The Earth gets hotter by 2.5C to 3.5C with each 100m depth. It’s what makes geothermal energy possible, anywhere. In the UK geothermal could meet the nation’s heat demands for at least 100 years, say Jon Gluyas, Andrew Crossland and Charlotte Adams of the Durham Energy Institute. Properly managed it could last indefinitely. Given that heat does not travel well, geothermal must be developed locally. Fortunately, accessible heat lies beneath or adjacent to most major UK population centres. It has around 23,000 abandoned coal mines, all deep heat sources, surrounded by the towns and cities built to serve them, amounting to 40% of UK housing stock. The authors estimate 36,000,000TWh of heat storage capacity exists in those mines. Given the UK’s total electricity consumption in 2018 was a little over 300TWh, geothermal could be a game changer.

The Earth is hot, very hot. It is hot enough in places for rock to melt and to be erupted as lava from volcanos. The source of the Earth’s heat is in part residual, created when the planet formed around 4.5 billion years ago and in part heat released from the decay of naturally occurring, long half-life, radiogenic elements within the Earth. This internal heat is critical for supporting life on Earth such as creating the magnetic field which prevents the atmosphere being stripped away by the solar wind. Coal, oil and gas are also products of the hot Earth, generated by burial and heating the remains of ancient life.

In an effort to reduce greenhouse gas emissions and secure future energy supplies most nations are now trying to reduce their dependency on those fossil fuels. This “transition” is dependent upon developing a suite of alternative low carbon sources within the international energy mix – from electricity to aviation fuels. Within electricity, most widely developed renewable energy sources are hydro, wind and solar for the generation of electricity. The growth of the latter has been nothing but remarkable, for example a combination of existing nuclear fission reactors together with development of wind and solar have now deliver over 50% of UK annual electricity supply. This has been mirrored by a reduction in the number and use of coal fired power stations which now meet less than 6% of annual supply (down from over 40% in 2012), reconfiguration of other coal fired power stations to use biofuels and efficiency measures to bring down demand. The carbon intensity of electricity generation itself has fallen from about 510 g CO₂/kWh in 2007 to less than 280 g CO₂/kWh in 2018, with most embedded carbon coming from the use of natural gas. We now need the same efforts to decarbonise our heat demand.

The Importance of heat: 50% of total energy demand

For the UK and much of the industrially developed northern Europe and large parts of North America, heat makes up a much greater portion of our energy demand than does electricity generation. Heat generation in the UK accounts for 50% of our total energy demand and almost all of the heat generated in the UK comes from burning fossil fuels, particularly natural gas. Using government figures, we have calculated that that 77% of all heat required in the UK comes from fossil fuel combustion; about 66% is from burning gas (and minor quantities of oil) in domestic and other small-scale boilers and a further 11% comes indirectly from burning fossil fuels in power stations and using electricity in the home to generate heat. That heat demand is seasonal, consuming up to three times the power of electricity during the winter, but only a fraction of that during the summer.

Reliance upon gas for heat is of concern because the UK has been a net importer of gas for over a decade, leaving the cost and security of energy supplies at great risk to geopolitical shocks. Burning fossil fuels to produce electricity for heating is extremely profligate in energy terms because of the losses involved.

The UK government has placed a moratorium on new gas connections from 2025. The expectation is that consumers will shift to electrical heating but increase the use of fossil fuels for electricity generation if that electricity is not derived from low carbon sources.

Adding geothermal into the heat mix

There is another way to tackle heat supply on a national scale; decarbonisation of heat through a mix of alternative or higher efficiency interventions. This heat mix, combining electrical, geothermal and waste heat could reduce stress on the power system and make better use of other resources available to us.

The Earth gets hotter by 2.5C to 3.5C with each 100 m depth. This heat can be extracted to provide space heating. The UK has a commercially operating city-centre geothermal heat network in Southampton in the south of England where water at 75C is abstracted from a deep saline aquifer at a depth of 1.8km. Customers for the heat include the local hospital, university and commercial premises.

