Numerical investigation of the long-term thermal performance of a novel thermo-active foundation pile coupled with a ground source heat pump in a cold-climate

The potential to drastically reduce the high upfront cost and the additional land requirement of installing the ground heat exchanger of a ground source heat pump (GSHP) system has led to an increasing interest in thermo-active foundation piles. This study numerically investigates the long-term thermal performance of a novel offset pipe thermo-active foundation pile in a GSHP system for residential space heating and cooling. The pile is a fully enclosed helical steel pile that is 18.288 m deep and 0.139 m in diameter with eccentrically placed inlet and outlet pipes. A thoroughly validated numerical model coupled with realistic normalized building load profiles was employed for the analysis. Four installation configurations (pile beneath a basement, pile beneath a surface-level concrete slab, pile installed at the ground surface level, and pile buried underground) were considered. In addition, the potential of the pile to meet three realistic normalized building load profiles (maximum heating load: 0.3 tons, 0.4 tons, and 0.6 tons) was investigated. The long-term study results indicate that the ground thermal imbalance causes an average ground temperature drop of −0.18 K/year, −0.25 K/year, −0.41 K/year for the three load profiles. Moreover, the ground thermal imbalance causes the system heating COP to decrease by 0.006/year, 0.011/year, and 0.021/year for the 0.3-ton, 0.4-ton, and 0.6-ton maximum load profiles, respectively. Results further show that installing the pile under the basement gives the best overall performance regarding heating COPs and heat pump operability. When used as a retrofit solution for an existing building, burying the pile underground gives the best performance compared to installing it at the ground surface level.