Geothermal and hydrothermal energy

Geothermal energy is the energy associated with the heat in the interior of the earth. The common usage of the term refers to the thermal energy relatively near the surface of the earth that can be utilized by humans. Hydrothermal energy is the energy associated with hot water, whereas geothermal is a more general term. Geothermal energy has been exploited since early history. It is a source of renewable energy with a low pollution potential that can be used for producing electricity as well as for heating and cooling and helping with a number of other needs.

Background

A geothermal system is made up of three elements: a heat source, a reservoir, and a fluid that transfers the heat. The heat source can be a magmatic intrusion or Earth’s normal temperature, which increases with depth. The reservoir is a volume of hot permeable rock from which circulating fluids extract heat. Fluid convection transports the heat from the higher-temperature low regions to the upper regions, where it can be accessed and used.

Causes of Geothermal Phenomena

While individuals in early mining operations may have noted the general increase in temperature with depth, not until the eighteenth century were subsurface temperature measurements performed. The results often showed an increase in temperature with depth. The rate of increase varied from site to site. An average value that is often used today is a 2.5 to 3 degree Celsius increase per 100 meters increase in depth from the surface. The geothermal gradient suggested that the source of Earth’s heat was below the surface, but the exact cause of the heat was open to discussion for many years. It was not until the early part of the twentieth century that the decay of radioactive materials was identified as the primary cause of this heat. The thermal energy of Earth is very large; however, only a small portion is available for capture and utilization. The available thermal energy is primarily limited to areas where water or steam carries heat from the deep hot regions to, or near, the surface. The water or steam is then available for capture and may be put to such uses as electricity generation and heating.

The interior of the earth is often considered to be divided into three major sections, called the crust, mantle, and core. The crust extends from the surface down to about 35 kilometers beneath the land and about 6 kilometers beneath the ocean. Below the crust, the mantle extends to a depth of roughly 2,900 kilometers. Below, or inside, the mantle is Earth’s core. The crust is rich in radioactive materials, with a much lower density in the mantle and essentially none in the core. The radioactive decay of these materials produces heat. Earth is also cooling down, however. The volume of the mantle is roughly forty times that of the crust. The combination of the heat generated from the decay of radioactive materials and the cooling of Earth results in the flow of heat to Earth’s surface. The origin of the total heat flowing to the surface is roughly 20 percent from the crust and 80 percent from the mantle and core.

The outermost shell of the earth, made up of the crust and upper mantle, is known as the lithosphere. According to the concept of plate tectonics, the surface of the earth is composed of six large and several smaller lithospheric regions or plates. On some of the edges of these plates, hot molten material extends to the surface and causes the plates to spread apart. On other edges, one plate is driven beneath another. There are densely fractured zones in the crust around the plate edges. A great amount of seismic activity occurs in these regions, and they are where large numbers of volcanoes, geysers, and hot springs are located. High terrestrial heat flows occur near the edges of the plates, so Earth’s most important geothermal regions are found around the plate margins. A concentration of geothermal resources is often found in regions with a normal or elevated geothermal gradient, as well as around the plate margins.

History of Development

The ancient Romans used the water from hot springs for baths and for heating homes. China and Japan also used geothermal waters for bathing and washing. Similar uses are still found in various geothermal regions of the world. Other uses of thermal waters were not developed until the early part of the nineteenth century. An early example occurred in the Larderello area of Italy. In 1827, Francesco Larderel developed an evaporation process that used the heat from geothermal waters to evaporate the thermal waters found in the area, leaving boric acid. Heating the water by burning wood had been required in the past.

Also in the early nineteenth century, inventors began attempting to utilize the energy associated with geothermal steam for driving pumps and winches. Beginning in the early twentieth century, geothermal steam was used to generate electricity in the Larderello region. Several other countries tried to utilize their own geothermal resources. Geothermal wells were drilled in Beppu, Japan, in 1919, and at The Geysers, California, in 1921. In the late 1920s, Iceland began using geothermal waters for heating. Various locations in the western United States have used geothermal waters for heating homes and buildings in the twentieth century. Among these are Klamath Falls, Oregon, and Boise, Idaho.

