Geothermal energy originates from heat generated by naturally occurring processes deep within the Earth. This heat radiates from the Earth’s core to the surface, cooling on its 2,900 km journey. The change in temperature with depth is known as the geothermal gradient, this varies regionally with an average of around 25°C per km in continental settings. Harnessing geothermal energy involves tapping into this heat. This is generally achieved by drilling boreholes that may extend a few hundred metres to several km into the Earth’s crust.

In some areas of the world where there are active volcanoes or where the tectonic plates that form the Earth’s crust meet and are moving, very high geothermal gradients exist (up to 200°C per km). Geothermal energy can be found at surface in the form of geysers, hot springs, and steam vents. For example, Iceland is known for its geothermal resources which exist because a boundary between two of the Earth’s plates runs right through the country. Geothermal heat and produced electricity meet most of Iceland’s energy demand.

Geothermal resources are available everywhere in the world and not just in countries that host active and dormant volcanoes. Such geothermal energy can be used to meet demands for heating, cooling and heat storage. Low temperature geothermal resources have many uses, heating homes, offices, warehouses, industrial processes, horticulture, agriculture and aquaculture.

The UK resource is significant, and many areas have geothermal gradients a little above the global average.  It has been calculated that heat from the Earth could displace burning fossil fuels for the entire UK heat demand for at least 100 years and essentially for ever if managed properly.  Power (electricity) can be generated from temperatures as low as that of a cup of tea but the efficiency of doing so is very low.  A temperature differential of 100°C and better still 150°C between the heat source at depth and the Earth’s surface is needed for efficient electricity generation and this would require drilling to 5 km or deeper in most parts of the UK. In Cornwall at the United Downs site and the Eden Project, the potential for electricity production from deep drilling into granite is currently being investigated.

Geothermal resources are usually classified by depth, deep and shallow being greater than or less than 500m respectively. The UK does not have any active volcanoes but is not far from a plate margin resulting in slightly higher than average geothermal gradients distributed across a wide range of geothermal settings including:

Repurposed infrastructure

The UK has exploited its fossil fuel reserves over many centuries, both the operational and legacy infrastructure has potential to be repurposed for geothermal applications. These industries have been criticised of late due to their associated carbon dioxide emissions but reusing this infrastructure for future low carbon energy supply has created exciting new opportunities.

Operating offshore and onshore oil and gas wells produce increasing amounts of hot water as oil production recedes. If hot enough, this production water can be used to generate electricity on the platform to offset fossil fuel consumption associated with powering the platform. Onshore produced hot water can be used for heating e.g. to supply a heat network.

Onshore disused wells can be converted to provide geothermal energy either by retrofitting them with a single pipe in pipe heat exchanger or using them as part of a doublet if more than one well is available or by drilling additional wells if necessary. They can then be used to provide heat to meet local demands.

The UK also has a significant number of disused mines with around 23,000 deep coal mines and numerous metal mines. Around one quarter of homes overlie coalfield areas, and many towns and cities owe their existence to coal mining.  This means that there is good spatial correlation between heat resource contained within the abandoned mines and heat demand. Many of these mines are now filled with water which is at temperatures (15-25°C), perfectly suited to heat production when combined with heat pumps to boost the temperature sufficiently to meet home-heating requirements. Where water in the mines is managed by pumping by the Coal Authority, it may be accessible at surface for heat extraction. Alternatively, boreholes may be drilled to access and return the water following heat extraction. This has recently been demonstrated by Gateshead Energy Company who drilled into disused mine workings 150m beneath the town that now supply up to 6MW of heat through a heat network that heats, homes, businesses, and municipal buildings.

Hot sedimentary aquifers

An aquifer is a rock body that is capable of both storing and transmitting water. This might be through natural fractures and/or pore spaces in the rock. Deeply buried and hence hot aquifers occur in sedimentary basins, the deepest of which is around 15 km.  Water within such basins may be up to around 200°C, though at shallow levels temperatures are much lower. The UK has extensive areas of buried sandstone and limestone aquifers at depths of up to 6 km and with maximum temperatures of a little over 100°C. These account for the majority of the UK’s deep geothermal resource. This means that plenty of potential exists. One example of a hot sedimentary aquifer geothermal system, and one of the UK’s earliest geothermal systems, is operational in Southampton.

Hot dry rocks

The UK hosts several deeply buried granites which have elevated geothermal gradients but do not contain or flow significant quantities of fluid. Heat can be extracted from these rocks by stimulating  them to allow water to flow and collect heat. This is achieved by injecting cool water that becomes heated before returning to surface where energy is recovered before returning the water underground. For these systems to work, there needs to be flow paths between the points of injection and abstraction. Hot dry rock systems are of interest because they have the potential to produce both electricity and heat.

