Geothermal Energy Explained

Geothermal energy is a renewable and sustainable energy source that is still largely untapped. Read about the different types of geothermal plants and which countries are installing the most capacity. Also, why progress has stalled, including the risk of earthquakes associated with the most promising developments in geothermal engineering.
rafla geothermal power station Iceland
Krafla Geothermal Power Station, Iceland, has operated since 1977. Photo: CC BY-SA 3.0

Geothermal energy is a form of renewable energy which has a relatively small carbon footprint. That makes it an attractive power source for the future. There is no doubt that geothermal will play a greater role in limiting climate change as we transition to sustainable energy sources, but how soon and to what extent, is still unclear.

The Grand Prismatic Hot Spring at Yellowstone park
The Grand Prismatic Hot Spring at Yellowstone park is the world’s third largest hot spring. The band of colors yellow, oranges and blue come from the heat loving thermophile bacteria living around the spring water. Photo: Brocken Inaglory/CC BY-SA 3.0

What is Geothermal Energy?

If you were to drill a hole straight down into the Earth, you would discover that the temperature gets hotter the deeper you go. This is because Earth is full of heat, and that heat is called geothermal energy. Geo– means ‘Earth’ and thermal means ‘connected with heat’.

Geothermal heat can be found at relatively shallow depths but becomes extremely hot when we reach molten rock called magma. At it’s core, Planet Earth is an estimated 5,500 °C – about as hot as the surface of the sun.

Geothermal energy has been used for thousands of years by the Romans, Chinese and Native Americans for the purpose of heating, cooking and bathing in hot springs.

Currently, we only have the engineering capability to tap into the upper lithosphere – a few miles down. But that’s usually deep enough, at least in certain locations!

Geothermal energy can be used by humans in two ways. Either directly – with no transformation – for heating. Or it can be used to generate electricity by means of a geothermal power plant.

Below are two diagrams to explain how both systems work. However, for the remainder of this article we will focus our attention on geothermal power plants.

(a) Direct Heat Using Geothermal Heat Pumps

These devices pump warm water from below the ground surface into homes, buildings and swimming pools. Some municipals even have pipes under public paving to keep them ice-free in winter. In the summer, some heat pumps act as air conditioning and cool buildings by running in reverse.

Diagram: How heat pumps work in a house
(1) Water or a refrigerant moves through the pipes. (2) When the weather is cold, the liquid heats up as it travels through the part of the pipe that is buried underground. (3) Once it gets back above ground, the warmed liquid transfers heat into the building. (4) The liquid cools down after its heat is transferred. It is pumped back underground where it heats up once more, starting the process again. (5) On a hot day, the system can run in reverse. The liquid cools the building and then is pumped underground where extra heat is transferred to the ground around the pipes. Image: EPA.gov

(b) Electricity From Geothermal Power Plants

Boreholes or wells are drilled up to 2 miles deep into ground. Hot water or steam rises through these pipes to the surface which is then used to turn a turbine and generate electricity.

What is a Geothermal Power Plant?

It’s a plant which uses geothermal energy to generate electricity. Hot water is extracted through wells from up to 2 miles deep in the Earth’s crust. Geothermal plants essentially work in the same way as a coal or nuclear power plant, the main difference being in the heat source. Geothermal replaces the boiler in a coal plant and the reactor in a nuclear power station.

Geothermal is a mature generation of technology. Like hydropower, and organic biomass, geothermal is a competitive form of ‘always on’ energy. In contrast, solar energy and wind power are intermittent sources of energy which are vulnerable to shifting weather patterns. When developed, renewable marine energies such as wave power and tidal power are likely to be ‘always on’. Another next-generation energy source is hydrogen power, which can also be stored. Green hydrogen in particular is attracting attention.

There are certain regions of the world which are more attractive for the generation of geothermal electricity. Usually those locations are near the boundaries of tectonic plates where hot magma is close to the surface. For example, the Puna Geothermal Venture (PGV) plant on Hawaii’s Big Island near Pahoa.

Geothermal plants are often built along the so-called Ring of Fire. A horseshoe-shaped area around the Pacific Ocean, it experiences a lot of volcanic eruptions and earthquakes. As geothermal power plants require very hot temperatures (300°F to 700°F), anywhere near an area with lots of volcanic activity, hot springs or geysers (vents in the Earth’s surface which erupt boiling water) is a possible candidate. They are also found in mid-oceanic ridges, such as Iceland and the Azores and rift valleys such as the East African Rift.

