Study on the influence of borehole heat exchanger structure and soil thermal property on soil heat storage characteristics of the HST-BHE system

Abstract

To effectively solve the problem of soil thermal imbalance in cold regions, a new soil heat storage system that absorbs heat from ambient air through heat source tower (HST) and injects the heat into soil through borehole heat exchanger (BHE) in the non-heating season has been proposed in author’s previous research. However, the influence of structure parameters of the BHE and soil thermal properties on soil heat storage characteristics is not deeply studied. In this paper, heat and mass transfer mathematical model of the integrated system of HST and BHE (named as HST-BHE system) was built based on the developed models of BHE and HST firstly. Then, the influence of BHE structural parameters and soil thermal properties on soil heat storage characteristics was studied. The results show that the soil heat storage capacity of the HST-BHE system and temperature difference between the inlet and outlet of BHE increase with the increases of borehole length, buried pipe spacing or soil thermal conductivity, and decrease with the increases of borehole radius under specific conditions. While the borehole wall temperature of the BHE increases with the increase of buried pipe spacing and decreases with the increase of borehole depth, borehole radius or soil thermal conductivity. Unusually, increasing the water flow velocity in BHE will decrease the temperature difference but increase the soil heat storage capacity. These soil heat storage characteristics are controlled by the heat transfer thermal resistances between water and soil in the BHE as well as water and air in the HST, and all of the impact parameters should be optimized in actual engineering applications when considering the economy of the HST-BHE system.