Numerical investigation on thermal performance of horizontal backfill coupled heat exchangers with different sizes and quantities in mine stopes

The geothermal exploitation in a deep mine not only alleviates the problem of energy shortage, but also realizes the prevention and control of the underground heat hazard. Based on the functional backfill technology of the mine, the heat exchange of horizontal heat exchange tubes significantly influences the performance of heat exchange system in a mine. A model coupled heat transfer of horizontal heat exchange tube (H-HET) and cemented paste filling body was established in this study. The effect of H-HET length and quantity on the thermal performance of a deep mine backfilling-heat extraction system (DMBHES) was analyzed. The results demonstrated that although increasing the length of H-HET could increase the total heat removal, the effect of heat extraction per unit length became worse. Considering the average energy cost index (ζ), when the rock temperature was 30 °C, the advised length of the H-HET should not exceed 600 m, for rock temperatures ranging from 40 to 80 °C, the appropriate length range for the tubes is suggested to be 700 m to 1000 m. With the number of buried pipes increased, the outlet temperature of the pipes, total nominal heat recovery as well as the average efficiency coefficient were improved, but the slope gradually flattened. For the backfilling area in this study, the appropriate number of heat exchange tubes was five, and 30.63 × 10⁵ MJ of heat energy could be extracted during a heating season. The research results provide theoretical guidance for the parameter design and performance optimization of DMBHES.