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Geothermal Basics

What is geothermal energy?

The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is heat from within the earth. This heat can be recovered as steam or as hot water, and it can be used to heat buildings or to generate electricity.

Geothermal energy is a renewable energy source because the heat is continuously produced inside the earth.

Geothermal energy is generated deep inside the earth

Image of the earth's interior, from the outside to the inside, with the crust, the mantle of magma and rock, the outer core of magma, and the innermost core of iron.

Source: Adapted from a National Energy Education Development Project graphic (public domain)

Geothermal energy is generated in the earth's core. Temperatures hotter than the sun's surface are continuously produced inside the earth caused by the slow decay of radioactive particles, a process that happens in all rocks. The earth has a number of different layers:

  • The core has a solid iron core and an outer core made of hot melted rock called magma.
  • The mantle surrounds the core and is about 1,800 miles thick. The mantle is made up of magma and rock.
  • The crust is the outermost layer of the earth. The crust forms the continents and ocean floors. The crust can be 3 to 5 miles thick under the oceans and 15 to 35 miles thick on the continents.

The earth's crust is broken into pieces called plates. Magma comes close to the earth's surface near the edges of these plates. This is where volcanoes occur. The lava that erupts from volcanoes is partly magma. The rocks and water absorb the heat from this magma deep underground. The rocks and water found deeper underground have higher temperatures.

People around the world use geothermal energy to heat their homes and to produce electricity by drilling deep wells and pumping the hot underground water or steam to the surface. People can also make use of the stable temperatures near the surface of the earth to heat and cool buildings.

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Where Geothermal Energy is Found

Image of a map of the world titled Ring of Fire. The ring of fire goes around the edges of the Pacific. The map shows that volcanic activity occurs around the Pacific rim.

The Ring of Fire lines the edges of the Pacific Ocean.

Source: Adapted from National Energy Education Development Project (public domain)

Geothermal reservoirs are naturally occurring areas of hydrothermal resources. They are deep underground and are largely undetectable above ground. Geothermal energy finds its way to the earth's surface in three ways:

  • Volcanoes and fumaroles (holes where volcanic gases are released)
  • Hot springs
  • Geysers

Most geothermal resources are near tectonic plate boundaries

The most active geothermal resources are usually found along major tectonic plate boundaries where earthquakes and volcanoes are located. One of the most active geothermal areas in the world is called the Ring of Fire. This area encircles the Pacific Ocean.

U.S. geothermal resource map
U.S. Geothermal Resource Map
Click to enlarge »

Source: U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy

When magma comes near the earth's surface, it heats ground water trapped in porous rock or water running along fractured rock surfaces and faults. Hydrothermal features have two common ingredients, water (hydro) and heat (thermal).

Geologists use various methods to find geothermal reservoirs. Drilling a well and testing the temperature deep underground is the most reliable method for locating a geothermal reservoir.

U.S. geothermal power plants are located in the West

Most of the geothermal power plants in the United States are located in western states and Hawaii, where geothermal energy resources are close to the earth's surface. California generates the most electricity from geothermal energy. The Geysers dry steam reservoir in northern California is the largest known dry steam field in the world and has been producing electricity since 1960.

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Use of Geothermal Energy

Some applications of geothermal energy use the earth's temperatures near the surface, while others require drilling miles into the earth. There are three main types of geothermal energy systems:

  • Direct use and district heating systems
  • Electricity generation power plants
  • Geothermal heat pumps

Direct use and district heating systems

Direct use and district heating systems use hot water from springs or reservoirs located near the surface of the earth. Ancient Roman, Chinese, and Native American cultures used hot mineral springs for bathing, cooking, and heating. Today, many hot springs are still used for bathing, and many people believe the hot, mineral-rich waters have natural healing powers.

Geothermal energy is also used to heat buildings through district heating systems. Hot water near the earth's surface is piped directly into buildings and industries for heat. A district heating system provides heat for most of the buildings in Reykjavik, Iceland.

Industrial applications of geothermal energy include food dehydration, gold mining, and milk pasteurizing. Dehydration, or the drying of vegetable and fruit products, is the most common industrial use of geothermal energy.

Geothermal electricity generation

The United States leads the world in the amount of electricity generated with geothermal energy. Geothermal electricity generation requires water or steam at high temperatures (300° to 700°F). Geothermal power plants are generally built where geothermal reservoirs are located within a mile or two of the earth's surface. In 2015, U.S. geothermal power plants produced about 16.8 billion kilowatthours (kWh), or 0.4% of total U.S. electricity generation. In 2015, seven states had geothermal power plants.

