Digging deep: how geothermal power could become a significant part of the UK energy mix

Digging deep: how geothermal power could become a significant part of the UK energy mix


Geothermal energy hit the headlines recently when three projects in Cornwall were awarded the first-ever Contracts for Difference (CfD) for this technology, in the latest government auction to secure future low-carbon power.

But how likely is geothermal generation to become a significant part of the UK energy mix?

Research carried out by Durham Energy Institute (DEI), which is based at Durham University, estimates the UK has sufficient deep geothermal resources to meet the population’s heat demand for at least 100 years.

However, while geothermal offers a potential source of secure and ultra-low carbon energy, the logistics of harnessing it are by no means simple – or cheap.

Digging 3 miles+ deep
In countries that have volcanic activity, harnessing geothermal energy is far more straightforward, as the heat is much closer to the surface. That’s why Iceland is a world leader in geothermal energy, generating 80% of its power and heat from this natural resource.

But in the UK, accessing sufficient geothermal heat to generate power for commercial use requires drilling a main borehole to between two and three miles into the earth. 

Alongside this, a shallower borehole is used to inject water to flow down through naturally hot granite rocks, before being extracted via the deeper borehole. 

This hot water could then be used to drive turbines to make electricity, and also for local heating purposes.

Potential sites in northeast, Wales and the south
The DEI has identified 45 sites around the UK with deep geothermal potential. The research was commissioned by MP Dr Kieran Mullan and forms the basis of his report to Parliament Dig Deep, which promotes investing in a UK geothermal industry. 

The sites with the most potential have mostly been identified in the northeast of England, although there are also some in Wales and the south too. 

And while geothermal plant penetrates a deep vertical area, the surface land area has a relatively small footprint. 

For example, the US Office of Energy Efficiency & Renewable Energy estimates the geothermal plant use around 404m2 per gigawatt hour (GWh) of generation, compared to:

  • Coal: 3,642 m2
  • Wind: 1,335 m2
  • Solar photovoltaic (PV) power stations: 3,237 m2
That said, the deep heat extraction process can create a risk of local small-scale seismic activity. This means that plant ideally need to be located away from populated areas.

Cornwall home to UK’s first geothermal plant
The UK’s first deep geothermal plant is already nearing completion near Redruth in Cornwall. 

The United Downs project is reported to cost £35 million and once operational, is expected to generate between one to three megawatt (MW) of electricity for grid supply, with plans to distribute excess thermal energy via a local district heating scheme.

Two other Cornish sites are then expected to then follow, collectively generating 12 MW of capacity for the UK grid.

350 MW at estimated £1.6 billion cost
Dr Kieran Mullan’s Digging Deep report estimates that to develop 30 separate sites of around 350 MW / 0.35 gigawatt (GW) of installed capacity could cost £1.6 billion spread over an estimated period of 32 years.

For comparison, the planned £3.3GW Sizewell C nuclear reactor is estimated to cost upwards of £35 billion, and will take 10 to 11 years to build. 

Geothermal plant also take a much shorter time period to build. For example, the Redruth site in Cornwall started drilling in 2019 and expects to become operational by 2025. 

Consumers to foot the bill
As with other energy infrastructure projects, consumers are likely to be expected to foot the bill – either upfront, or via higher prices once operational. 

But if the funding model continues via CfD, then it potentially offers better value than a levy such as the now-closed Renewables Obligation (RO).

For example, the 12 MW Cornish geothermal projects awarded a CfD in the September 2023 auction will receive a £119/MWh strike price over 15 years from 2026/7. This compares to £52.29 awarded for onshore wind, £47 for solar PV and £198 for tidal stream generation.

But unlike these other technologies, deep geothermal is still very much a pilot project. 

However, neighbouring countries with comparable geology have already developed deep geothermal sectors including France, Germany and the Netherlands. 

So it is possible. Watch this space.
Sally Phillips

Sally Phillips

Sally is the Director of Corporate and Strategic clients for the I&C (Industrial & Commercial) division with responsibility for the commodity energy sales and service, cash collection, as well as metering services and export contracts for small generators. Prior to this, Sally was Head of Transformation for I&C energy sales and solutions. The purpose of this division was to manage transformation of E.ON UK, following the acquisition of Innogy in 2019. Sally’s previous roles included: Head of Sales for the B2B solutions division, delivering low-carbon solutions to I&C clients and Head of Sales of I&C Sales with accountability for all commodity sales and growth. She currently lives in Warwickshire with her husband and two daughters.

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