Jun 9, 2008
Fresh, clean water is going to be increasingly in short supply. Despite the recent heavy rains across Southern Europe, the building of desalination plants in such regions, and the shipping in of water supplies from elsewhere is likely to increase in coming years, while desertification will maintain its dehydrating crawl and some regions of the developing world will continue to die of thirst in hotter dry season, while squandering the precious harvest of the rainy season.
So, what’s the answer? Solar-powered desalination certainly, or perhaps the extraction of the hydrate component of abundant desert minerals such as gypsum (calcium sulfate dihydrate, 20% water by weight)?
In a forthcoming issue of the International Journal of Global Environmental Issues (2008, 8, 224-232), M. Whisson of Subiaco, in Western Australia, discusses two serious alternatives for providing even the most parched lands with unlimited fresh water. Both approaches are reminiscent of ancient, old world technologies, but could provide a modern solution. Whisson explains the problem:
The world water crisis may be more serious than generally appreciated. One reason for this is that the main response has been to increase storage of rain rather than to increase the amount of fresh water. Another is that fossil groundwater has been widely seen as inexhaustible.
Storage and redistribution of rain water, of course requires processing plants while those suffering debilitating and ultimately fatal arsenic poisoning on the Indian sub-continent are all too familiar with the effects of the desiccation of aquifers.
“There are two, and only two, unlimited sources of water: the sea and the air,” says Whisson. The Earth has 1.26 x 1021 litres of water, of which 98% is seawater. The surface is acted on by solar radiation, turbulence and wind, which liberates water into the atmosphere ensuring that the lower 1 kilometre of the atmosphere (volume of 5 x 107 cubic kilometres contains 1 x 1015 kg of water, which turns over with a half life of a few days.
Harvesting of water from the air on a very small but socially important scale has a long history, as does desalination of seawater but, says Whisson, in the context of current and growing world needs, these approaches will provide nothing but a short delay in the onset of global life-threatening water scarcity. This is especially so, given their small, centralised scaling as industrial units.
Instead, Whisson suggests two fresh water collecting systems and argues that they have no ultimate limitations, either because of the availability of water or because of environmental constraints. The first system is the Water Road, a macro-engineering concept, which produces fresh water from seawater without the energy and processing demands of conventional desalination. It also offers a distributed network system that precludes many of the issues facing an industrial-style desalination plant. This system uses a large surface area to allow a non-fresh water supply to be distilled by solar and wind energy and trapped as fresh clean water.
The distillation would occur during the transfer of seawater inland (essentially given a kickstart by tidal surges) to the area of need, explains Whisson. This seems counter-intuitive, but immediately provides a high surface area, while the slow flow rate through a wide pipeline under a transparent heat-insulating cover means a large surface area of water is exposed to the sun over several days, with wind turbulence on the seawater surface acting like the natural process of transfer of surface water to the air over the open sea. This system of evaporation also avoids the inhibitory effect of water vapour saturation of the evaporating air. The concentrated seawater formed as a byproduct could be used to produce common sea salt at much lower cost and efficiencies than traditional drying pools.
The second system is a Water from Air system that uses a wind turbine to extract moisture from the wind. Whisson points out that at a relative humidity of 60%, a temperature fall from 20 to 5 Celsius would only require 10 grams of water per cubic metre of air. However, once it is recognised that a wind-driven turbine with an aperture of 10 square metres facing into a moderate breeze of 10 kilometres per hour would acquire 100000 cubic metres of air containing 1000 kg water every hour. Even with an efficiency of just 20% that would be a useful system, especially given that thousands of turbines could be installed in dry regions.
The two systems are seen as complementary,” explains Whisson, the Water Road providing water to large arid geographic areas, such as Western Australia, and the Water From Air units providing dispersed multiple water collection from the air wherever it is needed, whether on high industrial buildings, farm buildings, coastal cliff-tops, remote sand hills or small isolated communities.
Whisson, M. (2008). Two proposals for unlimited fresh water. International Journal of Global Environmental Issues, 8(3), 224. DOI: 10.1504/IJGENVI.2008.018637