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Geothermal energy is a term for the heat of the earth, usually from produced natural steam; heat recovered from circulated water; or direct heat-to-energy conversion (research is ongoing in this field).
Heat from the earth
The geothermal energy of the Earth's crust originates from the original formation of the planet (20%) and from radioactive decay of materials (80%). The geothermal gradient—the difference in temperature between the core and the surface of the planet—drives a continuous conduction of thermal energy in the form of heat from the core to the surface. Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core–mantle boundary can exceed 4000 °C (7,200 °F). The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of mantle, which is lighter than the surrounding rock, to convect upward. In the crust, rock and water is heated, sometimes up to 370 °C (700 °F).
Geothermal energy produced in hot springs has been used for bathing since Paleolithic times and for space heating since ancient Roman times, but today it is best known for electricity generation. Worldwide, 11,700 megawatts (MW) of geothermal power is online in 2013. An additional 28 gigawatts of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications in 2010.
Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but historically it has been limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications like home heating, opening a potential for widespread exploitation. Geothermal wells release greenhouse gases trapped deep within the earth, but the emissions are lower per energy unit than those of fossil fuels, meaning geothermal power has the potential to help mitigate global warming if widely used to replace fossil fuels.
Theoretically, the Earth's geothermal resources are more than adequate to supply the world’s energy needs, but only a small fraction may be profitably exploited. Drilling and exploration for deep resources is very expensive. Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, subsidies, and interest rates. Pilot programs like Eugene Water & Electric Board’s (EWEB) customer opt-in Green Power Program show that customers would be willing to pay a little more for a renewable energy source like geothermal. But as a result of government assisted research and industry experience, the cost of generating geothermal power has decreased by 25% over the past two decades. In 2001, geothermal energy cost between two and 10 US cents per kWh.
In the 20th century, demand for electricity led to the consideration of geothermal power as a generating source. On July 4, 1904, Prince Piero Ginori Conti tested the first geothermal power generator at the same Larderello dry steam field where geothermal acid extraction began. It successfully lit four light bulbs. Later, in 1911, the world's first commercial geothermal power plant was built there. It was the world's only industrial producer of geothermal electricity until New Zealand built a plant in 1958. In 2012, it produced 594 megawatts. Lord Kelvin invented the heat pump in 1852and Heinrich patented the idea of using it to draw heat from the ground in 1912, but it was not until the late 1940s that the geothermal heat pump was successfully implemented. It’s thought that the first was Robert C. Webber's homemade 2.2 kW direct-exchange system, but sources disagree as to the exact timeline of his invention. J. Donald Kroeker designed the first commercial geothermal heat pump to heat the Commonwealth Building in Portland, Oregon, and demonstrated it in 1946. Professor Carl Nielsen of Ohio State University built the first residential open loop version in his home in 1948. (The technology became popular in Sweden as a result of the 1973 oil crisis, and has been growing slowly in worldwide acceptance since then. The 1979 development of polybutylene pipe improved the heat pump’s economic viability.)
In 1960, Pacific Gas and Electric began operating the first successful geothermal electric power plant in the United States at The Geysers in California. The original turbine lasted for more than 30 years and produced 11 MW net power. The binary cycle power plant was first demonstrated in 1967 in the USSR and then introduced to the US in 1981. This technology allows the generation of electricity from much lower temperature resources than previously. In 2006, a binary cycle plant in Chena Hot Springs, Alaska, came on-line, producing electricity from a record low fluid temperature of 57 °C (135 °F). In 2010, the United States led the world in geothermal electricity production with 3,086 MW of installed capacity from 77 power plants. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California. The Philippines is the second highest producer, with 1,904 MW of capacity online. Geothermal power makes up approximately 27% of Philippine electricity generation.
