Geothermal aquifers are naturally-occurring underground reservoirs of hot water that can be found in deep porous rocks. The heat energy contained in aquifers can be used via boreholes extracting it to be used for heating or (where temperatures are higher than 150oC) for electricity generation. A geothermal aquifer is currently used for the district heating system in Southampton.
Geothermal hot dry rock energy involves the extraction of heat through the injection of water into otherwise dry rock formations. Water is pumped down one well to induce hydraulic fracturing, creating a reservoir. The liquid is then circulated under pressure through these fractures, absorbing heat, before returning to the surface via one or more production wells. This technology is under investigation in a number of countries,
Ground source heat pumps are sometimes to referred to as geothermal heat pumps, although they exploit solar heat absorbed in soil and rock close to the surface rather than heat from the earth’s core.