Geothermal literally means ‘Earth's heat’. Geothermal energy is derived from the thermal energy generated and stored in the Earth’s interior. The geothermal resource is renewable, as there is a constant heat flow to the surface and atmosphere from the immense heat stored within the Earth while the water is replenished by rainfall and circulation within the crust. Where natural water circulation is limited, fractures may be created and / or stimulated to create an Engineered Geothermal System (EGS). Successful development of EGS may multiply the potential for geothermal energy.
Nowadays, geothermal exploitation is divided into three technological aspects: the extraction of heat for power generation, the extraction of heat for direct use and the extraction of heat from shallow resources by means of ground source heat pumps. Power generation and distribution of heat may be combined in co-generation power plants and cascade utilisation may increase the total efficiency of the energy extraction process.