Proximity to tectonic plates
Traditionally, geothermal electric plants were built exclusively on the edges of tectonic plates where high temperature geothermal resources are available near the surface. The development of binary cycle power plants and improvements in drilling and extraction technology enable enhanced geothermal systems over a much greater geographical range. Demonstration projects are operational in Landau-Pfalz, Germany, and Soultz-sous-Forêts, France, while an earlier effort in Basel, Switzerland was shut down after it triggered earthquakes. Other demonstration projects are under construction in Australia, the United Kingdom, and the United States.
The thermal efficiency of geothermal electric plants is low, around 10–23%, because geothermal fluids do not reach the high temperatures of steam from boilers. The laws of thermodynamics limit the efficiency of heat engines in extracting useful energy. Exhaust heat is wasted unless it can be used directly and locally, for example in greenhouses, timber mills, and district heating. System efficiency does not materially affect operational costs as it would for plants that use fuel, but it does affect return on the capital used to build the plant. Because geothermal power does not rely on variable sources of energy, unlike, for example, wind or solar, its capacity factor can be quite large – up to 96% has been demonstrated. The global average was 73% in 2005.
Types of geothermal energy
Geothermal energy comes in either vapor-dominated or liquid-dominated forms. Larderello and The Geysers are vapor-dominated. Vapor-dominated sites offer temperatures from 240-300 C that produce superheated steam.
Liquid-dominated reservoirs (LDRs) were more common with temperatures greater than 200 °C (392 °F) and are found near young volcanoes surrounding the Pacific Ocean and in rift zones and hot spots. Flash plants are the common way to generate electricity from these sources. Pumps are generally not required, powered instead when the water turns to steam. Most wells generate 2-10MWe. Steam is separated from liquid via cyclone separators, while the liquid is returned to the reservoir for reheating/reuse. As of 2013, the largest liquid system is Cerro Prieto in Mexico, which generates 750 MWe from temperatures reaching 350 °C (662 °F). The Salton Sea field in Southern California offers the potential of generating 2000 MWe. Lower temperature LDRs (120-200 C) require pumping. They are common in extensional terrains, where heating takes place via deep circulation along faults, such as in the Western US and Turkey. Water passes through a heat exchanger in a Rankine cycle binary plant. The water vaporizes an organic working fluid that drives a turbine. These binary plants originated in the Soviet Union in the late 1960s and predominate in new US plants. Binary plants have no emissions.
Lower temperature sources produce the energy equivalent of 100M BBL per year. Sources with temperatures from 30-150 C are used without conversion to electricity for as district heating, greenhouses, fisheries, mineral recovery, industrial process heating and bathing in 75 countries. Heat pumps extract energy from shallow sources at 10-20 C in 43 countries for use in space heating and cooling. Home heating is the fastest-growing means of exploiting geothermal energy, with global annual growth rate of 30% in 2005 and 20% in 2012.
Approximately 270 petajoules (PJ) of geothermal heating was used in 2004. More than half went for space heating, and another third for heated pools. The remainder supported industrial and agricultural applications. Global installed capacity was 28 GW, but capacity factors tend to be low (30% on average) since heat is mostly needed in winter. Some 88 PJ for space heating was extracted by an estimated 1.3 million geothermal heat pumps with a total capacity of 15 GW.
Geothermal heating is more cost-effective at many sites than electricity generation. At natural hot springs or geysers, water can be piped directly into radiators. In hot, dry ground, earth tubes or downhole heat exchangers can collect the heat. But even in areas where the ground is colder than room temperature, heat can often be extracted with a geothermal heat pump more cost-effectively and cleanly than by conventional furnaces. These devices draw on much shallower and colder resources than traditional geothermal techniques. They frequently combine functions, including air conditioning, seasonal thermal energy storage, solar energy collection, and electric heating. Heat pumps can be used for space heating essentially anywhere.
Iceland is the world leader in direct geothermal energy applications. About 93% of its homes are heated with geothermal energy, saving the country over $100 million per year in oil import costs. Reykjavík, Iceland, has the world's biggest district heating system. Once known as the most polluted city in the world, it is now one of the cleanest.
