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Geothermal power (from the Greek words geo, meaning earth, and therme, meaning heat) is energy generated by heat stored beneath the Earth's surface. Prince Piero Ginori Conti tested the first geothermal power plant on 4 July 1904, at the Larderello dry steam field in Italy.^[1] The largest group of geothermal power plants in the world is located in The Geysers, a geothermal field in California.^[2] As of 2007, geothermal power supplies less than 1% of the world's energy.^[3]

Contents 1 Electricity generation 1.1 Dry Steam Power Plants 1.2 Flash steam 1.3 Binary-cycle 1.4 Enhanced Geothermal Systems 2 Advantages 3 Disadvantages 4 Potential 5 History of development 6 Development around the world 6.1 Africa 6.2 Australia 6.3 Iceland 6.4 New Zealand 6.5 Portugal 6.6 Philippines 6.7 United Kingdom 6.8 United States 7 See also 8 References 9 External links

Three different types of power plants - dry steam, flash, and binary - are used to generate electricity from geothermal energy, depending on temperature, depth, and quality of the water and steam in the area.^[4] In all cases the condensed steam and remaining geothermal fluid is injected back into the ground to pick up more heat. In some locations, the natural supply of water producing steam from the hot underground magma deposits has been exhausted and processed waste water is injected to replenish the supply. Most geothermal fields have more fluid recharge than heat, so re-injection can cool the resource, unless it is carefully managed.

Steam plants use hydrothermal fluids that are primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine. (Also eliminating the need to transport and store fuels!) This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904, and is still very effective. Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal power. These plants emit only excess steam and very minor amounts of gases.^[5]

Flash steam power plants use hot water above 182 °C (360 °F) from geothermal reservoirs. The high pressure underground keeps the water in the liquid state, although it is well above the boiling point of water at normal sea level atmospheric pressure. As the water is pumped from the reservoir to the power plant, the drop in pressure causes the water to convert, or "flash", into steam to power the turbine. Any water not flashed into steam is injected back into the reservoir for reuse.^[4] Flash steam plants, like dry steam plants, emit small amounts of gases and steam.^[6]

Flash steam plants are the most common type of geothermal power generation plants in operation today. An example of an area using the flash steam operation is the CalEnergy Navy I flash geothermal power plant at the Coso geothermal field.

The water used in binary-cycle power plants is cooler than that of flash steam plants, from 107 to 182 °C (225-360 °F)^[6]. The hot fluid from geothermal reservoirs is passed through a heat exchanger which transfers heat to a separate pipe containing fluids with a much lower boiling point.^[4] These fluids, usually Iso-butane or Iso-pentane, are vaporized to power the turbine.^[7]. The advantage to binary-cycle power plants is their lower cost and increased efficiency. These plants also do not emit any excess gas and, because they use fluids with a lower boiling point than water, are able to utilize lower temperature reservoirs, which are much more common. Most geothermal power plants planned for construction are binary-cycle.^[7]

Main article: Hot dry rock geothermal energy

Enhanced Geothermal Systems (EGS), also known as Hot-dry-rock systems, involve pumping water into hot rocks in the earth, rather than harvesting hot water already in the earth. This type of geothermal system has many advantages over the others, as it can be used anywhere, not just in tectonically active regions. However, it requires deeper drilling than the other forms of geothermal energy harvesting.^[8]

The Northern California Power Agency will use solar energy to help generate geothermal energy at the Geysers geothermal field north of Calistoga. The agency will install 6,300 solar modules on an existing water pumping station that takes wastewater from Lake County and places it deep underground. Earth's heat turns the water into steam, which power plants on the surface use to generate electricity. The agency operates two power plants at the Geysers. They are using wastewater to generate geothermal power, and using solar to power the wastewater pump. The $8.2 million project will be designed and built by SPG Solar of Novato and should be finished by September 2008.^[9]

Geothermal energy offers a number of advantages over traditional fossil fuel based sources. From an environmental standpoint, the energy harnessed is clean and safe for the surrounding environment.^[10] It is also sustainable because the hot water used in the geothermal process can be re-injected into the ground to produce more steam. In addition, geothermal power plants are unaffected by changing weather conditions.^[11] Geothermal power plants work continually, day and night, making them base load power plants. From an economic view, geothermal energy is extremely price competitive in some areas and reduces reliance on fossil fuels and their inherent price unpredictability.^[12] Given enough excess capacity, geothermal energy can also be sold to outside sources such as neighboring countries or private businesses that require energy. It also offers a degree of scalability: a large geothermal plant can power entire cities while smaller power plants can supply more remote sites such as rural villages.^[13]

