Thermal interference of backfill heat exchangers in heat storage/release processes in deep mines

The abundant underground space created during the mining process of the deposits provides the basic conditions for the large-scale storage of renewable energy such as solar energy. A novel method of realizing seasonal storage of renewable energy, such as solar energy, is the construction of backfill heat exchangers(BFHEs)with a heat storage/release function by inserting ground heat exchangers into backfill bodies in mines. However, thermal interference issues can affect their storage/release performance and should not be overlooked. In this study, the COMSOL software was used to create a validated three-dimensional unsteady heat transfer model for a typical serpentine tube BFHEs, and the effects of tube spacing, tube diameter, backfill body's thermal conductivity and specific heat were quantitatively investigated on the thermal interference of tube-to-tube and layer-to-layer of BFHEs using the thermal interference coefficient of tube-to-tube(I_tub)and the thermal coefficient of layer-to-layer(I_lay),respectively. Additionally, the I_tub and I_lay's sensitivities to the investigated parameters were discussed by reference to the relative sensitivity parameters. The findings indicate that the I_tub presents a decreasing and then increasing trend over time, but the overall change is small(0.92-1.00). This reflects that the thermal interference of tube-to-tube has a minor impact on single-layered serpentine BFHEs. The I_lay falls monotonically and considerably, and is lower than 0.25 and 0.49 at the end of heat storage/release processes, respectively. This means that the thermal interference of layer-to-layer deteriorates seriously with the heat storage/release for multi-layered serpentine BFHEs. The sensitivity analysis reveals that I_lay is noticeably more sensitive to the investigated parameters than I_tub. The sensitivities of I_lay to specific heat, tube spacing, thermal conductivity and tube diameter decrease in descending order, with the first two parameters having positive effects and the latter two having negative effects. During the most time of heat storage/release, the I_tub is most sensitive to tube spacing. The effects of key parameters on thermal interference in BFHEs were quantified in this study, and the relative findings provide a basis for optimizing the serpentine tube arrangement and preparing backfill materials to reduce the effect of thermal interference.