Geothermal energy is a renewable energy source that taps the thermal energy beneath the Earth’s surface. The hot springs of Iceland and the geysers of Yellowstone National Park are examples of geothermal reservoirs that have broken through the Earth’s crust. Humans have used geothermal resources for millennia.

As governing bodies like the U.S. Department of Energy seek renewable means of energy production, they incentivize utilities, corporations, and individuals to tap Earth’s heat as a means of powering industrial processes and generating electricity. This has led to a recent surge in geothermal technologies.

Geothermal energy can produce electricity in geothermal power plants. You can also put it to direct use as a home heating source via geothermal heat pumps.

• Geothermal electricity production: Most electrical power plants create electricity by boiling water to produce steam. The steam then rotates massive turbines which produce an electrical current. Geothermal electricity plants also use this method, only instead of producing steam using fossil fuels like coal or natural gas, they use naturally heated water from underground reservoirs. There are variations on geothermal power generation (including the dry steam method, the flashed method, and the binary cycle system), yet all exploit the naturally high temperatures of geothermal fluid.
• Geothermal heating systems: Geothermal heating systems use geothermal water to heat radiators in homes, offices, and factories. In some places, including much of Iceland, geothermal water from hot springs even comes out of the faucet for home use. Geothermal systems can also function as cooling systems by using geothermal heat pumps to move heat from a building into the ground. The system then replaces that hot air with low-temperature air from beneath the earth’s surface.

Geothermal energy is useful for a variety of reasons.

1. It’s potentially available throughout the world. With significant investment, geothermal energy can be tapped nearly anywhere on Earth. Although the source of geothermal heat is the earth’s core and the movement of tectonic plates, all usable geothermal reservoirs are found in the Earth’s crust—sometimes just a few feet below the surface.

2. It provides constant baseload power. Once tapped, a geothermal source provides continuous power and does not need batteries to operate efficiency, as is the case with other renewable energy sources like solar and wind.

3. It produces lower greenhouse gas emissions than fossil fuels. Tapping underground sources of heat does involve the release of greenhouse gasses like methane (CH4) and carbon dioxide (CO2), but it does not approach the level of greenhouse gas emission that comes with the burning of coal, oil, or natural gas.

4. It can make use of existing fossil fuel infrastructure. As industries and utilities transition away from fossil fuels, those sources can facilitate the introduction of geothermal in the same regions. Oil wells and gas wells have already completed the requisite drilling needed to tap geothermal heat, which has led many engineers to propose using the same wells for fossil fuels and geothermal power.

Despite several clear advantages over fossil fuels, geothermal energy systems have not yet claimed a significant market share. There are two primary reasons for this.

1. The start-up costs are high. Geothermal electricity production and geothermal heating can be cost-effective once the infrastructure is in place. However, creating that infrastructure is expensive. To reach geothermal reservoirs, crews must break through dense bedrock. The drilling is labor-intensive and noisy. Some municipalities and homeowners associations do not allow it.

2. New technology is required in many locations. For geothermal systems to provide energy efficiency, there must be a natural flow of fluid from a geothermal field to the air above it. Only about 10 percent of the earth’s land area is conducive to this fluid flow. A new technology called enhanced geothermal systems (EGS) can get around this problem by breaking up hot rocks deep below the earth’s surface. These rocks are injected with water, which becomes steam that rises to the surface for energy use. Used, low-temperature water is then returned to the earth via injection wells. While such deep drilling can produce carbon dioxide and methane emissions, geothermal projects can eventually get around this by cycling their geothermal fluid in a closed loop.