The Heat is On: Harnessing Nature’s Underground Oven
By Allen Huang
It’s World Geothermal Energy Day on October 17 – probably not a date you have marked in your calendar, vying with Halloween but perhaps we do need to give this latent energy source a lot more love.
In the quest for renewable energy, we’ve built massive dams, constructed towering wind turbines, and covered fields with solar panels. Yet, amidst these efforts, are we overlooking a powerful energy source right beneath our feet? Geothermal energy remains largely untapped, waiting to be harnessed.
The Science Beneath Our Feet: Understanding Geothermal Energy
Perhaps the simplest and cleanest description of geothermal comes from the U.S. Energy Information Administration (EIA): Geothermal energy arises from the decay of radioactive particles in the Earth’s core—specifically uranium, thorium, and potassium. This decay generates immense heat, creating a continuous ‘thermal engine’ that we can tap into for generating heat and electricity
When you witness lava flowing from a volcano or a geyser erupting with steam and hot water, you’re observing the powerful forces of natural geothermal processes at work.
Volcanic eruptions occur when magma—molten rock formed from intense heat and pressure deep within the Earth—forces its way to the surface. This magma originates from the melting of rocks in the mantle, which occurs due to the Earth’s core generating immense heat from decaying uranium, potassium and thorium. As our planet’s tectonic plates move and shift, the magma can erupt through cracks in the Earth’s crust – volcanoes!
Geysers, on the other hand, are formed from underground water reservoirs near these molten rocks, and they erupt when the water is boiled into steam.
a well-known sight all over the world from Yellowstone park in the U.S to the Great Geysir in Iceland to El Tatio, in Chile.
A Brief History of Human Use
Geothermal has been used by human beings in one form or another for thousands of years; ten thousand years ago it was believed that American Paleo-Indians used the boiling water from natural hot springs to cook and clean. In fact geothermally created natural hot springs have been utilized all over the world throughout human history, including in China, and by the Roman Empire. By the Second Industrial Revolution, engineers had managed to harness geothermal energy to heat the inside of buildings with specially designed hot water pipelines. These pipelines were first used in Boise, Idaho, in a facility opened in 1892 and which still runs today to power the State House and City Hall.
The first industrial application of geothermal energy for electricity came from the small village of Larderello in Tuscany, Italy. In 1904, Italian engineer Piero Ginori Conti used the natural steam from the geothermal to power an engine, and they soon built a commercial power station entirely based on geothermal energy.
After the end of World War II, many more countries have seized on the opportunity to develop their own geothermal facilities. The Geysers, the first geothermal power plant in the United States, located in the Mayacamas Mountains in Northern California, was built in 1960. Today, it remains the largest geothermal power plant system in the world, with 725 megawatts of installed generating capacity, which is enough to generate the entire city of San Francisco.
It is clear that geothermal energy has been harnessed by humans for millennia, but today, advancements in technology are unlocking geothermal’s true potential, which is to create electricity, and not just provide a ton of hot water!
Why Is Geothermal a Renewable Energy?
“Geothermal energy is renewable because the Earth has retained a huge amount of the heat energy that was generated during the formation of the planet. In addition, heat is continuously produced by decay of radioactive elements within the Earth. The amount of heat within the Earth, and the amount that is lost through natural processes (e.g. volcanic activity, conduction/radiation to the atmosphere), are much, much more than the amount of heat lost through geothermal energy production….Over time, it is commonly necessary to drill additional wells in order to maintain energy production as temperatures and/or reservoir fluid pressures decline.” Drew L. Siler, PhD, Geothermal Geologist, American GeoSciences Institute
As a renewable energy, the most significant benefit for geothermal energy is the reduced level of greenhouse gas emissions (GHG) it is responsible for in contrast with fossil fuels, when used to generate electricity. On average, geothermal electricity power plants, produce 122 grams of CO2 per kilowatt-hour (g/kWh), which is about 73% lower than typical natural gas power plant emissions (450 g/kWh) and 86-90% lower than coal-fired power plants (900-1300 g/kWh).
How is Geothermal Energy Extracted & Turned Into Electricity
The conventional extraction of geothermal energy involves drilling wells into geothermal reservoirs, typically located 1–5 kilometers beneath the surface.
