Geothermal energy from flooded mines: Modeling of transient energy recovery with thermohaline stratification

Geothermal energy from flooded mines is a high-potential clean energy resource that can provide heating to large communities with power comparable to small-scale power plants. The transient energy recovery process for utilizing this energy resource, however, has not been well understood, especially those involving the heat transfer in mine water with the widely-observed layering phenomenon where the temperature and salinity are stratified. To better understand the transient energy recovery process considering such a layering phenomenon, this study presents a numerical analysis of transient heat extraction from a flooded mine shaft with mine water dominated by thermohaline stratification. The numerical analysis is conducted based on a realistic case using an open-loop heat pump system. The simulation results show that, when normal pumping rates are used, the water temperature available to heat pumps almost keeps unchanged because the transient energy recovery using an open-loop system only leads to a temperature reduction of 0.2-0.3 K. By comparison, the simulation results in this study are consistent with those measured from real demonstration projects, showing the accuracy of the simulations and confirming the high efficiency and reliability of this energy innovation. The modeling results in this study also reveal that heat extraction does not affect the stability of thermohaline stratification when normal pumping rates, e.g.,0.0014-0.03 m(3)/s, are adopted, but will break thermohaline stratification with pumping rates over a hundred times of the commonly-used ones. These findings provide guidelines for future applications at different scales, and the methodology reported in this study can be used to assist the design of the energy recovery systems.