Stratified thermal response test measurement and analysis

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An improved thermal response test method called the stratified thermal response test (STRT) was proposed and a corresponding system was developed in this study. The difference between the STRT and other methods is that the temperature was measured by sensors placed inside the borehole heat exchangers (BHEs) at different depths to obtain the virtual thermal conductivity in each stratum. The method was validated based on the measurements and determination of the thermal conductivities for two different sedimentary geological conditions using three approaches and was applied to determine the key heat transfer layer and the optimum buried depth of the vertical BHEs. The discrepancies between the thermal conductivities obtained by STRT and the conventional thermal response test were 2.5% and 2.7% respectively, which validates the reliability of the STRT. The positions of the key heat transfer layers of the two boreholes were 50–60 m and 60–80 m respectively; the location of the key heat transfer layer was affected by the intense groundwater activities. At the optimum buried depths of 80 m and 90 m for the two boreholes, the highest heat transfer rate per meter was obtained at the minimum cost. It is expected that the proposed STRT method and system will become increasingly useful for optimizing the depths of BHEs installed in heterogeneous strata because the installation cost can be minimized.