Long-Term Performance Analysis of Foundation Pile and Vertical Borehole Heat Exchangers for Ground Source Heat Pump Systems in Cold Climates

Abstract

Ground source heat pump systems (GSHPs) have received considerable interest from researchers for the decarbonization of energy use in buildings owing to their higher efficiency. The ground heat exchanger (GHE) is the primary component of a GSHP. Vertical borehole heat exchangers (VBHE) and foundation piles (FP) are common GHEs that researchers are developing and optimizing to reduce capital cost and seasonal ground thermal imbalance associated with GSHP. While VBHEs have been a subject of research for decades, FPs are emerging as an excellent replacement for VBHEs because the initial cost associated with installation is lower, and their installation does not require complex drilling equipment and expertise. In this study, a numerical investigation was undertaken to characterize the performance of a VBHE and an FP in a GSHP system under the same operating conditions. Realistic time-varying building energy loads were used for a residential building in Calgary, Alberta. To verify the reliability of the developed model, the results from the model were compared with experimental data from the literature, yielding excellent agreement. The results of this study indicate the potential for ground freezing due to continuous heat extraction in the vicinity of the FP and VBHE if the peak building energy load capacity exceeds 1.1 kW (0.3 tons) per FP and 5.6 kW (1.5 tons) per 150 m VBHE, respectively. Overall, using 5.6 kW per 150 m VBHE performs better, with a lower ground temperature decline, the highest heating mode coefficient of performance (COP), and a less pronounced decline in outlet temperature after the fifth operation cycle. This study provides valuable insights for optimizing GHEs, enhancing system efficiency, and ensuring long-term thermal sustainability.