What is Geothermal Energy?

Learn everything you need to know about geothermal energy in this full breakdown article.

What is Geothermal Energy - ESRC - Environmental Sustainability Resource Center

If there’s anything that reminds most people of energy, it’s heat – from the crackle of superhot lightning to the glowing rays of the sun, heat and power just seem to go together. And though bright sources of heat are easily recognizable (lightning, the sun, even the dancing flames of a campfire), there is one excellent heat source right beneath our feet. Literally.

Remember clear back in your elementary years when you learned that the center of the Earth wasn’t just dirt? Although learning about the molten magma floating around Earth’s core likely dashed your plans to dig all the way to the center of the planet, you might have also found it fascinating that you were essentially living on top of a superheated, glowing ember. And not just any ember, but an ember the size of, well, a planet.

There are actually three different sources of heat buried deep in the earth:

  • The residual heat left over from when the planet was first formed
  • Heat caused by the friction of earth’s core materials moving around one another
  • Heat produced by the decay of radioactive elements

All of that adds up to a ton of heat, and a massive source of potential energy known as geothermal power (geo, from the Greek gê, meaning earth; and thermal, from the Greek thermós, meaning hot). But how do you harness all that heat energy, and is the process even safe enough to be viable? Keep reading as we dive deep into the Earth in our quest, not for metals or jewels, but for energy.

The History of Geothermal Power

Ancient America

Hot springs dot sections of North America, and early tribes of those regions quickly realized the benefit of such areas. Hot Lake in Oregon is one area where members of warring nations could come together and bathe in peace. Early tribes also used natural hot springs for warmth, cooking, and even medicinal purposes.

Greece and Rome

The ancient Greeks and Romans also used hot springs as places of sacred healing power. Hippocrates, the famous Greek physician who wrote the Hippocratic oath taken by modern-day medical professionals, encouraged the use of hot springs for regular bathing. And the geothermal baths at Pompeii remain excellently preserved, allowing archaeologists to see the sophisticated mechanisms by which these ancient civilizations utilized geothermal energy.


The ancient Nabataean city of Petra is probably best known for its appearance in Indiana Jones and the Last Crusade. While the treasury featured in the film is truly stunning, the city of Petra is a sprawling and fascinating archaeological site that consists of numerous buildings, temples, and yes, geothermal baths.

On top of one of the cliffs overlooking the main city of Petra, its citizens constructed a complex and ingenious heated spa complex. Although it’s unclear whether the main function of this location was religious or simply social, the heated baths at Petra rival those of ancient Rome. In fact, the Nabataeans drew upon both Roman and Egyptian engineering in the design and construction of their massive city in the desert.

Chaudes-Aigues, France (14th century)

In the early 14th century, the small French village of Chaudes-Aigues became the true birthplace of widespread, urban heating. The town was already located on a hot spring powered by the volcanoes of Auvergne, with the water being some of Europe’s hottest at an average temperature of 80-82 degrees… Celsius. That’s nearly 180 degrees Fahrenheit!

Using the rapid flow of the hot springs and the fact that they were located above the village, a simple network of wooden pipes was constructed to move the hot water from the spring and into the homes throughout the village. No pumps were required; gravity did all the work. And the heat of the water was regulated using simple taps, much like we use in our plumbing today. The hot water flowed through the pipes beneath the floors of the houses, providing consistent, ambient heat, even in the middle of the winter. And this marvel of engineering took place in the middle of a small, French village, all while the Black Death was sweeping across Europe.

Warm Springs Avenue – Boise, Idaho (1892)

It would be hundreds of years before geothermal power was used to heat homes, and once again, the homes in question were located in a small town conveniently placed near hot springs. Warm Springs Avenue is a small street in Boise, Idaho, and its homes made history in the late-1800s when they became some of the first in the world to use geothermal energy as a heat source for their homes. The houses on Warm Springs Avenue were heated using warm water pumped into them from the nearby Kelly Hot Springs (which also provided water for the city’s fire hydrant system). Some of the original houses built in 1892 are still standing today.

Larderello, Italy (1904)

The next major leap in geothermal power usage occurred in Larderello, Italy. An experimental power plant was developed to see if a geothermal location could do more than simply heat a nearby village. The first geothermal electric power plant was designed – a plant that could take the heat of Earth’s core and convert it into electricity.

