Geothermal basics

What is geothermal energy?

Geothermal energy is heat found inside the earth. The name comes from the Greek words geo (earth) and therme (heat). Geothermal energy is a renewable energy source because the earth constantly produces this heat. People use geothermal heat for many purposes, including:

• Bathing in hot springs
• Heating buildings
• Generating electricity

Where does geothermal energy come from?

Geothermal energy comes from deep inside the earth. It’s produced by the slow breakdown of radioactive particles in the earth's core. This process happens naturally in all rocks.

The earth has four main layers:

• Inner core: A solid ball of iron about 1,500 miles wide
• Outer core: Hot melted rock called magma about 1,500 miles thick
• Mantle: Magma and rock surrounding the outer core about 1,800 miles thick
• Crust: Outermost layer of solid rock that forms the continents and ocean floors; about 15 miles to 35 miles thick under the continents and 3 miles to 5 miles thick under the oceans

Heat within the earth

The earth's inner core is incredibly hot, about 10,800 degrees Fahrenheit (°F), which is as hot as the sun’s surface. Temperatures in the mantle range from about 392°F near the crust to about 7,230°F closer to the core.

Rocks and water absorb heat from magma deep underground. The deeper you go, the hotter the rocks and water.

Tectonic plates and magma

The earth's crust is broken into large pieces called tectonic plates. Magma gets close to the surface where these plates meet, which is where volcanoes occur. The molten rock that erupts from volcanoes is called lava. Lava is essentially magma that has reached the earth's surface.

Where geothermal energy is found

How does geothermal heat reach the surface?

Geothermal reservoirs are natural underground areas where heat and water are found together. These reservoirs are usually deep underground and cannot easily be seen.

Geothermal energy comes to the earth's surface in three ways:

• Volcanoes and steam vents called fumaroles (where volcanic gases are released) bring heated air to the surface.
• Hot springs produce naturally heated water from underground.
• Geysers are rare hot springs that send powerful eruptions of hot water and steam into the air.

Where are most geothermal resources found?

Most major geothermal resources are found near the edges of the earth’s major tectonic plates, which is also where most volcanoes are located. One of the most active geothermal areas in the world is called the Ring of Fire, which circles the Pacific Ocean.

When magma is close to the earth's surface, it heats ground water. This water may be trapped in porous rock or running through fractured rocks or faults. Hydrothermal features have two common ingredients: water (hydro) and heat (thermal).

Geologists use various methods to find geothermal reservoirs. The most reliable method is to drill a well and test the temperature deep underground.

Where are most U.S. geothermal power plants located?

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

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 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

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.

The United States leads the world in the amount of electricity generated with geothermal energy. In 2021, there were geothermal power plants in seven states, which produced about 16 billion kilowatthours (kWh), equal to 0.4% of total U.S. utility-scale electricity generation.

In 2021, 27 countries, including the United States, generated a total of about 92 billion kWh of electricity from geothermal energy. Indonesia was the second-largest geothermal electricity producer after the United States—nearly 16 billion kWh of electricity—and equal to 5% of Indonesia’s total electricity generation. Kenya was the eighth-highest geothermal electricity producer at about 5 billion kWh. This was equal to about 43% of Kenya's annual electricity generation, which was the largest percentage share among all countries with geothermal power plants.

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 power plants

Geothermmal resources

Geothermal power plants use special underground areas called hydrothermal resources, which have both water (hydro) and heat (thermal).

These power plants need very hot water or steam—from 300 degrees Fahrenheit (°F) to 700° F—found in either dry steam wells or from hot water wells. We get these resources by drilling wells into the earth and then piping the steam or hot water to the surface. The hot water or steam then powers a turbine that generates electricity. Some geothermal wells are as much as 2 miles deep.

Types of geothermal power plants

There are three basic types of geothermal power plants:

• Dry steam plants use steam directly from a geothermal reservoir to spin generator turbines. The first geothermal power plant, built in 1904 in Italy, used the natural steam that came out of the ground.
• Flash steam plants, the most common type, use high-pressure hot water from deep inside the earth. When this hot water reaches the surface, its pressure drops, and some of it flashes (instantly turns) into the steam that drives a generator turbine. Once the steam cools, it turns back into water and is injected back into the ground to be used again.
• Binary-cycle power plants transfer the heat from geothermal hot water to another liquid with a lower boiling point. The heat from the geothermal water causes the second liquid to turn to steam, and the steam drives a generator turbine. In this type of plant, the geothermal water never touches the turbine and is returned to the earth. This closed system produces no air emissions.

Geothermal heat pumps

Geothermal heat pumps heat and cool buildings

Although air temperatures above ground change throughout the day and with the seasons, the temperature of the earth 10 feet below ground is surprisingly steady, between 50 degrees Fahrenheit (°F) and 60°F. For much of the United States, the soil temperature is usually warmer than the air temperature in winter and cooler than in summer. Geothermal heat pumps use the earth's steady temperature to heat and cool buildings.

Geothermal heat pumps transfer heat into or out of buildings

In winter, geothermal heat pumps draw heat from the warmer ground (or nearby water) into buildings.

In summer, the process is reversed. The system pumps unwanted heat from inside buildings into the cooler ground.

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 friendly, and cost-effective systems for heating and cooling buildings. Many types of buildings can use geothermal heat pumps, including homes, office buildings, schools, and hospitals.

Geothermal energy and the environment

Clean energy

The environmental effects of geothermal energy depend on how it’s used. Direct-use applications (like heating buildings) and geothermal heat pumps have almost no negative effects on the environment. In fact, they can be good for the environment because the more we use geothermal resources, the less we use energy sources that are more harmful to the environment.

Geothermal power plant emissions

Unlike power plants that burn fossil fuels, such as coal or natural gas, geothermal power plants do not burn fuel to generate electricity. Although they may release small amounts of sulfur dioxide and carbon dioxide, their emissions are far lower than fossil fuel-fired power plants:

• About 97% less sulfur compounds (which cause acid rain)
• About 99% less carbon dioxide (a greenhouse gas)

Geothermal power plants use scrubbers to remove hydrogen sulfide, a gas naturally found in geothermal reservoirs. Most geothermal power plants also inject the steam and water they use back into the earth. This recycling helps renew the geothermal resource and further reduces emissions from the power plant.

Protecting natural geothermal features

Many beautiful geothermal features, such as geysers and fumaroles in Yellowstone National Park, are protected by law because they are considered natural treasures.