Around a quarter of UK homes sit above the country's old mining tunnels. Following decades of disuse, they have gradually flooded. Warmed by the earth, this liquid offers one answer to renewable energy needs. It can be pumped up and used to power homes and buildings in the local area.
With an estimated 2 billion cubic metres of warm water, geologists believe that Britain's mine shafts hold one of the biggest underused sources of clean energy. Tapping into the heat from water in the mines has the added benefit of boosting the economies of some of the communities most badly hit by deep coal mining closures in the 1980s.
Mine water gets warmer the deeper it goes. Temperatures typically range from 10 to 20C but can reach up to 45C at depths of 1km. The water often contains toxic compounds, but as a thermal source it is a valuable resource that can be harnessed through drilling boreholes which bring it to the surface. The water is then directed through heat pumps and extractors which compress the liquid, raising it to a much higher temperature before distributing it through heating networks. Once its heat has been absorbed in surrounding buildings, the water can be poured back into the mine system where it will be warmed up again and can be used all year round. Temperatures aren't affected by the seasons, and can also be used to cool homes. Exploratory stages are already being carried out in Glasgow, Scotland.
Using its mining history to create new geo-energy assets, the UK is following in the footsteps of other European countries, like the Dutch city of Heerlen which opened its first mine water power station in 2008. Connected to 500 houses and commercial facilities, it helps cut the area's carbon emissions from heating by almost two thirds