Geothermal energy can be used to generate electricity and heat for a wide range of commercial, residential and industrial applications. It uses heat generated from the ground, so is a resource that’s easily accessible, reliable and won’t run out.
The geothermal fields in the Taupo Volcanic Zone and in Ngawha in Northland are best suited for electricity generation, as subsurface temperatures are higher than 200°C.
Wells deeper than 2 km tap geothermal water and steam in underground reservoirs. The water and steam drive turbines that in turn drive generators to produce electricity.
There are two types of geothermal electricity generating plants that operate in New Zealand:
- Flash steam plants. These pull deep, high-pressure hot water (240°C-290°C) into lower-pressure tanks, using the resulting steam to drive turbines. The largest examples are the Wairakei and Kawerau power stations.
- Binary plants. These pass 75°C-220°C geothermal water by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to ‘flash’ into vapour, driving the turbines. Examples can be found at Te Huka, Wairakei, Kawerau, Rotokawa, Mokai and Ngawha power stations.
Geothermal energy is ideally suited to direct heating. Hot water near the earth's surface is piped directly into facilities for bathing, heating buildings and greenhouses, heating water for fish and prawn farming and for industrial processes such as timber drying.
Geothermal heat pumps use energy stored in the ground, groundwater and surface water to centrally heat and cool buildings.
GHPs are very efficient. Heat pumps transfer heat from lower temperature energy sources such as soil to heat and cool buildings. A GHP system consists of a heat exchanger - pipes buried in a shallow ground loop near the building - a heat pump and a heating/cooling distribution system in the building.
GHPs can be used in the commercial and industrial sectors, where high installation costs can be offset quickly by greatly reduced operating costs. They are also used for central heating at the higher end of the residential market.