What Is Geothermal Energy?

Geothermal energy is thermal energy stored within the Earth that can be extracted and used for electricity generation, direct heating, and cooling. The word combines the Greek roots geo (earth) and thermos (heat), and the resource originates from the residual heat of planetary formation combined with ongoing radioactive decay of isotopes in the mantle and crust. Unlike solar or wind energy, geothermal energy is available continuously regardless of weather or time of day, which makes it a firm, dispatchable power source in the renewable energy portfolio.

The United States leads the world in installed geothermal generating capacity, with just over 4 gigawatts of electrical output concentrated in western states where the thin crust brings magmatic heat closer to the surface. Iceland generates roughly 30 percent of its total electricity from geothermal resources and uses direct heat for nearly all residential space heating. Globally, the U.S. Department of Energy estimates that technical advances in enhanced geothermal systems could enable at least 90 gigawatts of domestic generating capacity in the United States alone by 2050, a figure that would represent a 22-fold increase over current installed capacity.

Resource Types and Temperature Regimes

Geothermal resources span a wide temperature range, from shallow, low-temperature aquifers at 10 to 30 degrees Celsius used in heat pumps, to high-enthalpy hydrothermal reservoirs exceeding 250 degrees Celsius where steam can be extracted directly. Hydrothermal systems, the basis for most existing power plants, require three elements: adequate heat, sufficient fluid to carry that heat to the surface, and permeable pathways through which the fluid can move. Where natural permeability is limited, enhanced geothermal systems (EGS) create artificial reservoirs by injecting water under pressure to stimulate fracture networks in hot dry rock. Petrothermal resources, which tap heat stored in impermeable rock at depth, represent the largest long-term potential category. Geothermal heat pumps exploit the stable temperature of shallow ground (typically 10 to 15 degrees Celsius at a few meters depth) as a thermal exchange medium for building heating and cooling at any location worldwide. The DOE Geothermal Technologies Office coordinates research across all three of these resource categories, including demonstration projects for enhanced geothermal systems.

Direct Use and District Heating

Beyond electricity generation, geothermal heat is applied directly in a broad range of industrial and community settings. District heating networks, pioneered in Iceland and replicated in parts of France, the United States, and China, distribute geothermally heated water through insulated pipelines to residential and commercial buildings. Agricultural applications include greenhouse heating, aquaculture pond warming, and food drying. Spa and balneological uses are among the oldest recorded applications of geothermal heat, documented at Roman-era sites in Europe and in pre-Columbian settlements in the American Southwest. Direct-use systems require lower reservoir temperatures than power plants and therefore access a geographically wider resource base, placing viable projects in regions far from active volcanic systems.

Environmental Considerations

Geothermal power carries a smaller surface footprint per unit of energy than most thermal generation technologies and emits substantially lower greenhouse gas volumes than coal or natural gas plants. However, some reservoirs release hydrogen sulfide and dissolved carbon dioxide dissolved in the extracted fluid, requiring gas abatement systems at plant facilities. Fluid reinjection, practiced at most modern plants, returns cooled water to the reservoir to sustain pressure and minimize subsidence risk. The University of Michigan Center for Sustainable Systems factsheet on geothermal energy reports that geothermal plants produce lifecycle greenhouse gas emissions roughly 10 to 40 grams of CO2-equivalent per kilowatt-hour, compared to approximately 820 grams for coal generation.

Applications

Geothermal energy has applications in a wide range of disciplines, including:

• Baseload electricity generation at utility scale
• District heating for urban residential and commercial buildings
• Greenhouse and aquaculture heating in agriculture
• Industrial process heat for food drying and mineral processing
• Lithium and mineral extraction from geothermal brines
• Space conditioning via ground-source heat pump systems