After World War II, many countries became interested in geothermal energy; geothermal resources of some type exist in most countries. Geothermal energy was viewed as an energy source that did not have to be imported and that could be competitive with other sources of electricity generation. In 1958, New Zealand began using geothermal energy for electric power production. One of the first power plants in the United States began operation at The Geysers, California, in 1960. Mexico began operating its first geothermal power plant at Cerro Prieto, near the California border, in 1973.

By 2015, the United States was a leading country in electric power production from geothermal resources, with 3,548 megawatts of installed electrical capacity (28 percent of the world's total). As of that year, Costa Rica, El Salvador, Iceland, Kenya, and the Philippines had significant geothermal energy outputs that accounted for at least 15 percent of each country's energy production—as much as 51 percent in the case of Kenya, where it has become the main power source. Nonelectric uses of geothermal energy occur in most countries. In 2000, the leading nonelectric users of geothermal energy in terms of total usage were, in descending order, China, Japan, the United States, Iceland, Turkey, New Zealand, the Republic of Georgia, and Russia.

Classification of Geothermal Resources

Geothermal resources are classified by the temperature of the water or steam that carries the heat from the depths to, or near, the surface. Geothermal resources are often divided into low temperature (less than 90 degrees Celsius), moderate temperature (90 to 150 degrees Celsius), and high temperature (greater than 150 degrees Celsius). There are still various worldwide opinions on how best to divide and describe geothermal resources. The class or grouping characterizing the geothermal resource often dictates the use or uses that can be made of the resource.

A distinction that is often made in describing geothermal resources is whether there is wet or dry steam present. Wet steam has liquid water associated with it. Steam turbine electric generators can often use steam directly from dry steam wells, but separation is necessary for the use of steam from wet steam wells. In various applications, the water needs to be removed from wet steam. This is achieved through the use of a separator, which separates the steam gas from liquid hot water. The hot water is then re-injected into the reservoir; used as input to other systems to recover some of its heat; or, if there are not appreciable levels of environmentally threatening chemicals present, discharged into the environment after suitable cooling.

Exploration

The search for geothermal resources has become easier in the twenty-first century than it was in the past because of the considerable amount of information and maps that have been assembled for many locations around the world and because of the availability of new instrumentation, techniques, and systems. The primary objectives in geothermal exploration are to identify geothermal phenomena, determine the size and type of the field, and identify the location of the productive zone. Further, researchers need to determine the heat content of the fluids that are to be discharged from the wells, the potential lifetime of the site, problems that may occur during operation of the site, and the environmental consequences of developing and operating the site. Geological and hydrological studies help to define the geothermal resource. Geochemical surveys help to determine if the resource is vapor- or water-dominated, as well as to estimate the minimum temperature expected at the resource’s depth. Potential problems later in pipe scaling, corrosion, and environmental impact are also determined by this type of survey. Geophysical surveys help to define the shape, size, and depth of the resource. The drilling of exploration wells is the true test of the nature of the resource. Because drilling can be costly, use of previous surveys in selecting or siting each drill site is important.

Electricity Generation

The generation of electrical energy from geothermal energy primarily occurs through the use of conventional steam turbines and through the use of binary plants. Conventional steam turbines operate on fluid temperatures of at least 150 degrees Celsius. An atmospheric exhaust turbine is one from which the steam, after passing through the turbine, is exhausted to the atmosphere. Another form of turbine is one in which the exhaust steam is condensed. The steam per kilowatt-hour produced for an atmospheric exhaust unit is about twice that for a condensing unit, but atmospheric exhaust units are simpler and cheaper.

The Geysers has one of the largest dry-steam geothermal fields in the world. Steam rises from more than forty wells. Pipes feed steam to the turbogenerators at a temperature of 175 degrees Celsius. Some of the wells are drilled to depths as great as 2,700 meters. The geothermal field at Wairakei on North Island of New Zealand has been a source of electric power for several decades. The hot water (near 300 degrees Celsius) rises from more than sixty deep wells. As the pressure falls, the hot water converts to steam. The flashing of hot water to steam is the major source of geothermal energy for electric power production.

Binary plants allow electricity to be generated from low- to medium-temperature geothermal resources as well as from the waste hot water coming from steam/water separators. Binary plants use a secondary working fluid. The geothermal fluid heats the secondary fluid, which is in a closed system. The working fluid is heated, vaporizes, drives a turbine, is cooled, condenses, and is ready to repeat the cycle. Binary plant technology is becoming the most cost-effective means to generate electricity from geothermal resources below 175 degrees Celsius.

