Reduction of Thermal Resistance of Ground Heat Exchangers using Large Grain Size Materials

Abstract

In this research, thermal response tests (TRTs) and laboratory tests were carried out to investigate the applicability of gravels as the filling material of ground heat exchangers (GHEs). In the TRTs, we filled a double U-tube GHE with gravels of 5-10mm grain size and carried out two TRTs, once using single U-tube and once using double U-tube. The TRT interpretation showed that the thermal resistance of the GHE was 0.109 mK/W and 0.069 mK/W in the single and double U-tube test, respectively, which is much smaller than those obtained when using mortar or small-grain sand. This suggested that high hydraulic conductivity of the gravel enhanced the natural convection of groundwater, which improved the heat transfer in the filling material. To confirm the advantage of gravels as filling materials, we then carried out laboratory experiments using a sand layer model of 0.6 m thick. A stainless steel casing of 100 mm diameter is placed at the center of the sand layer and is used as a GHE model. In the experiments, we carried out circulation tests of heat medium using a coaxial GHE, placed at the center of the GHE model, to evaluate the thermal resistances (Rth) under different heating/cooling conditions. Filling materials were changes as water, gravel and silica sand. Through the circulation tests, smallest Rth was observed when using water due to the active natural convection of water in the annular space between the GHE and the well wall. Gravels showed smaller Rth than silica sand due to the difference in the permeability of gravel (3.34x108 m2) and silica sand (1.18x1010 m2) due to the higher permeability, indicating that gravels of large grain size are more preferable as filling materials than materials of small grain size. The evaluation of Rayleigh numbers also confirmed the results of experiments.