The geothermal potential of the UK is not limited to Southampton. A recent evaluation by BritGeothermal has demonstrated that UK geothermal resource could meet the nation’s heat demands for at least 100 years and properly managed could last effectively indefinitely and with a low carbon intensity. Heat does not travel well and so district heat schemes need to be developed. Thankfully, accessible heat lies beneath or adjacent to most of our major population centres. The Southampton District Energy Scheme (SDES) extracts heat from the Wessex Basin. Similar basins occur beneath Glasgow and Edinburgh, the east coast of England from Newcastle to The Wash, adjacent to Liverpool, Manchester, and Birmingham.

At present, geothermal heat is highly underdeveloped in Britain. Development of the SDES was down to one person, the visionary and opportunist Mike Smith who was the city council’s accountant when in the mid-1980s the UK government drilled a bunch of geothermal appraisal wells in the wake of the oil crisis of the early 1970s. By the time the wells were drilled the UK had become a petro-economy on the back of North Sea oil and gas. The well drilled in central Southampton was about to be abandoned and Smith stepped in and rescued the well which became the basis of the SDES. The gas boom led Britain to chase alternative forms of heat, but that pioneering work by Mike Smith could now be the saviour for a country which is increasingly dependent for winter survival on unsustainable gas supplies.

Easy wins: old coal mines are ready heat sources

To develop geothermal energy in the UK, we could take advantage of the easy wins associated with existing energy supply infrastructure that is about to be or has been abandoned such as petroleum wells and coal mines. Unbeknown to many, the UK has a small onshore fossil fuel industry which started 100 years ago at Hardstoft in Derbyshire. We even had a couple of mini oil booms; Lincolnshire and Nottinghamshire in the late 1930s and 1940s and then in Dorset in the 1970s. These oil fields now produce very little oil but they do produce warm water. For example, the unwanted but co-produced hot brine from Wytch Farm oilfield in Dorset contains 40 times more heat than the single well in Southampton. This heat could readily be supplied to the nearby towns of Poole and Southampton.

The UK also has around 23,000 abandoned coal mines, all now flooded and containing tepid to warm water with a truly massive heat content. This heat can be extracted close to areas of demand because the UK built its town and cities around its coalfields – around 40% of UK housing stock lies directly above these relatively easily accessible geothermal heat sources.

36bn GWh, more than the UK will ever need

Heerlen in Limburg, Netherlands has already demonstrated that mine energy can be used to decarbonise heating on a city scale over a period of more than a decade. In contrast, The Coal Authority, the body who manage abandoned UK coal infrastructure, has to pump water from some mines to prevent minewater emergence and in doing so is producing and dissipating 80 MW of heat – effectively underutilising a valuable heat resource which could support local communities. Calculations of the heat capacity of water in flooded mines suggest 36 billion GWh of heat storage capacity exists – orders of magnitude more than annual electricity demand.

Reviving ex-mining communities

Not only has the UK the capacity to decarbonise heat using its heritage of abandoned coal mines coupled with abstraction of water from sedimentary basins but there are also indications that society will support it. Work undertaken by Durham Energy Institute in former mining villages of the Durham Coalfield has found a great desire to rebuild communities, fractured by the closure of the mines, around a shared, low-carbon geothermal heat resource. Development of the UK’s geothermal resource would go a long way towards meeting our carbon budget, improve our energy security by reducing reliance upon gas imports and maybe just persuade the next generation that we can spare them the legacy of a carbon-intensive energy industry.

The UK heat strategy is at a crossroads. It could chase electrified heat through heat pumps and create headaches for the electricity industry in working out how to supply increased and highly seasonal power demand in a low carbon manner. Alternatively, a mixed approach could be adopted including better use of our onshore geothermal resources to reduce our heat demand and again provide heat in a more localised way. Britain’s abandoned coal mines have often been viewed within the lens of the negativity of the 1970s and 1980s. We now believe these hold the key to a new community responsible approach to decarbonisation.

***

Prof Jon Gluyas is Executive Director, Durham Energy Institute

Dr Andrew Crossland is Associate Fellow, Durham Energy Institute, and a Director of Advance Further Energy Ltd.

Dr Charlotte Adams is Fellow, Durham Energy Institute