At this point in time, water and wind had already been harnessed to create electricity using the Law of Electromagnetic Induction first introduced by a British scientist named Michael Faraday. He discovered that spinning a magnet through a coil of wire would induce an electrical current, which could then be stored or transferred immediately into homes. Wind and water power were fairly natural variations on this technology; after all, both wind and water turbines had been spinning for thousands of years. But geothermal power? That source took several additional decades to be explored.

In 1913, the first commercial, geothermal electrical plant finished construction in Larderello. It produced 250 kilowatts of electricity – enough to power 2500 light bulbs. And not only did the complex at Larderello create electrical energy, it was also used to extract borate compounds (a useful mineral) from the superheated fluids.

Reykjavik, Iceland (1930s)

One of the worlds largest and most-famous geothermal heating systems was built in the early 1930s in Reykjavik, Iceland. 99% of the city began receiving space heating from the geothermal plant upon construction, and the city is still one of the largest centers of geothermal heating in the world. In fact, the original geothermal production was so successful that an entire company called Reykjavik Geothermal was founded in 2008 to help spearhead geothermal efforts around the world.

California Geysers (1960)

In the mid-1960s, a large geothermal field simply called ‘The Geysers’ was constructed in the middle of California’s Mayacamas Mountains. It is still one of the world’s largest geothermal fields, and is actually composed of over 22 individual power plants. It was also the very first U.S. geothermal electric power plant to be commercially operated, and has enough capacity to power 900,000 homes.

Modern Day

As the demand for renewable energy sources has grown, so, too, has the incidence of geothermal power. By 2015, more than 80 countries were using some form of geothermal energy. While some of that energy is converted to electricity, lots of countries also use geothermal power as a convenient heat sources for homes and businesses. Geothermal has taken its place in the renewable energy revolution, and offers plenty of opportunity for use around the world.

So, What Exactly Can Geothermal Power Do?

Even though it should be clear by now that geothermal power can do, well, kind of a lot, what exactly are its limits? Luckily, the innumerable uses of geothermal power can be broken down into three primary categories: direct-use applications, geothermal heat pumps, and electricity generation.

Direct-Use Applications

Some of the earliest uses of geothermal power in the world were perfect examples of ‘direct-use’ applications. These are applications that use the heated water straight from the ground without any specialized equipment needed. The earliest baths (and even the heated houses in 14th century, France) were examples of direct-use applications of geothermal power.

There are plenty of modern examples of direct-use geothermal applications, too. There are still hot spring recreation areas that draw hundreds of thousands of visitors to enjoy the warm, soothing water. Some greenhouses and aquaculture ponds around the world are heated using geothermal power. And different cultures and locations can use hot springs for cooking, industry, snow-melting, and even milk pasteurization!

Geothermal Heat Pumps (GHP’s)

Heat pumps are a truly fascinating use of thermodynamic technology. Technically, a heat pump is any system that uses electricity to move heat from a hot space to a cold one. While heat pumps can be found in all sorts of places, especially in industrial applications, they work wonders when paired with geothermal power.

That’s because geothermal heat pumps tap into the stable temperature of the first 1,000 feet or so of Earth’s crust, which remains constant at about 50 to 60 degrees Fahrenheit. When it’s super cold outside in the winter, a heat pump can bring some of the warmth found in Earth’s crust up to the surface to warm homes or businesses. In the summer when the outside temperature is hot, the heat pump can be reversed and draw the hot air from a building and down below the surface, where it circulates and cools down.

Geothermal heat pump systems include a pump and a heat exchanger, which is basically a long loop of pipes buried beneath the surface. The pump circulates air, and the heat exchanger transfers heat energy between the ground and the air in the house. Most of the time, the heat exchanger is filled with fluid (a combination of water and antifreeze) to capture and distribute as much heat energy as possible.

Geothermal heat pumps are extremely efficient, using up to 50% less electricity than other heating and air conditioning systems. They also produce less pollution than air conditioning systems or other heat systems.

Electricity Generation

One of the biggest scientific breakthroughs of geothermal power came when it was first paired with an electrical generator to produce electricity. Traditional fossil fuels produce electricity by being burned as fuel, which then heats water into steam; in contrast, the water in hot springs is so warm that it produces steam naturally, no extra fuel required. Hot spots at the Earth’s surface can be converted into geothermal electricity plants, where they can capture the heat and steam produced at these locations and turn it into electrical power. In addition, wells can be drilled to tap into deeper warmth closer to the Earth’s core.

