Turning steam into power
New Zealand’s geothermal power stations produce electricity using the following process:
- Geothermal fluid – a naturally occurring mineralised mixture of pressurised water and steam heated to between 200º and 300ºC – is drawn from a geothermal field by production wells at depths of 1–3 kilometres. Temperatures as high as 326ºC have been recorded at Mōkai, which is thought to be New Zealand’s hottest geothermal field.
- The high-pressure hot water is separated into steam and water, and the dry steam is used to spin turbines. The spinning of the turbines generates electricity.
- Modern geothermal power plants such as Rotokawa, commissioned in 1997, have secondary (binary) turbines. Low-pressure exhaust steam heats pentane (a hydrocarbon with the low boiling point of 34ºC), producing the gas that spins the binary turbines.
- All waste fluids are injected back into the geothermal field to help replenish it and to avoid contaminating surface waters with dissolved chemicals.
Geothermal energy potential
Despite the success of the Wairākei project, which has proved to be a cost-effective and reliable contributor to New Zealand’s electricity supply system, it was not until the late 1980s that further geothermal power stations were built, at Ōhākī and Kawerau. The pioneering zeal of the 1950s and 1960s was followed by a lull during the 1970s and 1980s, when attention turned to the large Māui natural-gas field. However, this is expected to run out before 2010, and interest in geothermal power, along with other renewable sources such as wind and solar energy, has been revived.
There was a rapid growth in the production of electricity between 1995 and 2000 in response to the 1993 deregulation of electricity supply and generation. In 2002 New Zealand had seven geothermal power stations. Six were in the Taupō Volcanic Zone, and one at Ngāwhā in Northland. Geothermally generated electricity provided about 7% of New Zealand’s total electricity.
New Zealand’s geothermal energy potential is considered large in comparison to other renewable energy sources, and could supply a third of our total electricity needs if fully developed. However, there are significant costs to the environment.