Field Test-Based Comparative Analysis of Heat Exchange Performance in Vertical and Horizontal Ground Heat Exchangers for Geothermal Applications

Abstract

In Saga Prefecture, Japan, geothermal air conditioning systems have been implemented in public facilities as part of the efforts to achieve carbon neutrality. At Saga International Airport, both vertical and horizontal boreholes have been drilled to utilize geothermal energy. This study investigates the heat exchange performance of vertical and horizontal ground heat exchangers (GHEs) through thermal response tests (TRT). In the field experiments, one vertical borehole and two horizontal boreholes were investigated. A double U-tube was installed in the vertical borehole, which extends to a depth of 100 meters, while a single U-tube and a double U-tube were installed in two horizontal boreholes, each extending 100 meters horizontally at a depth of approximately 6 meters. The horizontal boreholes were constructed using horizontal directional drilling (HDD) method. Water was heated at 4 kW and circulated at a constant flow rate for two days. The ground temperature during circulation, as well as during recovery over the subsequent two days, was measured using fiber-optic thermometers. Heat transfer in the ground was modeled as conduction only, with subsurface temperature estimated using a cylindrical heat source function, and the thermal conductivity along GHEs was determined through a non-linear regression method. The outlet temperatures, and ground temperature were calculated and compared with measured data. The matching results for both vertical and horizontal systems showed good agreement between the model and the measurements. The results indicated that the heat exchange ability was highest in the vertical double U-tube GHE, followed by the horizontal double U-tube and the horizontal single U-tube GHEs. Furthermore, estimated thermal conductivity in the vertical borehole increased with depth, and the thermal conductivity at a depth of 6 meters in the vertical borehole was similar to that of the horizontal boreholes　which were drilled at 6 meters.