Coal mines as energy storage

Why are the UK coal mines, that have contributed so greatly to climate change issues, now being seen as a source of clean energy? Well, the Coal Authority, an executive non-departmental public body that manages around £3 billion worth of coal mining issues, believes this is the case.

While previously considered an historical liability, the mining legacy is now viewed by the Coal Authority as an asset of strategic importance to the UK. For over two hundred years, UK coal was intensively mined with over 100 billion miner and machine hours employed in its extraction. This vast industrial undertaking has left behind an extensive network of tunnels, roadways and fractured ground.

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: what can it be used for?

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 energy 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.

This is a highly efficient process, for every 1KW of electricity 5-10KW of heat equivalent are produced (depending on source temperature), this represents a cost and carbon advantage over public supply gas. If the electricity used in the heat pumps is supplied from renewables, the mine heat would be virtually carbon free.

Mine water heat would not be subject to external factors that cause energy price fluctuations; this could provide commercial advantages to horticultural growers and in attracting new industries to the coalfields. Another opportunity is the redistributing money from local heating into the local economy, as at the Heerlen Mine Water Heat Scheme in Holland. This means the communities built on the coalfields, once disadvantaged by their closure, have an asset beneath their feet that can tackle fuel poverty, energy resilience, price instability, climate change and be a catalyst to create new jobs. There is something quite poetic about this.

Cross-seasonal storage of energy in mines

UK mines come in all shapes and sizes. Some are ideal as a continuously recharging source of geothermal energy. Others could see a reduction in temperature if the rate of withdrawal exceeds natural recharge. This is where storing of energy in the mine comes into its own as a means of holding significant quantities of energy across-seasons for later use. This concept comes into its own as a means of assisting the balancing the electricity grid, not only across seasons, but also through daily peaks.

Sources of energy for storage could come from Energy from Waste Plants, sewage, industry, renewables and spare grid capacity. The Coal Authority are particularly interested in promoting a district heating scheme using thermal panel on residents roofs as a source of heat for long term storage. Thermal panels are 95 per cent efficient and can produce up to 60 per cent of a home’s heat requirement per annum.

The difficulty is they produce high temperature hot water mostly during the summer and most efficiently between peak usages. If this heat was stored in mines, residents would benefit from be able to draw their heat back when they most need it.

Storing energy in mine shafts

There are over 134,000 known mine shafts in the UK. Many have been backfilled, but where not they have potential to be converted into large heat storage flasks.

The Coal Authority is currently working with Midlothian Council, Scotland, on the use of an abandoned mine shaft as a heat storage vessel of some 12,000cum being charged from surplus heat from an Energy from Waste Plant currently under construction. Stored heat will feed into a district heating scheme providing operational and energy savings to the scheme and plant operator.

The future?

The Coal Authority is keen to see the building of commercial scale pilot schemes to kick-start an industry. One of the first to be the Bridgend Council, Wales, Caerau district heating scheme in which pilot boreholes have found 20.5°C in a roadway at just 220m below ground level.