Underground geothermal reservoirs are relatively rare. More normally hot underground rock is not permeated by water and so there is no medium naturally available to bring the heat energy to the surface.
Where hot rock exists close to the surface, it is possible to create a man-made hydrothermal source. This is accomplished by drilling down into the rock and then pumping water through the borehole into the subterranean rock. If water is pumped under high pressure it will cause the rock to fracture, creating faults and cracks through which it can move. (In fact underground rock often contains natural faults and fractures through which the water will percolate.) If a second borehole is drilled adjacent to the first, then water which has become heated as it has percolated through the rock can be extracted and used to generate electricity.
The first attempt at this hot-dry-rock technique was carried out by scientists from the Los Alamos laboratory in New Mexico in 1973. Since then experiments have been carried out in Japan, the UK, Germany and France. The most recent project is the European Hot Dry Rock Research project at Soulez-sous-Forets in France. Here boreholes have been drilled to 5 km below the surface and temperatures of 201°C have been found. The next stage of this project involves construction of a system with a power plant that can extract 30 MW of thermal energy from the hot rock.2
Estimates suggest that a hot-dry-rock system will need to provide 10-100 MW of energy over at least 20 years to be economical. The technology is still in an early stage of development and it is likely to be 10-20 years before commercial exploitation is possible.
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