Geothermal energy, the heat stored in the Earth, is an enormous, underused resource that is accessible (available anywhere), reliable (available 24/7), and renewable (naturally replenished).
Geothermal resources range from shallow ground to hot water and rock several kilometres below the Earth's surface.
Geothermal energy can be used to generate electricity, and for a wide range of commercial, residential and industrial heating and cooling applications.
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In New Zealand, geothermal fields in the Taupo Volcanic Zone and at Ngawha are best suited for generating electricity. In these locations, the subsurface temperatures exceed 200°C.
Wells over 2km deep are drilled into underground reservoirs to tap geothermal water/steam, which is used to drive turbines that in turn drive generators to produce electricity.
Two types of geothermal electricity generating plants are operating in New Zealand:
- Flash steam plants pull deep, high-pressure hot water (240 - 290°C) into lower-pressure tanks and use the resulting flashed steam to drive turbines. The largest examples are Wairakei and Kawerau Power Stations.
- Binary plants 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 to vapour, which then drive 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 Earth's surface can be piped directly into facilities and used for bathing, heat buildings, grow plants in greenhouses, heat water for fish/prawn farming and industrial process heating. Overseas, geothermal heat is also used in district heating applications (piped hot water to heat buildings in whole communities).
Geothermal heat pumps use shallow energy stored in the ground, groundwater or surface water to centrally heat and cool buildings.
A GHP is very efficient. Heat pumps can efficiently transfer heat from lower temperature energy sources (such as soils) to useful higher grade energy (and heat buildings), using a small amount of electrical energy.
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.
In winter, heat from the relatively warmer ground goes through the heat pump into the house. In summer, hot air from the house is pulled through the heat pump into the relatively cooler ground. Heat removed during the summer can be used as no-cost energy to heat water.
Geothermal heat pumps are a very efficient way to heat and cool buildings, including homes, businesses, schools, and other public and private facilities. However, their most economic use is expected to be the commercial and industrial sectors, where the high capital installation cost will be quickly offset by greatly reduced operating costs. Likewise, they may be attractive for whole-home central heating/cooling solutions for the high end of the residential market.
Read more about geothermal heat pumps on the GNS Science website.