A geothermal energy heat exchange system suitable for abandoned mines and its optimization

Significant potential exists for developing geothermal energy from abandoned mines. In order to extract geothermal energy from abandoned mines, a heat exchange system customized for abandoned mines is proposed and optimized, providing a new approach to utilizing geothermal resources in mines. Ground source heat pump technology is utilized for extracting geothermal energy, and a flow-heat coupling mathematical model for the abandoned mine heat exchange system is established through the use of pipeline flow and heat conduction equations. Based on this model, a numerical simulation calculation of a single U-pipe is performed and the applicability of the numerical model is validated through comparison with micro heat transfer testing using a single pipe. A numerical model of the heat exchange system in group pipe mode is then established, and optimization of the heat exchange performance is carried out considering three elements: single-row borehole angle, multi-row row spacing, and arrangement mode. Accordingly, the following conclusions are obtained: (1) The single-row borehole angle has a significant impact on heat transfer performance, and it is necessary to increase the angle between boreholes as much as possible while not exceeding 30° when designing. (2) The fork arrangement significantly outperforms the in-line arrangement for row distances under 4 m, but the advantage diminishes as the distance increases. Therefore, the fork arrangement should be selected wherever possible in the design. (3) The row spacing design for both in-line and fork arrangements should be greater than 4 m.