New Zealand’s geothermal resources have a long history of utilisation. Geothermal areas are important to Maori who use the heated waters for cooking, washing, bathing, warmth, preserving, ceremonial use and healing. Maori also use geothermal minerals as paints, wood preservatives and dyes.
New Zealand’s geothermal features are also major tourist attractions. The Whakarewarewa geothermal field near Rotorua is the most visited tourist attraction in New Zealand. Geothermal fields also have significant scientific and conservation value.
The high temperature fluids carry dissolved minerals and gas which can have commercial value. Internationally there are geothermal developments that extract boric acid or CO2 from geothermal fluids. Trials have been undertaken in New Zealand on silica extraction with a view to use as a paper filler.
The wide range of other uses for geothermal energy can be divided into direct use and electricity generation. While moderate temperature resources can be used for electricity generation, it is unlikely to be economically competitive in New Zealand except in the special case of off-grid generation in remote areas, so that commercial generation is confined to high temperature resources. Direct use involves using geothermal heat directly (without a heat pump or power plant) for such purposes as heating of buildings, industrial processes, domestic heating, greenhouses, aquaculture, public baths and pools. Direct use can utilise high and moderate to low temperature geothermal resources. Existing developments have generally been on an individual basis rather than capturing the benefits of combined use for district heating or industrial parks.
Very low temperature resources can be also be exploited for heat applications using ground-source heat pumps to provide space heating and cooling. This is a practical technology that can be used almost everywhere in New Zealand where some groundwater occurs, and offers real opportunities for electricity conservation, but it cannot be used for electricity generation. A GNS study has shown that ground-source heat pumps can be attractive where there are cold winters or hot summers, where electricity is over 12c/kWh, where gas is not available and where new construction is taking place.
For more information on uses of geothermal energy, including NZ case studies – visit GNS Science’s Earth Energy webpages.
Most high temperature geothermal resources produce a mixture of water and steam at temperatures up to 300ºC. A few resources, or more often limited sectors within an large system, produce dry steam. The energy accessed can be used to generate electricity in geothermal power plants or used directly for a range of purposes. Commercial geothermal energy operations access geothermal fluids by drilling wells typically 1000 to 3000m deep, while domestic wells for heating are usually only around 100m deep. When hot water above 90 to 100ºC flows up the well, the release of pressure allows some of the fluid to boil into steam, resulting in two-phase mixture at the surface. The water and steam are physically separated before use.
Potential energy use at various indicative field temperatures is:
|70-140ºC||space and water heating, drying|
|140-220ºC||drying, process heat, binary electrical plant|
|220+ºC||steam turbine and binary electricity or process steam|
There have been alternative proposals for developing geothermal resources. One involves drilling into hot dry rock (HDR) followed by artificial induction of permeability and introduction of water. An attempt to research the HDR concept in Australia has led to the discovery of a very large, hot, permeable reservoir. New Zealand scientists and engineers were considering investigating magmatic resources, and some corrosion trials were carried out on White Island for the purpose.
The main use of geothermal energy in New Zealand is for electricity generation. About 15% or 120 PJ of New Zealand’s primary energy supply comes from geothermal resources, which contribute about 10% of electricity supply. There are a wide range of direct uses in New Zealand for which consumer energy totals 10 PJ, a figure almost identical to electricity generation in 2005 (see Direct Heat Report). The largest geothermal direct user in the world is the Norske Skog Tasman pulp and paper mill at Kawerau. With an installed capacity of 210 MWt and an annual energy use of 5500 TJ, the plant uses geothermal fluids to generate clean process steam for paper drying, a source of heat in evaporators, timber drying and electricity generation.
Use of geothermal energy is expected to increase substantially over the next 25 years, in fact it will be difficult to meet renewable energy growth targets without it.
Stages Of Processing Of Geothermal Energy
|Extraction: Water is directed from a natural spring or from a drilled well with flow rates controlled by piping, orifice plates and valves.|
|Treatment: Steam and water are separated and other impurities are removed.|
|Heat exchange/use: Steam is used to drive a turbine or supply process heat. Hot water can be put through a heat exchanger for space or water heating, or to drive a binary cycle power plant (uses low boiling point fluid), or used directly (e.g. bathing). Ground source heat pumps require buried pipes to exchange heat with the ground or ground water.|
|Disposal: Used fluids are usually reinjected via wells into the field or (rarely) discharged to land or waterways. These fluids may need treatment (e.g. anti-scalant) prior to discharge. Gas is dispersed into the air.|