The word geothermal comes from the Greek words geo (earth) and therme (heat), and means the heat of the earth. Earth's interior heat originated from its fiery consolidation from dust and gas over 4 billion years ago and is continually regenerated from the decay of radioactive elements that occur in all rocks.


It is almost 6,500 kilometers (4,000 miles) from the surface to the center of the Earth, and the deeper you go, the hotter it gets. The outer layer, the crust, is three to 35 miles thick and insulates us from the hot interior.

From the surface down through the crust the normal temperature gradient (the increase of temperature with the increase of depth) in the Earth’s crust is 17 - 30°C per kilometer of depth (50-87°F per mile). Below the crust is the mantle, made of highly viscous, partially molten rock with temperatures between 650 and 1,250°C (1,200-2,280°F). At Earth's core, which consists of a liquid outer core and a solid inner core, temperatures may reach 4,000-7,000°C (7,200 to 12,600°F).

Since heat always moves from hotter regions to colder regions, the Earth’s heat flows from its interior toward the surface. This outward flow of heat from Earth’s interior drives convective motion in the mantle rock which in turn drives plate tectonics -- the "drift" of Earth's crustal plates that occurs at 1 to 5 cm per year (about the rate our fingernails grow). Where plates move apart, magma rises up into the rift, forming new crust. Where plates collide, one plate is generally forced (subducted) beneath the other. As a subducted plate slides slowly downward into regions of ever-increasing heat, it can reach conditions of pressure, temperature and water content that cause melting, forming magma. Plumes of magma ascend by buoyancy and force themselves up into (intrude) the crust, bringing up vast quantities of heat.

Where magma reaches the surface it can build volcanoes. But most magma stays well below ground, creating huge subterranean regions of hot rock sometimes underlying areas as large as an entire mountain range. Cooling can take from 5,000 to more than 1 million years. These shallow regions of relatively elevated crustal heat have high temperature gradients.

Perhaps the best known of these volcanic regions are in the countries that border the Pacific Ocean -- the geologically active area known as the Ring of Fire -- where the oceanic plates are being subducted under the continental plates. Other volcanic chains form along mid-ocean or continental rift zones (where plates move apart) -- in places such as Iceland and Kenya, or over hot spots (magma plumes continuously ascending from deep in the mantle) such as the Hawaiian Islands and Yellowstone.


In some regions with high temperature gradients, there are deep subterranean faults and cracks that allow rainwater and snowmelt to seep underground -- sometimes for miles. There the water is heated by the hot rock and circulates back up to the surface, to appear as hot springs, mud pots, geysers, or fumaroles.

If the ascending hot water meets an impermeable rock layer, however, the water is trapped underground where it fills the pores and cracks comprising 2 to 5% of the volume of the surrounding rock, forming a geothermal reservoir. Much hotter than surface hot springs, geothermal reservoirs can reach temperatures of more than 350°C (700°F), and are powerful sources of energy.


If geothermal reservoirs are close enough to the surface, we can reach them by drilling wells, sometimes over two miles deep. Scientists and engineers use geological, electrical, magnetic, geochemical and seismic surveys to help locate the reservoirs. Then, after an exploration well confirms a reservoir discovery, production wells are drilled. Hot water and steam shoot up the wells naturally (or are pumped to the surface) where -- at temperatures between around 120-370°C (250-700°F) -- they are used to generate electricity in geothermal power plants. Shallower reservoirs of lower temperature -- 21-149°C (70-300°F) -- are used directly in health spas, greenhouses, fish farms, and industry and in space heating systems for homes, schools and offices.


In geothermal power plants, we use the natural hot water and steam from the earth to turn turbine generators to produce electricity. Unlike fossil fuel power plants, no fuel is burned. Geothermal power plants give off water vapor, but have no smoky emissions. (See Environmental Aspects, below.)

Flashed Steam Plants. Most geothermal power plants operating today are "flashed steam" power plants. Hot water from production wells is passed through one or two separators where, released from the pressure of the deep reservoir, part of it flashes (explosively boils) to steam. The force of the steam is used to spin the turbine generator. To conserve the water and maintain reservoir pressure, the geothermal water and condensed steam are directed down an injection well back into the periphery of the reservoir, to be reheated and recycled.

Dry Steam Plants. A few geothermal reservoirs produce mostly steam and very little water. Here, the steam shoots directly through a rock-catcher and into the turbine. The first geothermal power plant was a dry steam plant, built at Larderello in Tuscany, Italy in 1904. The power plants at the Larderello dry steam field were destroyed during World War II, but have since been rebuilt and expanded. That field is still producing electricity today. The Geysers dry steam reservoir in northern California has been producing electricity since 1960. It is the largest known dry steam field in the world and, after 40 years, still produces enough electricity to supply a city the size of San Francisco.