In cascaded systems, the output water from one system is used as the input heat source to another system. Such systems allow some of the heat in waste water from higher temperature systems to be recovered and used. They are often used in conjunction with electric generation facilities to help recover some of the heat in the wastewater or steam from a turbine.

Space Heating

Space heating by geothermal waters is one of the most common uses of geothermal resources. In some countries, such as Iceland, entire districts are heated using the resource. The nature of the geothermal water dictates whether that water is circulated directly in pipes to homes and other structures or (if the water is too corrosive) a heat exchanger is used to transfer the heat to a better fluid for circulation. Hot water in the range from 60 to 125 degrees Celsius has been used for space heating with hot-water radiators. Water with as low a temperature as 35 to 40 degrees Celsius has been used effectively for heating by means of radiant heating, in which pipes are embedded in the floor or ceiling. Another way of using geothermal energy for heating is through the circulation of heated air from water-to-air heat exchangers. Heat pumps are also used with geothermal waters for both heating and cooling.

In district heating, the water to the customer is often in the 60 to 90 degrees Celsius range and is returned at 35 to 50 degrees Celsius. The distance of the customers from the geothermal resource is important. Transmission lines of up to 60 kilometers have been used, but shorter distances are more common and desirable. When designing a district heating system, the selection of the area to be supplied, building density, characteristics of the heat source, the transmission system, heat loss in transmission, and heat consumption by customers are all important factors.

There are more than six hundred geothermal wells serving a variety of uses in Klamath Falls, Oregon. Utilization includes heating homes, schools, businesses, and swimming pools as well as snow-melting systems for sidewalks and a section of highway pavement. Most of the eastern side of the city is heated by geothermal energy. The principal heat extraction system is the closed-loop downhole heat exchanger utilizing city water in the heat exchangers. Hot water is delivered at approximately 82 degrees Celsius and returns at 60 degrees Celsius.

Hot water from springs is delivered through pipes to heat homes in Reykjavík, Iceland, and several outlying communities. This is the source of heating for 95 percent of the buildings in Reykjavík. Hot water is delivered to homes at 88 degrees Celsius. The geothermal water is also used for heating schools, swimming pools, and greenhouses and is used for aquaculture.

Greenhouse Heating

Using geothermal resources to heat greenhouses is similar to using it to heat homes and other buildings. The objective in this case is to provide a thermal environment in the greenhouse so that vegetables, flowers, and fruits can be grown out of season. The greenhouse is supplied with heated water, and through the use of radiators, embedded pipes, aerial pipes, or surface pipes, the heat is transferred to the greenhouse environment. Forced air through heat exchangers is also used. The United States, Hungary, Italy, and France all have considerable numbers of geothermal greenhouses.

Aquaculture

One of the major areas for the direct use of geothermal resources is in aquaculture. The main idea is to adjust the temperature of the water environment in a production pond so that freshwater or marine fish, shrimp, and plants have greater growth rates and thus reach harvest age more quickly. There are many schemes to regulate the temperature of the pond water. For supply wells where the geothermal water is near the required temperature, the water is introduced directly into the pond. For locations having a well-water temperature too high, the water is spread in a holding pool where evaporative cooling, radiation, and conductive heat loss to the ground can all be used to reduce the temperature to a level in which it can be added to the main production pond.

Industrial Applications

The Tasman Pulp and Paper Company, located in Kawerau, New Zealand, is one of the largest industrial developments to utilize geothermal energy. Geothermal exploration started there in 1952; it was directed toward locating and developing a geothermal resource for a pulp and paper mill. In 1985, the company was using four wells to supply steam to the operations. The steam is used to operate log kickers directly, to dry timber, to generate clean steam, and to drive an electricity generator. Geothermal energy supplies about 30 percent of the total process steam and 4 percent of the electricity for the plant. Geothermal energy in the form of steam is used to dry diatomaceous earth in Námafjall, Iceland. The diatomaceous earth is dredged from the bottom of a lake and pumped 3 kilometers by pipeline to a plant where it is dried.

Numerous other industrial applications of geothermal resources exist in the world. These range from timber drying in Japan to salt production from evaporating seawater in the Philippines, vegetable drying in Nevada, alfalfa drying in New Zealand, and mushroom growing in Oregon.