Hot springs and places where water is pumped to the surface can provide accessible sources of geothermal energy. However most geothermal projects require drilling boreholes or wells to access heat and fluids. They may be operated closed loop where fluid is circulated around a sealed pipe but does not mix with the surrounding rocks or fluid underground or open loop where underground water is pumped from and returned to the subsurface allowing it to pick up heat as it circulates.

Closed loop systems generally have lower risk of failure because they do not rely upon water being present and flowing at depth. Closed loop systems may comprise a single well that contains a pipe in pipe type array where cooler water is pumped down the outer pipe, becomes heated by the surrounding rock formation and returns to surface via the inner pipe. An example of this has been demonstrated by Cera Phi at Kirby Misperton in Yorkshire by repurposing a former gas well to produce geothermal heat.

Open loop systems extract naturally occurring geothermal water directly from the target formation and reinject it back underground at a different location following heat extraction. More generally at least two boreholes or a doublet are drilled allowing for abstraction and re injection of water at different points. Around an order of magnitude more heat energy can be recovered from open loop doublet systems rather than single closed loop systems.

Geothermal energy has a wide range of uses including electricity generation, heating and cooling for homes and businesses, particularly food industries, heat networks, heating for horticulture and aquaculture, spas and leisure facilities, drying products and snow melting. An emerging use for geothermal is in energy storage using the Earth as a thermal battery. Cascade use of geothermal involves using one geothermal source to supply a range of end users, it may also improve system economics through the reuse of waste heat. This is achieved by matching the temperature requirements of the end user against supply temperature following successive phases of heat extraction (each phase of heat extraction will likely drop temperatures by 5-10°C). For example, a geothermal power plant could produce both electricity and hot water. The heat at 100°C could then be used to supply a heat network connected to homes, the residual heat at 90°C could then be used to heat a spa complex then an area of glasshouses could use hot water at 70°C, then aquaculture with the cooled water eventually returned underground.

Geothermal energy provides a continuous, controllable, and secure source of low carbon energy. It delivers water at stable temperatures which leads to predictable energy supply and an ability to set secure pricing. For geothermal heat developments, the benefits can be realised locally. Geothermal energy installations generally have very low visual impact.

It is difficult to predict how much energy you can produce before you drill and test wells meaning there can be high upfront cost and risk associated with geothermal development. In the case of geothermal heat, beyond heat networks which may extend several km, heat is generally not as portable as electricity. The upside of this is that its value can be realised locally and that resources cover much of the UK.

There would be two main consequences if we did not develop our geothermal resources. Firstly, the nation is becoming increasingly reliant upon energy imports which, as an island nation, leaves us at risk from geopolitical shocks that could impact supply and cost of our energy. Secondly it would be more difficult to meet net zero targets without using geothermal to decarbonise some of our heat demand which is arguably a tougher challenge than decarbonising electricity. Over the past decade our electricity supply has decarbonised by approximately 50% however far less heat has been decarbonised and geothermal could provide part of the solution.

There are several reasons why other countries have developed more geothermal energy than the UK.  The UK has since the Industrial Revolution began, had access to coal and then petroleum (oil and gas).  Until 2003, the UK was a net exporter of fossil fuels, geothermal energy was simply not needed.

Energy security in the absence of home-produced fossil fuels has led to geothermal energy development in many nations. Nations with strong commitments to reduce their emissions of greenhouse gases have also developed geothermal energy.  As of 2023 about 30 nations have installed geothermal electricity production and 83 use geothermal heating and cooling. Several countries also have policies and support mechanisms in place that encourage geothermal uptake. For example, the geothermal sector in the Netherlands grew 50% over a 5-year period due to them having a risk insurance scheme, feed-in tariff for heat production and open access to geological and other subsurface data.

This means that there is a huge opportunity for the UK to learn and benefit from nations with more developed resources.

The National Geothermal Centre is a not-for-profit company supported by Durham University, the Reece Foundation, The Net Zero Technology Centre  and Shift Geothermal. It was established as an independent, collaborative, and inclusive entity to supercharge the UK’s untapped geothermal resources. Our vision is that geothermal energy will make a key contribution to the future low carbon energy mix.

At the National Geothermal Centre, we would like to see installed capacities of 10GW of geothermal heat and 1.5GW of geothermal electricity by 2050. This would avoid 10 million tonnes of carbon emissions annually. We believe that these targets are achievable and comparable with the aspirations of the wider EU vision for 250GW installed geothermal capacity by 2040.

By 2050 the National Geothermal Centre has estimated that 50,000 direct jobs and many more indirect jobs will be created through the supply and sale of geothermal energy. Opportunities exist for crossover and business diversification for energy companies, consultancies, drilling and exploration companies and across the wider supply chain.

We also see opportunities for research and industry cooperation to support innovation from conception to deployment. Many businesses are considering geothermal to reduce their energy costs and keep them in UK and many have ambitious net zero top targets and can see new business opportunities for supplying products with a lower carbon footprint.