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Krafla Geothermal Station in northeast Iceland

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]

 Electricity generation

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.

 Dry Steam Power Plants

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

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.

Binary-cycle

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]

 Enhanced Geothermal Systems

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]

 Advantages

The Nesjavellir Geothermal Power Plant in Iceland

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]

 Disadvantages

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]

 Potential

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