Experimental study of horizontal ground heat exchangers embedded in the backfilled mine stopes

Abundant geothermal resources are stored in underground mines in China. In order to realize the green cooperative development of deep mineral resources and geothermal resources, the heat storage functional backfill technology can be used to form artificial thermal reservoir, and the geothermal energy can be extracted through the buried pipes, so as to expand the auxiliary value and reduce the cost of backfill mining. It's important to investigate the heat storage/release behavior of the backfill body with buried pipes and its influencing factors for improving the geothermal energy utilization efficiency. In view of the limitations of field experiments, a scale model test platform for backfill coupled heat exchanger of mine was established in this paper. According to the heat transfer differential equations, the similarity criterion number and similarity constant that should be satisfied between the model and the prototype were deduced, and the effects of different pipe arragement, flow rate of heat-carrying fluid, surrounding rock temperature on the heat transfer performance of the backfill coupled heat exchanger were studied. Based on the heat balance, the total energy efficiency of the backfill coupled heat exchanger system was calculated. The experimental results showed that with the increase of the distance between pipes, the pipe length, the flow rate and the surrounding rock temperature, the heat extraction and water outlet temperature of the system all increased. With the increase of the velocity of heat-carrying fluid, the water outlet temperature decreased. The increase in the number of pipes in the backfill body and the decrease in the distance between pipes lead to the increase in thermal interference and thus the decrease in heat extraction, but the decrease value is less than the increase value of heat charging caused by the increase in pipe length (serpentine) and flow rate (double U-type). The total energy efficiency of the system was the highest, which was 47.9%, when the serpentine pipe arrangement was adopted, the working fluid velocity was 0.7 m/s, and the surrounding rock temperature was 35C. According to the scale model, 1.06 x 109 kJ of heat can be obtained from the mines a year by applying BCHE system in engineering practice.

Automated summary

Abundant geothermal resources can be extracted from underground mines in China using the heat storage functional backfill technology. This study established a scale model test platform to investigate the heat transfer performance of a backfill coupled heat exchanger system. The results showed that the distance between pipes, pipe length, flow rate, and surrounding rock temperature had significant effects on the heat extraction and water outlet temperature. The highest total energy efficiency of 47.9% was obtained with a serpentine pipe arrangement, a working fluid velocity of 0.7 m/s, and a surrounding rock temperature of 35°C.