Binary Power Plants. In a binary power plant, the geothermal water is passed through one side of a heat exchanger, where it's heat is transferred to a second (binary) liquid, called a working fluid, in an adjacent separate pipe loop. The working fluid boils to vapor which, like steam, powers the turbine generator. It is then condensed back to a liquid and used over and over again. The geothermal water passes only through the heat exchanger and is immediately recycled back into the reservoir.

Although binary power plants are generally more expensive to build than steam-driven plants, they have several advantages: 1) The working fluid (usually isobutane or isopentane) boils and flashes to a vapor at a lower temperature than does water, so we can generate electricity from reservoirs with lower temperatures. This increases the number of geothermal reservoirs in the world with electricity-generating potential. 2) The binary system uses the reservoir water more efficiently. Since the hot water travels through an entirely closed system it results in less heat loss and almost no water loss. 3) Binary power plants have virtually no emissions.

Hybrid Power Plants. In some power plants, flash and binary processes are combined. An example of such a hybrid system is in Hawaii, where a hybrid plant provides about 25% of the electricity used on the Big island.

About 2850 megawatts of geothermal generation capacity is available from power plants in the western United States. Geothermal energy generates about 2% of the electricity in Utah, 6% of the electricity in California and almost 10% of the electricity in northern Nevada. The electrical energy generated in the U.S. from geothermal resources is more than twice that from solar and wind combined.


Shallower reservoirs of lower temperature -- 21-149°C (70-300°F) -- are used directly in health spas, greenhouses, fish farms, and industry and in space heating systems for homes, schools and offices

It is only during the last century that we have used geothermal energy to produce electricity. But using geothermal water to make our lives more comfortable is not new: people have used it since the dawn of mankind. Wherever geothermal water is available, people find creative ways to use its heat.

Hot Spring Bathing and Spas (Balneology)

For centuries, peoples of China, Iceland, Japan, New Zealand, North America and other areas have used hot springs for cooking and bathing. The Romans used geothermal water to treat eye and skin disease and, at Pompeii, to heat buildings. Medieval wars were even fought over lands with hot springs. Today, as long ago, people still bathe in geothermal waters.

In Europe, natural hot springs have been very popular health attractions. The first known "health spa" was established in 1326 in Belgium. (One resort was named "Espa" which means "fountain." The English word "spa" came from this name.) All over Eurasia today, health spas are still very popular. Russia, for example, has 3,500 spas.

Japan is considered the world’s leader in balneology. The Japanese tradition of social bathing dates back to ancient Buddhist rituals. Beppu, Japan, has 4,000 hot springs and bathing facilities that attract 12 million tourists a year. Other countries with major spas and hot springs include New Zealand, Mexico and the United States.


Geothermal resources are used worldwide to boost agricultural production. Water from geothermal reservoirs is used to warm greenhouses to help grow flowers, vegetables and other crops. For hundreds of years, Tuscany in Central Italy has produced vegetables in the winter from fields heated by natural steam. In Hungary, thermal waters provide 80% of the energy demand of vegetable farmers, making Hungary the world’s geothermal greenhouse leader. Dozens of geothermal greenhouses can also be found in Iceland and in the western United States.


Geothermal aquaculture, the "farming" of water-dwelling creatures, uses natural warm water to speed the growth of fish, shellfish, reptiles and amphibians. This kind of direct use is increasing in popularity. In China, for example, geothermal aquaculture is growing so fast that fish farms cover almost 2 million square meters (500 acres). In Japan, aqua farms grow eels and alligators. In the U.S. aquafarmers in Idaho, Utah, Oregon and California grow catfish, trout, alligators, and tilapia -- as well as tropical fish for pet shops. And Icelanders hope to raise as many as two and a half million abalone a year.


The heat from geothermal water is used worldwide for industrial purposes. Some of these uses include drying fish, fruits, vegetables and timber products, washing wool, dying cloth, manufacturing paper and pasteurizing milk. Geothermally heated water can be piped under sidewalks and roads to keep them from icing over in freezing weather. Thermal waters are also used to help extract gold and silver from ore and even for refrigeration and ice-making.

Heating/District Heating

The oldest and most common use of geothermal water, apart from hot spring bathing, is to heat individual buildings, and sometimes entire commercial and residential districts.

A geothermal district heating system supplies heat by pumping geothermal water -- usually 60° C (140°F) or hotter -- from one or more wells drilled into a geothermal reservoir. The geothermal water is passed through a heat exchanger which transfers the heat to water in separate pipes that is pumped to the buildings. After passing through the heat exchanger, the geothermal water is injected back into the reservoir where it can reheat and be used again.