Where Geothermal Energy Is Found Map
Global map of where geothermal energy is used. Source: IGA International Geothermal Association.

Top 10 Geothermal Countries by Installed Electricity Capacity (MW)

CountryCountry Capacity (MW)
USA3676
Indonesia2133
Philippines1918
Turkey1526
New Zealand 1005
Mexico962.7
Italy944
Kenya 861
Iceland 755
Japan 601
The Top 10 Geothermal Countries 2019 – based on installed electric generation capacity (MWe). Source: ThinkGeoEnergy – Geothermal Energy News

In 2020, geothermal power plants were operating in 29 countries with a total installed power generation capacity of 15.4 GW at the year-end 2019. 1.

Geothermal has a significant share of electricity demand in countries like Kenya, Iceland, Philippines, El Salvador, New Zealand and Kenya. It also provided more than 90 percent of heating demand in Iceland. 2

Top 10 Countries With Highest Share of Electricity Generation by Geothermal

CountryShare of National Generation %
Kenya 51.0
Iceland30.0
Philippines 27.0
El Salvador25.0
New Zealand 14.5
Costa Rica 14.0
Nicaragua 9.9
Indonesia3.7
Mexico 3.0
Italy 1.5
USA0.3
Source: 2020 www.statista.com
Installed Geothermal Capacity in 2019

Types of Geothermal Power Plants

The heat available in geothermal field will determine the type of technology used. Geothermal power generation currently is based on the following 3 technology options:

(1) Flash Steam Power Plants

This is the most common type of geothermal plant. Flash plants use geothermal reservoirs of water with temperatures greater than 182°C (360°F). This very hot water naturally flows up the wells in the ground under its own pressure. As the water moves upward, the pressure decreases and some of the hot water turns into steam.

The steam is then separated from the water and used to power a turbine/generator to produce electricity. Any water and condensed steam leftover is injected back into the reservoir, making it a highly sustainable method of production.

(2) Binary Cycle Power Plants

Binary cycle power plants operate where reservoir water temperatures are lower, about 107°-182°C (225°-360°F). They are different to their flash and dry steam counterparts, in that the water or steam which is extracted does not come into contact with the turbine. Instead the heat is transferred to a ‘working fluid’, often butane, which vaporizes (flashes) at lower temperatures.

Pressure from the vapor rotates the turbines and triggers electricity production. The water is injected back into the ground to be reheated and the process repeated. Meanwhile the fluids condense back into liquid and are collected and reused. It is important to note that the reservoir water and liquids never come into contact to ensure there is no emissions and risk of contamination to the environment.

(3) Dry Steam Plants

Dry steam power plants are the least common type of geothermal plant. They are located in areas than can directly draw underground resources of steam, which are hard to find. The steam is pumped from underground wells into a turbine/generator.

Dry steam plants are the oldest types of geothermal power plants in the world – the first one was built back in 1904 in Italy.

In the United States, there are only two underground resources of steam that we know about: The Geysers in northern California and Yellowstone National Park in Wyoming, where there is a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plant in the country is The Geysers which was first drilled in 1924.

Dry steam power plants typically have higher efficiencies than other types of geothermal power plants due to high temperature sources. However, use is limited due to lack of sources. Additionally, running hot steam from the earth increases the corrosion on turbine blades which increase operation expenses.

The Status of Geothermal

Geothermal is a minor player in world energy production, accounting for less than 1 percent of global energy output. 3 It has been chugging along in the background for decades but was eclipsed by solar and wind power back in early 2000s. So it remains a ‘niche’ energy source, except in certain countries such as Kenya, Iceland and New Zealand where it plays a major role in electricity power generation.

Globally, geothermal electricity generation increased by an estimated 3 percent in 2019, reaching 15.4 gigawatts installed capacity. This was less than the average growth of the 5 previous years. 4 1

Most of this growth was in emerging economies because they have abundant and untapped resource availability. However, according to the IEA’s Sustainable Development Scenario (SDS), an annual growth rate of 10 percent is required to meet climate change targets by 2030. We are way off track.

