Influence of ocean tides on apparent ground thermal conductivity inferred from thermal response tests

This study investigated the influence of tide-induced groundwater flow on the thermal performance of deep horizontal ground heat exchangers (HGHEs) installed using horizontal directional drilling (HDD) for ground-source heat pump (GSHP) systems. Heating cable-based thermal response tests (TRTs) were first conducted in three 20 m deep vertical boreholes at coastal sites in Saga Prefecture, Japan. Tidal effects on ground temperature were observed at two sites, and one site was selected for further investigation. Four HGHEs were subsequently installed in approximately 110 m long boreholes along two lines parallel to the coastline, with horizontal sections at depths of 10 and 15 m. Initial 3-day TRTs yielded apparent thermal conductivities of 1.55–1.88 W/(m·K). Longer-duration TRTs on M-15 (15 m depth, landward line) and S-10 (10 m depth, seaward line) yielded higher apparent thermal conductivities of 3.43 and 2.98 W/(m·K), respectively. A coupled groundwater-flow and heat-transfer numerical model, evaluated against measured groundwater levels and TRT temperatures, was used to investigate the effects of tidal range and distance from the coastline. Simulations showed that increasing tidal range reduced the HGHE outlet temperature and improved the coefficient of performance, whereas coastline distance had a relatively small effect within the investigated range of 5–40 m. These findings suggest that tide-induced groundwater movement can enhance HGHE thermal performance and that coastal areas with large tidal ranges offer promising conditions for HDD-based HGHE applications.