Geothermal Energy Provides a Stable and Reliable Heat Supply

 Low enthalpy (t>60°C) geothermal energy can effectively drive a sea or brackish water desalination unit in order to produce fresh water for drinking and/or irrigation. As a geothermal plant, whether used for power generation or for space heating or other applications, has large quantities of available heat at low cost, the most cost effective method for seawater desalination is to provide directly geothermal heat to a MED (multi effect distillation) plant.

Geothermal energy provides a stable and reliable heat supply 24 hours a day, 365 days a year, ensuring the stability of the thermal processes of desalination. Geothermal production technology, i.e. to extract hot water from underground aquifers, is mature. Low temperature MED desalination technology is also mature. Geothermal desalination yields fresh water of high quality. MED desalination method has low energy requirements maximizing the fresh water output from a given low enthalpy geothermal potential and minimizing the corresponding costs.

Geothermal desalination is cost effective, as fresh water costs of less than 1 Euro/m3 are possible. Geothermal desalination is friendly to the environment, as only renewable energy is used with no emissions of air pollutants and greenhouse gasses. Geothermal desalination aids local development and improves employment perspectives. Geothermal desalination saves foreign currency as no imported fossil fuels are used. Geothermal desalination has been successfully demonstrated on the island of Kimolos, Greece through a project supported by the European Commission.

MED powered by geothermal energy is preferred due to lower energy requirement in comparison with other desalination processes. MED method is based on the multi-effect distillation rising film principle at low evaporation temperatures (less than 70°C) due to low, almost vacuum, pressure prevailing in the vessels. The rising effect principle takes advantage of the fact that the inner tube surfaces are always covered by a thin film of feed water that prevents scale formation.

Evaporation through multiple-effect is a very energy efficient technology, as in each vessel (effect) the feed water boils utilizing the heat released by condensing vapor from the previous effect. A pilot unit to demonstrate the feasibility of exploiting the low enthalpy geothermal potential of the island for the production of fresh water through geothermal water desalination with the objective to achieve water sufficiency for the island .

MED results in excellent water quality with a salinity level close to 10 ppm. Fewer stages (effects) are needed in an installation compared to an MSF system, resulting in lower costs per m3 of produced fresh water. The total production of fresh water is approximately 80 m3/day. The produced water cost is estimated of the order of 1.6 €/m3 (including only annual operation costs), which is satisfactory for a small unit if this size; desalination costs are expected to be considerably lower in large scale geothermal desalination plants.