Effects of ground heat exchanger variable pipe separation in ground-source heat pump systems

Closed loop ground-source heat pump (GSHP) systems can efficiently provide clean and renewable energy for heating and cooling purposes using direct geothermal energy. These systems use Ground Heat Exchangers (GHE) to transfer heat to and from the ground. Vertical GHEs contain loops, pipes with circulating fluid, which transfer energy between the ground and the fluid. One very common assumption made in designing GSHP systems is that, when installed, the loops containing the circulating fluid remain straight and evenly separated along the length of the GHE. However, this is rarely true, as the high-density polyethylene (HDPE) pipes can flex within the GHE before being grouted into position. This can result in thermal interference not accounted for accurately in the design, with the worst case scenario represented by direct contact between the inlet and outlet pipes, leading to a negative impact on the performance of the system. This paper investigates the effect of this interference and the implications of ignoring it in design.