Geothermal comes from the Greek words ‘geo’ (earth) and ‘therma’ (heat). Temperature in the earth increases typically by 10–50ºC/km around the planet. For example, the crust of the Earth is around 30km deep and has temperatures ranging from 15ºC near the surface to 540ºC near the base of the crust. The bulk of this heat is generated from the radioactive decay of naturally occurring potassium, thorium and uranium isotopes in minerals in the upper crust of the earth.
Geothermal energy sources include:
Geothermal energy is being used to generate power in over 20 countries around the world, including Iceland, United States, Italy, France, New Zealand, Mexico, Nicaragua, Kenya, Costa Rica, Russia, Philippines, Indonesia, People's Republic of China and Japan.
In Australia, geothermal energy is currently being generated at Birdsville in Queensland, where a 120 kilowatt demonstration plant has been operating using 98ºC groundwater from the town bore since 1999.
Regions of interpreted high crustal temperature at depths over 3 km form the main exploration targets for EGS in Australia. Typically the high temperatures are caused by granites (granites have higher levels of naturally radioactive minerals) or radioactive ore bodies buried by sedimentary cover which insulates the heat source and traps the produced heat over geologic time.
EGS exploration involves exploring under the insulating sedimentary cover to discover and then drill anomalously hot basement rocks, which are then fracture-stimulated so that water can be circulated via deep injector wells into the heat source. This heated water is recovered from deep production wells and circulated to the surface to a heat exchanger and used to generate electricity.