Seawater Greenhouses

http://en.wikipedia.org/wiki/Seawater_Greenhouse
http://www.seawatergreenhouse.com/

This particular technology will work well in arid locations by the ocean.  The technology is called Ocean Thermal Energy Conversion (OTEC), and promoted in Marshall Savages, Millennial Project: Colonizing the Galaxy in Eight Easy Steps (See Millennial Project 2.0, Living Universe Foundation and their blog).  It's kind of the reverse of geothermal, using deep cold ocean water as the differential, rather than hot underground land water.  Combining the prospects of Deep Geothermal and OTEC could do a lot to bring the world vast amounts of clean energy and water, whether it's along the Sahara coastline, or in central Iowa.

"Water Production and Water Savings

The Seawater Greenhouse converts sea water into fresh water, providing a unique local desalination capability. The water is condensed from water vapour in the air, in much the same way as dew. It is pure distilled water, produced without chemical treatment. The quantity produced depends on the climate - the hotter and sunnier, the more water.

The air entering the Greenhouse is both cooled and humidified. High humidity and low temperatures (the Greenhouse operates at approx. 90% relative humidity) reduces plant transpiration substantially, by up to 80%. This reduces irrigation requirements. The irrigation rate in Tenerife averaged 1.2 litre/day/m2 against 8 litres/day/m2 used by local farmers.

The impact of a new source of water on a local area can be highly beneficial. In Tenerife, a barren area 'turned green' as seepage from irrigation reversed saline intrusion and enabled new plant growth.

Of even greater importance is the effect the Seawater Greenhouse can have on reducing demand for mains water and reserves of ground water. Around 8-10 litres per m2 per day can be saved which, on a macro scale, will have an immense impact, freeing existing water supply for other uses."

Construction and Materials - http://www.seawatergreenhouse.com/construction_and_materials.htm

The Seawater Greenhouse has a specific function - to produce fresh water and cool air while allowing maximum light penetration. The choice of materials is guided by the level of performance required - and cost. Various specifications of Seawater Greenhouse are available.

Low-cost solutions give excellent results. The design requires a light steel structure with polythene covering, cardboard evaporators and a plastic condenser. ABS and MDPE plastics are used for plumbing. Polythene films are cheap and effective. They are specially treated to incorporate ultraviolet-reflecting and infrared-absorbing properties and can be 100% recycled at the end of their useful life.

The cardboard evaporators are strengthened by a surprisingly effective process. They crystallise calcium carbonate from the sea water and harden like sea shells. The process is controllable and the results indicate that the life of the evaporators can be extended almost indefinitely."