Environmental Impact

The environmental impacts associated with the use or conversion of geothermal resources are typically much less than those associated with the use or conversion of other energy sources. The resource is often promoted as a clean technology without the potential radiation problems associated with nuclear energy facilities or the atmospheric emissions problems often associated with oil and coal electric plants. Nonetheless, although associated environmental problems are low, there are some present. In the exploration and development phases of large-scale geothermal developments, access roads and platforms for drill rigs must be built. The drilling of a well can result in possible mixing of drilling fluids with the aquifers intersected by the well if the well is not well-cased. Blowouts can also pollute the groundwater. The drilling fluids need to be stored and handled as waste.

Geothermal fluids often contain dissolved gases such as carbon dioxide, hydrogen sulfide, and methane. Other chemicals, such as sodium chloride, boron, arsenic, and mercury, may also be associated with the geothermal water. The presence of these gases and chemicals must be determined, and appropriate means must be selected to prevent their release into the environment. In some cases, this problem is reduced by the re-injection of wastewater into the geothermal reservoir.

The release of thermal water into a body such as a stream, pond, or lake can cause severe ecosystem damage by changing the ambient water temperature, even if only by a few degrees. Any discharge of hot water from the geothermal site needs to involve a means of cooling the water to an acceptable level—one that will not cause environmental damage. This result is often achieved through the use of holding ponds or evaporative cooling. The removal of large volumes of geothermal fluid from the subsurface can cause land subsidence. This is irreversible and can cause major structural damage. Subsidence can be prevented by the re-injection of a volume of fluid equal to that removed.

Noise pollution is one of the potential problems with geothermal sites where electricity generation is conducted. Noise reduction can require costly measures. Because many geothermal electric generation sites are rural, however, this is often not a problem. The noise generated in direct heat applications is typically low.

Economics

The initial cost of a geothermal plant is usually higher than the initial cost of a similar plant run on conventional fuel. On the other hand, the cost of the energy for operating a geothermal plant is much lower than the cost of conventional fuels. In order to be economically superior, the geothermal plant needs to operate long enough to at least make up for the difference in initial cost.

Cascaded systems can be used to optimize the recovery of heat from the geothermal water and steam and therefore to decrease the overall costs. Systems can be cascaded such that the wastewater and heat from one is the input heat source to the next. An example is the cascading of systems used for electricity generation, fruit drying, and home heating. Finally, the distance between the geothermal source and the plant or user should be minimized, as there can be significant transmission losses in heat as well as high costs for pipe, pumps, valves, and maintenance.

Electricity: Current and Future Prospects

The United States leads the world in electrical generating capacity, and in the production of geothermal electricity. The US installed geothermal electrical generating capacity has moved from 2,534 megawatts in 2005 to 3,965 megawatts in 2022. This US generating capacity is spread over seven states but is concentrated in California, which had 66.6 percent of the total geothermal electricity generation in the country in 2023. The Geysers, the United States' first geothermal energy site, was still home to the largest concentration of geothermal power plants in the world in the 2020s. The other states with geothermal electrical generating capacity are Nevada, with a 26.1 percent share of US geothermal electricity generation; Utah (3.2 percent); Hawaii (2.1 percent); Oregon (1.3 percent); Idaho (0.5 percent); and New Mexico (0.2 percent).

In the 2010s and 2020s, the United States also had a number of projects at least at stage one of development; that is, they had secured rights to the resource and had begun initial exploratory drilling. Many of the projects were farther along than that, with some in the facility construction and production drilling stage. In addition to expansions of geothermal capacity in Idaho, California, Alaska, Utah, Nevada, Oregon, and New Mexico, geothermal energy projects have begun or have been planned in Arizona, Colorado, Washington, Texas, North Dakota, Louisiana, Montana, Mississippi, and Wyoming.

Total worldwide geothermal power generation (based on installed capacity) rose from 8,933 megawatts in 2005 to 15.9 gigawatts in 2021, according to the International Renewable Energy Agency. As of 2023, the United States was the world’s top generator, followed by Indonesia (2,418 megawatts), the Philippines (1,952 megawatts), Turkiye (1,691 megawatts), New Zealand (1,042 megawatts), Kenya (985 megawatts), Mexico (976 megawatts), Italy (916 megawatts), Iceland (754 megawatts), and Japan (576 megawatts), and fifteen other nations.

