As mines are abandoned, pumps switched off and mines flood. Geothermal heat, transmitted from the earth's centre find the mining legacy ideal conduits and warms this mine water to temperatures typically 20-40°C. Calculations suggest that the flooded mine workings are now a source of heat – with an estimated 2.2 million gigawatt hours of harvestable heat being produced per annum. That is enough to heat five times the number of homes in the UK, and with over one quarter of UK homes being in the coalfields the opportunity is enormous.
Mine heat is low grade, which means it is unsuitable for generating electricity as steam, but ideal for cost effectively heating homes, warehouses, leisure centres and offices. With approximately 45 per cent of heat in England and Wales used for heating, and 55 per cent in Scotland, this is a significant proportion of UK energy demand. To harvest the heat requires the use of heat exchangers and pumps. These work the same as the refrigerators we have at home. They use electricity to remove heat and cool the heat source.