In the Paris basin in France, historic records show that geothermal water from shallow wells was used to heat buildings over six centuries ago. An increasing number of residential districts there are being heated with geothermal water as drilling of new wells progresses.

The first district heating system in the United States dates back to 1893, and still serves part of Boise, Idaho. In the western United States there are over two hundred and seventy communities that are close enough to geothermal reservoirs for potential implementation of geothermal district heating. Eighteen such systems are already in use in the U.S. -- the most extensive in Boise, Idaho and San Bernardino, California.

Because it is a clean, economical method of heating buildings, geothermal district heating is becoming more popular in many places. Besides France and the U.S., modern district heating systems now warm homes in Iceland, Turkey, Poland and Hungary. The world's largest geothermal district heating system is in Reykjavik, Iceland, where almost all the buildings use geothermal heat. The air around Reykjavik was once very polluted by emissions from reliance on fossil fuels. Since it started using geothermal energy, Reykjavik has become one of the cleanest cities in the world.

Geothermal Heat Pumps

Another geothermal technology that helps keep indoor temperatures comfortable by using Earth's heat is the geo-exchange system, or geothermal heat pump. Geothermal heat pumps do not use geothermal reservoirs, so they can be used almost everywhere in the world -- in areas with normal as well as high temperature gradients. By pumping fluid through loops of pipe buried underground next to a building, these systems take advantage of the relatively constant temperature 7 - 13°C (45 - 55°F) of the Earth right beneath our feet to transfer heat into buildings in winter and out of them in summer.

Geothermal heat pumps reduce electricity use 30-60% compared with traditional heating and cooling systems, because the electricity which powers them is used only to move heat, not to produce it. There are about 300,000 heat pump installations in the U.S.; Switzerland and several other countries are implementing heat pump programs. The U.S. Environmental Protection Agency rates geothermal heat pumps among the most efficient of heating and cooling technologies.


Earth’s heat is continuously radiated from within, and each year rainfall and snowmelt supply new water to geothermal reservoirs. Production from individual geothermal fields can be sustained for decades and perhaps centuries. . The U.S. Department of Energy classifies geothermal energy as renewable.


When we use renewable geothermal energy for direct use or for producing electricity, we conserve exhaustible and more polluting resources like fossil fuels and uranium (nuclear energy). Installed geothermal electricity generation capacity around the world is equivalent to the output of about 10 nuclear plants.

Worldwide, direct uses of geothermal water avoids the combustion of fossil fuels equivalent to burning of 830 million gallons of oil or 4.4 million tons of coal per year. Worldwide electrical production from geothermal reservoirs avoids the combustion of 5.4 billion gallons of oil or 28.3 million tons of coal.


With all sources of energy, developers and consumers must work to protect the environment. The challenges differ with the type of energy resource, and the differences give geothermal energy certain advantages. Geothermal direct use facilities have minimal or no negative impacts on the environment. Geothermal power plants are relatively easy on the environment. They are successfully operated in the middle of crops, in sensitive desert environments and in forested recreation areas.

Protection of the Air and Atmosphere. Hydrogen sulphide gas (H2S) sometimes occurs in geothermal reservoirs. H2S has a distinctive rotten egg smell that can be detected by the most sensitive sensors (our noses) at very low concentrations (a few parts per billion). It is subject to regulatory controls for worker safety because it can be toxic at high concentrations. Equipment for scrubbing H2S from geothermal steam removes 99% of this gas.

Carbon dioxide (a major climate change gas) occurs naturally in geothermal steam but geothermal plants release amounts less than 4% of that released by fossil fuel plants. And there are no emissions at all when closed-cycle (binary) technology is used.

Protection of Groundwater. Geothermal water contains higher concentrations of dissolved minerals than water from cold groundwater aquifers. In geothermal wells, pipe or casing (usually several layers ) is cemented into the ground to prevent the mixing of geothermal water with other groundwater.

When highly-mineralized geothermal water needs to be stored at the surface, such as during well testing, it is kept in lined, impermeable sumps. After use, the geothermal water is either evaporated or injected back to its deep reservoir, again through sealed piping.

Visual Protection. No power plant or drill rig is as lovely as a natural landscape, so smaller is better. A geothermal plant sits right on top of its fuel source: no additional land is needed such as for mining coal or for transporting oil or gas. When geothermal power plants and drill rigs are located in scenic areas, mitigation measures are implemented to reduce intrusion on the visual landscape. Some geothermal power plants use special air cooling technology which eliminates even the plumes of water vapor from cooling towers and reduces a plant profile to as little as 24 feet in height.

