Experimental study on thermal resistance of vertical ground heat exchangers

Thermal resistance values in ground heat exchangers (GHEs) were evaluated using the results of six thermal response tests (TRTs) with two methods. In the first method, Kelvin's line source theory is applied on the temperature performance in TRT results. In the second method, the thermal resistance is calculated on the basis of well completion information. Thermal resistance values estimated from the two methods showed good agreements. Then, the thermal resistances of various well completion types were compared, using the second method. Thermal resistance increased in the order of co-axial GHE, double U-tubes, and single U-tube. As grouting materials, silica sand was the most effective in reducing thermal resistance. The evaluation result also showed that a borehole without grouting could minimize thermal resistance, when natural convection exists in the GHE.