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Geothermal energy

by  Carol Stewart

Bubbling mud, erupting geysers and hot mineral pools are some of the surface features of New Zealand’s geothermal systems. Their energy has been tapped to feed geothermal power stations, producing electricity. The benefits are substantial, but they do not come without a cost to the environment.


Heat from the earth

The word ‘geothermal’ comes from the Greek and means ‘heat from the earth’. Deep inside the earth heat is released by the decay of radioactive elements such as uranium and thorium. Geothermal systems occur where circulating groundwater is heated and rises as a column of hot water to the surface.

There are two main types of geothermal system:

  • Low-temperature systems, which range from 30ºC to 100ºC, are associated with areas of extinct volcanism, or with active faults.
  • High-temperature systems are associated with active volcanism. They are heated by shallow reservoirs of molten rock (magma), and temperatures typically reach 200–300ºC.

The surface features of a geothermal system may be an isolated hot spring, mud pool, geyser, or area of steaming ground. A set of these features grouped together is called a geothermal field.

New Zealand’s geothermal features are world famous. In particular, the Taupō Volcanic Zone has one of the greatest concentrations of geothermal activity in the world, and is rivalled only by Yellowstone National Park in the United States.

Early uses

Before Europeans arrived, Māori used hot springs for heating, cooking and preserving food, and for their medicinal and therapeutic properties. These traditional uses did not affect or modify geothermal features greatly.

European settlers soon discovered the scenic charms and healing benefits of thermal springs, and spa bathing became the basis of a rapidly growing tourism industry. Bathhouses and treatment centres were set up in Rotorua from about 1870. Between 1891 and 1904 the number of spa baths taken each year by visitors increased from 10,000 to 100,000. At first this demand could be met by the natural springs, but eventually shallow wells had to be drilled to increase the hot-water supply.

A geothermal hotel

Rotorua’s Millennium Hotel makes full use of its location. Steam from under the ground is used to heat rooms, tap water, and the swimming and spa pools. It is also used for cooling and air conditioning. Cooling can be produced when geothermal heat evaporates a low-boiling-point liquid.

Town heating

Geothermal waters have been used for many years in Rotorua, and to a lesser extent in Taupō, to heat homes, businesses and institutions. It would have been efficient to develop municipal heating systems, but this was hindered by a lack of capital and political will. Instead, individuals and organisations drilled their own shallow bores, using small-scale, primitive heating systems that wasted a lot of the heat.

There were severe electricity shortages in the 1950s and restrictions were imposed. This encouraged people in Rotorua to drill wells to heat their homes. By the 1970s it became apparent that drawing off hot water was depleting the Rotorua reservoir and damaging local geysers and springs. Since 1991 geothermal extraction has been managed to protect surface geothermal activity. Recent trends have been towards communal systems, with 10 or more households typically sharing a well.

A major use of geothermal energy in Rotorua is pool heating. Swimming pools can contain clean, fresh water warmed by heat exchangers. Mineral pools use the geothermal waters.


Wairākei geothermal power station

Harnessing underground steam to generate electricity is one of the world’s more unusual engineering feats. The earliest experiments were carried out at Larderello in Italy, where the world’s first geothermal power station was opened in 1913. New Zealand army engineers serving in Italy during the Second World War were sent to inspect the station, but when they arrived in June 1944 it had been destroyed by retreating German forces.

New Zealand engineers visited Larderello again in 1948, when the power station had been rebuilt and was producing over 140 megawatts of electricity. Back in New Zealand, two dry years in a row had meant that hydroelectric dams could not produce the country’s energy requirements. Another source of power, independent of imported oil, was becoming imperative.

Pioneering days at Wairākei

In 1949 exploratory drilling began at Wairākei, just north of Taupō. This site was chosen because the Department of Scientific and Industrial Research (DSIR) had already succeeded in obtaining steam for the Wairākei Tourist Hotel by drilling to 170 metres; cooling water was available from the nearby Waikato River; and the land was undeveloped.

Initial explorations were encouraging, and the power station was built between 1958 and 1963. It was only the second in the world, and the first to attempt to harness wet steam (a mixture of steam and hot water, in contrast to Larderello’s use of dry steam). Engineers invented a steam–water separator, and had to pioneer ways of overcoming numerous other problems. As a result, New Zealand expertise became highly sought-after by countries interested in developing geothermal resources.

A flirtation with nuclear power

The Wairākei geothermal project was initially a joint venture between the New Zealand government and the United Kingdom Atomic Energy Authority (UKAEA) to produce power and heavy water. Heavy water (in which both hydrogen atoms have been replaced by their heavier isotope deuterium) is used to slow down the nuclear fission process that occurs in thermal nuclear reactors. It can be made by distilling ordinary water, but this process uses a lot of energy; geothermal heat was thought to be an ideal energy source. The idea was first suggested at a conference in Rotorua in 1946, and in 1954 funding was approved. However, the costs proved to be prohibitive, and the UKAEA pulled out of the project in 1956.


Geothermal power production

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.


