Deep geothermal energy in the UK

Materials World magazine
2 Jun 2015

Tony Bennett, Operations Director of EGS Energy Limited, argues for an increase in the development of deep geothermal heat and power plants.

Exploiting heat from dry rock sounds almost too good to be true, but it has a proven record. Attempts began in the early 1970s, with a series of pilot projects at Fenton Hill in New Mexico, USA. From the late 1970s to the early 1990s a world-leading R&D project was carried out at the Hot Dry Rock Geothermal Energy Project at Rosemanowes Quarry in Cornwall, funded by the EU and the UK Government and run by the Camborne School of Mines. Since then, research has continued at a European-led project at Soultz-sous-Forets in eastern France, and two commercial deep geothermal power plants have been successfully commissioned at Landau and Insheim in western Germany.

Now, the concept of extracting geothermal heat from a non-conventional source of hot fractured rock, such as granite, is known as an engineered geothermal system (EGS). Basically, it requires at least two deep deviated wells to be drilled to a target depth that is sufficient to achieve a downhole temperature of approximately 170°C. The system may be either forced fluid flow, where cold water is pumped down one well and is forced through the natural fractures in the hot rock, from which it gains heat before returning to surface via the production well, or may comprise far-field fluid contribution. The hot production fluid is passed through a heat exchanger, which either feeds a closed-loop power generation system, such as that based on the Organic Rankine Cycle, or supplies direct heat, before being returned into the injection well. The thermal output from this system can be used for power generation and direct heat uses.

Bringing benefits

Plants of this type require a relatively high up-front capital cost (more than £5m/MWe gross) compared with other sources of renewable energy. However, the operational costs are low. The big advantages of deep geothermal energy are:

the flexibility to be dispatched according to demand

virtually no CO2 emission

a low footprint per MWe

low visual impact

a >90% availability within a projected plant life of at least 25 years. 

As with many novel technologies, there has been reluctance from the private sector to provide the financial investment needed for the first plants, therefore public funding is likely to be required if development in deep geothermal is to be initiated in the UK. The system relies on using a hot fractured rock, such as the granite batholiths that underlie parts of the UK. The resource from this type of energy is vast – according to a report by Sinclair Knight Merz in 2012, it could be as high as 20% of the UK’s annual average electricity generation capacity requirement. 

Deep geothermal ticks many boxes at a strategic level. It has the potential to make a significant contribution towards the non-fossil fuel commitment of EU nations and towards the reduction of greenhouse gas emissions – a headline target and focus of the Europe 2020 Strategy and an aspiration for a wide range of national and regional policies. The exploitation of geothermal energy could create a variety of new, sustainable high-value jobs, both direct and indirect, as well as bring investment into the local communities, and its low environmental impact makes it particularly attractive for regional regeneration schemes.

Project proposals

Cornwall and west Devon are among the most attractive locations in the UK to carry out the initial development of deep geothermal energy. The granite mass that lies beneath much of this region, known as the Cornubian Batholith (dated at 290–270Ma), is recorded to have a high heat flow (at 120mW/m/s). Assuming a linear geothermal gradient of 35–40°C/km, a temperature of 170°C would be attained at a depth of 4,500 metres. Although the permeability of granite is low, the evidence indicates that the Cornubian Granite contains major natural permeable structures and fracture zones that are favourably aligned in relation to the regional stress regime. 

The technology and equipment required to drill deep deviated wells in hard rock is largely based on standard practice used by the oil and gas sector. However, specialist expertise is essential to deal with the unique problems that can arise in the challenges of a deep, hard rock environment. 

There are currently two separate proposals in Cornwall to develop the first deep geothermal heat and power plants in the UK – a two-well 4MWe plant at the Eden Project and a three well 10MWe plant at United Downs, near Redruth. Public funding is likely to be required to drill the initial wells to prove that favourable hydrogeological conditions exist at the target depth of 4,500 metres, de-risking each project to a level that is attractive for private investment. 

Joining the bandwagon

During the 1980s, the UK was at the forefront of R&D of this technology, but lack of continued investment has allowed other countries to take the lead. Iceland, New Zealand, the Philippines and the western USA are well-known for their geothermal power generation industry, where the heat is derived from hydrothermal reservoirs. EGS offers the potential to substantially increase the geothermal resource by exploitation in areas where natural hydrothermal reservoirs are not available.

The USA is a global leader in installed geothermal capacity, most of which is located in California, and supplies nearly 7% of the state’s electricity. It is estimated that more than 100 GWe of economically viable capacity may be available from geothermal within the USA. The US Department of Energy is currently involved in five active EGS demonstration projects, of which the largest is the Newberry Project in central Oregon. The long-term goal is to generate cost-competitive electricity from EGS, but in the near-term it is intended that R&D and demonstration projects will validate technological readiness. Another large EGS project is Cooper Basin, in South Australia, where temperatures of 280°C have been proven at a depth of 4,900m. 

In today’s market, geothermal energy is a large natural resource that has the potential to meet much of the UK’s renewable energy requirements at a cost that is comparable with other energy technologies. Given the targets that the UK has to achieve during the next few decades in CO2 and fossil fuel reduction, together with increasing concerns over energy security, now is the ideal opportunity to develop a technology that has the potential to make a significant contribution to the country’s energy needs.