With the move towards sustainable, low carbon energy in the UK, alongside a need for long-term energy security, a range of renewable energy sources are under consideration by national and devolved governments in the UK. Some, like wind energy, are well-developed, while others have barely entered the public consciousness. One such source is geothermal energy. In this article, Professor Cathy Hollis explores the untapped potential of geothermal energy and outlines policy recommendations to expedite this.

• The UK has the geological environment to produce warm water, which could be used to heat buildings. There is also the potential to produce some power from geothermal energy in parts of the country.
• The temperature of water supplied from the subsurface is variable, but even cooler waters can be used to reduce the amount of energy used to heat water for domestic, leisure, agricultural and commercial properties.
• There is some progress in small geothermal projects, but there is currently no national framework in place to incentivise and manage exploration and production of geothermal heat.

Geothermal energy is probably best known for its capability to provide power in volcanically active settings, such as Iceland. However, extraction of heat from the subsurface in less geologically active countries is also possible, by drilling to depths of up to 5km in the Earth’s crust using conventional drilling technologies. This allows warm water, which resides in pores within the rock, to be pumped to the surface. This water is naturally heated by the Earth, as the temperature of the Earth increases with depth – the ‘geothermal gradient’ – usually by around 20 to 50^oC per kilometre.

Warm water in the UK

In the UK, the average geothermal gradient is approximately 25^oC per kilometre, which means that water pumped from 2km depth will be warmer than 50^oC. Although it is often not hot enough to produce power, the water could provide a very valuable source of heat for domestic, agricultural buildings, commercial properties and leisure facilities. A recent assessment by the British Geological Survey indicates that geothermal energy resources in the UK could supply sufficient heat for the entire UK for about 100 years, as well as contributing to nearly 10% of current electricity demand. Once wells have been drilled, the surface footprint and operational disruption, for example from noise or traffic, is minimal.

Natural warm springs occur across the UK, most famously in Bath where water rises to the surface at approximately 40^oC. In the Pennines, warm spring water is used for spas and swimming pools without treatment at Buxton and Matlock. A small number of projects in the UK have proven the viability of deep geothermal (i.e. heat from water produced from over 500 metres) on a larger scale.  In Cornwall, engineered geothermal systems at the United Downs project have been drilled to produce heat and power by injecting water and heating it naturally within fractured granite. As part of the City of Southampton Energy Scheme, a small geothermal facility produces water from an underlying sandstone layer at approximate 75^oC, suitable for heating a hospital, shopping centre and civic centre. This same sandstone formation is present at depth elsewhere in the UK, for example in the Cheshire Basin.

Utilising other sources of geothermal energy

Another potentially important source of geothermal water, up to approximately 80^oC, is fractured Carboniferous Limestone. This formation is present in many landscapes in the Peak District, Yorkshire and Cheddar Gorge, and is therefore familiar to many of us. The same rock occurs at depths of up to several kilometres beneath many UK cities, including Manchester, as well as several countries in northern Europe (e.g. Belgium and the Netherlands), where it is already being exploited for geothermal heat. These fractured carbonates have been well studied by our team at The University of Manchester, providing a robust understanding of their physical properties, as well as their distribution in the deep subsurface in northern England.

An immediate next step would be the use of a deep well, to test the ability of these potential geothermal aquifers to flow water at sustainable rates. In some areas, rather than drilling new wells, disused oil and gas wells could also be used to produce warm water from target formations. These wells were drilled onshore in eastern and southern England for oil production in the 20^th century and could potentially be repurposed to produce warm water.  Our industrial legacy means we could also provide warm water from abandoned coal mines, which remain close to many UK cities, including Manchester, Liverpool, Glasgow and Swansea. The water produced from these mines is relatively cool, usually less than 40^oC, but this heat can be extracted using heat pumps, reducing the energy required to heat water for radiators.  A project is already underway in north-east England which hopes to exploit minewater heat for a new residential district (Innovation – Seaham Garden Village).

Geothermal going forward?

Although the geological expertise and engineering capability is in place for the exploitation of geothermal heat, there are some key steps that need to be taken to realise its potential.  Geologically, the greatest risk is associated with the placement of wells.  Geological formations are complex and it is important to target those layers that will provide a sustainable flow of water. To do this, it is critical that research funding and publicly accessible databases are made available, to optimise our understanding of the deep subsurface onshore UK. This will ensure that we understand the physical properties of deep aquifers that will supply water for decades, and ensure that the risk of inducing seismic events is minimised.

There is growing national interest and expertise from engineering companies, training organisations and universities to grow our knowledge, expertise and workforce in geothermal energy. These organisations are being represented by the Geothermal Energy Advancement Association, of which The University of Manchester is a founding member.  Currently, however, there is no national framework for licensing land for geothermal drilling or streamlining permissions. The British Geological Survey recommends this be addressed and that long-term financial incentives and publicly funded risk insurance are provided, to encourage geothermal drilling.

Finally, there is work to be done to understand the public acceptance of geothermal energy and to communicate the risks and opportunities around deep drilling to produce warm water. Deep geothermal does not require fracking for water production since the water flows through natural pore space in the rock.  However, produced water will be re-injected to the subsurface to sustain flow and there is some risk of induced seismicity during this process and during drilling. Much of this movement takes place at depths of 2km or more, and the magnitude is small (usually around 2 or less on the Richter scale) compared to some natural earthquakes in the UK (which can be magnitude 3-4).  Nevertheless, public concern is understandable, and the science and engineering community need to improve engagement on these issues.  Lower temperature, minewater geothermal would not require deep drilling and so carries a much lower risk.  More practically, geothermal energy provides a local heat source as transport of hot water requires expensive pipe insulation, increasing the cost of the heat.  Therefore, finding land that is suitable for drilling within urban areas could be challenging, although many brownfield sites that could be connected to a local district heating network would be suitable.  Overall, there are enormous benefits to the UK of geothermal heating to rapidly develop a local, secure and sustainable low-carbon supply of heating that can be established rapidly with existing knowledge within the science and engineering community.

Policy recommendations

• Make research funding available, as well as publicly accessible databases, to optimise our understanding of the UK onshore subsurface and to increase the probability of success during deep drilling and the placement of wells.
• Introduce long-term financial incentives and publicly funded risk insurance, to meet the growing national interest in geothermal energy.
• Focus on understanding public acceptance and communicating the risks and opportunities around drilling for geothermal energy.

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