Determining Thermal Conductivities of Boreholes by Immersed Probes Using the Constant Temperature Test

Abstract

In designing of Ground Source Heat Pumps (GSHP), Thermal Response Tests (TRT) provide a profound basis for the final implementation. With a TRT, the effective thermal conductivity and thermal resistance of a borehole can be obtained. Until today, several TRT types have been introduced as Constant Heat Flux TRT, Constant temperature TRT, Enhanced GRT and more. In this study, we introduce a new simple thermal conductivity obtaining method that can be used for boreholes. Generally, TRTs are applied with the circulation of water inside pipes or immersing probes in water. In the case of applying water circulation in deep boreholes, different heat flux values will correspond to different layers. Moreover, giving constant heat flux similar to conventional TRT into a water-filled borehole or a water-filled pipes results in convective movements of water and this may disrupt the results. However, using immersed probes as a heat source to keep the temperature constant might prevent water convection cells inside a pipe or borehole. In order to keep the water temperature constant along the borehole, application of different heat fluxes will be required for different layers. These heat flux values mainly depend on the thermal conductivity values of the layers. Therefore, the thermal conductivity of each layer can be approximated by using the heat flux values obtained from the test. This method is investigated in this study and it is shown that thermal conductivities of layers can be obtained by applying a constant temperature test by immersed probes.