Deep geothermal energy: a renewable option for the UK?

The UK is not known for its hot springs, geysers or volcanoes. This has traditionally made it an unlikely candidate for geothermal energy – the use of the Earth’s natural heat to provide power and heating. 

However, according to a 2018 report, if you dig deep enough, the UK has enough geothermal energy potential to heat all our homes for 100 years, and to provide a fifth of our electricity needs. These figures are still early-stage estimates, but they make it clear that, given the right technologies, deep geothermal could be an important resource. It is also a renewable form of energy, meaning it could play a vital role in tackling climate change. And unlike other renewables such as wind and solar, it is constantly available. “It is the missing piece of the jigsaw,” says Augusta Grand, CEO of the Eden Project’s Eden Geothermal Ltd. “It is a renewable energy source on a very small piece of land that does base load, clean power and heat… and it’s just waiting for us.”

Heating alone accounts for more than 40% of the UK’s energy usage and is responsible for a third of our greenhouse gas emissions, so a renewable energy source that could cover all our heating requirements is obviously not to be sniffed at.

Getting deeper: the UK's pioneering deep geothermal projects

The problem traditionally has been that the UK’s geothermal potential is buried too deep to be economically viable. Geothermal heat increases proportionately with depth. The Earth’s core is as hot as the surface of the sun, and temperatures rise the deeper you dig, at an average rate of 27ºC per kilometre in the UK. There are currently over 55,000 ground source heat pumps in the UK. These use small amounts of energy that are obtained by drilling tens of metres into the ground to supply heat to individual buildings. But for bigger projects, deep geothermal energy is required, which is defined in the UK as energy stored at depths greater than 500 metres. For projects that will heat thousands of homes you need to dig at least 2 kilometres down, and that costs money.

“This is the hairy thing about drilling,” says Dr Ryan Law, a geologist and CEO of GEL Energy, a UK-based geothermal energy company. “You’re paying a daily rate for all that equipment, including the drilling rig… Every day, you’re probably burning through £55,000. You really need to get that right. A 10-day mistake is half a million pounds.”

It is these high costs that have discouraged investment in deep geothermal energy in the UK. But things are changing. Several pioneering new projects are showing how the UK’s geothermal potential can be tapped. And a raft of new technologies are paving the way for what could be a deep geothermal revolution.

One of these pioneers is Cornwall’s Eden Project. The vast geodesic domes housing its rainforest and Mediterranean biomes and plant nurseries are heated by geothermal energy, thanks to a 5.3-kilometre-long well drilled into Cornwall’s granite bedrock by Eden Project spinout company, Eden Geothermal. At this depth it can access hot water trapped in fractures in the rock. The water, which reaches temperatures of 190ºC, is pumped to the surface where it is run through a heat exchanger that delivers heat at 76ºC to the biomes. The water is then returned underground to be naturally reheated by the Earth. This circulating system of water using one well is known as a coaxial system. Although it is over 5 kilometres deep, it takes up limited surface ground. The well took 162 days to drill and £22 million to develop and was funded by Cornwall Council, the European Union and an institutional investor.

The Eden Project’s well has been heating its biomes since 2023. It was only designed to produce heat, and its output is a modest 800 kilowatts, requiring a gas top-up during peak times. However, the initial well was only intended as a proof of concept. Eden Project has far more ambitious plans for the future, including wells that can produce electricity. “We’ll have another 5.2-kilometre well,” says Grand. “So that will be the next 5 megawatts, and that will be electricity. And then we’ll do another six wells [three double wells at 5 megawatts each], so that will be another 15 megawatts on top.” 15 megawatts is enough energy to power 15,000 to 20,000 homes. It will be provided by double well systems: one well that extracts the hot water and another, shorter, well that returns the used water to the Earth to be naturally reheated.

It’s no coincidence that one of the UK’s first deep geothermal projects is situated in Cornwall. The UK’s most southerly county sits atop a 300-million-year-old granite spine that has some very special geological properties that bring its heat closer to the surface. This extra heat occurs because granite has another, hidden, source of energy stored within it. 

Some of this extra heat is produced by radioactive elements in the granite including uranium, potassium, and thorium. “It’s not like you take a handful of granite and it’s radioactive,” says Law, “but across hundreds of kilometres, all these tiny radioactive elements produce heat.”

Law’s company, GEL, runs the United Downs project just outside Truro, which has tapped into Cornwall’s hot granite by digging the deepest well in the country. Technically, the Eden Project’s well pips it for length at 5,277 metres, but this takes into account its deviation from vertical. When true vertical depth is calculated, United Downs comes in as the deepest at 5,057 metres, compared to the Eden Project’s 4,871 metres.

GEL started drilling in 2019. It received the funding to dig two wells, which means it can produce electricity as well as heat. GEL’s double well is called a geothermal doublet system. It pumps the 190ºC water up from fractures in the rock. The resulting steam is then used to produce electricity in the same way as a conventional power station: spinning turbines attached to generators that transform the mechanical energy into electricity. The water is then pumped back down the second, shorter, injection well, where it percolates down through the granite, being naturally reheated as it goes, until it returns to the aquifer and is ready to be reused.

GEL’s well and power plant are now fully completed. It expects to produce its first power output by the end of 2025. The plant will produce 3.15 megawatts of electricity, enough to power 10,000 to 15,000 homes. As with the Eden Project, the first well is to demonstrate the concept and will pave the way for more in the future. “We have other sites in Cornwall that have planning permission for larger power production,” says Law. “Those sites are up to 5 megawatts, and we’ll just keep growing it as we prove it more and more.”

