Critical insights into thermohaline stratification for geothermal energy recovery from flooded mines with mine water

Geothermal applications with waste water in abandoned mines are a sustainable way of recycling wastes in abandoned facilities for utilizing clean energy. Thermohaline stratification in mine water is significant to this energy application, because it dominates the heat and mass transport in the mine-water-geologicformation system and consequently determines the efficiency and sustainability of geothermal energy systems. This study addresses six unresolved issues for understanding the formation and evolution of thermohaline stratification via multiphysics simulations, including effects of key transport parameters on thermohaline stratification; mechanisms underlying layer-merging; effects of the buoyancy ratio on thermohaline stratification, and predictions of the initial distributions of temperature and salinity for thermohaline stratification. Our results showed that the effective kinematic viscosity is the most dominant transport parameter to determine the layer-merging speed and layer number of thermohaline stratification, where seven more thermohaline stratification layers could be observed if two orders of magnitude of this parameter are increased. For layer-merging, relatively weak interfaces, which have a small buoyancy ratio across the neighboring layers, disappear and are eroded first. Our results also revealed that the buoyancy ratio determines the layer number, where increasing the buoyancy ratio from 2.16 to 4 can induce twenty more layers. The initially linear temperature and salinity distributions in mine water are needed for predicting the present and future thermohaline stratification, especially the energy recovery. To meet this need, an approach was proposed to accurately predict such initial distributions via back-calculating field measurements. This study provides insights into understanding the key energy transport mechanisms in mine water and recommendations for facilitating future implementations of geothermal energy recovery with mine water dominated by thermohaline stratification. (C) 2020 Elsevier Ltd. All rights reserved.