These wells tap into both low-temperature resources (below 150°C) and high-temperature resources (above 150°C). Drilling geothermal wells requires specially designed rotary drilling equipment to handle the harder rock formations and higher temperatures encountered, unlike conventional oil and gas wells.
Once the wells are completed, geothermal fluids—hot water, steam, or a mixture of both—are extracted from the reservoir and brought to the surface through a system of pipelines. These fluids rise through the well due to pressure differences and natural convection.
It’s the steam created which can be used directly to drive turbines for electricity generation, while the hot water can be sent to a heat exchanger to transfer its thermal energy.
How Many Methods Are There To Turn Heat into Electricity?
Currently, there are four separate methods to transform the extracted geothermal power from the crust for electricity generation. The conventional extraction methods, known as dry steam, flash steam, and binary-cycle, make up the majority of the traditional geothermal power generation across the world. In recent years, enhanced geothermal systems (EGS), which involves human made inductions of geothermal energy through drilling wells and creating reservoirs, has made geothermal power more widely available.
Dry Steam Power Plants utilize steam from geothermal reservoirs to directly turn turbine generators. In this type of system, superheated steam is drawn from underground reservoirs that naturally contain minimal water. Since the steam emerges from the Earth in a “dry” state, with little or no liquid water mixed in, it can be piped directly to a steam turbine, which spins to generate electricity.
This method is highly efficient because there is no need to first separate water from steam or flash the steam into existence. This method is suitable for geothermal fields that produce dry steam with minimal water content, such as The Geysers in Northern California. However, due to rigorous requirements for suitability, dry steam geothermal fields are rare and only exist in a few locations worldwide.
Flash Steam Power Plants extract high-pressure hot water from deep within the Earth and convert it to steam by reducing the pressure in a process known as “flashing.” In a flash steam plant, geothermal fluid is pumped from underground reservoirs that contain both hot water and steam, typically at temperatures exceeding 182°C (360°F). As the pressurized water is brought to the surface, it enters a flash tank, where a sudden reduction in pressure causes some of the water to vaporize, or “flash,” into steam. This steam is then used to drive turbine generators, producing electricity.
Flash steam plants are highly versatile because they can work with geothermal reservoirs that contain a mixture of steam and water, unlike dry steam plants which require pure steam. Additionally, any liquid water left after the flashing process can be injected back into the Earth or used in a secondary flash tank for additional energy production. Flash steam power plants are the most common type of geothermal plant because they can efficiently harness high-temperature geothermal resources, which are found in a wider range of geothermal fields compared to dry steam reservoirs.
Binary-Cycle Power Plants transfer heat from geothermal water to a secondary fluid with a lower boiling point, usually easily vaporizable organic compounds like hydrocarbons, through a heat exchanger. The vaporized secondary fluid drives the turbines. This method is highly effective for lower temperature resources between 107°C (225°F) and 182°C (360°F) and operates in a closed-loop system, keeping the geothermal water and secondary fluid separate.
This minimizes emissions and prevents the release of harmful gasses, making it a cleaner energy solution compared to other geothermal technologies. Due to its ability to harness moderate-temperature geothermal resources, binary-cycle plants can be used in a wider range of locations. A prominent example of a binary-cycle power plant is the Chena Hot Springs Plant in Alaska, which operates efficiently using relatively low-temperature geothermal resources.
Enhanced Geothermal Systems (EGS), also known as Hot Dry Rock (HDR), artificially stimulate and enhance geothermal reservoirs by injecting water to create fractures in hot dry rock formations in a process known as stimulation. This method expands geothermal energy potential to areas without natural hydrothermal resources, usually deeper beneath the surface, and could vastly increase the accessible geothermal energy worldwide.
It’s important to point out that, unlike fracking, Enhanced Geothermal Systems (EGS) stimulation, which is used to generate geothermal energy, typically uses much less water with much smaller pressures. In many cases, the water is recycled within a closed-loop system, minimizing waste and contamination risks. Additionally, EGS does not involve extracting fossil fuels or producing harmful byproducts like methane, which further reduces the risk of water pollution.
Plus Geothermal Energy Can Still Be Used for Heating and Cooling
When not being used to generate electricity, the extracted geothermal energy can be turned into a steady source for heating and cooling different places, including factories, agriculture fields, as well as industrial factories. Applying geothermal energy relies on two methods: direct use applications and geothermal heat pumps (GHPs), replacing the fossil fuels of natural gas and coal that have been traditionally used to generate heat.
