Modelling the U-pipe heat exchanger to estimate the borehole thermal resistances from distributed thermal response test data

The correct estimation of the parameters used by the design methods of the borehole heat exchangers (BHEs) is critical to achieve the desired performance of ground-source heat pumps (GSHPs). The present study provides methods for estimating these parameters from a distributed thermal response test (DTRT) for single and double U-tube BHEs. The focus is on the total internal borehole thermal resistance Ra, which is often overlooked by conventional analysis methods. Still, Ra becomes increasingly important as the borehole length increases. Parameter estimates have been obtained with the related uncertainties from a DTRT by using analytical models applied to both a measured data set and numerically simulated DTRT data sets. Results indicate that a uniform borehole-wall heat flux model outperforms a uniform borehole-wall temperature model in estimating the borehole and ground thermal properties. In addition, results demonstrate that using the transient inlet, outlet and bottom fluid temperatures instead of the complete vertical temperature profile in a DTRT is sufficient to give an accurate estimate of the borehole and ground properties, suggesting a more cost-effective measurement approach.