Across the UK, 23,000 coal mines lie abandoned. Carved into the subsurface, they now rest beneath 25 per cent of the country’s built environment. But these once-productive voids are not dormant. Former coal mines are flooded with water that has been heated by geothermal energy radiating from the Earth’s core. If this water could be extracted, experts predict that it could produce enough geothermal energy to meet the UK’s heating needs for the next century.
Through the BritGeothermal Research Partnership, researchers at Durham University are now working on this proposition. Currently, the vast majority of UK homes are heated by gas, which is both a carbon emitter and a potentially insecure resource given that more than 50 per cent of the gas used to generate heat is imported. A mine energy system would work by extracting the tepid water in abandoned mines and passing it through a heat exchanger at the surface. The exchanger would transfer the heat from the water to another fluid, such as brine or anti-freeze, which would then circulate through individual homes. Meanwhile, the original water would return back underground to be reheated.
Dr Charlotte Adams, an assistant professor at Durham University, explains that mine energy systems could significantly decrease the UK’s carbon emissions. While the process isn’t carbon neutral – the water in most mines isn’t hot enough to use directly and so a heat pump, powered by electricity, would be required to boost the temperature – Adams adds that for every kilowatt of electricity used to run the pump, around four kilowatts of heat would be produced. ‘Although it’s not carbon zero, it is low carbon,’ she says. ‘And, with the greening of the grid, the carbon content of electricity has dropped significantly. If you were to swap a domestic house boiler for a heat pump, you’d probably reduce the carbon emissions by around 75 per cent.’
There is another option which would avoid this problem altogether. Engineers could drill deeper geothermal wells, where temperatures are higher. However, there are risks to this approach. Not least that it is not easy to predict how much water is present at depths beneath one kilometre, meaning deeper geothermal projects carry an increased risk of lack of water flow. By using mines, the infrastructure is already in place and the water flow is almost guaranteed.
There are 30 sites globally which currently utilise heat from mines. The UK has just one commercial-scale project in Gateshead, where geothermal energy is used to heat the warehouses of a wine supplier. For a bigger roll-out, heat network infrastructure would be required and the gas boilers in most homes would need to be replaced with heat exchangers and heat pumps. Nevertheless, this transition is one the researchers think makes a lot of sense, and it is already being debated in Parliament. ‘I think a lot of people don’t realise how much water there is in these abandoned mines, and how extensive the resource is,’ says Adams. ‘A lot of what we’ve been doing is raising awareness about this huge potential.’