Abstract
Seasonal storage and extraction of heat in legacy coal mines could help decarbonize the space heating sector of many localities. The modelled evolution of a conceptual mine-water thermal scheme is analysed in this study, involving cyclical storage of heat in an enclosed underground coal mine. Conductive heat transport simulations are performed in a 3D model of a flooded room-and-pillar panel, based on typical mine layouts, to quantify the maximum thermal recovery from the host rock in different scenarios. We show that, by optimizing the seasonal management, from 25% to 45% of the energy transferred to the subsurface could be potentially recovered at the end of the first operational year. The modelled heat retrieval, achieved by subsurface cold-water circulation, does not consider the potentially enhancing effect of local advection around mine voids and applies to cases of relatively low dispersal of heat by the regional groundwater flow. The cumulative heat recovered from the modelled host rock could equal the thermal energy provided by the “mined” coal in less than 70 years. A comparison of the value of the original coal “mined,” at today’s prices, to a representative value for the heat recycled in the space created by its extraction, suggests that within less than 3 decades of thermal cycling similar monetary values are reached for the specific conditions modelled.
Highlights
One of the major challenges for the ongoing energy transition is the decarbonisation of the space heating sector, a major CO2 emission source (Sansom, 2015; Watson et al, 2019) which amounts for 51% of the global energy consumption (REN21, 2019)
While the model was designed with typical dimensions and geological properties found in legacy coal mines, this study does not intend to reproduce the settings of a particular site; instead, it aims to model conditions that could be considered prospective for potential mine-water schemes, e.g., a large inter-connected void space and limited interaction with surface recharge
It is in this anthropogenically altered rock volume where advection would have the largest impact on the performance of the mine-water system; outside this area, heat conduction would be the main mechanism for thermal transfer and advection would have a much smaller role proportional to the regional groundwater flow (Ferguson, 2015)
Summary
One of the major challenges for the ongoing energy transition is the decarbonisation of the space heating sector, a major CO2 emission source (Sansom, 2015; Watson et al, 2019) which amounts for 51% of the global energy consumption (REN21, 2019). The exploitation of geothermal resources in large-scale open-loop schemes requires a considerable groundwater flow (Bertermann et al, 2015) and the proximity of the heat resource to the potential users (Menéndez et al, 2020). Of the seven mine-water geothermal plants in operation globally, the two schemes with the highest installed capacity (>500 kW) are located in flooded coal mines of Asturias, Spain, and Heerlen, Netherlands (Menéndez et al, 2020). The minewater project of the Barredo Shaft in Asturias (~3,500 kW) was installed in 2012 for space heating and cooling of a hospital located at 2 km from the coal mine, with an average mine-water temperature of 23°C. The potential expansion of this type of systems is restricted by the natural heat resource in place, typically with associated Carboniferous formations at shallow depths and groundwater temperatures below 30°C (Peralta Ramos et al, 2015)
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