Abstract:

Using solar energy to power evaporative processes has found various applications in desalination, wastewater treatment, and power generation among other fields. Due to its green-energy sources and energy-efficient conversions, it has gained increasing attention. However, as one of the oldest approaches, the application of solar evaporation is still limited by issues such as low evaporation efficiency, fouling, and the rapid degradation of solar absorbers. During solar evaporation processes, the solar absorbing materials are directly heated by sunlight and the generated heat is transferred to the water around the material. Within this process, in situ photo-thermal conversion is realized by absorber materials at the air-water interface. After the water is heated, it vapors continuously. Therefore, the material for solar absorption and photo-thermal conversion is key to improving the efficiency of solar evaporation processes. Currently, many approaches are being developed to achieve high-efficient solar evaporation, such as the photon management, nano-scale thermal regulation, the development of new photo-thermal conversion materials, and the design of efficient light-absorbing solar stiller. Carbon-based materials including carbon nanotubes, graphene, carbon black, graphite, etc. have broad light-absorption profiles over the entire solar spectrum, which makes them the outstanding photo-thermal conversion materials. Herein, we design a new structure and house-like solar still on the basis of carbon-based materials to achieve high light absorption, efficient photo-thermal conversion, and continuous desalination. We use the chemical vapor deposition (CVD) technique to fabricate a reticulated carbon-nanotube solar evaporation membrane. Stainless steel mesh (SSM) is used as a reticulated skeleton, providing porous structures and increasing the mechanical strength of the membrane. Then the carbon nanotubes (CNTs) are grown on the reticulated skeleton to function as solar conversion structures due to their wide range of light absorption capacities. The CVD grown CNTs reticulated membrane (CGRM) is fixed in a house-like device with a sloped ceiling to condense and collect water vapors ensuring the continuous desalination of water. Our experiments show that the fabricated CGRM is hydrophobic with an average contact angle of 133.4° for a 100.0 g·L^-1 NaCl solution, only allowing water vapors to pass through while rejecting salts. When the light intensity was 1 kW·m^-2, the surface temperature of the membrane increased rapidly and stabilized at 84.37 ℃. The salt rejection rate of the system could reach up to 99.92%.To perform a comparative study, we also prepared a mechanically-filled CNTs reticulated membrane (MFRM1, MFRM2) for solar evaporation tests, which showed an inferior performance to that of the growing structure of the CGRM. Therefore, it was determined that our system might provide a potential way to harvest freshwater readily with portable-type equipment.

Key words: Microstructure design, Reticulated carbon nanotube evaporation membrane, Interface evaporation, Photo-thermal conversion, Solar still, Heavy brine