Improving ground heat exchangers for geothermal heat pump systems with a groundwater-filled borehole and a thermally enhanced double U-pipe

ABSTRACT: Ground heat exchangers (GHEs) used for geothermal heating and cooling systems in North America, more specifically ground-coupled heat pump systems, are commonly made of a single U-pipe installed in a borehole backfilled with grout. This design can be improved to minimize the borehole thermal resistance and increase the system performance. A backfill free GHE with a double U-pipe made of thermally enhanced high-density polyethylene and allowing internal free convection of the groundwater inside the borehole was installed at INRS laboratories to verify this alternative design inspired by Scandinavian methods. A thermal response test was performed to evaluate the borehole thermal resistance. Analysis revealed, in best operating conditions, a borehole thermal resistance equal to 0.05 m K W-1, which is 43 % lower than the thermal resistance inferred for a single U-pipe GHE installed at the same site with the conventional design method.