Direct use applications involve tapping into geothermal reservoirs that provide naturally heated water or steam from the Earth. This method requires minimal energy conversion, making it incredibly efficient. The geothermal water, typically ranging from 70°F to 300°F (20°C to 150°C), can be used directly for various heating applications. When the water can exceed the atmospheric boiling point of 212°F (100°C) due to the high pressure deep underground, making it an even more versatile energy source.
One of the most common uses of direct geothermal energy is for building heating. Geothermal fluids are piped through a heat exchanger to warm buildings through radiant floor systems or radiators. In some areas, entire neighborhoods benefit from district heating systems, where one geothermal source supplies heat to multiple buildings.
Geothermal energy is also used in agriculture, where it provides warmth to greenhouses, extending the growing season and improving crop yields. Aquaculture facilities use it to maintain ideal water temperatures for fish farming, which can boost growth rates and reduce energy costs. In industrial processes, geothermal heat is employed in various applications, such as food dehydration, milk pasteurization, and lumber drying, providing an affordable and reliable heat source.
While direct use applications rely on naturally hot water, geothermal heat pumps (GHPs), also known as ground-source heat pumps, use the Earth’s stable underground temperature for both heating and cooling. The ground or groundwater maintains a constant temperature year-round, which makes it an ideal source of heat exchange.
In the winter, GHPs extract heat from the ground and transfer it into buildings. A water-based solution circulates through a series of pipes, called a “ground loop,” buried below the surface. This fluid absorbs heat from the Earth, and the GHP system uses this heat to warm the building. In the summer, the process is reversed. The GHP extracts heat from the building and releases it into the cooler ground, providing efficient cooling.
So How ‘Hot’ Can Geothermal Heat Actually Get?
Just how much heat can be produced by geothermal depends on something called the geothermal gradient. This is the rate at which the Earth’s temperature rises with increasing depth—typically about 25–30 degrees celsius per kilometer down into the crust.
Understanding geothermal energy requires an idea of the earth’s basic structure and composition. So here goes: below our feet lies the Earth’s crust, averaging about 30–70 kilometers thick on land and 5–10 kilometers thick under the oceans.
Beneath the crust is the mantle, extending down to about 2,900 kilometers, composed of hot, semi-solid silicate rocks rich in iron and magnesium. The core lies even deeper, with temperatures estimated between 5,000 and 6,000 degrees Celsius, comparable to the surface temperature of the Sun.
Aside from contributing less GHG, which helps to mitigate climate change, unlike solar and wind, which depend on the weather of the day to function properly, geothermal energy is not dependent on weather conditions.
No matter the method of generation, geothermal energy is based on the constant heat generated from the Earth’s core, making it a reliable source of base-load electricity that operates 24/7, regardless of external weather conditions.
Most energy power plants, such as coal or nuclear power plants, require large volumes of water for cooling, but geothermal plants, especially those using dry-cooling systems, can operate with minimal water use. This reduction in water dependency not only preserves vital freshwater resources but also helps avoid the negative environmental impacts associated with the heavy water usage in other energy sectors.
As they both use similar technologies and rely on drilling, some abandoned oil and natural gas wells in the West have been retrofitted to become sites for geothermal production. The abandoned wells are surveyed, evaluated, reconfigured, and then used to establish a heat exchange system to develop into a new geothermal power plant.
In recent years, the Department of Energy has actively pursued the development of geothermal energy from abandoned oil wells through its Geothermal Technologies Office (GTO) and the Wells of Opportunity (WOO) initiative. The WOO initiative funds various projects across the United States, from improving well permeability in Nevada to harvesting waste heat from existing wells in Texas and California. By tapping into the millions of inactive wells across America, this approach significantly reduces expensive drilling costs, and addresses environmental concerns by giving new life to abandoned infrastructure.
The economic benefits of geothermal energy extend beyond its operational efficiencies. Take heat pumps used for residential heating for example, compared to conventional HVAC systems, geothermal heat pumps can reduce energy consumption by 25% to 50%, which can become a full replacement of HVACs in many climates. They are popular in both residential and commercial buildings, offering a sustainable alternative for heating and cooling. Additionally, geothermal heat pumps have long operational life spans—often lasting upwards of 25 years—and require less maintenance, as they are protected from outdoor weather conditions and do not rely on fuel combustion.
