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What is Geothermal Energy? How Does it Work?

Geothermal energy is a type of renewable energy taken from the Earth’s core. It comes from heat generated during the original formation of the planet and the radioactive decay of materials. This thermal energy is stored in rocks and fluids in the centre of the earth.

Geothermal Energy - Geyser EruptingThe difference between the temperature in the earth’s core and the surface drives a continuous conduction of thermal energy from the centre to the exterior of the planet.

High temperatures of over 4000°C cause some of the rock in the centre of the Earth to melt and form hot molten rocks called magma. These heats also cause the mantle to behave plastically and portions of it to convect upwards, since it is lighter than the surrounding rock. 

The rock and water in the Earth’s crust can reach heats of around 370°C.

Thermal energy contained in the rocks and fluids can be found from shallow depths right down to several miles below the Earth’s surface.


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The Meaning of Geothermal Energy

The word ‘geothermal’ has Greek roots with γη (geo), meaning earth, and θερμος (thermos), meaning hot.

How is it Used?

It has been used in some countries for thousands of years for cooking and in heating systems. The underground geothermal reservoirs of steam and heated water can be used for electricity generation and other heating and cooling applications.

One example of heating and cooling at a constant temperature is the installation of a geothermal heat pump about 10 feet underground. These pipes are filled with water or an antifreeze solution.  The water is pumped around the closed loop of pipes. These ground source heat pump systems help to cool buildings in summer and maintain warmth in summer. This occurs by absorbing the earth’s heat as the water and steam circulates back into the building.

Geothermal water has been used to help grow plants in greenhouses, for district heating in homes and businesses. It can also be piped under roads to melt snow.

How is Geothermal Energy Produced?

Wells of up to a mile deep or more are drilled into underground reservoirs to tap into the geothermal resources. These resources can be exploited from naturally occurring heat, rock and water permeability or through enhanced geothermal systems, which enhance or create geothermal resources through a process called hydraulic stimulation. These geothermal resources, whether natural or enhanced, drive turbines linked to electricity generators.

The first recorded instance of geothermal heat being used for producing electricity was in Larderello, Italy in 1904. Yet, geothermal heat has been used for bathing since the Palaeolithic Age. Monkeys in Japan have also been shown to use heated water from hot springs to keep warm during winter months in mountainous regions.

How Does Geothermal Energy Work?

Geothermal power plants come in three different designs; dry steam, flash and binary:

  • The oldest type is dry steam, which takes steam directly from fractures in the ground to drive a turbine.
  • Flash plants pull high pressure hot water from underground and mix it with cooler low pressure water. This, in turn, creates steam that is used to drive a turbine.
  • Binary plants use hot water passed through a secondary fluid that has a lower boiling point than water. The secondary fluid is turned into vapour which drives a turbine. Most future geothermal power plants are expected to be binary plants.

The United States is the world’s largest producer. They also have the largest geothermal development in the world, situated at The Geysers north of San Francisco, California. Despite the name, there are no geysers there and the energy used is all steam rather than hot water.

The first power generation well was sunk in 1924, with more wells drilled in the 1950s and further development taking place from the 1970s on.

Other nations, such as Iceland, are well placed to exploit geothermal resources, which they have done since 1907. With 25 active volcanoes and 600 hot springs, 25% of Iceland’s energy comes from five geothermal power plants.

What are the Advantages and Disadvantages?


As a source of renewable energy, the main advantages of geothermal energy are environmental. It produces just one-sixth of the carbon dioxide emitted by a clean natural-gas power plant.

Geothermal is also cheaper than conventional energy, with savings of as much as 80% compared with fossil fuels.

Unlike other renewable energy sources, such as solar and wind, it is constantly available.


Despite being inexpensive, sustainable and environmentally-friendly, geothermal is not without its drawbacks.

Firstly, production is limited to areas near tectonic plate boundaries. In addition, some locations may cool down after decades of use.

Although it is cheaper than fossil fuels once a plant has been built, the drilling and exploration of these sites is expensive. This is in part due to the amount of wear experienced by drills and other tools in such aggressive environments.

Geothermal plants can release hydrogen sulphide, a gas that smells like rotten eggs. Finally, some geothermal fluids contain low levels of toxic materials which need to be disposed of.

Geothermal and TWI

Due to TWI’s expertise in coatings and material properties, in addition to our extensive work in oil and gas, we are perfectly placed to use this expertise in the field.

For example, TWI’s knowledge of wear-resistant materials provides potential solutions for geothermal drilling. In addition, our knowledge of structural integrity and integrity management, as well as our expertise in joining have direct applications to geothermal projects.

TWI has already been working on a number of projects alongside some of the world’s leading geothermal technology and energy businesses.


Geothermal energy comes from heat in the Earth's core which was generated by the radioactive decay of materials when the planet formed. This thermal energy, stored by rocks and fluids at the centre of the Earth, can be used as a renewable energy resource.

This clean and renewable energy resource is cheaper than many alternatives, while geothermal energy also has the benefit of always being available, unlike resources such as solar and wind.

Frequently Asked Questions (FAQs)

Where can geothermal energy be used?

Geothermal energy can be used in any location with a geothermal resource, such as areas with hot springs, geysers, and volcanic activity. Some countries, such as Iceland and New Zealand, have a higher concentration of geothermal resources and utilise this energy source for a significant portion of their electricity needs.

What environmental effects does geothermal energy have?

Compared to fossil fuels, geothermal energy has a much smaller carbon footprint and does not release harmful pollutants into the air. However, the drilling and injection of fluids into the earth can cause small earthquakes and the release of geothermal fluids can contain trace amounts of harmful chemicals.

How reliable is geothermal energy?

Geothermal energy is a reliable source of energy as it is not dependent on weather conditions like solar and wind energy. Geothermal power plants can operate 24/7 and have a high capacity factor, which means they can generate electricity at a high percentage of their maximum capacity.

How does geothermal energy compare to other forms of energy?

Geothermal energy is a renewable energy source that has a smaller carbon footprint than fossil fuels. It is also more reliable than solar and wind energy as it is not dependent on weather conditions. However, the initial costs of building a geothermal power plant can be higher than other forms of renewable energy.

Is geothermal energy safe?

Geothermal energy is generally considered safe for the environment and human health. However, the drilling and injection of fluids into the earth can cause small earthquakes and the release of geothermal fluids can contain trace amounts of harmful chemicals. Proper monitoring and management practices can minimise these risks.

Is geothermal energy good for the earth?

Geothermal energy is a renewable energy source that has a much smaller carbon footprint than fossil fuels. It also has the potential to reduce greenhouse gas emissions and mitigate the effects of climate change. However, the drilling and injection of fluids into the earth can have some environmental impacts, so it is important to manage these risks carefully.

What are geothermal heat exchangers?

Heat exchangers in geothermal systems are used to transfer heat between the ground and the fluid circulating through the system. The fluid absorbs heat from the ground through the heat exchanger and then transfers that heat to the heat pump, which then distributes it throughout the building. There are different types of heat exchangers used in geothermal systems, such as closed-loop and open-loop systems. Closed-loop systems use a fluid that circulates through a closed loop of pipes buried underground, while open-loop systems use groundwater as the heat exchange fluid.

More Information

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