Geothermal energy

Based on Wikipedia: Geothermal energy

Beneath your feet, right now, the Earth is hot enough to boil water. Dig deep enough almost anywhere on the planet, and you'll hit rock at temperatures that would make an oven jealous. This isn't some exotic phenomenon found only near volcanoes—it's everywhere, a vast reservoir of heat that has been accumulating since our planet formed four and a half billion years ago. We're sitting on top of a nuclear reactor the size of a world, and we've barely begun to tap it.

The numbers are staggering. The Earth's interior contains about ten to the thirty-first power joules of thermal energy. To put that in perspective, humanity's entire annual energy consumption is roughly five hundred exajoules. The planet holds enough heat to power human civilization for millions of years. And unlike fossil fuels, which we're burning through in centuries, this heat continuously replenishes itself through the radioactive decay of elements like uranium and thorium scattered throughout the crust and mantle.

The Oldest Energy Source

Humans figured out geothermal energy long before we understood what it was. Paleolithic peoples bathed in hot springs. The Romans, with their genius for engineering comfort, conquered the town they called Aquae Sulis—now Bath, in England—and promptly built public baths fed by naturally heated water. They even installed underfloor heating. The admission fees for those Roman baths may represent humanity's first commercial use of geothermal energy.

In France, the town of Chaudes-Aigues has been running a geothermal district heating system since the fifteenth century. It's still operating today, over five hundred years later. Think about that: a renewable energy system that has outlasted empires, survived world wars, and continues to heat homes without interruption. No solar panel or wind turbine can claim that kind of track record yet.

The industrial age brought new applications. In 1827, entrepreneurs in Larderello, Italy, began using steam from geysers to extract boric acid from volcanic mud. Boric acid was valuable for making ceramics and medicines, but the real breakthrough came later: Larderello would become the birthplace of geothermal electricity.

Lightning from the Earth

On July 4, 1904—Independence Day in America, though this happened in Italy—Prince Piero Ginori Conti connected a small generator to a steam vent at the Larderello field. The machine produced enough electricity to light four bulbs. It was a modest beginning, but it proved something revolutionary: you could turn the Earth's heat directly into electrical power.

Seven years later, the world's first commercial geothermal power plant opened at the same site. For nearly half a century, Larderello remained the only place on Earth generating industrial quantities of geothermal electricity. New Zealand built the second plant in 1958. California's Geysers field followed in 1960, with a turbine that ran for over thirty years.

Today, geothermal power plants operate in twenty-six countries, with a combined capacity of about fifteen gigawatts. That might sound like a lot—and it is—but it represents less than one percent of global electricity generation. The potential is vastly larger than what we're currently using.

Why the Earth Is Hot

To understand geothermal energy, you need to understand why our planet is hot in the first place. The answer involves two sources of heat, both operating on timescales that dwarf human history.

About twenty percent of Earth's internal heat is primordial—leftover energy from the violent collisions that formed the planet. When countless asteroids and planetesimals smashed together to create Earth, the energy of those impacts converted to heat. Much of that original warmth remains trapped inside, slowly leaking out over billions of years.

The remaining eighty percent comes from radioactive decay. Uranium, thorium, and potassium isotopes scattered throughout the Earth's interior are constantly breaking down, releasing energy in the process. This is the same principle that powers nuclear reactors, except the Earth's reactor is unimaginably larger and runs at a much slower pace. A deep borehole in Cornwall, England, recently found granite unusually rich in thorium, helping explain why that region's rocks are hotter than expected.

At the boundary between Earth's core and mantle, temperatures exceed four thousand degrees Celsius—hotter than the surface of the Sun. This heat drives convection currents in the mantle, causing solid rock to flow like extremely thick syrup over millions of years. These currents are what move tectonic plates, cause earthquakes, and push magma toward the surface.

The Gradient of Temperature

If you dig straight down almost anywhere on Earth, the temperature rises by about twenty-five to thirty degrees Celsius for every kilometer of depth. This is called the geothermal gradient. It means that at three kilometers down—about the depth of a typical oil well—rock temperatures reach one hundred degrees or more, enough to boil water.

But averages don't tell the whole story. Near tectonic plate boundaries, where the crust is thinner and magma rises closer to the surface, the gradient can be much steeper. Iceland sits atop the Mid-Atlantic Ridge, where two plates are pulling apart; geothermal heat is so accessible there that the country generates roughly thirty percent of its electricity from it and heats about ninety percent of its buildings. El Salvador, Kenya, the Philippines, and New Zealand also derive significant portions of their power from geothermal sources.