One of the biggest benefits to producing electrical power through geothermal processes is that the entire system is considered renewable. The heat provided by the Earth is an effectively endless supply, and any water that is used can be condensed and then reheated to be reused again and again. The systems are incredibly efficient, and are also more reliable than other electrical generation methods.

So, How Exactly Does Geothermal Power Work?

While it’s fairly simple to explain how hot springs can be useful (after all, most of us have used hot water from the tap at some point in our lives, and hot springs aren’t all that different), the conversion of Earth’s subterranean heat into electricity is a bit more complex. In order to understand how geothermal electrical plants actually work, it’s first important to review some basic science: welcome to geology (a brief review).

First, let’s review some geology

In order to understand how geothermal power comes into existence, we have to understand what exactly is going on beneath our feet. The Earth is made up of four distinct layers, each with a different composition.

The Inner Core

At the very center of Earth is its inner core, a solid metal ball that’s about ¾ the size of the moon in radius. The core is made mostly of iron and nickel, but even though the inner core is solid, it’s still ridiculously hot: temperatures hover around 5,400 degrees Celsius, or 9,800 degrees Fahrenheit (fun fact: that’s almost as hot as the surface as the sun!)

The Outer Core

The outer core is also made of iron and nickel, but isn’t solid like the inner core. Instead, Earth’s outer core is liquid, and is still blisteringly hot. The outer core is heated by the radioactive decay of different elements, and that constant heating means that the superheated liquid moves in massive currents. The motion of these currents actually generates electrical currents, which then induce Earth’s magnetic field.

The Mantle

The mantle is probably the reason you decided not to tunnel through to the center of the Earth as a kid. It’s composed of melted rock, mostly a mix of iron, magnesium, and silicon. It’s super dense and thick, and circulates just like the outer core below it (only much more slowly). Diamonds are formed in the intense heat and pressure of the mantle, and then come to the surface embedded in volcanic rock.

The Crust

Earth’s crust is the part of our planet that we’re most familiar with. After all, we live here! The crust is super-thin compared to the rest of Earth’s layers, and is also extremely cold and brittle in comparison. Lighter elements make up the crust (mostly silica, aluminum, and oxygen). Some parts of the crust (especially under the oceans) can be as thin as 3.1 miles thick, while parts of the crust beneath continents can stretch from 18.6 to 43.5 miles in thickness. The crust is broken into tectonic plates, which float very slowly across the mantle below. As these plates move past one another, you get massive movement on Earth’s surface in the form of volcanoes and earthquakes.

Energy Recovery

All of that heat contained within the Earth is what we refer to as geothermal energy. In fact, scientists estimate that we can recover 15,000,000 (that’s 15 million!) exajoules of geothermal energy; an amount equal to roughly three times the total energy consumed by the world each year. And because geothermal energy is renewable, all that energy isn’t going to disappear after we use it. We can pull from the heat of Earth’s core again and again.

How is all that heat energy captured?

Now, it’s one thing to have a practically endless well of available power, and it’s another thing entirely to actually tap into it. There are three main methods used to convert geothermal energy into electrical energy: dry steam, flash steam, and binary-cycle processes.

Dry Steam

Dry steam power plants are nice and simple: they collect steam that rises from the ground at hot spots on Earth’s crust. The hot steam is then funneled straight into a turbine that spins and drives an electrical generator.


Simple; cheap to set up; clean


Severely limited by location

Flash Steam

Flash steam power plants require a bit more work to set up, but can produce much more electricity. Pumps draw super-hot water from beneath the surface and into ‘flash tanks.’ Here, the sudden decrease in pressure instantly vaporizes the liquid water into steam, which then powers the turbine and generator.


Reuses water; doesn’t require any extra fuel; great electricity production


Slightly limited by location; requires more maintenance


In binary-cycle power plants, a closed loop of pipes containing some sort of working fluid (usually ammonia and hydrocarbons) taps into Earth’s heat. The working fluid is heated by geothermal water drawn up through a second set of pipes, where all the energy contained in the water is then transferred to the working fluid. Switching from water to a different fluid can increase the efficiency of the machine: water needs to be heated above 347 degrees Fahrenheit to produce economical electric power, but certain fluids only need to reach 185-194 degrees Fahrenheit in order to produce electricity effectively.


Incredibly efficient; numerous locations available; produces lots of electricity


Complex; requires extra materials to run

How is all that energy moved around and used?

