A Comparative Study of the Long-Term Performance of Vertical U-Tube Borehole Heat Exchanger and Foundation Piles in a Cold Climate

Ground source heat pumps (GSHP) are gaining traction as a reliable and environmentally friendly solution for space heating and cooling. In cold climates, like Canada, heating loads are greater than cooling loads, which makes the amount of heat extracted from the ground each year greater than heat injected back. This causes a soil temperature decline, gradual performance reduction and possibly eventual failure. The associated ground thermal imbalance and high initial cost of a GSHP are a major challenge to wider adoption of this clean energy technology. This study aims to evaluate and compare the long-term thermal performance of the foundation pile and the conventional borehole heat exchanger of a ground source heat pump system for space heating and cooling in a residential building in Calgary, Canada. A meticulously validated and verified computational fluid dynamics model was developed using Ansys Fluent. The validated model was extended and coupled with transient building energy loads obtained from OpenStudio/EnergyPlus through a developed user defined function. The foundation pile, 20 m deep, 0.139 m diameter with enhanced bentonite grout as backfill material and vertical borehole heat exchanger, 150 m deep, 150 mm diameter were examined for a period of 5 years. The results show that the borehole heat exchanger handling 1.5 tons of the total building load gives better performance with the highest average cooling COP of 3.37 at the end of the 5th year with minimal impact on the ground temperature.