophobic surfaces were also easier to clean with >99% of the original direct transmittance restored using only water.