There are several environmental concerns behind geothermal energy. Construction of the power plants can adversely affect land stability in the surrounding region. This is mainly a concern with Enhanced Geothermal Systems, where water is injected into hot dry rock where no water was before.^[14] Dry steam and flash steam power plants also emit low levels of carbon dioxide, nitric oxide, and sulfur, although at roughly 5% of the levels emitted by fossil fuel power plants.^[13] However, geothermal plants can be built with emissions-controlling systems that can inject these gases back into the earth, thereby reducing carbon emissions to less than 0.1% of those from fossil fuel power plants.^[8]

Although geothermal sites are capable of providing heat for many decades, eventually specific locations may cool down. It is likely that in these locations, the system was designed too large for the site, since there is only so much energy that can be stored and replenished in a given volume of earth. Some interpret this as meaning a specific geothermal location can undergo depletion, and question whether geothermal energy is truly renewable, but if left alone, these places will recover some of their lost heat, as the mantle has vast heat reserves. The government of Iceland states: "It should be stressed that the geothermal resource is not strictly renewable in the same sense as the hydro resource." It estimates that Iceland's geothermal energy could provide 1700 MW for over 100 years, compared to the current production of 140 MW.^[15]

If heat recovered by ground source heat pumps is included, the non-electric generating capacity of geothermal energy is estimated at more than 100 GW (gigawatts of thermal power) and is used commercially in over 70 countries.[4] During 2005, contracts were placed for an additional 0.5 GW of capacity in the United States, while there were also plants under construction in 11 other countries.^[16]

Estimates of exploitable worldwide geothermal energy resources vary considerably. According to a 1999 study, it was thought that this might amount to between 65 and 138 GW of electrical generation capacity 'using enhanced technology'.^[17]

A 2006 report by MIT, that took into account the use of Enhanced Geothermal Systems (EGS), concluded that it would be affordable to generate 100 GWe (gigawatts of electricity) or more by 2050 in the United States alone, for a maximum investment of 1 billion US dollars in research and development over 15 years.^[16]

The MIT report calculated the world's total EGS resources to be over 13,000 ZJ. Of these, over 200 ZJ would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements - sufficient to provide all the world's energy needs for several millennia.^[16]

The key characteristic of an EGS (also called a Hot Dry Rock system), is that it reaches at least 10 km down into hard rock. At a typical site two holes would be bored and the deep rock between them fractured. Water would be pumped down one and steam would come up the other. The MIT report estimated that there was enough energy in hard rocks 10 km below the United States to supply all the world's current needs for 30,000 years. There seems no reason why the steam should not feed an existing coal, oil or nuclear fired generating plant.

Drilling at this depth is now routine for the oil industry (Exxon announced an 11 km hole at the Chayvo field, Sakhalin. Lloyds List 1/5/07 p 6). The technological challenges are to drill wider bores and to break rock over larger volumes. Apart from the energy used to make the bores, the process releases no greenhouse gases.

Other important countries are China, Hungary, Nicaragua, Iceland, and New Zealand. There is also a planned site in Adelaide, Australia that is over 1km long.

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Geothermal steam and hot springs have been used for centuries for bathing and heating, but it wasn't until the 20th century that geothermal power started being used to make electricity.

Prince Piero Ginori Conti tested the first geothermal power plant on 4 July 1904, at the Larderello dry steam field in Italy.

The first Geothermal power plant in the United States was made in 1922 by John D. Grant at The Geysers Resort Hotel. After drilling for more steam, he was able to generate enough electricity to light the entire resort. Eventually the power plant fell into disuse, as it was not competitive with other methods of energy production.^[18]

In 1960, Pacific Gas and Electric began operation of the first successful geothermal power plant in the United States at The Geysers. It lasted for more than 30 years and produced 11 MW net power.^[18]

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Geothermal power is generated in over 20 countries around the world including Iceland, the United States, Italy, France, Samogitia (Lithuania), New Zealand, Mexico, Nicaragua, Costa Rica, Russia, the Philippines, Indonesia, the People's Republic of China and Japan. Chevron Corporation is the world's largest producer of geothermal energy. Canada's government (which officially notes some 30,000 earth-heat installations for providing space heating to Canadian residential and commercial buildings) reports a test geothermal-electrical site in the Meager Mountain-Pebble Creek area of British Columbia, where a 100 MW facility could be developed.

Geothermal power is very cost-effective in the Rift area of Africa. Kenya's KenGen has built two plants, Olkaria I (45 MW) and Olkaria II (65 MW), with a third private plant Olkaria III (48 MW). Plans are to increase production capacity by another 576 MW by 2017, covering 25% of Kenya's electricity needs, and correspondingly reducing dependency on imported oil.

