The effect of shank-space on the thermal performance of shallow u-tube ground heat exchangers

Abstract

Reducing energy consumption in buildings via the use of energy-efficient technologies has become essential. The Ground Source Heat Pump, capable of delivering space conditioning with enhanced energy-efficiency compared to conventional air sourced systems, is one such technology. An important aspect dictating the performance of said technology is the Ground Heat Exchanger (GHE), consisting of a single or a series of U-tubes encased within a borehole backfilled with thermally conductive grout through which a circulating liquid absorbs or rejects heat to the ground. One parameter which may affect the exchanger’s thermal performance, is the shank-space, the centre-to-centre distance between the two branches of a U-tube. Generally, in order to ensure the maximum heat transfer surface possible, most of the research carried out has focused on boreholes with depths of 100m and over. Reaching such depths is not always possible and other design limitations, such as, the requirement for limited interaction with the water table, may require that significantly shallower U-tubes are used. Specific research on shallow ground heat exchangers is however limited. To address this aspect, a 3D steady-state CFD model of a U-tube ground heat exchanger was used to investigate the influence of varying shank-space on the thermal performance of two isolated vertical shallow U-tube GHE, one 20m deep and the other 40m deep. To facilitate the computational process, the 3D steady-state CFD model makes use of an innovative approach, whereby the U-junction at the bottom of the U-tube is eliminated. To ensure confidence in the results obtained, the simplified model was validated using available experimental and numerical studies performed for full U-tube models. As expected, the results show that the temperature drop of the circulating fluid varies for different shank-spaces and is lowest for the closest shank-space and highest for the widest shank-space. It is however, observed that this temperature drop is not linear with increases in shank-space and that for both modelled setups, the thermal performance improvement drastically diminishes with increasing shank-space, although with higher values being obtained for the 40m deep borehole. Such results indicate that for shallow U-tube, the temperature drop across the system is more dependent on the length of the pipework than the effect of shank-space.