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Geothermal desalination is a process under development for the production of fresh water using heat energy. Claimed benefits of this method of desalination are that it requires less maintenance than reverse osmosis membranes and that the primary energy input is from geothermal heat, which is a low-environmental-impact source of energy.
Around 1995, entrepreneur Douglas Firestone from Nevada came up with an idea to use geothermal water directly as a source for desalination. In 1998, several individuals began working with evaporation/condensation air loop water desalination. The experiment was successful and a proof of concept, proving that geothermal waters could be used as process water to produce potable water in 2001.
In 2005 to 2009 some testing was done in a sixth prototype of a device referred to as a delta t device, a closed air loop, atmospheric pressure, evaporation condensation loop geothermally powered desalination device. The device used filtered sea water from Scripps Institute of Oceanography and reduced the salt concentration from 35,000 ppm to 51 ppm.
A total of six prototypes and six modifications demonstrated that, with process water approaching 210 degrees Fahrenheit (100 degrees Celsius) and a chill source about 35 °F (2 °C), a full-size device would produce about one acre foot (600 m³) of water per day. Salt concentration in the wastewater would only be about 10% above the level of the original water, thus, from, say, 35,000 to about 38,000 parts per million, well within the ability of osmoregulators to adjust.
See also 
- European Renewable Energy Council - Geothermal Desalination
- ScienceDirect - Heat Transfer and Evaporation in Geothermal Desalination Units