Main article: Geothermal energy exploration in Central Australia

Main article: Geothermal power in Iceland

Iceland is situated in an area with a high concentration of volcanoes, making it an ideal location for generating geothermal energy. Over 26% of Iceland's energy is generated from geothermal sources. In addition, geothermal heating is used to heat 87% of homes in Iceland.^[19]

Main article: Kawerau geothermal power station

Portugal has a geothermal power plant in the São Miguel Island, in the Azores islands.

The US Geothermal Education Office and a 1980 article entitled "The Philippines geothermal success story" by Rudolph J. Birsic published in the journal Geothermal Energy (vol. 8, Aug.-Sept. 1980, p. 35-44) note the remarkable geothermal resources of the Philippines. ^[20][21] During the World Geothermal Congress 2000 held in Beppu, Ōita Prefecture of Japan (May-June 2000), it was reported that the Philippines is the largest consumer of electricity from geothermal sources and highlighted the potential role of geothermal energy in providing energy needs for developing countries.^[22]

According to the International Geothermal Association (IGA), worldwide, the Philippines ranks second to the United States in producing geothermal energy. As of the end of 2003, the US has a capacity of 2020 megawatts of geothermal power, while the Philippines can generate 1930 megawatts. (Italy is third with 790 megawatts). ^[23] Early statistics from the Institute for Green Resources and Environment stated that Philippine geothermal energy provides 16% of the country's electricity.^[24] More recent statistics from the IGA show that combined energy from geothermal power plants in the islands of Luzon, Leyte, Negros and Mindanao account for approximately 27% of the country's electricity generation.^[23] Leyte is one of the islands in the Philippines where the first geothermal power plant started operations in July 1977.^[21]

Main article: Geothermal power in the United Kingdom

Main article: Geothermal energy in the United States

The United States is the country with the greatest geothermal energy production.^[25]

The largest dry steam field in the world is The Geysers, 72 miles (116 km) north of San Francisco. The Geysers began in 1960, has 1360 MW of installed capacity and produces over 750 MW net. Calpine Corporation now owns 19 of the 21 plants in The Geysers and is currently the United States' largest producer of renewable geothermal energy. The other two plants are owned jointly by the Northern California Power Agency and the City of Santa Clara's municipal Electric Utility (now called Silicon Valley Power). Since the activities of one geothermal plant affects those nearby, the consolidation plant ownership at The Geysers has been beneficial because the plants operate cooperatively instead of in their own short-term interest. The Geysers is now recharged by injecting treated sewage effluent from the City of Santa Rosa and the Lake County sewage treatment plant. This sewage effluent used to be dumped into rivers and streams and is now piped to the geothermal field where it replenishes the steam produced for power generation.

Another major geothermal area is located in south central California, on the southeast side of the Salton Sea, near the cities of Niland and Calipatria, California. As of 2001, there were 15 geothermal plants producing electricity in the area. CalEnergy owns about half of them and the rest are owned by various companies. Combined the plants have a capacity of about 570 megawatts.

The Basin and Range geologic province in Nevada, southeastern Oregon, southwestern Idaho, Arizona and western Utah is now an area of rapid geothermal development. Several small power plants were built during the late 1980s during times of high power prices. Rising energy costs have spurred new development. Plants in Nevada at Steamboat near Reno, Brady/Desert Peak, Dixie Valley, Soda Lake, Stillwater and Beowawe now produce about 235 MW.

• Hot-dry-rock geothermal power
• Geothermal power in Iceland
• Geo-exchange
• Geothermal exchange heat pump
• Geothermal desalination
• The Geysers
• Category:Geothermal power and heating plants
• World energy resources and consumption

• Australian National University - Hot Rock Energy
• Bassfeld Technology Transfer - Introduction to Geothermal Power Generation (3.6 MB PDF file)
• Chevron Corporation Geothermal Production
• Energy and Geoscience Institute at the University of Utah
• Energy Efficiency and Renewable Energy - Geothermal Technologies Program
• Enhanced Geothermal Resources
• Geothermal Education Office
• Geothermal Energy Association
• Geothermal Energy-Renewable energy resource
• Geothermal Resources Council
• Idaho National Laboratory - Geothermal Program
• International Geothermal Association
• Los Alamos National Laboratory - Hot Dry Rock Geothermal Energy Technology
• MIT - The Future of Geothermal Energy (14 MB PDF file)
• National Renewable Energy Laboratory - Geothermal Technologies Program
• Oregon Institute of Technology - Geo-Heat Center
• Southern Methodist University - Geothermal Lab
• Info on other types of energy