Identification of Stressors Leading to Degradation of Antisoiling Coating in Warm and Humid Climate Zones

Abstract

We experimentally identified significant stressors that degrade the hydrophobic (contact angle > 90°) antisoiling coatings on exposure to Mumbai weather conditions, where losses due to dust deposition go up to 50% in three months. The contact angle of the antisoiling coatings changed from hydrophobic to hydrophilic (contact angle < 90°) in 12 weeks of field exposure. By testing the weakest performing coating on photovoltaic (PV) modules, no significant reduction of rate of dust deposition was observed on the coated PV module throughout the year. Soiling loss of the coated module went back to zero after every cleaning run (not seen in the uncoated module), which signifies ease of cleaning in the coated module. This resulted in a 2.2% lower soiling loss than the uncoated weekly cleaned module. This trend was reversed after four months of heavy rainfall, after which the uncoated module shows lower soiling loss than the coated module. Signatures of stressors identified by the field exposure test were: 1) rainfall—frequent natural cleaning during rain caused significant abrasion on the coated surface, and the acidic component of the rainfall reacted with the coating (which was verified by the indoor individual stress test), and 2) frequent manual cleaning runs, which lead to abrasion by dust and water (verified by the indoor individual stress test). The effect of frequent cleaning on the field was more severe, as, in real field condition, the combination of stressors acted concurrently. The effect of UV exposure and condensation was also studied by the indoor stress test, where the combination of UV exposure and condensation showed a statistically significant decrease in the contact angle (for all coatings). The rate of decrease in the contact angle was the highest for coating C, making it completely hydrophilic after 32 kWh/m2 of UV dose and 441 h of condensation. However, in indoor stress tests with individual stressors (UV exposure and water immersion), very high stress levels are required for producing a similar extent of degradation.