Thermal response tests of horizontal ground heat exchangers under tidal influences
In GSHP systems, Horizontal Ground Heat Exchangers (HGHEs) are widely used in areas with abundant land availability. However, the heat exchange rate of the HGHEs is generally lower than that of vertical GHEs because the shallow ground lacks groundwater flow and is subject to seasonal temperature changes. In this research, we carried out field tests to investigate the performance of deep (> 5 m) HGHEs drilled using Horizontal Directional Drilling (HDD) technology. The advection effect of groundwater flow caused by tidal fluctuations was also considered for further improvement of efficiency. We first drilled vertical GHEs of 20 m deep at 3 locations near the coastline in Saga Prefecture, western Japan, where tideinduced groundwater movement is expected and performed Thermal Response Tests (TRTs) at these GHEs. The measured ground temperatures showed sinusoidal increases at 2 locations. Based on the measured temperatures, the vertical distributions of apparent thermal conductivity (λ) were calculated in all sites and a site in Karatsu City was selected for further investigation due to high λ and tidal influences. In this site, we drilled 4 HGHEs of 101 m long using HDD method. The HGHEs were drilled along 2 lines parallel to the coastline at 10 m and 15 m deep and were completed with double Utube HDPE pipes. The TRTs consisted of a heating of 3 days with a heating load of 20 W/m. The interpretation of TRTs showed that the λ values ranged from 1.55 to 1.80 W/m/K based on the temperature behavior between 1 day and 3 days. Then, considering the chance of near-well formation damage while drilling, the late-time λ values were also calculated using temperature between 2 days and 3 days. The interpreted λ showed an increase of over 30%, suggesting the importance of performing longer TRTs in GHEs with possible formation damages.