Geothermal Energy: Where we are, and where we need to be

Geothermal Growth Projections Chart
IEA’s Sustainable Development Scenario (SDS) with reference to geothermal energy. The dark blue bars represent the targets (in TWh) that geothermal energy needs to achieve in 2025 and 2030.

Limitations of Geothermal Power

So what’s the problem, why is geothermal not keeping up with other renewables? After all, it has considerable potential for growth.

The amount of heat within 10,000 meters of the earth’s surface contains an estimated 50,000 times more energy than all oil and gas resources worldwide 5. According to the ARPA-E project AltaRock Energy, just 0.1 percent of earth’s thermal heat could supply humanity’s total energy needs for 2 million years.

All we need to do is tap into it. But that’s where the problems begin.

Heat Can’t Be Stored or Transported

Geothermal heat cannot be transported or stored; it must be used where it is found. Reservoirs above 100°C are usually necessary for most large geothermal plants, and these reservoirs are only found in specific locations, usually near tectonic plate boundaries or hot spots far away from urban centers. There are few places where geothermal energy is as readily accessible as Iceland. Geothermal resources in places such as the Kamchatka (Russia), Andes (South America) and Indonesia remain largely unexploited for this reason.

It’s Too Expensive

The cost of deploying a geothermal power plant is three times the price of a wind or solar farm. 6. Even compared to combined-cycle gas plants, geothermal energy is 4 to 6 times as expensive initially. The cost is mainly upfront and is associated with the difficulties and cost of research and drilling. Like oil, these costs sometimes dampen investors, not to mention the fact that the exploration phase has a significant potential for failure.

Less Efficient Over Time

Another important reason why geothermal energy remains underdeveloped, is due to the technical problem of silica scaling. 7.

Water pumped from beneath the earth contains high levels of dissolved minerals – mainly silica, or silicon dioxide – which can clog pipes much in the same way limescale builds up in a domestic kettle. This problem frequently causes disruption of electric generation, especially for binary cycle systems.

One study tracked the effects of silica scaling in a geothermal power plant located in Turkey. 8 The results revealed a continual drop of performance of the power plant over time. A 270 kW decrease was recorded in 2009 and by 2012, the decline was 760 kW. A great deal of research has been conducted to address the silica scaling problem with little success.

Geothermal Sites Cool Down Over Time

Although geothermal sites are capable of providing heat for many decades, eventually specific locations may cool down. In some cases, plants experience significant reductions in power output due to the depletion of their reservoirs. This is mostly the case where initial studies overestimated the reservoir potential, perhaps driven by the desire for a fast recovery of investment.

Risk of Earthquakes

Geothermal energy runs the risk of triggering earthquakes, or seismic disturbances called seismicity. This is due to alterations in the Earth’s structure as a result of digging.

This problem is more common with the emergence of enhanced geothermal system (EGS) systems which force water into the ground to purposely cause fissures (cracks). This type of operation – similar to fracking in the oil and gas industry – is highly controversial, and has subsequently been linked to a 5.5 magnitude earthquake in Pohang, South Korea in 2017. More on this below.

Eruptions

There is a risk of hydrothermal eruption, much in the same way there is a risk of explosion at gas or oil refineries. Consider that geothermal plants are often built on high-grade geothermal hot water deposits. 9. However, this risk is considered minimal and danger to life is minimized because most plants are built far from large population areas.

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The Benefits of Geothermal Energy

Continuous Energy, Not Intermittent

Geothermal energy is ‘always on’, available 365 days a year. It is thus considered to have a ‘high load factor’. It is not weather dependent like other renewables such as solar and wind power, which require the sun to shine and the wind to blow. Geothermal heat energy is continuous rather than intermittent. For this reason, is is capable of supplying base-load electricity, or electricity without interruption. It also means that it has a higher capacity factor. Each megawatt (Mw) of capacity generates significantly more electricity during a year than a megawatt of solar or wind capacity.

Sustainable

As a resource which is naturally replenished on a human time-scale, geothermal energy is a renewable and sustainable energy. It is not impacted by global depletion of resources or rising fossil fuel prices. Unlike oil and gas reserves which could be gone in the next 50 years, and coal in the next 100 years – Earth’s internal heat will not run out for another estimated 5 billion years, when the Earth itself expires. 10 11

Low Environment Impact

The environmental impact of geothermal development includes changes in land use associated with exploration and plant construction. There is also noise and visual pollution. There can also be foul odors due to the release of hydrogen sulfide, a gas that smells like rotten egg at low concentrations. Another concern is the disposal of some geothermal fluids, which may contain low levels of toxic materials. Most of these effects, however, can be mitigated with current technology so that geothermal uses have a minimal impact on the environment.

