What is geothermal energy and how does it work?

Geothermal energy is a renewable source that harnesses the Earth’s internal heat to generate electricity and provide heating. Unlike solar or wind power, geothermal energy offers a constant and dependable output, making it an attractive option for a sustainable energy future. In the United Kingdom, interest in geothermal energy has grown in recent years, particularly in areas such as Cornwall and parts of the Midlands, where geological conditions may allow for viable geothermal projects.

This energy exists in the form of hot water or steam beneath the Earth’s surface. Through modern technology, this heat can be accessed and converted into useful energy. Geothermal energy is both clean and efficient, offering a way to reduce reliance on fossil fuels while helping to cut greenhouse gas emissions.

Although geothermal energy is still a developing sector in the UK, government bodies, universities, and private companies have begun to explore its potential. Understanding how geothermal energy works, its benefits and limitations, and how it fits into the UK’s broader energy strategy is key to evaluating its future role.

The Origin and Use of Earth’s Internal Heat

The heat used to generate geothermal energy originates from two main sources: the residual heat from the Earth’s formation and the ongoing radioactive decay of elements such as uranium, thorium, and potassium deep within the planet.

In the UK, potential geothermal resources are primarily located in regions with favourable geology, such as Cornwall, which is home to granite formations capable of retaining heat. The United Downs Deep Geothermal Power Project in Cornwall, for example, is the most advanced geothermal project in the country and serves as a pilot for future developments.

To harness this energy, deep wells are drilled into the Earth to access reservoirs of hot water or steam. Once extracted, the thermal energy is brought to the surface and used either directly for heating or to generate electricity through specially designed power plants.

Historically, hot springs and spas, such as those in Bath, have made use of geothermal heat for recreational and therapeutic purposes. Today, the focus has shifted towards using this naturally occurring resource as part of a decarbonised, modern energy system.

How Geothermal Energy Works

Geothermal energy systems operate by tapping into naturally heated underground reservoirs. In the UK, locating these reservoirs involves geological surveying, drilling test boreholes, and conducting heat flow analysis. Once a suitable site is confirmed, deep wells—often over 4,000 metres—are drilled to access water heated by the Earth’s core.

Depending on the temperature and pressure of the reservoir, different methods are used to convert the heat into energy. In high-temperature systems, hot steam can be used directly to turn turbines and generate electricity. In lower-temperature regions, the heat is transferred to a secondary fluid with a lower boiling point in a binary cycle plant, where it evaporates and drives a turbine.

The cooled water is then reinjected into the ground through a separate well to maintain reservoir pressure and ensure long-term sustainability. This closed-loop system makes geothermal energy particularly environmentally friendly.

In the UK context, most existing and proposed applications focus on direct-use geothermal energy rather than electricity generation. This includes district heating networks, which supply heat to homes, businesses, and public buildings through underground pipes. Cities such as Southampton already operate geothermal district heating schemes, demonstrating its feasibility on British soil.

Types of Geothermal Power Plants and Applications

There are three main types of geothermal power plants, each suited to specific geological and thermal conditions: dry steam, flash steam, and binary cycle. In the UK, binary cycle plants are the most appropriate due to the moderate underground temperatures typical of the region.

Dry steam plants use underground steam directly to drive turbines. This is the simplest form of geothermal generation but is only suitable in locations with naturally occurring steam fields, which are not present in the UK.

Flash steam plants involve pumping high-pressure hot water to the surface, where a drop in pressure causes the water to vaporise into steam, which then powers a turbine. Again, this requires very high subsurface temperatures, which are uncommon in most parts of Britain.

Binary cycle plants, the most feasible option for the UK, use a heat exchanger to transfer heat from the geothermal water to a secondary fluid with a lower boiling point. This fluid vaporises and drives the turbine without direct contact with the geothermal fluid. These systems are highly efficient and can operate with lower-temperature resources.

Beyond power generation, geothermal energy is used for space heating, greenhouses, aquaculture, and swimming pools. In Britain, the focus is primarily on district heating and ground-source heat pumps, particularly in urban developments aiming to reduce carbon emissions. The potential to combine geothermal heat with other technologies, such as solar thermal or heat storage, adds further value to the system.

Advantages of Geothermal Energy Over Other Sources

Geothermal energy offers several distinct advantages that make it a compelling addition to the UK’s renewable energy portfolio. One of its key strengths is reliability. Unlike solar or wind, geothermal systems provide a constant energy output, offering baseload capacity that enhances grid stability.

Another important benefit is its low environmental impact. Emissions from geothermal plants are minimal, especially in closed-loop systems where the geothermal fluids are reinjected into the Earth. The land footprint is also small, which is beneficial in a country with dense population centres and competing land uses.

