Geothermal Energy Explained: From Volcanic Power Plants to Your Home Heat Pump

Two Completely Different Technologies Share One Name

When someone says "geothermal energy," they might be talking about two completely different technologies that happen to draw heat from the same source: the Earth. Understanding the distinction is essential, because the practical implications for where each technology is viable, what it costs, and what it can do differ enormously.

Utility-scale geothermal electricity is only viable in tectonically active regions where volcanic heat comes close to the Earth's surface — Iceland, the US West, New Zealand, Kenya. It produces continuous electricity from steam or hot water pumped from the ground.

Ground-source heat pumps (also called geothermal heat pumps) work almost anywhere in the world. They exploit the fact that the ground temperature at 10–15 feet depth stabilizes at roughly 50–60°F year-round, regardless of surface weather. They do not generate electricity — they use electricity very efficiently to move heat.

Utility Geothermal: Tapping the Earth's Volcanic Heat

The Earth's core temperature runs approximately 10,800°F (6,000°C). This heat, generated partly by the decay of radioactive isotopes and partly left over from planetary formation, conducts slowly outward through the mantle and crust. Most of the time, this heat is too diffuse and too deep to access economically.

In tectonically active regions, heat concentrates close enough to the surface to use. Magma intrusions push heat upward; permeable rock allows water to circulate near the heat source; and the result is a hydrothermal reservoir that can be tapped with wells.

A conventional geothermal power plant works simply: drill wells to 1–3 km depth, pump hot water or steam to the surface, run it through a turbine to generate electricity, and reinject the cooled fluid back into the reservoir to maintain pressure. The three main plant types — dry steam, flash steam, and binary cycle — differ in how they handle the temperature and phase characteristics of the geothermal fluid.

Where Geothermal Electricity Works

Iceland is the most dramatic example: roughly 65% of Iceland's electricity comes from geothermal, supplemented by hydropower. The country sits directly on the Mid-Atlantic Ridge, with readily accessible volcanic heat almost everywhere.

The United States has 3.7 GW of geothermal electricity capacity, concentrated almost entirely in California (The Geysers complex, the world's largest geothermal field), Nevada, and Hawaii. These regions have the volcanic geology that makes shallow hydrothermal resources accessible. Most of the eastern United States lacks this geology and has essentially no utility-scale geothermal electricity potential with current drilling technology.

Geothermal electricity is especially valuable because it is baseload power — it generates continuously, day and night, regardless of weather. Unlike wind and solar, which are intermittent, a geothermal plant has a capacity factor of 80–95%. Iceland demonstrates what a grid with abundant geothermal looks like: extremely stable electricity, minimal storage requirements, low carbon emissions.

Enhanced Geothermal Systems: The Next Frontier

Most of the Earth's accessible rock is hot but dry — no natural water to circulate. Enhanced Geothermal Systems (EGS) aim to fix this by injecting water into fractured rock at depth (3–10 km), creating an artificial hydrothermal reservoir. The potential resource is enormous — the US Department of Energy estimates EGS could provide 90 GW of baseload power from US resources — but the technology remains expensive and technically challenging.

Fervo Energy, based in Houston, has been developing EGS projects in Nevada and Utah with DOE support. Their Cape Station project in Utah is targeting commercial power delivery in 2026. EGS drilling costs run $5–$15 million per well, and multiple wells are required per project — current economics are not competitive with wind or solar without significant cost reductions.

Ground-Source Heat Pumps: The Technology You Can Actually Use Anywhere

Below the frost line — typically 10–15 feet in most of the continental United States — ground temperature stabilizes at roughly 50–60°F and stays there year-round. In Minnesota, the ground at 15 feet depth is 50°F in January. In Arizona, it's about 60°F in August. The surface temperature swings wildly; the subsurface does not.

A ground-source heat pump exploits this stable temperature as a heat exchange medium. In winter, it extracts heat from the ground (which is warmer than the outside air) and moves it into your home. In summer, it removes heat from your home and deposits it into the ground (which is cooler than the outside air). It is not generating heat from combustion — it is moving heat from one place to another using a refrigerant loop buried in the ground.

The Coefficient of Performance Difference

A gas furnace burns fuel and converts it to heat at 80–98% efficiency — a COP (coefficient of performance) of 0.8–0.98. For every unit of energy in the gas, you get slightly less than one unit of heat.

A ground-source heat pump's COP is 3–5. For every unit of electrical energy consumed, it delivers 3–5 units of heat energy into the building. The extra energy is not created — it is extracted from the ground. This is why heat pumps can dramatically reduce energy bills even though electricity costs more per unit than natural gas: you use far less of it.

Air-source heat pumps work on the same principle but exchange heat with outdoor air rather than the ground. They are cheaper to install but their efficiency falls significantly in cold weather — when outside air drops below 20–25°F, air-source heat pump efficiency drops to COP 1.5–2. Ground-source heat pumps maintain COP 3+ through the coldest winters because the ground stays at 50°F regardless of surface weather.

System Configurations

The ground loop — the buried pipe through which refrigerant or glycol solution circulates — can be installed in several configurations:

• Horizontal closed-loop: Pipes buried 4–6 feet deep in long horizontal trenches. Requires substantial land area (typically 1,500–2,000 square feet of trenching per ton of system capacity) but is the least expensive to install where space permits.
• Vertical closed-loop: Pipes run in boreholes drilled 150–400 feet deep. Requires much less surface area — suitable for suburban lots — but drilling costs more.
• Pond/lake loop: Pipes submerged in a pond or lake. Requires a large, deep water body near the home but is cost-effective where applicable.
• Open-loop system: Uses groundwater directly from a well, circulates it through the heat pump, and returns it to the aquifer or discharges it to a pond. Highly efficient but requires suitable groundwater and local regulatory approval.

Who Geothermal Heat Pumps Are Suited For

Ground-source heat pumps work in all 50 states — the stable ground temperature they exploit is available everywhere. But the upfront cost ($12,000–$32,000 installed, with $17,300 being a typical mid-range figure) means they make the most financial sense in specific situations: high-heating-load climates, homes replacing old inefficient heating systems, and properties with good geology for vertical boring or sufficient land for horizontal trenching.

The 30% federal tax credit available through 2033 significantly improves the economics. We cover the full cost and payback analysis in our companion piece on geothermal heat pump costs and savings. For comparison, residential solar panels follow a similar investment pattern and can be combined with a geothermal system for a highly efficient all-electric home — see our overview of how solar panels work.