Reasons To Love Less About Geothermal
As geothermal plants are usually built near regions closer to the earth’s crust, which are mostly situated on fault lines and are susceptible to seismic activity, the drilling, injection and extraction of fluids has been a long held concern from some scientists over the feasibility and safety of their widespread usage.
The development of enhanced geothermal systems (EGS) has raised further concerns about induced seismicity, which are earthquakes triggered by human activities. As EGS involves injecting fluids into hot dry rock reservoirs deep underground, this can change stress fields and cause the rock to fracture, potentially leading to seismic instability.
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A study by the National Academies of Sciences, Engineering, and Medicine titled “Induced Seismicity Potential in Energy Technologies” concluded that geothermal energy has a low likelihood of causing noticeable earthquakes. This is because the process involves both injecting fluid into and extracting it from underground reservoirs.
The practice of EGS in the United States is still under the experimental phase. The Frontier Observatory for Research in Geothermal Energy (FORGE) is the only active site for EGS research and development, and it is still being mostly used to test the potential of EGS instead of widespread utility implementation.
When fluid is injected, it increases pore pressure which reduces the effective stress clamping faults together. This pore pressure diffusion is one of the main mechanisms triggering injection-induced earthquakes. Temperature changes from injecting cold fluids into hot rock also play a role by causing thermoelastic stress changes.
Additionally, geochemical reactions between injected fluids and reservoir rocks can weaken faults through stress corrosion. The mechanisms of injection-induced seismicity are complex, involving coupled thermo-hydro-mechanical-chemical (THMC) processes, making it difficult to calculate.
While most injection-induced earthquakes tend to be small (below magnitude 3), occasionally larger events do occur, such as a magnitude 5.5 quake in Pohang, South Korea in 2017 associated with an EGS project. Concerns about such damaging induced seismicity have led to some EGS projects being canceled in the past. In the aftermath of the Pohang earthquake, scientists worked to develop solutions to mitigate risks in the future. They found that pressure from fluid injections had activated a previously unknown fault, leading to the large quake. To address this, researchers proposed improved monitoring techniques to detect hidden faults, along with advanced models to predict how injections alter subsurface stresses.
They also recommended implementing adaptive “traffic light” systems that can rapidly modify injection strategies if worrying seismic activity emerges. With such efforts, the goal is to enable safer EGS development going forward, as EGS has been widely shown to be the leader in the next generation of geothermal applications.
Aside from induced seismic activity water use, geothermal power plants can often require significant amounts of water for cooling and steam production too, which can strain local water resources, even though many geothermal power plants use close-loops to encourage water circulation.
Plus geothermal plants require substantial land for drilling and infrastructure, which can disrupt local ecosystems, biodiversity and communities; however, research has suggested that such land use is still relatively insignificant compared to development in oil and natural gas.
The upfront costs of geothermal projects, particularly for exploration and drilling, can also be significant; IRENA estimates that it costs $1,870 to $5,050 per kilowatt (kW) to construct a geothermal plant, with the cost heavily dependent on the geological conditions of the sites themselves. In comparison, larger wind farms cost only about $1,500 per kW to construct. This can sometimes make it harder to attract investment compared to more other renewable sources like wind and solar.
Geothermal Energy’s Current Status in The U.S
Currently, geothermal energy is hugely underutilized in the U.S., with the vast majority of heating demand—about 60%—being met by fossil fuels such as natural gas, propane, and oil, which accounts for 40% of total greenhouse gas emissions in the country. Geothermal heating systems, including ground-source heat pumps and district heating, account for only a small fraction of the U.S. heating market; according to the IEA, there are only 1.7 million geothermal heat pumps are used across the United States, and only 40% of them are used for residential purposes, which is 0.5% of all American households. The Department of Energy, using money allocated from the Inflation Reduction Act, has promised to install 28 million geothermal heat pumps across the country.
According to the U.S. Department of Energy, almost 4 gigawatts of electricity is produced using geothermal technologies, which is enough to power 3 million homes. While this is only 0.4% of all electricity generation, it remains the most amount of geothermal electricity generated in the world, with most of the current geothermal power plants located in California and Nevada.
The U.S. government has been seizing the opportunity of renewable energy development to aggressively expand the development of geothermal power plants in recent years. This process was enacted using two different means, both through congressional legislation and through investment plans from the federal government.
