Feasibility Study of Modifying Soil Thermal Conductivity for Ground Source Heat Pump Systems

The feasibility of design modification for Ground Source Heat Pumps (GTHP) with horizontal coiled loops (slinky coil) has been studied. The investigation indicated that the thermal conductivity of soil at the Optimum Moisture Content (OMC) could double that of natural soil. Subsequently, design optimisation by the simulation with Computational Fluid Dynamics suggested that the simple soil modification potentially had the largest economical advantage among various design ideas. Hence, this paper aims to evaluate the design strategy in terms of cost performance, required space and reliability of performance through the annual operation analysis. The case study was conducted for an office building in Nottingham, UK. The results revealed that the modification led to the larger heat extraction from the underground by up to 29.2 % per month. This technical benefit could bring smaller required area for the installation. However, the merit in the electricity consumption and payback period was largely affected by the COP curve of the heat pump. It was concluded that the rise of electricity price was unlikely to improve the feasibility into an acceptable range. Nonetheless, modification for retaining rainwater around the slinky coil to maintain the high thermal conductivity seemed to be a viable option. This strategy can be economically achieved with simple additional construction, such as installing membrane.