Investigation of thermal interaction between shallow boreholes in a GSHE using the FLS-STRCM model

Abstract

One of the highly efficient sources of renewable energy is the Ground Source Heat Exchangers (GSHE). Thermal interaction effects between boreholes cause a decrease in the efficiency of GSHEs. In the present study, the effects of the distance between boreholes, soil properties, grout heat conduction coefficient, and boreholes’ arrangement are investigated on the outlet fluid temperature in the GSHEs. The Finite Line Source (FLS) model is coupled with the Simplified Thermal Resistance-Capacity Model (STRCM) and a transient modified model called FLS-STRCM is developed. The FLS model considers the thermal interaction between boreholes while the STRCM model solves the flow field inside the boreholes. The obtained results of the FLS-STRCM are compared with the transient results of Computational Fluid Dynamics (CFD) simulations and available experimental data. To simulate the heating and cooling cycles of a building, the present model of GSHE is coupled with the water to air heat pump in the TRNSYS software. The available models of GSHEs in the TRNSYS software are not capable of considering different arrangements of boreholes. However, the present model, in addition to the transient simulation of GSHEs, can examine any desired layouts of boreholes. The results of the present model show that for a set of 6 boreholes in a rectangular arrangement, a reduction of the distance between boreholes from 7 m to 3 m results in 2.8% changes in the average temperature of outlet fluid. Also, the investigation of soil and grout properties shows that the soil properties has a dominant effect on the outlet fluid temperature. Considering the computational time and the acceptable accuracy of the presented model compared to the CFD methods, the FLS-STRCM model can be employed for the engineering applications in the construction of the GSHE system and the long-term simulations.