In Congress, after the passage of a similar bill in the House of Representatives, a group of bipartisan Senators have introduced the Geothermal Energy Optimization (GEO) Act of 2024 to streamline the permitting process in the same way it was done in the past to oil and gas permitting.
The current permit process, which follows strict guidelines, would result in reviews from multiple agencies and takes nearly a decade before any construction could even begin. The proposed changes in the bill will establish geothermal inspectors (ombudsmen) and strike teams, providing technical assistance and mediation for dispute resolution, ensuring efficient project development, as well as setting new targets for geothermal leasing on federal land, requiring the Bureau of Land Management to hold leasing auctions more frequently.
While most energy projects on federal lands, managed by the Department of Interior’s Bureau of Land Management (BLM) are strictly reviewed and regulated, the BLM has recently expedited the process of geothermal permitting through categorical exclusion of the National Environmental Policy Act (NEPA). In their press release to expedite geothermal permitting on federal lands, Tracy Stone-Manning, the director of BLM, called the technology “one of the technologies that can move our country toward a clean energy future.”
Following measures from Congress and the BLM, the Department of Energy has also released a new report to assess the practicality of massive commercial expansion of geothermal energy in the United States. With the money provided through the Inflation Reduction Act and the Bipartisan Infrastructure Law, the Department estimates that there will be potential to expand geothermal power to 90 gigawatts by 2050, a 22-fold increase from current capacity.
To reach what the report describes as a “liftoff,” the department needs to demonstrate market potential through successful deployment in greenfield conditions and validation projects across 5-10 different geologic settings using 20-25 billion dollars of startup investment. Once the startup investment is proven to be successful, large-scale geothermal projects that could utilize hundreds of billions of dollars for more widespread usage will be constructed across the country.
Global Hotspots: How Countries are Harnessing Geothermal Power
Kenya is a leader in geothermal energy in Africa and is one of the top producers globally. The Olkaria Geothermal Plant, part of the Hell’s Gate National Park, is a major facility contributing to the national grid, providing about 40% of the country’s electricity. The East African Rift System provides significant geothermal potential in the region.
Ethiopia is exploring its geothermal resources, particularly in the East African Rift. The Aluto-Langano geothermal plant has been operational since 1998, and more projects are in development to harness the country’s geothermal potential.
Italy has been the home to the world’s first geothermal power plant in Larderello, and it continues to harness geothermal energy for its energy grid, producing around 4% of its electricity from geothermal sources.
Many countries in Latin America is also looking to geothermal energy. For example, Mexico is a significant player, primarily in the Cerro Prieto Geothermal Power Station, one of the largest geothermal plants in the world. The country is actively expanding its geothermal projects as part of its commitment to renewable energy.
Chile is exploring its geothermal potential, particularly in the Andean region. While still developing its geothermal resources, the country has great potential due to its volcanic activity.
Australia is beginning to tap into its geothermal potential, especially in regions like the Cooper Basin and the hot rocks geothermal resource in South Australia. Although geothermal energy contributes minimally to Australia’s energy mix currently, there is significant interest and potential for growth, particularly with enhanced geothermal systems (EGS).
Globally, geothermal energy is gaining traction, proving its worth as a reliable renewable source.
Geothermal’s Future
Geothermal energy is poised to emerge as a crucial player in the global transition to clean energy. With its vast potential to significantly reduce greenhouse gas emissions, geothermal stands out as a reliable and sustainable resource. Unlike intermittent sources like wind and solar, geothermal offers consistent, base-load power generation, providing a stable energy supply regardless of weather conditions.
As technology advances and extraction methods improve, geothermal energy could play an even more pivotal role in diversifying the renewable energy portfolio. The ongoing development of Enhanced Geothermal Systems (EGS) opens up new possibilities, allowing us to tap into previously inaccessible geothermal resources, but it also requires drafting careful safety guidelines so it could prevent worsening seismic activity.
However, realizing this potential requires not only technological innovation but also supportive policies and investments. By fostering a regulatory environment that encourages environmentally sound geothermal exploration and development, governments can harness this underutilized resource.
With the right commitment and collaboration, geothermal energy could become as indispensable as wind and solar in our fight against climate change, driving us toward a cleaner, more sustainable future. The time to invest in geothermal is now—let’s unlock this powerful resource for generations to come.
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