Geothermal energy may have the potential to produce low-cost fresh water, says a new study .
University of Queensland’s Geothermal Energy Centre‘s director Hal Gurgenci said geothermal-powered desalination systems could be a boon for small towns facing water shortage.
‘This is a clever combination where desalination is coupled with an agricultural function which is both cost-efficient and environmentally-friendly,’ said Gurgenci.
Gurgenci said that while some of the geothermal resources may not be hot enough for power generation, they would be a perfect fit for thermal desalination of underground brackish aquifers.
Studies indicate that for plants in the range of one to 100 megalitres (megalitre is one million litres) per day, thermal desalination technologies are more suitable than reverse osmosis especially if there is a cheap and abundant supply of heat.
Gurgenci said that technology could also be used in smaller-scale applications, and in particular in agricultural settings, a university release said.
‘Geothermal heat can be used to heat and humidify a greenhouse and produce fresh water at the same time,’ Gurgenci said.
A schematic of the process is shown below.
The brackish water is pumped and filtered from a well and sent into a ground heat exchanger where it absorbs heat from a geothermal fluid. This heat exchanger can be built of polyethylene to conserve costs.
The heated brackish water is then fed in a cascade to the first evaporator then to the second evaporator. The brine can be circulated in the circuit several times until its concentration increases over an acceptable dissolved salt concentration.
The concentrated brine is finally collected in a tank, where it is stored for later treatment or processing or reinjection.
The evaporator is the entire front wall of the greenhouse structure. It consists of a cardboard honeycomb lattice and faces the prevailing wind. Hot brackish water trickles down over this lattice, heating and humidifying the ambient cooler air passing through into the planting area and contributing to the heating of the greenhouse. Fans draw the air through the greenhouse.
Air passes through a second evaporator and is further humidified to saturation point. Air leaving the evaporator is nearly saturated and passes over the passive cooling system with a condenser (IC) immersed in a water basin.
The fresh water condensing from the humid air is piped for irrigation or other purposes. This design can be scaled up to provide 10-20 kL/day while also helping greenhouse plant growing.
 Mahmoudi, H. … [et al.]. (2010). Application of geothermal energy for heating and fresh water production in a brackish water greenhouse desalination unit : a case study from Algeria. Renewable and sustainable energy reviews ; vol. 14, no. 1 ; p. 512-517. doi:10.1016/j.rser.2009.07.038