A Novel Thermal Response Test Using Heating Cables

Abstract

The in situ thermal conductivity of the subsurface has been measured with a novel thermal response test using heating cables inserted in a vertical ground heat exchanger. An electric current is applied along the cables to heat the borehole prior to measuring water temperature recovery at various depths inside the ground heat exchanger piping. The one-dimensional line-source equation, combined with the superposition principle accounting for the recovery period, is used to reproduce temperature measurements by adjusting the thermal properties of the subsurface. The adjusted subsurface thermal conductivity is independent of the borehole thermal resistance because the latter parameter is eliminated form the analytical solution describing temperature recovery. The analysis of the thermal response test can alternatively be performed with two- or three-dimensional numerical models to account for spatially-distributed heterogeneities of the subsurface, ambient groundwater flow and the geothermal gradient. The advantages of the heating cable test compared to conventional thermal response tests are that heat flow to the borehole is more constant and uniform because the heat source is located along the borehole and it is not affected by temperature changes at ground surface, the equipment required is more compact and easily installed, and the test can be fully automated to reduce time spent in the field. The test can also be conducted on any type of borehole, allowing investigation of in situ thermal properties when boreholes are drilled but ground heat exchangers are not installed.