No Air Pollution

The most visible of all the benefits of renewable energy, is a reduction in air pollution of all types. The energy sector – including its production, transportation and use – is the largest source of man-made air pollution.

Compared to coal as an electrical power source, geothermal power plant emissions differ significantly, not only in terms of carbon dioxide emissions, but also in terms of a wide range of other air pollutants such as particulate matter (PM2.5) and nitrogen oxides.

Overall, geothermal power plants emit less than 1 percent of the air pollutants emitted by coal-fired power plants of equal capacity. 12

Is Geothermal Energy Good for Climate Change?

The first thing to realize is that completely clean energy does not exist. The only clean energy is the one we do not use. Even energy generated from a renewable source like thermal heat, wind or sun will have a carbon footprint, to some degree.

Greenhouse gases are emitted in the exploration, drilling, manufacturing and installation of a power plant – whether that plant is intended to operate on renewable or fossil fuel resources. These emissions are routinely calculated during the so-called ‘life cycle assessment’ (LCA) of a plant or factory.

Life Cycle Assessment

One recent LCA of a double flash geothermal plant in Iceland, took a cradle-to-grave overview. 13 The analysis identified the main hot spots, such as the consumption of diesel for drilling, the use of steel for wells casing and the construction of the power plant.

The comparison showed that the carbon intensity of Hellisheiði is in the range of 15–24 g CO2-eq./kWh, which is similar to that of solar photovoltaic and hydropower, lower than other geothermal technologies and fossil-based technologies, but higher than nuclear and onshore wind.

Is Nuclear Energy A Replacement for Fossil Fuels?

Greenhouse Gas Emissions in Production

The geothermal sector is generally very low in greenhouse gas emissions, once a plant is up and running. However, as the sector has expanded, a wider range of geothermal resources have been brought into exploitation, and in rare instances, geothermal plants built in certain geological locations with different chemistry and thermodynamic conditions, can unwittingly release significant quantities greenhouse gases into the atmosphere. 14.

This is because the amount of greenhouse gas produced per kWh of geothermal generated electricity varies, depending on the reservoir characteristics.

Gases are naturally present in geothermal fluids. As those fluids are bought to the surface, some of those gases will be released into the atmosphere. Carbon dioxide typically constituents more than 95 percent of fluids and methane up to 1.5 percent. These are the greenhouse gases which contribute towards global warming and the climate crisis.

The New Zealand Geothermal Association reports that the average greenhouse gas emissions for geothermal power generation is 76g CO₂(equiv)/kWh.

For comparison, fossil fuel generation emissions range from 390 to 970g CO₂(equiv)/kWh for coal and gas combined cycle plants. 15

Kenya: Poster Child of Geothermal Power

Kenya has one of the most developed power sectors in sub-Saharan Africa, with an abundance of renewable energy resources, especially geothermal, wind and solar. In recent years, it has been a front runner in expanding into geothermal power. Back in 2010 fewer than 1 in 5 Kenyans had electricity. Now some 60 percent do — thanks mostly to geothermal power.

In 2004, Kenya had an installed energy capacity of 1,239 MW – of which 55 percent came from domestic hydropower, 33 percent oil fired thermal, 10 percent geothermal, and 2 imported hydropower. 16

Fast forward to 2020, and geothermal represents more than 50 percent of the electricity generated in the country. This is thanks mostly to geothermal power from the East African Rift, a 2,000 mile-long volcanic trench that’s slowly ripping the continent apart. Here you will find Hell’s Gate National Park, a steamy geothermal hotspot.

Geothermal generation – already a key component of Kenya’s energy mix – is identified as a key resource for medium- to long-term capacity additions. An estimated 656MW of geothermal generation is expected to come online by 2024. 17

Over the coming decade electricity demand in the country is projected to rise anywhere from 6 to 15 percent per year. How Kenya will meet that demand is a central question for its future.

