Application of hydrophobic fluorinated silicon oxide nanoparticle coating on electrodynamic screen (EDS) films for enhancing self-cleaning function of solar collectors
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Abstract
Optical surfaces, which are exposed to outdoor environmental conditions, are susceptible to dust deposition. Sunlight incident on the surface of photovoltaic (PV) modules is attenuated by the dust layer accumulation on the front surface of the modules. Energy-yield decrease by dust layer accumulation, called soiling losses, can be 5 to 40 percent annually unless the modules are cleaned frequently. Cleaning the optical surface with water is generally used in solar plants, which causes an unsustainable demand for freshwater in semi arid and desert areas. Adhesion of soil on the front surface of solar collectors plays a major role in the cleaning process. For low water cleaning, the module surface is often coated with an anti-soiling coating. If the surface is made hydrophilic, water can wet the surface most effectively making the cleaning process efficient, but requiring a low amount of water. If the surface is made hydrophobic, the surface energy is decreased reducing the adhesion force; the cleaning can be performed even with a lesser amount of water or with an application of an external removal force, such as wind. Major problems with the application of anti-soiling coatings are their environmental durability, poor adhesion of the coating on the surface and low resistance to abrasion. Since the removal of dust still requires spraying of water to the PV modules, the coatings get removed within a short period. Similarly, another dust removal method that does not require water for cleaning PV modules, is the Electrodynamic Screen (EDS) film, which consists of series of parallel transparent electrodes embedded within two transparent dielectric films and laminated on the surface of the solar collectors. When the electrodes are activated film by applied voltage pulses there will be a traveling electric field on the EDS surface, then the dust particles on the film surface gets charged electrostatically and are removed by Coulomb forces applied by the electric field. The EDS film application is an elective removal of the dust particles from the PV module surface without requiring any water or mechanical forces. However, the applied Coulomb force cannot overcome the dust adhesion force when the particles are smaller than 2 m in diameter or if the capillary adhesion forces are present because of the high humidity environment.
The objective of the thesis is to decrease the particle adhesion forces of the EDS film surface by the application of a hydrophobic coating so that the dust removal efficiency is enhanced for moving small particles. To improve EDS film based dust-removal performance it is necessary to have the coating non-conductive and optically transparent and hydrophobic for the effective application electrostatic removal mechanisms. The hydrophobic coating would reduce the surface energy and hence the dust adhesion force, assisting the self-cleaning effect, which would not require any water consumption. Since water-based cleaning is no longer needed for dust removal when the EDS film surface is modified with hydrophobic surface properties, the durability of the coating would improve. We describe here a synthesis of a hydrophobic fluorinated silicon oxide nanoparticle coating on the EDS films for enhancing the self-cleaning function of solar collectors. We used dip-coating method to apply a single-layer hydrophobic silicon oxide nanoparticle film with a large static water contact angle on the EDS film surface. silicon oxide was prepared by the sol-gel method using Tetraethylorthosilicate (TEOS) and ammonium hydroxide as a precursor anda catalyst. The suspension was treated with 3-aminopropyltriethoxysilane (APS) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS-17) to modify the surface of the coating, which greatly improved the hydrophobicity, and thus gave the coating the desired self-cleaning property. We used polyurethane (PU) as a binder between the substrate and coating to enhance the durability of the lm in outdoors applications. Experimental data on the optical transmittance, adhesion of the hydrophobic film on the glass surface of the EDS lm, and the dust removal efficiency are presented. The average optical transmission efficiency (TE) decreased from 93.43 to 89.78 percent and the output power restoration (OPR) of the solar panels laminated with EDS film with SH coating increased from 98.3 to 99.19 percent. Possible improvements to the hydrophobic coating process and its durability and future research needs are discussed.