Enhanced geothermal systems (EGS) actively inject water into wells to be heated and pumped back out. The water is injected under high pressure to expand existing rock fissures, enabling the water to freely flow in and out. The technique was adapted from oil and gas extraction techniques. However, the geologic formations are deeper and no toxic chemicals are used, reducing the possibility of environmental damage. Drillers can employ directional drilling to expand the size of the reservoir.
Geothermal power requires no fuel (except for pumps), and is therefore immune to fuel cost fluctuations. However, capital costs are significant. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet (extraction and injection wells) in Nevada can support 4.5 megawatts (MW) and costs about $10 million to drill, with a 20% failure rate.
In total, electrical plant construction and well drilling cost about €2–5 million per MW of electrical capacity, while the break–even price is 0.04–0.10 € per kW•h. Enhanced geothermal systems tend to be on the high side of these ranges, with capital costs above $4 million per MW and break–even above $0.054 per kW•h in 2007. Direct heating applications can use much shallower wells with lower temperatures, so smaller systems with lower costs and risks are feasible. Residential geothermal heat pumps with a capacity of 10 kilowatt (kW) are routinely installed for around $1–3,000 per kilowatt. District heating systems may benefit from economies of scale if demand is geographically dense, as in cities and greenhouses, but otherwise piping installation dominates capital costs. The capital cost of one such district heating system in Bavaria was estimated at somewhat over 1 million € per MW. Direct systems of any size are much simpler than electric generators and have lower maintenance costs per kW•h, but they must consume electricity to run pumps and compressors. Some governments subsidize geothermal projects. Geothermal projects have several stages of development. Each phase has associated risks. At the early stages of reconnaissance and geophysical surveys, many projects are cancelled, making that phase unsuitable for traditional lending. Projects moving forward from the identification, exploration and exploratory drilling often trade equity for financing.
Renewability and sustainability
Geothermal power is considered to be renewable because any projected heat extraction is small compared to the Earth's heat content. The Earth has an internal heat content of 1031 joules (3•1015 TW•hr), approximately 100 billion times current (2010) worldwide annual energy consumption. About 20% of this is residual heat from planetary accretion, and the remainder is attributed to higher radioactive decay rates that existed in the past. Natural heat flows are not in equilibrium, and the planet is slowly cooling down on geologic timescales. Human extraction taps a minute fraction of the natural outflow, often without accelerating it.
Geothermal power is also considered to be sustainable thanks to its power to sustain the Earth’s intricate ecosystems. By using geothermal sources of energy present generations of humans will not endanger the capability of future generations to use their own resources to the same amount that those energy sources are presently used. Further, due to its low emissions geothermal energy is considered to have excellent potential for mitigation of global warming.
Even though geothermal power is globally sustainable, extraction must still be monitored to avoid local depletion. Over the course of decades, individual wells draw down local temperatures and water levels until a new equilibrium is reached with natural flows. The three oldest sites, at Larderello, Wairakei, and the Geysers have experienced reduced output because of local depletion. Heat and water, in uncertain proportions, were extracted faster than they were replenished. If production is reduced and water is re-injected, these wells could theoretically recover their full potential. Such mitigation strategies have already been implemented at some sites. The long-term sustainability of geothermal energy has been demonstrated at the Lardarello field in Italy since 1913, at the Wairakei field in New Zealand since 1958, and at The Geysers field in California since 1960.
Fluids drawn from the deep earth carry a mixture of gases, notably carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4) and ammonia (NH3). These pollutants contribute to global warming, acid rain, and noxious smells if released. Existing geothermal electric plants emit an average of 122 kilograms (269 lb) of CO2 per megawatt-hour (MW•h) of electricity, a small fraction of the emission intensity of conventional fossil fuel plants. Plants that experience high levels of acids and volatile chemicals are usually equipped with emission-control systems to reduce the exhaust.
In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic elements such as mercury, arsenic, boron, and antimony. These chemicals precipitate as the water cools, and can cause environmental damage if released. The modern practice of injecting cooled geothermal fluids back into the Earth to stimulate production has the side benefit of reducing this environmental risk.
