Water Week

EWN Publishing

Sea-water into drinking water cheaper than desal, at 5-12 kW/cubic metre of water: new-idea, biomimetics;1.6 kwh/cubic metre

Posted by waterweek on 17 September 2007

“It’s often the case that green technology is considered to be commercially unattractive,” says Michael Pawlyn, an architect at Grimshaw, the firm behind the Eden project, a highly acclaimed biome structure in England, reported The Economist (8/9/07, p. 25).

Beetles the key?: That perception, he says, was wrong – and he has the designs to prove it. This meant finding a way to turn sea-water into clean drinking water without expending too much energy. Fog-basking beetles, which are found in Namibia, have an ingeniously simple way of doing this. They hide underground during the day so that when they come out at night, their dark backs are relatively cool compared with the ambient night air. As moisture-laden breezes roll in from the Atlantic, the water in the air condenses on the beetle’s back (just as a cold bottle of beer left on a table causes water in the air to condense on its surface). The beetles simply have to tilt their bodies to make the water trickle into their mouths.

Or camels?: A similar trick was also used by camels to prevent them from losing moisture as they exhale. Moisture secreted through the nostrils evaporated as the camel breathed in, cooling the nostrils in the process. When the camel breathed out, moisture within the air then condenses on the nostrils.

Inspired by camels’ nostrils: Inspired by this, Pawlyn and his colleagues have designed their theatre around the same principles. A series of tall, vertical evaptoration “gills” were positioned so that they face towards the sea and the incoming coastal breeze. Warm seawater, taken from close to the surface, would be pumped so that it trickles down these units. As the breeze blew through the gills some of the sea-water would evaporate, leaving salt behind. The clean, moist air would then continue down its journey until it encounters a series of vertical condensing pipes. These would be kept cold by pumping deep-sea water, from 1,000 metres below the surface, through them. As the moist, warm air hit the pipes the water condensed and trickled down to be collected.

1.6 kWh/cubic metre needed: “You get a very powerful desalination effect,” said Pawlyn. This system was able to supply enough water for the 70,000-square-metre complex. A traditional flash-distillation desalination plant consumed between five and 12 kilowatt hours (kwh) of energy per cubic metre of water. The biomimetic approach, however, required just 1.6 kwh per cubic metre. And since the water pumps would be mostly powered by a wind turbine, driven by the same prevailing winds that provide the plant’s airflow, the overall energy consumption of the site was reduced even further. In the process, the same system can also help to cool neighbouring buildings, said Pawlyn.


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