Controversy has grown over the use of non-renewable power to rising meet demand. Recently the construction of two coal-fired plants were announced, one of which however, is already in doubt (Lamu coal-fired plant) because it sits within a Unesco-listed World Heritage Site.

Kenya aims to be an ‘upper middle-income country’ by 2030. But according to its Nationally Determined Contribution (NDC), submitted under the Paris Climate Agreement, the country is supposed to increase its carbon emissions by less than 40 percent over their 2010 levels – instead of allowing them to double, as they would under a ‘business as usual’ scenario.

Future Prospects of Geothermal Energy

One of the most promising (and controversial) developments in geothermal engineering is the emergence of the enhanced geothermal system (EGS). As there are relatively few naturally occurring places with a reservoir of hot steam and water – ESG offers an alternative.

It involves pumping water into the ground, and creating fissures to release natural heat sources in areas where it would otherwise be impossible to harness. This fracking method is highly controversial and is associated with causing tremors and even earthquakes.

New research in Seismological Research Letters linked a 5.5 magnitude earthquake in Pohang, South Korea in 2017 to ESG. 18 The pressure caused by injecting fluid at high pressure into the ground affected nearby faults, triggering the bigger quake, the panel found.

It was the country’s second strongest and most destructive quake on modern record, injuring 135 people and causing an estimated US$290 million in damage. Previously, experts had believed ESG could not cause earthquakes of that magnitude.

On the plus side, ESG technology means you can effectively put a geothermal power plant anywhere, all you have to do is drill deep enough and you will find hot rock. The International Energy Agency predicts a 5 percent annual growth in the geothermal sector through 2023. A large part of that growth is expected to be ESG.

Low Carbon Energy of the Future

The latest hope for low carbon energy is Nuclear Fusion, a costly quest for limitless ‘always-on’ power that would revolutionize our energy system.

Further Reading

IEA Geothermal Energy Technology Collaboration Programme provides a framework for international co operation on geothermal energy R&D. Information on environmental impacts of geothermal development, deep roots of volcanic geothermal systems and emerging geothermal technologies.

Annual statistica review of world energy, including renewables used in the power sector – wind, solar, biomass and geothermal.

Life Cycle Analysis of a Geothermal Power Plant: Comparison of the Environmental Performance with Other Renewable Energy Systems. Riccardo Basosi et al. February 2020.

Geothermal technology: Trends and potential role in a sustainable future. Appl. Energy 2019. Austin Anderson and Behnaz Rezaie.

References

  1. International Geothermal Association (IGA) 2020 [][]
  2. “IRENA”. International Renewable Energy Agency. []
  3. “Sustainable Sizing of Geothermal Power Plants: Appropriate Potential Assessment Methods.” Alessandro Franco and Maurizio Vaccaro. May 2020 []
  4. IEA Tracking Report on Geothermal. 2020 []
  5. IRENA []
  6. Levelized Cost of Energy Comparison. Version 12. 2018 []
  7. “Kinetics of silica precipitation in geothermal brine with seeds addition: minimizing silica scaling in a cold re-injection system.” Felix Arie Setiawan et all 2019. []
  8. ” Multiple regression analysis of performance parameters of a binary cycle geothermal power plant.” M. Karadas et al. Geothermics 2015 []
  9. “Hydrothermal eruption dynamics reflecting vertical variations in host rock geology and geothermal alteration, Champagne Pool, Wai-o-tapu, New Zealand. “ Anna Gallagher et al. 2020 []
  10. “How long before fossil fuels run out?” – OurWorldInData []
  11. “Running out? Rethinking resource depletion.” Stuart Kirsch. July 2020 []
  12. “The role of geothermal and coal in Kenya’s electricity sector and implications for sustainable development.” Nov 2019 NewClimate Institute. []
  13. “The environmental impacts and the carbon intensity of geothermal energy: A case study on the Hellisheiði plant”. Andrea Paulillo, Aberto Striolo, Paola Lettier et al. 2019 []
  14. Greenhouse gases from geothermal production. ESMAP – Energy Sector Management Assistance Program. 009/16 []
  15. “Climate explained: why does geothermal electricity count as renewable?” The Conversation. []
  16. Thinkgeoenergy.com []
  17. “The joys of springs: how Kenya could steam beyond fossil fuel” National Geographic 2019. []
  18. “South Korea accepts geothermal plant probably caused destructive quake.” Nature. 2019 []
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