In the midst of a deepening climate crisis and the ongoing energy crunch, where price spikes are now commonplace and the future indeterminate, finding ways to source cheaper alternative energy, beyond just wind and solar power, has never been more pressing. Heat accounts for half of UK energy demands, with most currently derived from gas. Adding to the necessity for workable, clean and viable solutions is a 2025 deadline set by the government, after which there will be no gas or oil connections in new-build houses and businesses.
Some countries are fortunate enough to have natural resources that have been providing ‘free’ heat for many years. Around nine out of 10 homes in Iceland, for example, are heated by hot water springs. The good news is that the UK can do more than cast an envying eye at its northerly neighbour, for the UK, too, has its own thermal energy bank – and one that could prove much less disruptive, more easily available and potentially far cheaper: disused, flooded mines.
With around 23,000 collieries no longer in use in the UK, there is huge potential in former mines waiting to be unlocked. Following the closure of many underground coal mines, the pumps used to keep them dry were turned off. Subsequent refilling with groundwater has resulted in significant networks of coal seams and roadways filled with water. An estimated two billion cubic metres of warm mine water are believed to be occupying disused mine workings, enough to theoretically heat millions of homes. This would make mine water one of the UK’s largest clean energy sources.
Abandoned and mostly forgotten, these ‘holes in the ground’ are enjoying something of a rebirth in the quest for technology-ready alternatives that can play a substantial role in helping to supply Britain’s future energy needs. “Mine water can be accessed via drilled boreholes to target seams and roadways in former collieries,” says Gareth Farr, head of heat and by-product innovation at the Coal Authority. “In some areas, mine water is already at surface level – for example, at our mine water treatment schemes – and this offers another way to recover heat from mine water.”
The Coal Authority has calculated that the cumulative area of coal and non-coal workings across the British coalfields cover an area greater than 20,000km2, many holding water that could be used for mine water heat schemes.
How exactly is mine water ‘harvested’? Heat exchangers transfer the heat to a clean network of water for distribution via a heat network to homes and businesses, with source temperatures typically between 12-20°C, which can be boosted to more useable temperatures using a heat pump. “For every 1kW of electricity used to supply the heat pump, 3 to 4kW of heat can be produced, making this lower carbon than fossil-fuel heating,” adds Farr. “The mine water is then safely returned to the underground workings where the natural geothermal gradient will warm the water up, so it can be used again.”
While mine water in the UK is not hot by any standards, it does provide a great geothermal resource, owing to the exceptionally high flow rates that can be achieved. “Since the mine water is sourced from vast expanses of interconnected mine workings, the flow rates are eye-watering,” says David Walls, senior geothermal geologist at Edinburgh-based geothermal energy consultancy TownRock Energy. “For example, many of the pumping stations operated by the Coal Authority across the UK abstract over 100 litres per second.”
By way of example, he references Blindwells, East Lothian, where the treatment scheme pumps around 300L/s near to new housing developments. “It is this combination of high thermal energy availability and significant resource overlap with UK heating demands that make it so appealing. Utilising pumped mine water from a treatment scheme can significantly reduce the capital costs and risks associated with a mine water geothermal system, since no drilling is required.”
How does energy build up in these mines and how it is a harvested? The thermal energy in the mines has two significant thermal contributors, Walls explains. “At the shallow end of the spectrum – less than 100m below ground or so – we see temperatures which reflect average air temperatures [around 10-12°C]. However, the constant nature of these temperatures across the seasons means that a heat pump with a mine water source maintains its efficiency year-round. At deeper intervals, as deep as 1,000 metres, temperatures are controlled by the geothermal gradient of the area, which typically in the UK gets 25-30°C hotter per km depth; thus in some cases reaching 40°C at 1,000 metres depth. This is a result of thermal flux from the earth’s core.”