Direct geothermal heating systems contain pumps and compressors, which may consume energy from a polluting source. This parasitic load is normally a fraction of the heat output, so it is always less polluting than electric heating. However, if the electricity is produced by burning fossil fuels, then the net emissions of geothermal heating may be comparable to directly burning the fuel for heat. For example, a geothermal heat pump powered by electricity from a combined cycle natural gas plant would produce about as much pollution as a natural gas condensing furnace of the same size. Therefore the environmental value of direct geothermal heating applications is highly dependent on the emissions intensity of the neighboring electric grid.
Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei field in New Zealand. In Staufen im Breisgau, Germany, tectonic uplift occurred instead, due to a previously isolated anhydrite layer coming in contact with water and turning into gypsum, doubling its volume. Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. The project in Basel, Switzerland was suspended because more than 10,000 seismic events measuring up to 3.4 on the Richter Scale occurred over the first six days of water injection.
Geothermal has minimal land and freshwater requirements. Geothermal plants use 3.5 square kilometres (1.4 sq mi) per gigawatt of electrical production (not capacity) versus 32 square kilometres (12 sq mi) and 12 square kilometres (4.6 sq mi) for coal facilities and wind farms respectively. They use 20 litres (5.3 US gal) of freshwater per MW•h versus over 1,000 litres (260 US gal) per MW•h for nuclear, coal, or oil.
Some of the legal issues raised by geothermal energy resources include questions of ownership and allocation of the resource, the grant of exploration permits, exploitation rights, royalties, and the extent to which geothermal energy issues have been recognised in existing planning and environmental laws. Other questions concern overlap between geothermal and mineral or petroleum tenements. Broader issues concern the extent to which the legal framework for encouragement of renewable energy assists in encouraging geothermal industry innovation and development.
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Noteworthy papers in OnePetro
Anderson, D.N. 1972. Geothermal Development in California. Presented at the SPE California Regional Meeting, 8-10 November, Bakersfield, California, USA.SPE-4180-MS. http://dx.doi.org/10.2118/4180-MS.
Bayliss, B.P. Introduction to Geothermal Energy. 1972. Presented at the SPE California Regional Meeting, Bakersfield, California, USA, 8-10 November. SPE-4176-MS. http://dx.doi.org/10.2118/4176-MS.
Bodvarsson, S., Pruess, K., O’Sullivan, M.J. 1985. Injection and Energy Recovery in Fractured Geothermal Reservoirs. SPE-11689-PA. http://dx.doi.org/10.2118/11689-PA. SPE J. 25 (02): 303 – 312. http://dx.doi.org/10.2118/11689-PA.
Budd, C.F. Geothermal Energy for Electrical Generation. 1984. J Pet Technol 36 (02). SPE-12885-PA. http://dx.doi.org/10.2118/12885-PA.
Cubric, S. 1977. Recovery Of Geothermal Energy From Oil Reservoir Aquifers. SPE-6976-MS. https://www.onepetro.org/general/SPE-6976-MS.
Detournay, C., Hobbs, B. Numerical Simulations of Convection Cells in Sedimentary Basins with Application to Geothermal Energy. 2014. Presented at the 48th U.S. Rock Mechanics/Geomechanics Symposium, Minneapolis, Minnesota, 1-4 June, USA. ARMA-2014-7226. https://www.onepetro.org/conference-paper/ARMA-2014-7226.
Dorfman, M.H. 1982. The Outlook for Geopressured/Geothermal Energy and Associated Natural Gas. J Pet Technol 34 (09): 1,915 - 1,919. SPE-11250-PA. http://dx.doi.org/10.2118/11250-PA.
Hirakawa, S. 1982. Geothermal Field Development System. Offshore South East Asia Show, Singapore, 9-12 February. SPE-10432-MS. http://dx.doi.org/10.2118/10432-MS.
