Geothermal Energy: The Renewable That Ignores the Weather
Geothermal is the renewable that behaves like a conventional power station. Its capacity factor rivals nuclear, it occupies little land, and it runs at night in winter. It is also the one most tightly constrained by where you happen to be standing, which is why it supplies a large share of electricity in Iceland and Kenya and almost none in most other places.
Where the Heat Comes From
The interior of the Earth is hot for two reasons, and the split between them was settled only recently. About half the heat flowing out through the crust is primordial: energy released when the planet accreted and differentiated four and a half billion years ago, still working its way outward because rock conducts heat poorly.
The other half is radiogenic. Uranium-238, thorium-232 and potassium-40 are distributed through the mantle and crust and decay continuously, releasing heat as they do. The proportions were long uncertain until detectors began measuring geoneutrinos - antineutrinos emitted by those decays, which pass straight out through the planet and can be counted at the surface. Borexino and KamLAND provided the first direct measurements of radiogenic heat production, which is a rare case of particle physics settling a question in geology.
The total outward flow is roughly 47 terawatts, which sounds enormous and is diffuse: averaged over the planet it amounts to about 90 milliwatts per square metre, thousands of times weaker than sunlight. Geothermal energy is therefore never about intercepting the average flow. It is about finding the places where geology concentrates it.
Temperature rises with depth at about 25 to 30 degrees per kilometre in ordinary crust. In volcanic regions, rift zones and plate boundaries it can exceed 100 degrees per kilometre, putting usable temperatures within drilling range. That is why the map of geothermal power tracks the map of plate tectonics.
Two Technologies, One Name
A ground-source heat pump has almost nothing in common with a geothermal power station except the word. It circulates fluid through pipes a few metres down, where the ground stays near the local annual average temperature all year - cooler than summer air, far warmer than winter air. A heat pump then moves that heat into the building, delivering three to five units of heat per unit of electricity consumed.
This is not generation; it is efficient heat transfer, and it works essentially anywhere there is ground. It is also the overwhelming majority of what is called geothermal by installed count, and it is the part of the field with the broadest application.
Power generation is a different problem. It needs fluid hot enough to drive a turbine, which in practice means 150 degrees or more, and it needs that fluid to exist in permeable rock that will yield it at commercial flow rates. Where nature provides both - Iceland, New Zealand, Kenya's Rift Valley, California's Geysers field, Indonesia, the Philippines - plants have run for decades.
Three plant types cover the range. Dry steam plants, the oldest design, take steam directly from the ground. Flash plants take hot pressurised water and let it boil as pressure drops. Binary cycle plants, the most common new build, pass geothermal water through a heat exchanger to vaporise a secondary fluid with a lower boiling point, which allows them to work with resources as cool as 100 degrees and to keep the geothermal fluid in a closed loop.
The Reliability Advantage
The operational case for geothermal rests on a single number. Capacity factors of 80 to 90 percent put it alongside nuclear fission and far above wind at 35 to 50 percent or solar at 25 to 35. The heat does not stop at night, in winter, or during a still week.
That property is worth more in a grid than the raw energy figures suggest. A source that runs regardless of weather reduces how much storage and backup the rest of the system needs, and it does so continuously rather than on a schedule someone has to forecast. This is the same structural argument that drives interest in any continuously available energy source, and it is why geothermal is often described as a complement to solar and wind rather than a competitor.
Land use is the second advantage. A geothermal plant occupies a small footprint per megawatt, most of it wellheads and a turbine hall, and the wells can be drilled directionally from a single pad.
The resource is not strictly inexhaustible in the way sunlight is. Extracting heat faster than it is replenished cools a reservoir over decades, and several fields including The Geysers have seen output decline and required reinjection of water to recover. Managed properly the resource is renewable on the timescale of a plant's life; managed carelessly it is mined.
Enhanced Systems and What Would Change
The constraint on conventional geothermal is not heat. Hot rock exists under everywhere; what is missing in most places is water and permeability. Enhanced geothermal systems propose to supply both: drill into hot dry rock, fracture it to create pathways, inject water at one well and recover steam at another.
If this works reliably it removes the geography constraint entirely, which is why interest has risen sharply. Early projects met difficulties - induced seismicity led to the shutdown of a project in Basel in 2006 and contributed to one in Pohang, South Korea - and managing that risk is now central to the field. Recent work in Nevada has demonstrated commercial-scale flow in a horizontal well configuration borrowed from the oil and gas industry, and drilling costs have fallen for the same reason.
A related line of research targets supercritical conditions. Water above roughly 374 degrees and 22 megapascals enters a state that is neither liquid nor gas and carries far more energy per unit mass. The Iceland Deep Drilling Project has reached such conditions in a volcanic setting; a single supercritical well could in principle produce ten times the power of a conventional one, though the materials and corrosion problems are severe.
Geothermal today supplies a small share of world electricity and a locally decisive share in a handful of countries. Whether it becomes a global technology rather than a regional one depends almost entirely on whether enhanced systems can be made routine.
Frequently asked questions
Is geothermal energy really renewable?
On human timescales yes, provided extraction is managed. The Earth's heat flow is continuous and the reservoir replenishes, but drawing heat out faster than it arrives cools a field over decades - The Geysers in California experienced exactly this before water reinjection restored output. Properly managed it is renewable; over-exploited it behaves like a mined resource.
Can geothermal be used anywhere?
Ground-source heat pumps work almost anywhere, because they exploit the stable temperature a few metres down rather than deep heat. Electricity generation needs hot permeable rock within drilling reach, which restricts it to volcanic and tectonically active regions. Enhanced geothermal systems aim to remove that restriction.
How much of the Earth's heat is from radioactivity?
Roughly half of the 47 terawatts flowing out through the crust, the rest being primordial heat from the planet's formation. The proportion was uncertain for decades until geoneutrino detectors measured radiogenic heat directly, which is one of the few cases where particle physics answered a geological question.
Why is the capacity factor so high?
Because the resource is continuous. The heat below ground does not vary with time of day, season or weather, so a plant runs whenever it is not under maintenance. Capacity factors of 80 to 90 percent are typical, comparable with nuclear and two to three times those of wind and solar.
Does geothermal cause earthquakes?
Conventional plants using existing reservoirs rarely cause noticeable seismicity. Enhanced systems, which fracture rock to create permeability, have induced felt earthquakes - a project in Basel was shut down in 2006 for this reason. Managing induced seismicity through injection control and site selection is now a central part of enhanced geothermal engineering.
