Is commercially viable heat extraction from legacy mine workings in the UK sustainable without dynamic heat recharge?

There is an increasing interest in the use of the thermal energy within abandoned, flooded coal mines within the UK, which offers a potential seen as a low-carbon source that could support the decarbonizing of heating and contribute to the nation’s 2050 net-zero emission goal. With about 78% of UK dwellings currently using natural gas to fuel central heating, residential and commercial heating is responsible for 23% of the countries carbon emissions. Whilst all the underground coal mines are now closed, about 25% of the population still live above legacy mine workings, a proportion of which remains in deprived rural mining areas and is prone to be affected by energy poverty. Using an open-loop heat pump mine-water heating system, heat could be harnessed from the 12-20°C mine-water filling the underground mining voids to provide an economic low carbon heat resource that could directly benefit the local population, provided that the heat extracted does not exceed the heat in place.

Whilst the potential of flooded mine workings to provide sustainable heat energy has been extensively investigated, only a limited number of mine-water heating system are currently operating worldwide, such as Heerlen in the Netherlands. In order to aid the development of the resource in the UK, a better understanding of the sustainability and thermal footprint of heat extraction is required. However, generating an optimal production scenario through numerical modelling requires a thorough understanding of the geometry of mine workings. This is generally highly complex and subject to numerous uncertainties, due to the long history of mining, poor documentation of the mine workings and the inability to characterize the current state of the workings. Hence, no standard modelling approach to quantify the potential thermal resource of abandoned mine workings has yet been developed. In order to develop such a tool, it is essential to quantify the effects and uncertainties linked to the choice of a modelling approach, to the mine geometry or to the values of rock properties.  

Here, we focus the analysis on the relative importance of geometrical features in controlling the dynamic heat recharge and extraction rate from pillar-and-stall and longwall mines, using different modelling approaches. We show that the volume of the mining zone and the permeability contrasts between the caved and fracture zone are key controls on the thermal output and that equivalent porous models can reasonably reproduce the power output of more detailed models. A combination of georeferenced mining data, monitoring temperature, hydraulic data, and a range of typical rock property values for the coal measures is then used to develop a conceptual model of the Bilston Glen mine in the UK and provide a first assessment of its static heat potential, accounting for the uncertainty in the mined volume. Calibrated numerical models are finally developed and compared to the analytical solutions to get insights into the dynamic heat recharge of the system in the long-term, and support the development of a generic conceptual tool for the assessment of sustainable rate of heat extraction from mine workings.