Experimental study on enhanced heat transfer mechanism of U-shaped buried pipe by bio-microbial method

Thermal conductivity of soil is the key factor affecting utilization efficiency of ground source heat pumps (GSHPs). The microbially induced carbonate precipitation (MICP) technology has been proved to be an effective method to improve the thermal conductivity of soil. However, studies on using MICP treated soil as backfill to improve heat transfer performance of GSHP system is still rare. In this study, a small laboratory test device was developed to analyze the effects of different MICP treatment cycles on the heat transfer performance of GSHPs, taking into account four influencing factors: soil temperature, heat transfer and soil ultra-mild soil temperature recovery rate. The results show that MICP treated soil as backfill can improve heat transfer performance of GSHP to a certain extent, and the heat transfer amount in soil are 14.2 %, 34.9 % and 40.1 % corresponding to one to three MICP treatment cycles, which was higher than that of untreated soil. The longer the operation time, the more continuous heat dissipation or heat absorption of the system, the greater the heat transfer radius of the buried pipe temperature. For the influence of number of MICP treatment cycles, heat transfer effect of MICP-treated soil treated three times as backfill is the best. The temperature recovery rate of soil at the central test point of backfill area in summer is 73.15 %, followed by the temperature recovery rate of MICP-treated soil treated twice and treated once, which are 70.95 % and 69.6 % respectively. Considering heat transfer effect, treatment time and treatment cost, secondary MICP treated soil as backfill is the most cost-effective. Through scanning electron microscopy (SEM) image analyses, it is found that calcium carbonate acts as a “thermal bridge” between soil particles, increasing the contact surface between soil particles, resulting in improved heat transfer efficiency among soil particles, and thus improving the thermal conductivity of soil. The results provide good insight for the selection of GSHP layout, operation mode and environmental backfill in engineering practices.