Geothermal energy is heat derived from deep underneath the earth’s crust. In most areas, this heat reaches the surface in a very diffuse state. However, due to a variety of geological processes, some areas, including the western part of the USA, west and central eastern Europe, Iceland, Asia and New Zealand are underlain by relatively shallow geothermal resources.These are classified as low temperature (less than 90°C), moderate temperature (90° - 150°C) and high temperature (greater than 150°C).The uses to which these resources can be put depends on the temperature.The highest temperature is generally used only for electric power generation. Current global geothermal generation capacity totals approximately 8,000 MW. Uses for low and moderate temperature resources can be divided into two categories: direct use and ground-source heat pumps.

Geothermal power plants use the earth’s natural heat to vapourise water or an organic medium.The steam created powers a turbine which produces electricity. In New Zealand and Iceland, this technique has been used extensively for decades. In Germany, where it is necessary to drill many kilometres down to reach the necessary temperatures, it is only in the trial stages. Geothermal heat plants require lower temperatures and the heated water is used directly.