Field tests of large diameter shallow bore helical ground heat exchanger with simulated heating loads

In this paper, detailed experimental evaluation of the performance of a large diameter, shallow bore Ground Heat Exchangers (GHE) is presented. Four GHEs were installed at the test site located in Davis, CA USA. The core of the GHEs were back filled with fine sand. Since soil thermal properties vary with moisture, soil temperatures and moisture at various locations in the ground were recorded along with heat carrier fluid inlet and outlet temperatures and mass flow rates for individual GHE. The central GHE was installed with temperature and moisture sensors in the core (midway between helical and straight pipe), at the borewall, and at 1.11 m (43.7 in., r/rbore= 3.64) and 1.94 m (76.3 in., r/rbore= 6.36) radially from the center of the GHE. Temperature was measured at five depths while moisture was measured at three depths at these locations. An external gas water heater was used to simulate the load from a heat pump by providing hot water at the required temperature. In addition to the single GHE, tests were also carried out involving three GHEs in a straight-line configuration. For cases presented, test durations were varied from continous tests (for 21 hours), to intermittent (4 hours ON and 4 hours OFF). Soil properties (thermal conductivity and specific heat capacity) were measured from soil samples extracted from the site. The experiments were used to validate a Capacitance-resistance numerical model of helical GHEs.