It’s far from being energy on the cheap, however. Like many renewable energies, mine water geothermal is subject to high capital costs. Drilling and infrastructure expenditures are some of the main challenges to its uptake. “Mine water geothermal energy suits larger developments, in the range of hundreds of kiloWatts to multi-megawatts (kWth, MWth), since the flow rates can provide on this scale and the costs can be shared among multiple users or absorbed by a large single user,” adds Jake Diamond, senior geothermal engineer at TownRock.
The company has been closely involved in many projects to extract thermal energy from disused mines, including securing responsibility for the operation, maintenance, redevelopment and optimisation of a 3.6MW mine water heat pump scheme at wine merchant Lanchester Wines of Gateshead, replacing a natural gas supply (pictured, p9). TownRock has developed and put into practice an operations and maintenance (O&M) programme that includes key responsibilities, regulatory requirements and operational procedures for a mine water geothermal energy system.
Although flow rates from the abstraction wells were lower than expected at the point of adoption, the company is currently redesigning the subsurface components of the system step by step to increase the available flow rate, Diamond reports.
The company has also been contracted recently to determine the technical feasibility of utilising pumped mine water flow from a Coal Authority treatment scheme in North East England. TownRock has identified a mine water heat potential of up to 7.5MWth for diversion to heat pumps within an energy station. As a result, around 10MWth from the heat pumps could supply a heat network to surrounding buildings and homes. This site also gets electricity from a nearby solar farm, creating an opportunity for a zero-carbon 100% renewable heat supply to the network, if the two can be linked.
Across the protracted mining history of the UK, there were safety issues that had to be considered, with many miners lost to mine collapses and disasters over the years. So, can mine energy be exploited safely? Most certainly, believes Walls. “Thankfully, governing bodies and mining engineers have learned from these events, and subsidence incidents are now few and far between,” he points out. “We have a good understanding of the depths at which subsidence is likely to be an issue and we can design mine water geothermal systems to avoid these.”
As the vast energy resource lurking beneath our feet attracts increasing attention, the Coal Authority has also been heavily engaged in facilitating a number of local authorities and private companies across Great Britain that are investigating mine water heat networks. Notable amongst these is Gateshead Energy Company – part of Gateshead Council – where the authority is supporting the delivery of the company’s 6MW mine water heat scheme, the largest of its type in the UK. This involved drilling new boreholes up to depths of about 150m to access mine water, to recover and distribute heat, and to return the mine water safely to the underground workings.
The Coal Authority also has other opportunities for recovering heat from some of its 70-plus mine water treatment schemes. “Mine water treatment schemes’ primary role is to protect surface water and drinking water aquifers from poor quality mine water,” says the authority’s Gareth Farr. “However, in some cases, such as our Dawdon treatment scheme in County Durham, there is potential to also use this water as a heat source for new developments, such as the planned Seaham Garden Village.”
The Coal Authority owns the majority of subsurface coal mining infrastructure on behalf of the UK government and, as such, permits and licenses safe access to their underground property. “Our aspiration is to influence and enable four large mine water heat schemes by 2025,” adds Farr, “and this is reflected in our recent business plan.”
BOX: THE HEAT IS ON
The city of Southampton has the distinction of boasting a long-term district heating and cooling scheme since the 1980s. It integrates a geothermal well with combined heat and power (CHP) plants and a biomass facility. Within the well, water was found at a depth of almost 1,800m with a temperature of 76°C. The water rises naturally to within 100m of the surface, from where it is pumped to the heat station. After the scheme began to operate, a small combined-heat-and-power facility was integrated into the system, with more added later. District cooling was included in 1994 to meet the growing demand for chilled water and air conditioning.
The scheme has now been running for several decades at a total capital cost of £13 million, and includes 7MW of CHP, a 2MW geothermal well and 1MW of biomass energy. With the city council at the helm, alongside commercial partner Utilicom (now part of Engie), more than 40GWh of heat is provided each year, alongside 26GWh of electricity and 7GWh of chilled water. Operating reliability, according to the council, has been calculated at close to 100%.