Geothermal energy refers to the energy that is stored in the form of heat below the surface of the Earth. Given that more than 98% of the Earth's volume has temperatures in excess of 1000 °C, geothermal energy has the potential to contribute significantly to satisfying global needs for heating buildings and for electricity generation. However, although its potential is high, extracting geothermal energy is considered an emerging technology, such that today the number of successfully operating geothermal systems, in particular those producing electricity, is still small.
No electricity is currently being produced from geothermal sources in Switzerland. However, wells are being drilled by the exploration industry to test if this technology can be applied locally. We are conducting supporting research into geothermal energy as a member of the Swiss Competence Center for Energy Research – Supply of Electricity (SCCER–SoE), financed by the Swiss Federal Commission for Technology and Innovation (CTI).
Aquifer-hosted Geothermal Systems
Some of the sedimentary rocks beneath northern Switzerland are naturally porous and permeable, and they are know to contain slowly circulating groundwaters, i.e. they are aquifers. At several localities the hot groundwater is currently being extracted for heating applications (e.g. at Riehen near Basel). We are now investigating the potential of the hotter zones of these rocks as sources of geothermal electricity.
Fracture-hosted Geothermal Systems
Certain parts of the Swiss subsurface are cut by large faults that are naturally permeable and water-conducting. The hot water moving through such faults is already being used at some sites for heating purposes (e.g. Lavey-les-Bains, Brigerbad). We are investigating the potential of this type of geothermal resource for electricity production.
Owing to the geological history of Switzerland, much of the subsurface rock at temperatures above 120 °C is impermeable, without any naturally circulating groundwater. One approach to extract the heat from such rocks is to first enhance the permeability by hydraulically stimulating the deep rocks, then to inject riverwater and let it circulate through the engineered reservoir. Finally, the heated water is pumped up to the surface to generate electricity. In order for such "petrothermal systems" to work in Switzerland, where thermal gradients are rather low, the reservoir must be created at depths of 4–7 km where the rocks are at around 200 °C.
Mineral scaling and corrosion in geothermal systems
The cycle of artificially circulating water through hot, deep rocks and pumping it to the surface to extract its heat inevitably induces strong physicochemical changes in the water. These changes drive chemical reactions between the water and its surroundings. Minerals in the deep reservoir may be dissolved and new minerals may be precipitated, either enhancing or clogging the flow paths. Similarly, as the water is pumped to the surface it may precipitate minerals ("scales") and corrode the production well and the power plant. Dealing with mineral scaling and corrosion is typically the greatest financial cost in operating geothermal systems.