Other uses of geothermal energy

Industrial processes

Most of New Zealand’s geothermal energy goes to produce electricity, but it can be used for any processes where heat is required. The main non-electrical user is the Tasman Pulp and Paper Mill at Kawerau, which was built in 1957 and deliberately sited to take advantage of the underlying geothermal field. The heat is used for digesting wood pulp, drying timber and paper, and generating electricity.

Geothermal prawn farming

The world’s only geothermally heated prawn farm was established in 1987 on the banks of the Waikato River, next to the Wairākei power station. The first prawns were imported from Malaysia in 1988, and by 2005 the 5.8-hectare farm was producing about 20 tonnes per year. The farm heats its own water with heat exchangers, which draw heat from the power station’s waste water before it flows back into the Waikato River.

This is a good example of what is known as ‘cascade use’, where geothermal heat has a function past its primary purpose. Cascading improves the overall efficiency of a resource by using its waste products. In the case of the prawn farm, cascading also reduces the discharge of hot water into the river, where it can harm aquatic life.

Horticulture

Geothermal waters are used for heating greenhouses on a small scale (covering 10 hectares in total), especially for the commercial, out-of-season production of vegetables, flowers and fruit. This includes a large greenhouse (0.8 hectares) for growing orchids for export, and another set up to grow capsicums with heat from the Kawerau geothermal field.

Crop and timber drying

Drying lucerne (alfalfa) using geothermal energy was pioneered in Ōhākī in the 1970s. Geothermal heat from the Ōhākī power station has been used to make high-protein pellets to feed stock and to process dried juice into a protein concentrate. A timber-drying operation on site produces fence posts and poles, mainly for the local farming industry.

The Tasman Pulp and Paper Mill uses geothermal steam in heat exchangers to heat kiln air to 140ºC for timber drying.


Effects on the environment

Depletion of resources

The process of extracting geothermal fluids (which include gases, steam and water) for power generation typically removes heat from natural reservoirs at over 10 times their rate of replenishment. This imbalance may be partially improved by injecting waste fluids back into the geothermal system.

Damage to natural geothermal features

Natural features such as hot springs, mud pools, sinter terraces, geysers, fumaroles (steam vents) and steaming ground can be easily, and irreparably, damaged by geothermal development. When the Wairākei geothermal field was tapped for power generation in 1958, the withdrawal of hot fluids from the underground reservoir began to cause long-term changes to the famous Geyser Valley, the nearby Waiora Valley, and the mighty Karapiti blowhole. The ground sagged 3 metres in some places, and hot springs and geysers began to decline and die as the supply of steaming water from below was depleted.

In Geyser Valley, one of the first features to vanish was the great Wairākei geyser, which used to play to a height of 42 metres. Subsequently, the famous Champagne Pool, a blue-tinted boiling spring, dwindled away to a faint wisp of steam. In 1965 the Tourist Hotel Corporation tried to restore it by pumping in some three million litres of water, but to no avail. Geyser Valley continued to deteriorate, and in 1973 it was shut down as a tourist spectacle. This story has been repeated many times where there has been geothermal development.

Subsidence

Extracting geothermal fluids can reduce the pressure in underground reservoirs and cause the land to sink. The largest subsidence on record is at Wairākei, where the centre of the subsidence bowl is sinking at a rate of almost half a metre every year. In 2005 the ground was 14 metres lower than it was before the power station was built. As the ground sinks it also moves sideways and tilts towards the centre. This puts a strain on bores and pipelines, may damage buildings and roads, and can alter surface drainage patterns.

Polluting waterways

Geothermal fluids contain elevated levels of arsenic, mercury, lithium and boron because of the underground contact between hot fluids and rocks. If waste is released into rivers or lakes instead of being injected into the geothermal field, these pollutants can damage aquatic life and make the water unsafe for drinking or irrigation.

A serious environmental effect of the geothermal industry is arsenic pollution. Levels of arsenic in the Waikato River almost always exceed the World Health Organisation standard for drinking water of 0.01 parts per million. Most of the arsenic comes from geothermal waste water discharged from the Wairākei power station. Natural features such as hot springs are also a source of arsenic, but it tends to be removed from the water as colourful mineral precipitates like bright red realgar and yellowy green orpiment.

Air emissions

Geothermal fluids contain dissolved gases which are released into the atmosphere. The main toxic gases are carbon dioxide (CO2) and hydrogen sulfide (H2S). Both are denser than air and can collect in pits, depressions or confined spaces. These gases are a recognised hazard for people working at geothermal stations or bore fields, and can also be a problem in urban areas. In Rotorua a number of deaths have been attributed to hydrogen sulfide poisoning, often in motel rooms or hot-pool enclosures. Carbon dioxide is also a greenhouse gas, contributing to potential climate change. However, geothermal extraction releases far fewer greenhouse gases per unit of electricity generated than burning fossil fuels such as coal or gas to produce electricity.


External links and sources

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How to cite this page: Carol Stewart, 'Geothermal energy', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/geothermal-energy/print (accessed 16 July 2019)

Story by Carol Stewart, published 12 Jun 2006