The opposite of geothermal-rich areas are stable continental interiors, where thick, ancient crust insulates the surface from deep heat. In these places, you'd have to drill prohibitively deep to reach useful temperatures. But "prohibitively deep" is a moving target; as drilling technology improves, previously inaccessible heat becomes reachable.

Three Ways to Make Electricity

Geothermal power plants come in three main varieties, each suited to different underground conditions.

Dry steam plants are the oldest and simplest design. They work where superheated steam—water vapor above its boiling point—emerges directly from the ground. This steam spins a turbine, which drives a generator. Larderello and The Geysers in California are dry steam fields, offering temperatures between two hundred forty and three hundred degrees Celsius.

Flash steam plants handle a more common situation: reservoirs of extremely hot water under high pressure. When this water is brought to the surface and the pressure drops, some of it instantly "flashes" into steam. That steam drives the turbine, while the remaining water is injected back into the reservoir. Most of the world's geothermal plants use this technology. The largest flash steam installation is Cerro Prieto in Mexico, which produces seven hundred fifty megawatts from water at three hundred fifty degrees Celsius.

Binary cycle plants work with lower temperatures—as low as eighty degrees Celsius in some cases. Instead of using geothermal water directly, these plants pass it through a heat exchanger, where it warms a "working fluid" with a much lower boiling point than water. This fluid vaporizes, spins a turbine, condenses, and cycles back through. The geothermal water never touches the turbine and is reinjected underground, making binary plants essentially emission-free. A remarkable plant in Chena Hot Springs, Alaska, generates electricity from water at just fifty-seven degrees Celsius—lukewarm by hot spring standards, yet hot enough to produce power.

Binary cycle technology originated in the Soviet Union in the 1960s and now dominates new construction. It dramatically expands where geothermal electricity can be generated, since low-temperature resources are far more common than the volcanic hotspots needed for steam plants.

Engineering the Underground

What if you want geothermal power but don't have convenient hot springs or natural reservoirs? This is where enhanced geothermal systems, or EGS, come in. The concept is straightforward, if challenging in practice: drill down to hot rock, inject water under high pressure to fracture the rock and create pathways for fluid to flow, then pump water through those fractures to extract heat.

The technique borrows from oil and gas hydraulic fracturing, but with important differences. Geothermal wells are typically deeper, targeting formations at temperatures useful for power generation. And because no fossil fuels are involved, there's no need for the toxic chemicals sometimes used in hydrocarbon fracking. Instead, simple materials like sand or ceramic particles—called proppants—keep the artificial fractures open.

Small-scale EGS projects operate in France and Germany. They've proven the concept works, though not without complications. An early project in Basel, Switzerland, was shut down after it triggered small earthquakes—a reminder that injecting fluids underground can destabilize faults. This induced seismicity is a real concern for enhanced geothermal development, requiring careful site selection and monitoring.

An even newer approach is closed-loop geothermal, sometimes called advanced geothermal systems. Instead of pumping water through fractured rock, these systems circulate fluid through sealed pipes buried deep underground, like a giant underground radiator. The fluid absorbs heat through the pipe walls without ever contacting the surrounding rock. This eliminates concerns about water loss, contamination, and induced seismicity, though it also collects heat less efficiently. A Canadian company called Eavor has piloted closed-loop systems in Alberta, successfully producing thermal energy even in areas without exceptional geothermal gradients.

The Economics of Heat

Geothermal energy shares a crucial characteristic with wind and solar: fuel is free. Once you've built the infrastructure, operating costs are minimal. The challenge is that building the infrastructure—especially drilling the wells—is expensive and risky.

Drilling accounts for more than half the cost of a geothermal project. A typical well pair in Nevada—one well for extraction, one for reinjecting cooled water—produces about four and a half megawatts and costs around ten million dollars to drill. But here's the catch: roughly twenty percent of wells fail to produce useful resources. When you account for failures, the average cost of a successful well pair approaches fifty million dollars.

Geothermal drilling is harder than oil and gas drilling for several reasons. Geothermal reservoirs tend to occur in igneous and metamorphic rock—granite, basalt—which is much harder than the sedimentary rock where petroleum typically accumulates. This rock is often fractured, causing vibrations that damage drill bits. It's frequently abrasive, with high quartz content. The heat itself poses problems, limiting what electronics can survive downhole. And unlike oil wells, which are typically cemented only at the bottom, geothermal wells must be cemented along their entire length to handle thermal expansion and contraction.