It’s not just Cornwall that has great geothermal potential. Further north, the University of York has discovered that an area of Carboniferous limestone might run directly beneath its campus and hold the potential to supply its power and heat into the future. 

Carboniferous limestone is a source of hot underground water occurring in fractured aquifers. A study by the British Geological Survey indicated that the geology under the university could be rich in this type of rock, which can be a great source for deep geothermal energy. The same study estimated that Carboniferous limestone in the UK could provide up to 222 gigawatts of energy. While the study makes it clear that these are “tentative” early estimates, and that the presence of the Carboniferous limestone will only be confirmed by actual drilling, it is still an exciting find. “It’s five times more than the complete peak electricity supply of the UK,” says Paul Bushnell, Estates Works Manager at the University of York. “So, there’s an enormous resource there.”

York’s geothermal scheme is still in the preparatory stages. The plan was announced in April this year and has received £35 million funding from the Department for Energy Security and Net Zero. Drilling is expected to start in 2028. The project will come in two stages over a seven-year period; the first producing heat for the majority of the university’s 500 buildings, the second providing electricity. The scheme will form part of the university’s plans to cut its fossil fuel consumption by 78% by 2030. “It’ll be the quickest scheme done, certainly in the UK,” says Bushnell, “and possibly Europe.”

The new drilling breakthroughs making geothermal viable

Despite these three pioneering projects, the UK is behind many other countries when it comes to geothermal energy. The US produces the greatest amount of geothermal energy and yet still only taps into 1% of its total geothermal potential. It is closely followed by Indonesia, the Philippines and Turkey. In Europe, countries are setting ambitious geothermal heating targets, with Germany looking to provide 850 thermal megawatts by 2030, and the Netherlands a full thermal gigawatt in the same time frame. The EU has a goal of heating 25% of its houses with geothermal energy by 2030. In stark contrast, the UK has no set geothermal targets.

The rush towards geothermal in Europe has been inspired by several breakthroughs in drilling technology and techniques that have made it more economically viable. These include new polycrystalline diamond compact drill bits, which are much harder than traditional steel bits, increasing drilling rates by up to 70%. Other developments have enabled multi-well drilling, allowing for more wells at a single site; horizontal drilling, enabling more precise targeting of aquifers; and multistage perturbation, which enhances access to underground water by creating artificial fractures in the rock (but differs from the fracking technique used in the shale oil industry). As a result of these technologies, the International Geothermal Association expects deep geothermal energy to be competitive with market electricity prices by 2027 in the US.

Horizontal drilling has also enabled the construction of so-called closed loop systems in which circulating fluid is pumped through huge underground systems of pipes. These projects are being pioneered in countries like Germany and Canada by companies such as Eavor. Its Eavor-Loop system uses a proprietary fluid that is pumped through a closed loop of underground pipes to extract heat much in the same way as a car radiator. The Eavor-Loop can reach depths of over 5 kilometres and uses magnetic ranging technology to accurately intersect its wells at the target depth. The benefits of this system are that it avoids extracting water from the ground itself and doesn’t require expensive drilling techniques to target fractures in the rock. 

Other projects are developing their own proprietary drilling technologies, such as the EU’s DeepU, which is testing lasers combined with cryogenic gas to liquify and vitrify the rock, enabling faster drilling speeds. And US company, Quaise, is developing a technology borrowed from nuclear fusion research to blast rock using millimetre-wave radiation. Quaise believes this technology could enable drilling to depths of up to 20 kilometres where rock temperatures reach 500ºC. This could unlock terawatts of energy, enough to power billions of homes.

Not a silver bullet

There are still challenges to overcome if deep geothermal is to make a sizeable impact on the energy industry. One of the main issues in the UK is regulation. Drilling sites need access to road and electricity networks, all of which requires planning permission. Law says that the problem also lies with local councils, which are too understaffed to deal with all the documentation and data requests that go with the dozens of new renewable energy projects constantly landing on their desks.

However, despite the blockages there are some signs that the UK government is trying to clear the path towards a geothermal future. The British Geological Survey has just released the new Geothermal Platform, a one-stop digital database for the UK’s geothermal data, which should simplify the process of researching whether geothermal energy is a viable resource at any given location in the UK.

With more accessible data, deep geothermal energy could become an important part of the UK’s renewable energy solution. Grand points out how geothermal could contribute to our soaring energy demands: “The UK is expecting to need another 62 terawatt-hours of electricity for data centres by 2050. That’s about 8 gigawatts of geothermal [electricity],” explains Grand. Based on the anticipated performance of the new Eden Project wells, that’s about 3,200 wells, which with multi-well drilling may mean as few as 160 sites. “It’s doable. It’s not a big number.”

Dr Alison Monaghan, Head of Geothermal at the British Geological Survey is cautiously optimistic. She doesn’t believe that geothermal energy is a silver bullet that will solve all the UK’s energy needs. She points out that figures that calculate that it could heat all the UK’s homes for hundreds of years are more about theoretical potential than actual usable energy. With that caveat, however, she is hopeful that geothermal will be a significant part of the UK’s energy landscape far into the future. “I think geothermal energy has a really important role to play in heating and cooling… and a contribution towards electricity generation,” says Monaghan. “It is never going to be 100% of the UK’s heating requirements, but in many places, and with many heat users and demands, it could form a really important part of the net zero energy mix.”

Bushnell is more effusive, pointing out the huge potential of the Carboniferous limestone that the University of York hopes to tap into. “If it’s proven,” he says, “and this becomes a kind of exemplar then there’s a potential for kickstarting a whole industry, which is really exciting.”