From an economic perspective, geothermal systems have low operational costs once installed. They require no fuel, are not subject to price volatility, and deliver predictable returns over time. For district heating schemes, in particular, geothermal offers a cost-effective long-term solution.

Geothermal energy also aligns with the UK’s climate commitments under the Net Zero by 2050 strategy. By replacing gas boilers and other fossil-based heating systems, geothermal can play a crucial role in decarbonising the heating sector, which currently accounts for a significant portion of the country’s emissions.

Finally, the sector offers job creation opportunities in drilling, construction, plant operation, and maintenance, particularly in former industrial regions seeking sustainable redevelopment.

Disadvantages and Current Limitations

Despite its many benefits, geothermal energy in the UK faces several challenges that have limited its widespread adoption. One of the main issues is the high initial investment required for exploration, drilling, and system installation. These upfront costs are especially significant for deep geothermal projects, where returns are realised only in the long term.

Geological uncertainty is another factor. While areas like Cornwall and parts of northern England have potential, the geological data across much of the UK is incomplete. This makes exploration risky and deters private investment without government support or public-private partnerships.

From a policy standpoint, geothermal has historically received less attention and fewer subsidies compared to other renewables such as wind and solar. This lack of visibility has slowed technological development and limited public awareness of its potential benefits.

Environmental concerns are minimal but do exist. Deep drilling can, in rare cases, cause minor seismic activity. Additionally, poor management of fluids can lead to contamination risks, though modern systems are designed to mitigate these issues.

Finally, the limited number of trained professionals in geothermal engineering and system design poses a challenge. Expanding education and vocational training in this area will be necessary to support the sector’s growth.

Geothermal Energy in the UK: Current Status and Outlook

Geothermal energy is still an emerging industry in the UK, but momentum is building. Projects such as the United Downs Deep Geothermal Power and the Eden Geothermal project in Cornwall have demonstrated the feasibility of producing both electricity and heat from the country’s subsurface resources.

In Southampton, a geothermal well has been supplying a district heating network since the 1980s, making it one of the earliest adopters in the UK. More recently, the Seaham Garden Village project in County Durham is planning to integrate geothermal heat into a new community’s energy system.

Government support has increased through programmes like the Green Heat Network Fund, which provides capital funding for low-carbon heating technologies, including geothermal. Local councils and combined authorities are also beginning to include geothermal feasibility studies in their climate action plans.

Looking forward, experts suggest that the UK could develop up to 10GW of geothermal heating capacity, significantly contributing to national decarbonisation targets. To achieve this, the sector will need long-term investment, streamlined planning permissions, and stronger collaboration between academia, industry, and government.

The Global Future of Geothermal Energy

Globally, geothermal energy is experiencing renewed attention as countries seek dependable and low-emission energy sources. Nations such as Iceland, Italy, Indonesia, and Kenya are leading the way, using geothermal to supply large portions of their electricity and heating needs.

Technological innovations like Enhanced Geothermal Systems (EGS) are opening new possibilities. These systems allow for heat extraction even in regions without naturally permeable rock or underground water, vastly expanding the number of suitable locations for geothermal projects.

For the UK, these advances are particularly promising, as they may enable geothermal development beyond Cornwall and other high-potential zones. If the technology becomes commercially viable, it could transform geothermal into a nationwide solution.

International organisations like the International Energy Agency (IEA) and Geothermal Rising are promoting geothermal development through policy frameworks, funding mechanisms, and knowledge sharing. The UK stands to benefit by collaborating on research and adopting best practices from more mature geothermal markets.

In an increasingly electrified and decentralised energy landscape, geothermal energy provides a consistent, scalable, and low-carbon option that complements intermittent renewables. With the right strategy, it could become a vital part of the UK’s clean energy mix.

Frequently Asked Questions (FAQ)

1. How is geothermal energy different from other renewables?
Geothermal provides continuous, reliable energy regardless of weather, making it ideal for stable, baseload electricity and heating.

2. Can geothermal energy be used in UK homes?
Yes. Ground-source heat pumps are increasingly popular for heating and cooling homes efficiently, especially in new-build developments.

3. Is geothermal energy environmentally safe?
Yes. Emissions are minimal, and closed-loop systems reinject fluids to maintain environmental balance and avoid surface contamination.

4. Where in the UK has the most geothermal potential?
Cornwall, northern England, and parts of the Midlands are the most promising regions for deep geothermal development.

5. What is needed for geothermal energy to grow in the UK?
Greater investment, clearer policy support, expanded geological surveys, and training for specialists in geothermal system design and operation.