Despite these challenges, costs have fallen substantially. The price of geothermal power dropped by twenty-five percent during the 1980s and 1990s, and improvements continue. The United States Department of Energy estimated in 2021 that electricity from a new geothermal plant costs about five cents per kilowatt-hour—competitive with natural gas and cheaper than most new coal plants.

The Steady Power Source

One of geothermal energy's most valuable characteristics rarely gets the attention it deserves: reliability. Unlike solar panels, which produce nothing at night, or wind turbines, which sit idle on calm days, geothermal plants generate power around the clock, regardless of weather. The Earth's heat doesn't take breaks.

This "baseload" capability makes geothermal an excellent complement to intermittent renewable sources. A grid powered primarily by wind and solar needs something to fill the gaps—traditionally natural gas plants that can ramp up quickly. Geothermal could serve a similar stabilizing role while producing essentially zero carbon emissions. The average geothermal plant emits about forty-five grams of carbon dioxide per kilowatt-hour, less than five percent of what a coal plant produces, and binary cycle plants emit practically nothing.

Geothermal is also renewable in the truest sense. Heat extraction rates are trivial compared to the Earth's total thermal content. You're not depleting a finite resource; you're skimming a tiny fraction of energy that would otherwise slowly leak away anyway. Individual reservoirs can be depleted if overexploited—The Geysers in California has seen declining steam production—but proper management and water reinjection can maintain output indefinitely.

Heat Without Electricity

Generating electricity is impressive, but most of humanity's energy use is actually for heat: warming buildings, heating water, running industrial processes. Geothermal excels at this, and has for centuries.

District heating systems pump hot water from underground through networks of insulated pipes, warming entire neighborhoods. Reykjavik, Iceland's capital, heats almost all its buildings this way. Boise, Idaho, built America's first geothermal district heating system in 1892—it's still running. As of 2010, geothermal provided twenty-eight gigawatts of direct heating capacity worldwide, used for everything from residential heating to greenhouse agriculture to fish farming.

Even where geothermal gradients are modest, ground-source heat pumps can extract useful energy. Below about six meters depth, soil temperature remains constant year-round, close to the local average air temperature. A heat pump can move this low-grade heat into buildings in winter and dump unwanted heat back into the ground in summer. These systems aren't strictly "geothermal" in the volcanic sense, but they tap the same basic resource: the temperature difference between above and below ground.

The Jobs and the Future

Geothermal energy employed about one hundred thousand people worldwide as of 2019. A 2024 report from the World Bank's Energy Sector Management Assistance Program found that geothermal development creates an estimated thirty-four jobs per megawatt across various sectors—substantially more than wind or solar on a per-megawatt basis. The technology requires skilled workers for drilling, plant operation, and maintenance, contributing to local workforce development in ways that highly automated renewable sources sometimes don't.

Estimates of geothermal's ultimate potential vary wildly, from thirty-five gigawatts to two terawatts of generating capacity, depending on assumptions about drilling depth and enhanced geothermal technology. Even the conservative estimate represents several times current deployment. The optimistic scenario would make geothermal a major pillar of global energy supply.

The key variable is drilling technology. Oil and gas wells routinely reach three kilometers, but geothermal developers dream of reaching ten kilometers or more, where temperatures are high enough for efficient power generation almost anywhere on Earth. Such "superhot rock" geothermal systems could theoretically unlock continent-spanning resources far from any tectonic boundary.

Whether that future arrives depends on investment and innovation. In 2020, only thirty-two percent of geothermal financing came from private sources, compared to much higher private investment in solar and wind. Geothermal's high upfront costs and drilling risks make it harder to attract capital, even though the long-term economics can be excellent. Government support—loan guarantees, research funding, demonstration projects—has historically been crucial for advancing the technology.

A Planetary Inheritance

The Earth has been collecting and generating heat for four and a half billion years. That thermal inheritance flows continuously toward the surface at a rate of forty-four terawatts—more than twice humanity's total energy consumption. We've been using tiny sips of this heat since prehistory, first in hot springs, then for heating, and most recently for electricity.

The heat is there, essentially everywhere, waiting. The question is whether we can reach it economically and safely. Every advance in drilling technology, every improvement in heat exchangers and binary cycle plants, every successful enhanced geothermal project brings more of that planetary heat within reach.

There's something elegant about geothermal energy. It doesn't depend on sunshine or wind or the combustion of ancient organic matter. It's simply the Earth doing what the Earth does: being hot. And it will keep being hot for billions of years to come, long after the Sun has consumed all the fossil fuels we've chosen to burn instead.