By observing federal and state regulations, geothermal developers avoid interference with geysers and hot springs in areas set aside for their scenic beauty. Development in National Parks such as Yellowstone is specifically prohibited.


Since the 1970's the geothermal industry, with the assistance of government research funding, has overcome many technical drilling and power plant problems. Improvements in treatment of geothermal water have overcome early problems of corrosion and scaling of pipes. Methods have been developed to remove silica from high-silica reservoirs. In some plants silica is being put to use making concrete, and H2S is converted to sulphur and sold. At power plants n the Imperial Valley of California, a facility is being constructed to extract zinc from the geothermal water for commercial sale.

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. Research is currently underway to further improve exploration, drilling, reservoir, power plant and environmental technologies. Enhancing the recoverability of Earth’s heat is an important area of ongoing research.

Enhanced Geothermal Systems

Geothermal energy is accessible if there is sufficient heat, permeability, and water in a system, and if the system is not too deep. The available heat cannot be increased, but the permeability and water content can be enhanced. Private and government research projects in the United States, Japan and in Europe are improving the accessibility of geothermal energy by developing new technology to increase the permeability of the rocks and to supplement the water in hot, water-deficient rocks. Engineers estimate that by the year 2020, man-made geothermal reservoirs could be supplying 5 to 10% of the world’s electricity.

Enhancing Reservoir Water. One unique example of enhancing reservoir water is at The Geysers steam field in California, where treated wastewater from nearby communities is being piped to the steamfield and injected into the reservoir to be heated. This increases the amount of steam available to produce electricity. With this enhancement, reservoir life is increased while providing nearby cities with an environmentally safe method of wastewater disposal.

Enhancing Reservoir Permeability. Permeability can be created in hot rocks by hydraulic fracturing -- injecting large volumes of water into a well at a pressure high enough to break the rocks. The artificial fracture system is mapped by seismic methods as it forms, and a second well is drilled to intersect the fracture system. Cold water can then be pumped down one well and hot water taken from the second well for use in a geothermal plant. This "hot dry rock" technology is being tested in Japan, Germany, France, England and the U.S.


The outlook for geothermal energy use depends on at least three factors: the demand for energy in general; the inventory of available geothermal resources; and the competitive position of geothermal among other energy sources.

The Demand for energy will continue to grow. Economies are expanding, populations are increasing (over 2 billion people still do not have electricity), and energy-intensive technologies are spreading. All mean greater demand for energy. At the same time, there is growing global recognition of the environmental impacts of energy production and use from fossil fuel and nuclear resources. Public polls repeatedly show that most people prefer a policy of support for renewable energy.

The Inventory of accessible geothermal energy is sizable. Using current technology geothermal energy from already-identified reservoirs can contribute as much as 10% of the United States energy supply. And with more exploration, the inventory can become larger. The entire world resource base of geothermal energy has been calculated in government surveys to be larger than the resource bases of coal, oil, gas and uranium combined. The geothermal resource base becomes more available as methods and technologies for accessing it are improved through research and experience.

The Competitive Position depends primarily on cost:

Costs: Shorter and Longer Term. Production of fossil fuels (oil, natural gas and coal) are a relative bargain in the short term. Like many renewable resources, geothermal resources need relatively high initial investments to access the heat, hot water and steam. But the geothermal "fuel" cost is predictable and stable. Fossil fuel supplies will increase in cost as reserves are exhausted. Fossil fuel supplies can be interrupted political disputes abroad. Renewable geothermal energy is a better long term investment.

Costs: Direct and Indirect. The monetary price we pay to our natural gas and electricity suppliers, and at the gas pump, is our direct cost for the energy we use. But the use of energy also has indirect or external costs that are imposed on society. Examples are the huge costs of global climate change; the health effects from ground level pollution of the air; future effects of pollution of water and land; military expenditures to protect petroleum sources and supply routes; and costs of safely storing radioactive waste for generations. Geothermal energy can already compete with the direct costs of conventional fuels in some locations and is a clean, indigenous, renewable resource without hidden external costs. Public polls reveal that customers are willing to pay a little more for energy from renewable resources such as geothermal energy

Costs: Domestic and Importing. Investment in the use of domestic, indigenous, renewable energy resources like geothermal energy provides jobs, expands the regional and national economies, and avoids the export of money to import fuels.

Energy demand is increasing rapidly worldwide. Some energy and environmental experts predict that the growth of electricity production and direct uses of geothermal energy will be revitalized by international commitments to reduce carbon dioxide emissions to avert global climate change and by the opening of markets to competition.

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