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Part of the ORC system of the geothermal power plant in Soultz
Deep and hot rock layers can be used for energy production
Projektinfo 04/2009
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Geothermal electricity generation in Soultz-sous-Forêts

In summer 2008, a geothermal power plant, using the heat stored in the plutonic rock, started test operation close to the Alsatian town of Soultz-sous-Forêts (Upper Rhine Graben). As opposed to other geothermal power plants, which use natural hot water or steam sources, existing fractures in the 200 °C hot granite were expanded at a depth of 5,000 m by injecting water, and connected to form a geological heat exchanger.

Water can be circulated and heated via this underground heat exchanger through multiple boreholes. The thermal energy collected in this way can be used at the surface to generate electricity or to supply heat. This process is often also known as “Hot dry rock technology”. The advantage is that it can be applied in large areas, independently of sources of water or steam.


The Upper Rhine Graben is among the particularly interesting regions for geothermal energy use. Even for this region, Soultz is outstanding as a result of the unusually rapid increase in temperature as the drill depth increases. In multiple drilling phases, three boreholes have now reached the targeted final depth of 5,000 metres. The water circulating through the geological heat exchanger is pumped at temperatures up to approx. 175 °C, used for generating electricity in a power plant with special technology (ORC), and the cooled water is then pumped back down into the depths.

Thus, an international research project funded by the European Union, France, Germany, and initially also by Great Britain, was completed successfully. French and German energy supply companies and research teams from other countries were intensely involved in the project. In particular the experience gained in Soultz in stimulating dense rock formations will benefit HDR-process plants and those based on thermal water aquifers. This means that it will be possible to use geological heat reservoirs for energy supply in the future, which, until now, could not be considered, or could only be considered with restrictions due to lack of, or insufficient thermal water circulation.

Projektinfo 04/2009:
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EWIV Wärmebergbau