Calibration of thermal response test (TRT) units with a virtual borehole

This paper presents the development of a virtual borehole (VB) used to calibrate the ground thermal conductivity obtained from thermal response test (TRT) units. The VB is composed of an aboveground plate heat exchanger and chiller unit carefully controlled to mimic the thermal behavior of the ground by reproducing the time evolution of the mean fluid temperature for a user-selected ground thermal conductivity. During calibration, TRT units are connected to the VB just like if they were connected to a real borehole. The various components of the VB are described including the characterization of the heat exchanger, the implementation of a resistance-capacitance (RC) borehole model, and the required control algorithm. The VB concept is successfully tested by comparing the results obtained on a real borehole to those given by the VB for given conditions. An uncertainty analysis reveals that the ground thermal conductivity set by the VB is accurate to within ± 2.5%. The usefulness of the VB is then demonstrated by calibrating a commercially available TRT unit for two ground thermal conductivities, 1.0 and 3.0 W m-1 K -1. Results of this calibration indicate that the TRT unit evaluates ground thermal conductivities of 1.02 and 3.18 W m-1 k-1, respectively.