Hunsbedt, A., Kruger, P., London, A.L. 1977. Recovery of Energy From Fracture-Stimulated Geothermal Reservoirs. J Pet Technol 29 (08): 940 – 946. SPE-5875-PA. http://dx.doi.org/10.2118/5875-PA.
Lande, D.P. 1975. Geothermal Energy in California. Presented at the SPE California Regional Meeting, Ventura, California, USA, 2-4 April. SPE-5390-MS. http://dx.doi.org/10.2118/5390-MS.
Lichti, K.A., Ko, M., Julian, R., et al. 2013. The Application of Risk Based Assessment to Geothermal Energy Plant. Presented at CORROSION 2013, 17-21 March, Orlando, Florida, USA. NACE-2013-2438. https://www.onepetro.org/conference-paper/NACE-2013-2438.
Murphy, H.D., Lawton, R.G., Tester, J.W., et al. 1977. Preliminary Assessment of a Geothermal Energy Reservoir Formed by Hydraulic Fracturing. SPE J. 17 (04): 317 – 326. SPE-6093-PA. http://dx.doi.org/10.2118/6093-PA.
Polsky, Y., Blankenship, D., Mansure, A.J., et al. 2009. Enhanced Geothermal Systems Well Construction Technology Evaluation. Presented at the SEG Annual Meeting, Houston, 25-30 October. SEG-2009-4331. https://www.onepetro.org/conference-paper/SEG-2009-4331.
Prentice, J.K. 2009. Introduction And Geothermal Overview. Presented at the SEG Annual Meeting, Houston, 25-30 October. SEG-2009-4316. https://www.onepetro.org/conference-paper/SEG-2009-4316.
Reinicke, K.M., Oppelt, J., Ostermeyer, G.-P., et al. 2010. Enhanced Technology Transfer for Geothermal Exploitation Through a New Research Concept: The Geothermal Energy and High-Performance Drilling Research Program: Gebo. Presented at the SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. SPE-134436-MS. http://dx.doi.org/10.2118/134436-MS.
Stolz, N. 2010. New Precompetitive Data For Uranium And Geothermal Energy Exploration In Australia. Presented at the SEG Annual Meeting, Denver, 17-22 October. SEG-2010-1112. https://www.onepetro.org/conference-paper/SEG-2010-1112.
Thomas, L.K., Pierson, R.G. 1978. Three-Dimensional Geothermal Reservoir Simulation. SPE J. 18 (02): 151 – 161. SPE-6104-PA. http://dx.doi.org/10.2118/6104-PA.
Toralde, J.S.S. 2014. Offshore Geothermal Energy Utilisation: An Idea Whose Time Has Come? Presented at the Offshore Technology Conference-Asia, 25-28 March, Kuala Lumpur. OTC-25071-MS. http://dx.doi.org/10.4043/25071-MS.
Ullah, S.Z., R.S.B., Syed. 2008. Geothermal Reservoirs: A Renewable Source of Energy and an Extension of Petroleum Engineering. Presented at the CIPC/SPE Gas Technology Symposium 2008 Joint Conference, Calgary,16-19 June. SPE-114718-MS. http://dx.doi.org/10.2118/114718-MS.
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro
Bierman, S.L. 1978.Geothermal Energy in the Western United States : Innovation Versus Monopoly. New York : Praeger. Chandrasekharam, D. and Bundschuh, J. ed. 2002. Geothermal Energy Resources for Developing Countries. Oxford; Taylor & Francis. Dickson, M.H., Fanelli, M. 2005. Geothermal Energy: Utilization and Technology. Sterling, VA : Earthscan. Glassley, W.E. 2010. Geothermal Energy : Renewable Energy and the Environment. Boca Raton: Energy and the Environment series, CRC Press. Goodman, L.J., Love, R.N. 1980. Geothermal Energy Projects: Planning and Management. New York: Pergamon Policy Studies on Science and Technology, Pergamon Press. Wohletz, K., Heiken, G. 1992. Volcanology and Geothermal Energy. Berkeley ; Los Angeles ; Oxford : University of California Press.
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