A method to evaluate the performance of an open loop geothermal system for mine water heat recovery

Abstract

Purpose. To develop a method to evaluate hydrodynamic and thermal parameters of an open loop geothermal system with the discharge into surface water bodies as well as to test the method under real site conditions considering different technology options, geotechnical and thermodynamic factors. Methodology. We employed the relations of hydraulics and thermodynamics, performed an engineering review of open loop geothermal systems for mine water heat recovery, studied hydrodynamic and mining conditions of the colliery Novohrodivska No.2. The developed technique includes evaluating the temperature of rocks around flooded workings, the length of the hydraulic path and flow resistance of workings. Findings. The evaluated temperature of mine water entering on-ground heat exchangers ranges at 17.8 0.25 C, and the system thermal output is 1070 21 kW. Water temperature in flooded workings due to dilution with infiltration during the operation period of 25 years is expected to fall by 0.61.0 C, which decreases the thermal output by 5.68.3%. The estimated cooling of water during its rise in the shaft does not exceed 1C. The criterion of the geothermal system energy efficiency decreases from 1.8 when pumping close to the mine water level to 1.05 when pumping 460 m below the ground; the heat pump coefficient of performance (COP) reaches 5.0. Originality. The flow characteristics and hydraulic flow lengths at different horizons, the temperature of rocks around workings were found to be the dominant factors for the thermal output under steady flow. The pumping depth was proved to significantly affect the energy efficiency of the system. Practical value. The proposed method allows quantifying the energy criterion of an open loop geothermal system with the discharge into surface watercourses, which enables optimizing system performance indicators.