After generating electricity from geothermal power plants, the energy flows directly into the consumer power grid, where it joins with electrical energy from numerous sources. Most geothermal electricity is kept extremely local, which improves its efficiency. The Geysers geothermal plant in California, for example, meets nearly 60% of the electrical demand for the entire North Coast region of the state.

Is Geothermal Power Efficient?

One of the key numbers associated with any sort of energy resources is its efficiency. Basically, efficiency is a comparison of the work that goes into a system vs the work that comes out. When it comes to geothermal power, to judge its efficiency we look at how much energy is produced given the amount of work put into the system.

General Efficiency of Home Heat Pumps

Just as a quick refresher, geothermal heat pumps use the stable temperature of Earth’s crust to heat or cool a building. High-efficiency geothermal pumps are incredibly efficient: some estimates suggest they are 48% more efficient than gas furnaces, 75% more efficient than oil furnaces, and 43% more efficient than a comparable air conditioning unit.

While different home heat pump systems might produce different levels of individual efficiency, they’re generally considered to be a much better alternative than traditional heating or cooling mechanisms. And, as an additional bonus, home heat pumps produce substantially fewer emissions than regular furnace systems.

General Efficiency of Geothermal Electrical Plants

Just as with home systems, each geothermal electrical plant will have a different efficiency level (in fact, levels can vary day-to-day at the plant, depending on numerous factors). Some estimates, however, have suggested that geothermal power plants have a capacity factor of 73% (meaning they come close to their most efficient performance levels). In comparison, coal-fired power plants average around 60%, and natural gas plants net a measly 45%. So, compared to other electrical plants, geothermal seems pretty efficient.

How Long Do Geothermal Electrical Plants Last?

Even though the potential for energy from Earth’s heat is practically limitless, environmental strain can cause some geothermal plants to dip in efficiency the longer they operate. Most sites can run at full capacity for 20-30 years before their energy output decreases. Other locations, however, have been running for decades without even slowing down.

The Lardarello complex in Italy was first built in the early 1900s, and is still producing massive amounts of electricity. The Geysers in California have operated since 1960, and recent updates to the system have improved its overall efficiency. One of the largest geothermal sites in the world, Reykjavik Geothermal in Iceland has been running without any dip in production since the 1930s. And the geothermal plant at the Oregon Institute of Technology has been producing electricity consistently since the mid-1950s. While some geothermal plants might need some renovation after a few decades, there are plenty of older models still working just as well as when they were first built.

The Conclusion

In general, geothermal power is an incredibly efficient resource. It doesn’t waste excess materials, and it’s actually quite good at converting heat energy into electrical energy. Large-scale, industrial systems operate at good efficiency, and even small home heat pumps are more efficient than traditional heating and cooling systems. So, is geothermal power efficient? I think it’s safe to say that it is.

Where Exactly Is All This Geothermal Energy, Anyway?

Although it’s easy to drive down the highway and see huge wind farms, or take a look around your neighborhood in search of solar panels, you probably won’t see any geothermal plants in your neck of the woods. Even though geothermal energy can technically be tapped from numerous locations, in order to make the process economically viable there are only a few places in the world that develop large-scale geothermal plants.

Best Geographical Locations (and why)

Geothermal energy is best tapped at the natural breaks between Earth’s tectonic plates. Here, the crust is thin and humans have easier access to the heat of Earth’s mantle. Some of the most effective geothermal plants in the world are located at these plate boundaries. Each of the countries that produce the most geothermal power fall on or near the boundary of two tectonic plates:

United States

The U.S. is the leading producer of geothermal energy in the world, due in large part to the Geysers geothermal complex in California. Geothermal plants in Hawaii also help add to the total.


It’s been estimated that Indonesia may overtake the U.S. as the world’s leading geothermal power producer by 2027. Currently, the country has four of the world’s ten largest geothermal projects. The nation of islands lies along the edge of the Ring of Fire, a tectonically active section of the Pacific Ocean.


Also located in the Ring of Fire, the Philippines is the third largest geothermal producer in the world. It has several different complexes that produce energy for the nation.


Turkey rests on its own, tiny tectonic plate called the Anatolian Plate. This plate is particularly active, however, as it’s constantly squeezed between the massive Arabian and Eurasian plates. This motion has led to the formation of several hot springs in the country, which have been tapped to make the nation the world’s 4th largest producer of geothermal electricity in the world.