Share of U.S. geothermal electricity produced by each state, 2015:

California 74%
Nevada 20%
Utah 3%
Hawaii 1.4%
Oregon 1.1%
Idaho 0.6%
New Mexico 0.1%

Geothermal heat pumps

Geothermal heat pumps use the constant temperatures near the surface of the earth to heat and cool buildings. Geothermal heat pumps transfer heat from the ground (or water) into buildings during the winter and reverse the process in the summer.

Geothermal energy contributes a significant share of electricity generation in several countries

In 2013, twenty countries, including the United States, generated a total of about 70 billion kWh of electricity from geothermal energy.1 Indonesia was the second-largest geothermal power producer after the United States, at about 11 billion kWh of electricity, which equaled approximately 5% of Indonesia's total power generation. Iceland was the seventh-largest producer at about 5 billion kWh of electricity, but the largest share of its total electricity generation was from geothermal energy at 27%.

1Based on data published by U.S. Energy Information Administration in May 2016.

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Geothermal Power Plants

A geothermic power station
A geothermic power station.

Source: Stock photography (copyrighted)

Geothermal power plants use hydrothermal resources that have both water (hydro) and heat (thermal). Geothermal power plants require high-temperature (300°F to 700°F) hydrothermal resources that come from either dry steam wells or from hot water wells. People use these resources by drilling wells into the earth and then piping steam or hot water to the surface. The hot water or steam is used to operate a turbine that generates electricity. Some geothermal wells may be as deep as two miles.

Types of geothermal power plants

There are three basic types of geothermal power plants:

  • Dry steam plants use steam piped directly from a geothermal reservoir to turn generator turbines. The first geothermal power plant was built in 1904 in Tuscany, Italy, where natural steam erupted from the earth.
  • Flash steam plants take high-pressure hot water from deep inside the earth and convert it to steam to drive generator turbines. When the steam cools, it condenses to water and is then injected back into the ground to be used again. Most geothermal power plants are flash steam plants.
  • Binary cycle power plants transfer the heat from geothermal hot water to another liquid. The heat causes the second liquid to turn to steam, which is then used to drive a generator turbine.
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Geothermal Heat Pumps

Geothermal heat pumps use the earth's constant temperatures for heating and cooling

Although temperatures above ground change depending on time of day and season, temperatures 10 feet below the earth's surface are consistently between 50°F and 60°F. For most areas, this means soil temperatures are usually warmer than the air in winter and cooler than the air in summer. Geothermal heat pumps use the earth's constant temperatures to heat and cool buildings. Geothermal heat pumps transfer heat from the ground (or water) into buildings during the winter and reverse the process in the summer.

A type of geothermal heat pump system
diagram of a closed loop geothermal heat pump system

Source: U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy (public domain)

Geothermal heat pumps are energy efficient and cost effective

According to the U.S. Environmental Protection Agency (EPA), geothermal heat pumps are the most energy efficient, environmentally clean, and cost effective systems used for temperature control. Geothermal heat pumps can be used for all types of buildings, including homes, office buildings, schools, and hospitals.

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Geothermal Energy & the Environment

The environmental impact of geothermal energy depends on how geothermal energy is used or on how it is converted to useful energy. Direct use applications and geothermal heat pumps have almost no negative impact on the environment. Direct use applications and geothermal heat pumps can actually have a positive effect because they may reduce the use of other types of energy that may have greater negative impacts on the environment.

Grand Prismatic Spring, Yellowstone National Park, Wyoming
Grand Prismatic Sprinag, Yellowstone National Park, Wyoming

Source: Stock photography (copyrighted)

Geothermal power plants have low emission levels

Geothermal power plants do not burn fuel to generate electricity, so the levels of air pollutants they emit are low. Geothermal power plants release less than 1% of the carbon dioxide emissions released by a fossil fuel power plant. Geothermal power plants further limit air pollution through the use of scrubber systems that remove hydrogen sulfide. Hydrogen sulfide is naturally found in the steam and in the hot water used to generate geothermal power.

Geothermal plants emit 97% less acid rain-causing sulfur compounds than are emitted by fossil fuel power plants. After the steam and water from a geothermal reservoir are used, they are injected back into the earth.

Many geothermal features are national treasures

Geothermal features in national parks like geysers and fumaroles in Yellowstone National Park are protected by law.