Direct Use: Current and Future Prospects

More geothermal energy is directly used as thermal energy than is used to generate electricity, both in the United States and worldwide. Direct use of geothermal energy includes space heating (both district heating and individual space heating), cooling, greenhouse heating, fish farming, agricultural drying, industrial process heat, snow melting, and swimming pool and spa heating. The greatest direct use for geothermal energy in the United States, by a wide margin, is geothermal heat pumps.

Geothermal heat pumps are economical, energy-efficient, and available in most places. They provide space heating and cooling and water heating. They have been shown to reduce energy consumption by 20 to 40 percent. In 2015, geothermal heat pumps accounted for more than two-thirds of worldwide direct-use geothermal capacity, and more than half of worldwide use.

Enhanced geothermal systems constitute an emerging technology. Most current geothermal systems use steam or hot water that is extracted from a well drilled into a geothermal reservoir. Geothermal resources available for use can be expanded greatly, however, by using geothermal resources that do not produce hot water or steam directly but can be used to heat water to a sufficient temperature by injecting water into the hot underground region using injection wells and extracting it through production wells. The term “engineered geothermal system” is also used for this type of system. For this system, increasing the natural permeability of the rock may be necessary, so that adequate water flow in and out of the hot rock can be obtained. Estimates indicate that use of geothermal resources requiring enhanced geothermal systems would make more than 100,000 megawatts of economically usable generating capacity available in the United States.

Bibliography

Armstead, H. Christopher H. Geothermal Energy: Its Past, Present, and Future Contributions to the Energy Needs of Man. 2nd ed. Spon, 1983.

Batchelor, Tony, and Robin Curtis. “Geothermal Energy.” Energy: Beyond Oil. Ed. Fraser Armstrong and Katherine Blundell. Oxford UP, 2007.

Dickson, Mary H., and Mario Fanelli, eds. Geothermal Energy: Utilization and Technology. 1995. Reprint. Earthscan, 2005.

DiPippo, Ronald. Geothermal Power Plants: Principles, Application, Case Studies and Environmental Impact. 2nd ed. Butterworth-Heinemann, 2008.

"Geothermal Energy Factsheet." University of Michigan Center for Sustainable Systems, css.umich.edu/publications/factsheets/energy/geothermal-energy-factsheet. Accessed 14 Oct. 2024.

"Geothermal Explained." US Energy Information Administration, 3 Apr. 2024, www.eia.gov/energyexplained/geothermal/use-of-geothermal-energy.php. Accessed 14 Oct. 2024.

Glassley, William E. Geothermal Energy: Renewable Energy and the Environment. 2nd ed. CRC, 2105.

Gupta, Harsh K., and Sukanta Roy. Geothermal Energy: An Alternative Resource for the Twenty-first Century. Elsevier, 2007.

Lee, Sunggyu, and H. Bryan Lanterman. “Geothermal Energy.” Handbook of Alternative Fuel Technologies. Ed. Sunggyu Lee, James G. Speight, and Sudarshan K. Loyalka. Taylor, 2007.

Lienau, Paul J., et al. Reference Book on Geothermal Direct Use. Geo-Heat Center, Oregon Inst. of Technology, 1994.

Lund, John W. “Characteristics, Development and Utilization of Geothermal Resources.” Geo-Heat Center Quarterly Bulletin 28.2 (2007).

Lund, John W., and Tonya L. Boyd. "Direct Utilization of Geothermal Energy 2015 Worldwide Review." Proceedings of the World Geothermal Conference 2015. Bochum: Intl. Geothermal Assn., 2015.

Matek, Benjamin. 2015 Annual US & Global Geothermal Power Production Report. Geothermal Energy Assn., 2015.

McCaffrey, Paul, ed. U.S. National Debate Topic, 2008-2009: Alternative Energy. Wilson, 2008.

Rinehart, John S. Geysers and Geothermal Energy. Springer, 1980.

Simon, Christopher A. “Geothermal Energy. ” Alternative Energy: Political, Economic, and Social Feasibility. Rowman, 2007.

Slack, Kara. U.S. Geothermal Power Production and Development Update. Geothermal Energy Assn., 2008.

"Top Ten Geothermal Countries 2023—Power Generation Capacity." Australian Geothermal Association, 10 Jan. 2024, australiangeothermal.org.au/post/top-10-geothermal-countries-2023-power-generation-capacity. Accessed 14 Oct. 2024.