New Zealand

The gorgeous landscape of New Zealand was formed by the explosive volcanic action of the Ring of Fire. Most of the geothermal fields in the nation are located around the Taupo Volcano.


Most of Mexico’s geothermal power is produced by a single, large power station, which is located just south of the U.S. border. The complex consists of 5 separate plants that provide power to the surrounding region.


If you’ve heard of Pompeii, you probably know that Italy has plenty of volcanic activity. It also produces plenty of geothermal energy, mostly from the world-famous Larderello Geothermal Complex. This center was the first plant to produce electricity using geothermal energy, and is located in Tuscany. It provides electricity to nearly 2 million families across the country.


Iceland is known as the land of ice and fire, and although the country can be volcanically tumultuous, it does produce a huge amount of geothermal power. It was also one of the first nations to use geothermal energy to heat its homes.

General Location Requirements

While geothermal power can be found in nearly every country in the world, there are certain things that help determine whether a location is ideal for a geothermal plant or not:

Heat Source

Obviously, to have a geothermal plant, you’ll want a good source of natural, subterranean heat. Areas located at the boundaries of two tectonic plates often have natural hot spots. In some cases, islands (such as Hawaii) have formed above a hot spot in the oceanic crust below and can also make excellent locations for geothermal power.

Water Recharge Abilities

Rather than needing to pump in water or use a different liquid, you’ll want a location that naturally provides plenty of water to run through the geothermal system.


Having a good amount of water is great, but you also need a place for all that water to be stored. Underground, naturally occurring reservoirs work best, as they allow fluids to rise close to the surface without being pulled away by a subterranean stream.

Impermeable cap rock

This requirement can get a bit technical, but basically, you want rock that’s hard enough to keep water from getting through it. The cap rock effectively seals in both the underground water and the heat until you decide to tap into it.

Economically viable

It just doesn’t make sense to construct a complex geothermal plant at the bottom of the ocean. Maybe it could be done, but it’s just too expensive to ever really contemplate. Similarly, you don’t want to pick a spot for your geothermal plant that’s going to be super difficult to get to. The further away your plant is from civilization, the further the electricity you produce is going to have to travel to be useful (and the less efficient the plant will be).

Pros and Cons of Geothermal Energy

All types of energy production have pros and cons that go along with them, and geothermal is no exception. The question is always if the positives of the technology outweigh the negatives. Let’s take a look!

Geothermal Energy Pros

  • Geothermal energy is a ‘clean’ energy source – it doesn’t burn any fossil fuels, and still provides plenty of electricity.
  • Geothermal plants produce 1/6th the carbon dioxide levels produced by natural-gas powered plants. And binary geothermal plants produce practically no emissions of any sort.
  • Unlike solar or wind energies, geothermal is always available. You aren’t dependent on the weather or the time of year to harvest electricity.
  • Geothermal energy can provide nearly 80% in savings over traditional fossil fuels. That’s because geothermal plants don’t actually ‘use’ any fuel; they just reuse the same heat source over and over in an endless cycle.
  • One of the biggest political incentives of geothermal power is that it’s a domestic energy product. There is potential for geothermal energy production in numerous countries around the world; a complete switch would eliminate any dependence on foreign oil.

Geothermal Energy Cons

  • One of the biggest concerns with geothermal plants is that they can release sulfur and hydrogen sulfide into the atmosphere. While these gases aren’t as harmful to the environment as CO2, they smell terrible and can be dangerous in large amounts.
  • Similarly, geothermal fluids released by geothermal plants can contain low levels of toxic materials which would need to be disposed of properly.
  • Geothermal power plants can cause mini tremors in their operational area (similar to tremors produced by fracking).
  • Plants can have a high initial build cost.
  • Specific geothermal locations can cool down over time, which means new holes must be drilled to maintain efficiency of the plants.

The Future of Geothermal Energy

While there are still several bugs to work through when it comes to harnessing geothermal energy, the good news is that we’re just getting started. Dependance on geothermal plants to produce electricity is ramping up, and the more demand increases, the more technological advancements we’ll see as a result. Some companies are investigating what it might take to convert old oil and gas wells into geothermal plants, which would expand the usability of geothermal energy. Better systems will replace old technology, and the next several decades may even see a breakthrough in how we harness the vast well of energy located directly beneath our feet.

Updated On: November 10, 2021
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