Evaluating the long-term sustainability of geothermal energy utilization from deep coal mines

Deep underground mines offer a great potential as low-enthalpy geothermal resource to be adopted in direct regional heating. In this paper, a parallel horizontal ground heat exchangers (pHGHE) system is introduced to utilize the geothermal resource in deep coal mines, however, the sustainability of the system in a long-run performance has not been well addressed in the previous study. This work presented a quantitative evaluation on the long-term sustainability of pHGHE system installed in backfilled stopes of deep coal mines. A novel nu-merical model is applied to characterize the heat transfer occurring between the horizontal pipes and backfill stopes. The proposed model was first verified to an analytical solution to determine the model reliability and accuracy. The validated numerical model was further applied to a typical underground backfilled stope of Anju coal mine in China. The results demonstrated that the maximum sustainable specific heat extraction rate must not exceed 40 W/m to maintain a 50-year exploitation period. A total thermal capacity of 17 GWh could be realized during each extraction cycle under the current conditions, which could provide heating to more than 170,000 m2 of residential building area. Accordingly, sensitivity analysis was performed to identify the key parameters influencing system sustainability. In addition, the effects of groundwater flow on the thermal interaction and long-term sustainability of the pHGHE systems were examined. Groundwater flow can be beneficial to the long-term sustainability of the system. This work confirms the feasibility and sustainability of geothermal resource utilization in backfill stopes of underground coal mines. The insights gained in this study can provide technical guidance for the design of geothermal systems in similar coal mines.