Design and optimization of carbon-coated silica nanospring system for solar-driven evaporation for water desalination and purification

초록

The solar-driven evaporation of water presents a viable solution to global freshwater scarcity. This method harnesses solar irradiation as the sole and sustainable energy source, effectively minimizing the substantial energy consumption associated with traditional water purification technologies. Nevertheless, many solar evaporators experience low water evaporation rates or rely on expensive light absorbers and intricate processes. This study introduces a high-efficiency solar evaporator for producing fresh water by investigating the design and operating factors for a system that uses rods for solar water evaporation. The system integrates dense silica 1D nanosprings (NS) grown via the vapor-liquid-solid (VLS) mechanism on an alumina ceramic rod (ACR). Subsequently, a conformal layer of thin carbon, known as Graphene from the University of Idaho Thermolyzed Asphalt Reaction (GUITAR), is coated using atmospheric pressure chemical vapor deposition (APCVD) to increase solar energy absorption and lower contact angles of water. While GUITAR coating increased heat absorption and hydrophilicity, thick nanosprings prevented evaporation by keeping water out of the nanosprings, thus forming an insulating air layer between solid and water. Therefore, the optimum NS thickness was found, and it induced a faster wetting transition resulting in a higher heat transfer area and lower water insulation. The optimal operating condition was also found. Since water would be wicked along the rod surfaces not under water through capillary action along the nanospring surfaces, the center of the irradiation should be more toward the upper side of the rod as to irradiate both surface under and over the water. The irradiation should be sufficiently strong enough because the evaporation rate does not linearly change per irradiation flux. The integration of 1D NS onto the ACR results in a comparative evaporation rate increase of 16.7 %, when contrasted with the ACR that has only a GUITAR. With the optimal design and operation condition, the evaporation rate up to 5.8 kg m-2 h-1 was achieved. This study shed light on the importance of controlling the thickness of surface-built nanostructures and wetting transition when it is commonly believed that nanostructures increase the evaporation rate.

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