Validation of the operation efficiency criteria for geothermal probes in flooded mine workings

Abstract

Purpose. To develop and test the energy and cost criteria for evaluating the operation efficiency of a closed geothermal system using coaxial or U-shaped probes that can be installed in flooded workings of mines. Methodology. To justify the energy and cost criteria, we applied thermodynamic, hydraulic and cost-efficiency relationships, conducted the engineering analysis of closed geothermal systems, studied hydrogeological settings and geothermal conditions of the mines of the Selidovo group in Donbas. The developed criteria were examined within the ranges of key parameters such as the flow rate of the heat transfer fluid and the probe submerged length. Findings. We quantified the influence of the probe submerged length and the heat transfer fluid flow rate on the energy balance and the net present value NPV and identified the parameter combinations that allow achieving efficient heat recovery in terms of energy balance and cost-efficiency. The produced/spent energy ratio may reach 1.52.2 and the NPV a few dozen thousand when increasing the submerged depth to 500 m at the flow rate of 20m3/d. A higher flow rate may lead to a negative energy balance but the NPV remains positive within some ranges of the probe submerged length, thus, indicating the system profitability. The payback period can be shortened to a few years. Originality. The proposed energy criterion balances the thermal energy produced and the thermal equivalent of electric energy generated using fossil fuel and spent on system operation. This ratio as distinct from the usually applied COP parameter allows comparing the energies of the same nature and drawing more adequate conclusions on environmental acceptability of a geothermal system. Practical value. The proposed criteria can be used for prioritization of geothermal system installation and the operation efficiency evaluation among the number of potential sites in post mining areas.