Innovative approaches to solar distillation

Abstract

It is estimated that by 2025, two-thirds of the world's population will be living under water-stressed conditions. In many countries, the supply of fresh water from natural resources is already scarce. In such instances energy driven desalination of sea or brackish water is required. People living in remote or disaster areas may lack fresh water as well as energy supplies. A possible solution for such circumstances is renewable desalination which uses renewable energy to desalinate seawater. This study focused on solar stills, the basic constituent of solar desalination technologies. The design of a conventional solar still was modified by replacing the insulated opaque sides with transparent glass. This increased the condensation capacity and when tested under natural weather conditions the productivity was improved by around 43% to 5.5 litres/m2 .day. The improved simple still was used as a benchmark to compare other modified solar still configurations. The sides of the latter stills were also made of glass. In an attempt to create a thin film of water to enhance the evaporation process, a titanium dioxide evaporator was used. When irradiated with ultraviolet radiation, Ti02 exhibits photo-induced hydrophilicity. Two solar stills comprising an inclined evaporator were tested under natural weather conditions simultaneously: one with a Ti02 coating and the other painted black. They both reached the same productivity, however the Ti02 unit operated at a lower temperature due to the lower solar absorptance of Ti02. The improved wettability of titanium dioxide compensated for the lower temperature. However scale formation was observed and it is believed that this attenuated the photo-induced hydrophilic properties. The inclined evaporator stills reached an evaporator efficiency 8 .6% lower than that of the improved simple solar still. A solar chimney and externally water cooled copper condensers were used to improve the convection and condensation processes of a solar still. The buoyant pressure generated by the solar chimney was used to induce a convective current and carry the water vapour from the evaporation chamber to the condensers. This improved the evaporator efficiency by 8.9%. However, since the evaporator area was reduced by 14% to make space for ventilation grilles, the overall production rate was 6.3% lower than that of the improved simple still. The improved simple solar still reached a productivity which compares and exceeds many improvements described in literature. Further improving this gain without making use of latent heat recovery, proved to be difficult. A double effect solar still, incorporating the vertical glass sides, reached a productivity 20% higher than that of the improved simple still. This study can serve as a basis for future research projects attempting to further investigate the use of titanium dioxide and a solar chimney in solar distillation.