Assessment of Large Diameter Shallow Bore Ground Heat Exchangers for Residential Heat Pumps

Ground heat exchangers (GHE) for residential heat pumps have the advantage of rejecting (extracting) heat to a lower (higher) temperature ambient environment as compared with air source heat pumps. In this research, the thermal performance of a low cost shallow GHE with larger diameter and of helical shape is modelled using both a lumped parameter numerical model (called Capacitance Resistance Model or CaRM [1]) and a commercially available computational fluid dynamics (CFD) software. Measurements from a test site at a single-family residence are used to provide real-world data to calibrate the CFD model for soil properties. The CFD model is further used to validate and refine the CaRM model. The effect of moisture on the soil thermal property and the GHE performance is simulated by the improved version of CaRM. The results are validated by using detailed test site measurements (including moisture and temperature). The validated model is then used to perform a parametric study of helical GHEs coupled to a water-to-air heat pump serving the loads from a single-family home in two different climate zones of California (Sacramento and Riverside). GHE system performance parameters such as system electricity consumption, COP, and GHE installation costs were evaluated for varied GHE parameters including borehole diameter of the helical coil, depth of helical coil, spacing of GHE, and configuration of the bore-field. For all configurations considered, the GHE system consumed less electricity annually than the air-source case. The GHE with 40.6 cm helix diameter, 6.09 m helix depth borehole, 1.27 cm nominal tubing diameter with bore backfilled with sand resulted in the lowest installed cost relative to saved energy (kWh) for both analyzed climate zones. Building energy simulation tools such as EnergyPlus rely on resistance factors (called g-functions) to model GHEs. Therefore, the g-function for helical GHE were developed for different bore diameters, bore depth and helical pipe pitch. The g-functions were based on the mean fluid temperatures calculated by CaRM. An approximate method to calculate the traditional mean borehole temperature g-function using the mean fluid based g-function is also presented