Field Testing and Long-Term Numerical Evaluation of Horizontal Ground Heat Exchangers Installed via HDD Method at Saga Airport, Japan
This study investigates the long-term thermal performance of horizontally installed ground heat exchangers (GHEs) using horizontal directional drilling (HDD) at Saga Airport, western Japan, to improve the feasibility and cost-effectiveness of ground source heat pump (GSHP) systems. Building on field data from thermal response tests (TRTs), a numerical model of ground thermal behavior was developed using FEFLOW version 8.1 and was validated against measured temperatures, demonstrating its reliability for predictive long-term analysis. The validated model was then applied to five-year simulations of two GHE configurations (single-pass and double U-tube) and two layout arrangements (linear 1×20 and block 4×5) to assess thermal interference, heat extraction capacity, and subsurface thermal storage under realistic site conditions. The double U-tube configuration achieved slightly higher heat extraction rates than the single-pass configuration due to its larger heat transfer area; however, the coefficient of performance (COP) of the double U-tube was marginally lower, likely because of thermal interactions among adjacent U-tubes. The linear layout consistently outperformed the block-type layout, which exhibited more pronounced ground temperature rise and thermal accumulation. These results indicate that, under the studied conditions, thermal interference between GHEs has a stronger impact on system performance than subsurface thermal storage. The moderate performance difference between configurations suggests that simpler single-pass designs remain practical when installation cost and construction simplicity are prioritized. Overall, the results indicate that thermal interference between GHEs exerts a stronger influence on long-term performance than subsurface thermal storage. The study provides design-oriented insights for optimizing pipe configuration, spacing, and layout, and demonstrates the technical viability of HDD-installed horizontal GHEs as a practical option for shallow geothermal applications in cost-sensitive settings.