Evaluation of utilizing horizontal directional drilling technology for ground source heat pumps

Abstract The drilling cost of Ground Heat Exchangers (GHEs), which increases the capital cost, is one of the main obstacles to their widespread development. Horizontal Directional Drilling (HDD) is a lower cost no-dig drilling technology commonly used for drilling tunnels or for the installation of urban infrastructure such as pipes, cables, etc., using the prescribed underground path. Thus, using HDD technology for horizontal GHE (HGHE) is expected to improve the competitiveness of ground source heat pump (GSHP) systems, since its drilling cost and the requirement of surface land area are significantly smaller than those of conventional excavation. In this research, a horizontal U-shaped hole was drilled as an experimental GHE in Saga City, western Japan. This hole with diameter of 230 mm, length 63 m, and a maximum depth of 8 m was used to place a polyethylene pipe with a 52 mm ID as GHE, inside. The thermal response test (TRT) with a heat load of 35−45 W/m effectively demonstrated the good heat exchange ability of the HGHE due to the larger heat exchange surface and the lack of thermal interference between pipes. The developed numerical model in FEFLOW and based on GHE design as well as the field’s geological condition were validated through the history matching of the heat medium temperatures measured during the TRT. This model was then used to carry out a sensitivity analysis under different design conditions. The results showed that the heat exchange rates were significantly enhanced in larger diameter of GHEs, while the unit heat exchange rates declined in longer lengths of GHE. The deeper installation of GHE was found to be insignificant due to the delayed seasonal change of the ground temperature, not to mention the consequent higher drilling cost. In addition, the effect of groundwater flow indicated that faster groundwater velocities, and in perpendicular direction, show the maximum system improvement. Finally, an economic and comparative study was performed to calculate and compare the merits of HDD with equivalent conventional GSHPs including single and double U-tube, coaxial and slinky GHE. The results of economic analysis showed that HDD technology can reduce GHSP cost up to a maximum of 78 %.