Testing and analysis of the influence factors for the ground thermal parameters

To make full use of the ground heat exchanger, it is necessary to investigate the main factors affecting the thermal-physical property of soil. Thermal response tests (TRT) were conducted with different heating power and testing time in the same borehole in the present study. The results show that the change of the thermal conductivity is within 5% under different heating power. As some factors cannot be tested in practical engineering, a TRNSYS model of TRT was established based on duct ground heat storage model. The accuracy of the results from TRNSYS model was validated by testing. Changing the capacity of the water tank has little influence on the thermal conductivity, but affects the length of time heating fluid to reach the stable state. The rate of thermal conductivity change decreases gradually with the increase of the thermal conductivity of backfill material with constant borehole diameter. This indicates that the effect of backfill materials on the thermal conductivity of soil is limited. When the drilling depth is kept as constant, the borehole thermal resistance of double U-tube is reduced by nearly 70% compared to the single U-tube, while the thermal conductivity is increased by 8-10%.