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On 21 December 1991, the UK’s first commercial onshore wind farm at Delabole, north Cornwall, was brought online. Powering 2,700 homes a year, it was greeted with both hope and scepticism.

Fast forward a quarter of a century and onshore wind met 9% of the UK’s power needs in 2017, and generated enough clean power for more than 7 million homes. In the space of a single generation, politicians and the public at large had become cognisant of clean alternatives to coal and oil, leading to increased investment and rapid advancements in renewable technologies like wind and solar photovoltaics.

Nearly 30 years after it was originally trialled in the UK, geothermal also has the look and feel of an energy alternative whose time has finally come. Unbeknown to many, from the mid-1970s through to 1991, the £42m Hot Dry Rock (HDR) project looked at the potential of deep geothermal drilling in the south west of England, where granites contain elevated levels of uranium, thorium and potassium, naturally occurring radioactive elements that generate heat as they decay over time.

“One of the reasons the HDR project didn’t go any further is that the UK was in the middle of the ‘dash to gas’ and enjoying cheap energy, and the issue of carbon-intensive power systems wasn’t really on the political agenda,” explains Robin Shail, senior lecturer in geology at the Camborne School of Mines (CSM), which led the original project.

“However, energy costs have since changed, the whole decarbonisation agenda has taken off, green power is seen as good and the economics of geothermal, while not entirely straightforward, have also been transformed. So, the time is right from an energy policy point of view, but also politically in terms of people’s perception of energy and its environmental impact.”

Deep impact: inside the United Downs geothermal project

Geothermal energy generation converts vast stores of thermal energy deep within the earth’s crust – where temperatures can locally reach 1,000˚C – into power. Wells as much as two miles deep are drilled, and steam or hot water is piped to the surface, where it turns a turbine that generates electricity.

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Along tectonic plate boundaries and in volcanic regions, high temperatures are found closer to the surface, and countries such as Iceland, New Zealand and Indonesia are able to exploit geothermal energy by drilling shallow wells. In regions where temperatures are lower, geothermal fluids can be used to provide direct renewable heating.

Cornwall may be a long way from plate boundaries and volcanic areas, but it is underlain by large volumes of ‘high heat-producing’ granite, which have increased the geothermal gradient making it the most prospective area in the UK to exploit geothermal energy for both heat and power.

“If you drill a kilometre borehole anywhere in the UK, the typical geothermal gradient or temperature at the bottom will be just over 20˚C – in Cornwall it is close to 40˚C,” explains Shail.

In November 2018, Geothermal Engineering Limited (GEL) broke ground on the United Downs Deep Geothermal Project (UDDGP) near Redruth. Bankrolled by £10.6m grant from the European Regional Development Fund, plus £2.4m from Cornwall Council and £5m from private investors, the company is drilling two deep geothermal wells and plans to build a 1MW–3MW pilot power plant.

The aim is to demonstrate the technical and commercial viability of supplying both electricity and heat, and engineers hope there will be a large enough resource to provide electricity for 3,000 homes.

“Cornwall is being targeted partly because you can drill to shallower depths to achieve the same higher-temperature gradients,” says Shail. “Drilling is a massive part of the costs in terms of developing deep geothermal, so targeting areas that require the shallowest drilling is really important.”

“Drilling work at Redruth began last November and the production well is currently at a depth of just over 4km, with a terminal depth of just over 5km. The aim is to have a power plant that produces 1–3MW of power, equivalent to 20-60 litres of fluid per second going through the system.

“GEL aims to target rocks at a depth of 4.5-5km with a temperature of 190˚C, with fluids returning to the surface at around 175˚C.”

Forward thinking: Geothermal Power Generated from UK Granites (GWatt)

CSM is working with Heriot-Watt University and the British Geological Survey, in partnership with with GEL, GeoScience Limited and Cornwall Council, as part of a £1.8 million research project, Geothermal Power Generated from UK Granites (GWatt). It is funded by the Natural Environment Research Council (NERC) and led by the British Geological Survey.

CSM scientists will look at how the geological history of fault systems across south-west England has influenced the flow of geothermal fluids at depth. By mapping some of the larger fault systems where they pass through the predicted hotter parts of the granites, they hope to identify prospective areas for future deep geothermal.

“Ultimately, it comes down to deep fluid flow through fractures and assessing the parameters that are going to influence that over the 20-25 year lifecycle of heat production. For instance, permeability could be reduced by the growth of naturally occurring minerals,” says Shail.

He adds that geothermal techniques have moved on considerably since the HDR project in the 1980s, when scientists and engineers looked to create their own fracture network in the granite.

Now, the exploration focus has shifted to naturally occurring fault systems with much higher permeability. If suitable naturally occurring rock structures cannot be found at Redruth, will fracking be used to create man-made fractures, in the same way it does to extract hydrocarbons from shale rock?

“There is no intention to employ fracking at Redruth” says Shail. “The whole purpose is to target naturally occurring, high-permeability fault structures; this is in contrast to shale, which by its very nature is impermeable, and so has to be fracked.

“Also, politically, the UDDGP is about producing green rather than carbon-intensive energy, and not releasing anything that isn’t already there – so it is a completely different agenda.”

The future rocks: geothermal potential in the UK and worldwide

Shail cites a seminal report published in 2006 by Massachusetts Institute of Technology (MIT) as providing much of the impetus for deep geothermal initiatives such as the UDDGP in Cornwall.

‘The Future of Geothermal Energy’ pointed to the huge potential of geothermal resources located away from plate boundaries, if fluid flow and reservoir issues can be successfully addressed.

Overcoming technical challenges and de-risking projects will hopefully convince private investors to fund further research, allowing the nascent geothermal industry in the UK and worldwide to grow.

“We are still in the early stages of advanced geothermal roll out worldwide, but there is undoubtedly an increased interest in this,” says Shail “In Finland, for example, wells are being drilled at depths of up to 7km in areas with much lower geothermal gradients that we have here in the UK or Germany.

“The purpose of the United Downs project is to demonstrate that deep geothermal power production is technically feasible in south west England. Another 4-5km deep geothermal borehole may potentially be drilled by EGS Energy at the Eden Project in Cornwall. If these are successful, it will increase investor confidence and the wider development of deep geothermal energy across the region.”

Initiatives such as these will hopefully ensure that the work that began three decades earlier marked the beginning, not the end, for geothermal energy generation in the UK.