What is Geothermal Energy?

Geothermal is energy stored as heat within the Earth. This can be used for heating or electricity generation. The word “geothermal” is derived from the Greek words geo (Earth) and therme (heat) – thereby summarising what geothermal is – “warm earth”.

Geothermal is a sustainable and continuous source of low carbon, renewable heat that comes naturally from the Earth. This geothermal energy can be used to heat or cool our home and buildings. It can support industry and empower our way of life.

Available 24 hours per day, 365 days per year, whatever the weather, this energy source has a small footprint and once installed, has minimal visual impact. Geothermal energy provides “always on” decarbonised heat to buildings through the transfer of stored heat from the ground to buildings, either directly or via heat pump systems.

The Earth contains a huge amount of thermal energy which was generated during its formation and continues to be generated.

A man wearing GeoEnergy NI hard hat and hi visibility jacket testing the water in a stream.
Tetra Tech hydrogeologist testing water samples

With increasing depth, the rocks in the Earth’s crust become warmer – this increase in temperature with depth is called geothermal gradient. However, this gradient is not uniform across the globe and areas with volcanic activity can have much higher heat flow at or near the Earth’s surface. For example, Iceland, Indonesia, Turkey, Philippines, and New Zealand are some of the biggest geothermal generating countries because their proximity to volcanic activity results in temperatures high enough to be able to produce steam which can be used to generate power.

Even though Northern Ireland has no active volcanoes there is still potential for geothermal energy. Other countries such as the Netherlands, France and Germany where the geology is more similar to Northern Ireland are already using this lower temperature geothermal resource. In Northern Ireland, temperature data from deep boreholes shows geothermal gradients ranging from 20 to 34°C/kilometres, with the northeast of Northern Ireland being warmer than average at the uppermost end of this range.  If we assume an average surface temperature of 10°C, it means that waters in these rocks are likely to be up to 80°C at depths of 2,000 metres, which is still relatively hot, and suitable for large-scale direct heating use.

Shallow geothermal is particularly useful in parts of the world that experience strong variations in the temperature between seasons and the difference between the air temperature and the ground can enable a ground source heat pumps to heat or cool buildings. This type of geothermal is less reliant on the specific geology and the geothermal gradient.


Sweden has the highest number of ground source heat pumps (GSHP) installed in the world.


Geothermal technology is well proven, though it is still considered a niche energy technology, particularly in areas with no active volcanic activity.  However, many governments are becoming increasingly interested in integrating this 24/7, sustainable, low carbon, renewable energy source into their energy mix to create an overall system that delivers value through money, jobs and security of supply.

In 2021, the Department for the Economy, published the Energy Strategy – Path to Net Zero Energy. This strategy sets out a path to 2030 that will mobilise the skills, technologies and behaviours needed to take us towards net zero carbon and affordable energy by 2050.  The Strategy recognises that geothermal energy has a role to play in decarbonising our energy mix and commits to bringing forward geothermal demonstrator projects.

This is where geothermal systems are extremely useful. These systems can help eliminate the need for fossil fuels and thereby significantly reduce the emissions of greenhouse gasses into the atmosphere.

For example, in Munich, 50,000 homes are already supplied heating by geothermal energy from the south German Molasse Basin, achieving carbon savings of 75,400 tonnes per year compared with natural gas heating. The Free State of Bavaria has set a goal to generate 25% of heat from geothermal energy by 2050, and Stadtwerke München, Munich’s utility provider, aims for Munich to be the first European city supplied by a 100% geothermal energy district-heating network by 2040.


Geothermal energy is not new, and it has been used by people for thousands of years for heating and cooling. The Romans were bathing in the geothermal springs in Bath, England over 2,000 years ago.

Today it is used around the world – think of Geysers in the Mayacamas Mountains in San Francisco, which produces around 10% of the total renewable energy in California. In Paris, geothermal heat has been used since 1969 and currently around 250,000 homes are heated as a result of geothermal.


Archaeological evidence from North America, Asia, and Europe indicates human use of geothermal energy dating back 10,000 years!

Heat from deep within the Earth and from the Sun is stored within the rocks, sediment and waters of the Earth’s crust. Rocks under the surface that contain significant amounts of water in the space between grains or in cracks are called aquifers. Aquifers can be found at depths of tens to thousands of metres underground. Because the rocks and the waters will be at temperatures that relate to their depth, there are geothermal resources that can be used for a range of purposes and there are different technologies available to harness the heat, most of which include drilling into the ground.

The water is then brought to the surface where the heat energy is transferred and (if necessary) warmed to the optimum temperature using a heat pump. These heat exchange processes can also operate in reverse and transfer excess heat back to the subsurface to cool buildings.

In most cases, low temperature heat from a shallow depth is used by transferring heat from the rock and soil to a circulating fluid in a pipe, which can then be extracted and further warmed to the optimum temperature using a heat pump. As there is no interaction with the groundwater held in the rock, this type of geothermal use is known as ‘closed-loop’.

A graphical representation of the different types of geothermal available, the depths at which each need to be drilled to and what they can be used for.


Many countries have successfully and safely used geothermal technologies for decades, such as Sweden, Germany and the Netherlands.

In Northern Ireland, we are fortunate to have favourable geology, with huge untapped potential for useful geothermal energy right beneath our feet. Despite this, potential geothermal energy is currently underutilised within Northern Ireland. The GeoEnergy NI project will aim to prove the potential for geothermal energy within this region, thereby allowing us to further explore the possibilities of harnessing this renewable energy source and in turn to “reduce our reliance on fossil fuels whilst becoming more self-sufficient” as outlined in the NI Executive’s Energy Strategy.

The map highlights Sherwood Sandstone as a potential geothermal aquifer across Northern Ireland

Map of Northern Ireland showing where the geothermal gradient is high and thereby has good potential for future geothermal projects.

We know that almost all of Northern Ireland has rocks and sediments at shallow depth that could be used for geothermal heating and cooling with a Ground Source Heat Pump (GSHP). Many of these rocks and sediments have high thermal conductivity making them particularly suitable. Large areas of Northern Ireland have high quality aquifers where the water could be used for geothermal. In places where the aquifer is at shallow depth, the low temperature water would require use of heat pump technology. Other areas have aquifers buried to depths of several kilometres and high geothermal gradients mean that temperatures between 70°C and 90°C have been recorded. These warmer waters can be used directly to meet larger heat demands in a number of boreholes. (R. Pasquali, N. O’Neill, D. Reay, T. Waugh, The Geothermal Potential of Northern Ireland, 2010)

One of the most attractive aquifers for geothermal is the Sherwood Sandstone. This aquifer underlies Belfast and a number of other towns and cities across a large part of northeast Northern Ireland. Because it occurs at a range of depths, the water within the aquifer could provide geothermal energy for a range heating and cooling needs.

The main opportunities for utilising this energy source within Northern Ireland lie in its heating applications, including heating of buildings and greenhouses, as well as for applications in agriculture, industry and district heating. However, currently there are very few domestic geothermal systems in Northern Ireland. As of March 2023, data held by the Microgeneration Certification Scheme suggested that 386 small ground (and water) source heat pumps have been installed in Northern Ireland, representing around 0.05% of households.


Different types of rocks on a table, some with holes through the middle.
Selection of borehole rock samples

While geothermal energy is relatively new and rarely used in Northern Ireland, there are several large non-domestic shallow geothermal systems operating. Those who have been using this technology include the Lyric Theatre, some of the Health and Social Care Trusts, the School of Biological Sciences at Queen’s University Belfast (QUB) and the new conference centre at QUB’s Riddel Hall.

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