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One of the most pressing challenges currently being faced by humanity is the threat of water shortages and freshwater scarcity, without which we could face a complete destabilization of human civilization. At present 1.1 billion people suffer from freshwater scarcity, and this figure is expected to rise to 40% of the world’s population by 2050 [1–3]. Furthermore, the demand for freshwater from manufacturing and household is expected to rise by 400% and 130%, respectively. Solar steam generation (SSG) is a technology that could be used to remediate the current situation by utilizing solar energy to heat water to its latent heat of vaporization in order to produce water vapor/steam, which would then condense and be collected as freshwater [6,7]. However, there are significant limitations to this approach: high cost associated with complex geometrical design and lenses needed for appropriate magnification of sunlight, exotic coatings to increase solar absorptivity, and most importantly the non-localized warming of the water causing the thermal energy to dissipate into the bulk of the fluid [8–12]. Thus, this technology needs to be improved in order to serve as a viable solution remediation for the freshwater crisis. One such advancement that addresses the aforementioned shortcomings is interfacial solar steam generation. In this approach, a highly absorptive heat localization structure is floated at the surface of the fluid in order to intensely absorb solar radiation and translate it to the thermal energy of a thin film of water adsorbed within the heat localization structure. A heat localization material (HLM) is constituted of a strong visible light and NIR absorber layer, an optional support layer to improve the mechanical properties of the absorber, and a substrate which is typically needed for thermal isolation of the absorber form the fluid and for buoyancy. Such an approach is not reliant on expensive absorption strategies such as lenses or coatings, and most importantly, localizes the heating of the water only at the surface; thereby resulting in intense heating and the rapid generation of water vapor/steam and no loss to the bulk of the fluid and the external environment [13–18].
In this study the absorber layer is a novel nanocomposite of the layered graphitic carbon nitride (g-CN) and copper sulphide nanoparticles. A support layer of a porous mixed cellulose ester membrane, and a substrate of extruded polystyrene foam wrapped in airlaid paper.
g-CN, apart from being an inherent visible light absorber, will also allow for superior water transport and subsequent heat transfer to water due to its hydrophilic nature. Covellite nanoparticles display intense plasmonic absorption at the NIR region resulting in intense heating. Moreover, both copper sulfide and graphitic carbon nitride are non-toxic which is critical as the desired application solar desalination [19–21].
The practical suitability of the HLM was then tested in a solar still and compared to the performance four identical solar stills. This system had a transmittance in the range of 0 to 5, a water evaporation rate of 2.6 kg/m2, and an evaporation rate that is 1.5 times greater than that of pure water. The HLM modified solar still was the most effective in comparison to the control solar stills as it had the greatest yield of freshwater as well the highest quality of freshwater. Therefore, this literary work further validates the effectiveness of heat localization technology in providing a practical, productive and eco-friendly solution to the freshwater crisis, while introducing a new material with this functionality. It is a step towards a brighter future.
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