Geothermal energy

The word geothermal combines terms for Earth and heat. Heat below the surface is replenished in part by the decay of naturally occurring radioactive elements, as well as by heat retained from the planet's formation. Accessible resources vary with geology, depth, temperature and the movement of fluids through rock. Geothermal energy is therefore not one single technology: it includes electricity generation from deep, hot resources and direct heating or heat exchange using resources at lower temperatures.

Conventional geothermal power plants use naturally occurring hot water or steam from underground reservoirs. Wells bring the fluid or its heat to the surface, where steam or a secondary working fluid drives a turbine connected to an electricity generator. After heat is extracted, water may be reinjected underground, depending on the system design. These resources are most straightforward to develop where hot rock, fluid and permeable pathways occur together, although exploration and drilling are still required.

Enhanced geothermal systems (EGS) are designed to access hot rock where natural permeability or fluid circulation is insufficient. Operators create or improve pathways in the rock and circulate a working fluid through it, bringing heat back to the surface. This approach could broaden the range of locations that can be considered, but resource characterization, drilling cost, reservoir performance, water management and induced seismicity are important technical and regulatory considerations. Other developing approaches include closed-loop systems, which circulate fluid in sealed underground pipes rather than through a fractured reservoir. These designs remain under development and should not be treated as equivalent in maturity to established hydrothermal plants.

Geothermal heat is also used without first generating electricity. Direct-use systems bring underground hot water to buildings or industrial processes that need heat. Geothermal heat pumps work differently: they use the comparatively stable temperature of shallow ground as a heat source in colder conditions and a heat sink in warmer conditions. A heat pump transfers heat; it does not extract high-temperature geothermal fluid or generate electricity. District systems can connect multiple buildings to shared ground-source infrastructure.

Geothermal facilities can supply electricity independently of immediate sunlight or wind conditions, and geothermal heat can serve buildings or processes directly. Actual output and environmental performance depend on site and engineering choices. Projects face high upfront costs and uncertainty during exploration, and suitable conventional resources are geographically uneven. Development can also raise questions about water use, land disturbance, induced seismicity, permitting and community consent. Improved drilling and reservoir technologies may expand access, but technical potential estimates are not guarantees that resources can be developed economically or with no local effects.

Geologists assess heat, rock properties, fluids and permeability to estimate what a location may support. In the United States, the U.S. Geological Survey's 1975 assessment mapped conventional hydrothermal systems, and later assessments examined both identified and undiscovered resources. Newer assessments consider engineered systems as well. The International Energy Agency's 2024 report describes new technologies and oil-and-gas-derived drilling expertise as possible drivers of wider geothermal deployment, while emphasizing barriers such as project risk, licensing, environmental concerns and social acceptance. Estimates of future potential should be interpreted as conditional scenarios rather than present-day production.