Comparisons of test methods to determine the ground thermal conductivity for geothermal applications

In-situ thermal response tests (TRTs) are widely used to determine the ground thermal conductivity surrounding a borehole heat exchanger (BHE) for geothermal applications. In the present work, three test methods, including the constant heating-flux method (CHFM), the constant heating-temperature method (CHTM), and the lithology analysis method (LAM) based on the laboratory thermal probe test (TPT), were performed to determine and compare the ground thermal conductivity surrounding two vertical boreholes located in Caofeidian. Test results showed that the average ground thermal conductivities using the CHTM-based and CHFM-based TRT were in a good agreement, with a relative deviation of 3.6%. Compared with the TPT, however, the result by the CHTM-based and CHFM-based TRT was 20.3% and 23.9% higher, respectively. Further, the factors influencing the accuracy of test results were analyzed. It was found that the sampling disturbance by the core-drilling process was stronger in the sandy layer than in the clay and silt layer, which finally caused a relatively lower TPT result. In order to obtain more useful information on the ground thermal properties and to evaluate the effect of groundwater flow as possible, it is recommended that TRTs should be conducted based on the lithology analysis and the measurement of the natural ground temperature profile.