How Geothermal Energy Works

The Pohutu Geyser in New Zealand erupts with a natural blast of the Earth's interior heat.

­We depend on our cars to take us to work and get our children to school. We rely on our home heating systems to keep us warm in the winter. We take it for granted that we can easily switch on our computer, vacuum cleaner or oven.

Yet scientists say the sources of energy we need to power all these modern conveniences are running dangerously low. We could run out of oil in as little as 40 years, and out of natural gas soon after that [source: The Independent]. These fossil fuels have been percolating beneath the Earth for hundreds of millions of years, and once they're gone, they're going to take millions more years to replenish. Not only are we running out of fossil fuels, but they're adding to our environmental woes by releasing nasty byproducts that increase pollution and contribute to global warming.


­Scientists are running a race against time to find cleaner, more efficient, renewable sources of energy. One potential source that we've barely tapped is right underneath our feet. Deep inside the Earth lies hot water and steam that can be used to heat our homes and businesses and generate electricity cleanly and efficiently. It's called geothermal energy -- from the Greek words geo, or "earth," and therme, meaning "heat."

There is plenty of heat in the center of the Earth. The deeper you dig, the hotter it gets. The core, about 4,000 miles (6,437 kilometers) beneath the surface, can reach temperatures of 7,600 degrees Fahrenheit (4,204 degrees Celsius). Part of that heat is left over from the Earth's formation, about 4 billion years ago. The rest comes from the constant decay of radioactive isotopes inside the Earth.

­The heat inside the Earth is intense enough to melt rocks. Those molten rocks are known as magma. Because magma is less dense than the rocks surrounding it, it rises to the surface. Sometimes magma escapes through cracks in the Earth's crust, erupting out of volcanoes as part of lava. But most of the time magma stays beneath the surface, heating surrounding rocks and the water that has become trapped within those rocks. Sometimes that water escapes through cracks in the Earth to form pools of hot water (hot springs) or bursts of hot water and steam (geysers). The rest of the heated water remains in pools under the Earth's surface, called geothermal reservoirs.

How Can We Use Geothermal Energy?


Heat is sitting under the Earth -- we just need to tap it. Geothermal energy can be used in three ways:


­Direct geothermal energy. In areas where hot springs or geothermal reservoirs are near the Earth's surface, hot water can be piped in directly to heat homes or office buildings. Geothermal water is pumped through a heat exchanger, which transfers the heat from the water into the building's heating system. The used water is injected back down a well into the reservoir to be reheated and used again.

Geothermal heat pump. A few feet under the ground, the soil or water remain a constant 50 to 60 degrees Fahrenheit (10-15 degrees Celsius) year-round. Just that little bit of warmth can be used to heat or cool homes and offices. Fluid circulates through a series of pipes (called a loop) under the ground or beneath the water of a pond or lake and into a building. An electric compressor and heat exchanger pull the heat from the pipes and send it via a duct system throughout the building. In the summer the process is reversed. The pipes draw heat away from the house and carry it to the ground or water outside, where it is absorbed.

Geothermal power plant. Hot water and steam from deep underground can be piped up through underground wells and used to generate electricity in a power plant. Three different types of geothermal power plants exist:

  • Dry steam plants. Hot steam is piped directly from geothermal reservoirs into generators in the power plant. The steam spins turbines, which generate electricity.
  • Flash steam plants. Water that's between 300 and 700 degrees Fahrenheit (148 and 371 degrees Celsius) is brought up through a well. Some of the water turns to steam, which drives the turbines. When the steam cools it condenses back into water and is returned to the ground.
  • Binary cycle plants. Moderately hot geothermal water is passed through a heat exchanger, where its heat is transferred to a liquid (such as isobutene) that boils at a lower temperature than water. When that fluid is heated it turns to steam, which spins the turbines.


How Does Geothermal Compare to Other Energy Sources?

A drilling rig at the Newberry crater near LaPine, Ore., is accessing the heat below the Earth's surface.
­AP Photo/Don Ryan

Experts say geothermal energy is cleaner, more efficient, and more cost-effective than burning fossil fuels, and it can reduce our dependence on foreign oil.

Geothermal energy is clean because it can be generated without burning fossil fuels. Geothermal plants release a fraction of the carbon dioxide produced by fossil fuel plants, and they create very little nitrous oxide or sulfur gases [source: U.S. Department of Energy]. Reykjavik, Iceland, which heats 95 percent of its buildings using geothermal energy, is considered one of the cleanest cities in the world [source: International Geological Congress Oslo].


Because the energy is generated right near the plant, it saves on processing and transportation costs compared to other types of fuel. Geothermal plants are also considered to be more reliable than coal or nuclear plants because they can run consistently, 24 hours a day, 365 days a year.

The initial costs of geothermal energy are high -- wells can cost $1 to $4 million each to drill, and installation of a home geothermal pump system can run as much as $30,000 [sources: REPP, Consumer Energy Center]. However, a home geothermal energy pump can cut energy bills by 30 to 40 percent and will pay for itself within 5 to 10 years [source: Consumer Energy Center].

Geothermal energy is considered renewable because the heat is continually replaced. The water that is removed is put right back into the ground after its heat is used.

The world uses about 7,000 megawatts of geothermal energy, about 2,700 megawatts of which is produced in the United States (the equivalent of burning 60 million barrels of oil each year) [source: Geothermal Education Office]. Still, we're not using nearly as much geothermal energy as is available. That has to do with the limited geographic availability of geothermal energy, and the difficulty and expense of drilling down far enough to reach that energy. More advanced techniques being developed could allow for deeper drilling, potentially bringing geothermal energy to more people in more places.

­For right now, geothermal heat pumps are the most viable option. They can be used just about anywhere in the world because the temperature beneath the ground always remains constant.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Consumer Energy Center. "Geothermal Energy." (Feb. 2, 2009)
  • Energy Information Administration. Energy Kid's Page. (Feb. 2, 2009)
  • Geothermal Education Office. "Geothermal Energy Facts." (Feb. 2, 2009)
  • Geothermal Energy Association. "All About Geothermal Energy -- Basics." (Feb. 2, 2009)
  • Howden, Daniel. "World energy supplies are set to run out faster than expected, warn scientists." The Independent. June 14, 2007. (Feb. 7, 2009) out-faster-than-expected-warn-scientists-453068.html
  • REPP. Geothermal Resources. (Feb. 2, 2009)
  • "Underground Heat." Canada and the World Backgrounder. October 1999, pgs. 21-25.
  • U.S. Department of Energy, Geothermal Technologies Program. "Geothermal Basics." (Feb. 2, 2009)
  • Worden, Jessica. "Clean Heat: The Geothermal Energy Beneath Our Feet." House & Home. January/February 2005, Volume 16, Issue 1, pgs. 44-45.