Geothermal power is obtained by using heat from the Earth’s interior. Naturally the internal heat of the planet manifests itself in hot springs, geysers and, more dramatically, volcanoes and earthquakes. It originates from radioactive decay and solar energy absorbed through the surface of the planet.

The ancient Romans heated baths and houses with water from hot springs and similar uses are still found in some geothermally active regions of the world, such as Iceland, Turkey, and Japan. In Iceland geothermal heat is so abundant that nearly all buildings are heated by as are open air pools and even an entire lagoon despite the Nordic location of the country. Yet Iceland presently uses only a fraction of all available geothermal energy.

In Iceland rare geologic conditions bring the Earth’s internal heat close to the surface where it is easy to exploit. This is not the case in most of the world, where the geothermal energies are no less substantial, but significantly harder to reach. Usually deep holes are needed to reach high temperatures. Then either water is pumped down the hole to be heated up or natural reservoirs of superheated water or steam are tapped.

Closer to the surface heat pumps can be used to exploit tepid ground temperatures for heating. During winter temperatures even immediately under the surface are usually substantially higher than the air temperature and therefore can provide an economical source of heat for heat pumps.

Ground heat pumps are already commonly used in cold countries to heat buildings. Producing electricity with geothermal heat requires much higher temperatures and therefore either suitable geologic conditions or deep holes. Geothermal energy was first used in the production of electricity at Larderello, Italy, in 1904. Modern geothermal power plants are operated, among others, in Italy, New Zealand, Japan, Iceland, Mexico and the United States and many others are under construction in other countries. Worldwide, geothermal plants have the capacity to generate about 10 gigawatts of electricity as of 2007, and in practice supply 0.3% of global electricity demand.

Climate change and diminishing reserves of fossil fuels have increased interest in geothermal energy. Its use in space heating with heat pumps is growing quickly in many countries and new technologies for electricity generation are being studied around the world. The theoretical maximum for sustained geothermal energy production surpasses the total energy consumption of mankind.