Geothermal Energy: Hydrothermal, EGS, and Global Capacity

Earth's Interior Radiates 47 Terawatts Continuously

The Earth's interior radiates approximately 47 terawatts of heat to the surface continuously — energy generated by the radioactive decay of uranium, thorium, and potassium in the mantle and crust, plus residual heat from the planet's formation 4.5 billion years ago. Humanity's total primary energy consumption is approximately 18 terawatts. In principle, Earth's internal heat could power civilization many times over. The practical challenge is extraction: most of this heat flows from depths of 10–100 kilometers, far below the reach of practical drilling technology. Only in specific geological settings — where tectonic activity brings heat close to the surface — can geothermal energy be harvested economically with existing technology. The global installed geothermal power capacity reached approximately 15 gigawatts (GW) by 2023, with another 40+ GW of thermal energy used directly for heating — a small fraction of potential but a growing one as new technologies push beyond traditional volcanic settings.

Geothermal energy is the only renewable energy source that is available 24 hours a day, 7 days a week, regardless of weather, season, or time of day. This dispatchability — the ability to ramp output up and down on demand — gives geothermal a fundamentally different profile from solar and wind, which require storage or backup when the sun isn't shining and the wind isn't blowing. A geothermal power plant has a capacity factor (ratio of actual output to maximum possible output) of 90%+ — compared to roughly 25% for solar PV and 35% for wind. This makes geothermal electricity uniquely valuable in decarbonized grids that rely heavily on variable renewables.

Hydrothermal Systems: Traditional Geothermal

Conventional geothermal power plants exploit hydrothermal systems — naturally occurring reservoirs of hot water or steam trapped in permeable rock formations close to the surface, typically associated with volcanic regions, tectonic rifts, and hot spots. Three plant configurations are used, depending on reservoir temperature and pressure:

The Geysers in Sonoma and Lake Counties, California, is the world's largest geothermal complex — 22 power plants producing approximately 725 megawatts of electricity, enough to power 725,000 homes. It has operated since 1960. The Larderello field in Tuscany, Italy, was the world's first geothermal power plant (1904) and still generates approximately 600 megawatts today.

Iceland: The Geothermal Nation

Iceland sits directly atop the Mid-Atlantic Ridge — a divergent plate boundary where the North American and Eurasian plates pull apart at 2.5 centimeters per year, continuously generating volcanic and geothermal activity. This geological setting makes Iceland one of the world's most geothermally active countries per unit area, with roughly 200 active volcanoes and more than 800 hot springs.

• Approximately 90% of Iceland's homes and buildings are heated directly by geothermal hot water — a district heating system centered on Reykjavik that pipes naturally heated water directly from the ground to buildings without electricity generation
• Geothermal sources provide approximately 30% of Iceland's electricity (the remainder is hydropower); together, 100% of Iceland's electricity comes from renewable sources
• Iceland's per-capita geothermal energy consumption is the highest in the world: approximately 57 gigajoules per person per year, compared to a global average of less than 1 GJ
• The Hellisheiði power plant near Reykjavik (303 MW electrical, 400 MW thermal) hosts the Orca and Mammoth carbon capture plants operated by Climeworks, which use geothermal electricity and heat to capture CO₂ directly from the air and store it permanently in basalt rock

Enhanced Geothermal Systems: Unlocking Everywhere

The fundamental limitation of conventional geothermal energy is geographic: hydrothermal reservoirs exist only where tectonics bring heat and permeability close together. Enhanced Geothermal Systems (EGS) aim to eliminate this constraint by engineering artificial geothermal reservoirs anywhere on Earth where sufficient subsurface heat exists — which is essentially everywhere, at sufficient depth. EGS works by drilling wells 3–10 kilometers deep into hot (150–300°C) but dry or impermeable rock, then using high-pressure fluid injection (similar to fracking) to create or enhance fracture networks, followed by circulating water through the created reservoir to extract heat.

Fervo Energy's EGS Breakthrough

Fervo Energy, a Houston-based startup, achieved the first commercial-scale EGS power production in the United States in 2023 at its Project Red site in Nevada. Using directional drilling technology borrowed from the oil and gas industry — specifically horizontal drilling and distributed fiber-optic sensing along the wellbore — Fervo drilled two parallel horizontal wells 2.4 kilometers underground in hot granite. The wells, connected by engineered fractures, circulate water through the hot rock and return it to the surface at 180°C to generate electricity.

Fervo's Nevada pilot demonstrated 3.5 megawatts of power and signed a power purchase agreement with Google as part of Google's 24/7 carbon-free energy commitment — the first EGS power supply to a corporate clean energy buyer. In 2024, Fervo secured a larger project (Cape Station in Utah, targeting 400 MW) with construction underway. The U.S. Department of Energy's Enhanced Geothermal Shot initiative has set a target of reducing EGS costs by 90% to $45 per megawatt-hour by 2035 — a price competitive with new natural gas and solar-plus-storage projects. If achieved, EGS could transform geothermal from a niche resource in volcanic regions into a globally deployable baseload clean energy technology capable of providing dispatchable power to any electricity grid on Earth.