| Abstract |
The thermal response test (TRT) is a common method to investigate the crucial subsurface heat transport parameters to design sustainable ground-source heat pump (GSHP) systems. During the test, the borehole heat exchanger (BHE) is heated up with a defined amount of energy by circulating warm heat carrier fluid. The temperature change between BHE inlet and outlet is recorded, and it reflects the ability of the BHE to transfer heat or cold to the ambient ground. Based on the Kelvin line source theory, the effective thermal conductivity of the ground can analytically be derived. In grouted BHEs, which are typical in central Europe, the analytical line source can also be used to estimate the borehole resistance. However, the standard parameter estimation procedure has substantial limitations. A main shortcoming is that the Kelvin line source pretends that there is only conductive heat transport towards or from the BHE. The effective thermal conductivity thus is an apparent parameter, which diverges from the true one with increasing advective heat transport stimulated by flow of groundwater. In order to overcome this limitation, we present a new parameter estimation procedure, which utilizes the moving line source. Similar to the Kelvin line source, the proposed alternative is an efficient analytical method. As enhancement, it enables to separate the conductive and the advective heat transport component during thermal response testing. Due to the competitive character of both components, calibration reveals equally possible parameter combinations. To overcome this critical point an appropriate calibration procedure is necessary to scan all non-unique solutions. The applicability of the moving line source is verified by high-resolution numerical simulation as well as it is validated at a range of different field and laboratory case studies. The employed parameter estimation approach examines optimal and close-optimal solutions, and this way shows potential parameter correlations or value ranges. The findings from the validation are that (i) there is a distinct correlation between identified thermal conductivity and Darcy velocity of groundwater, (ii) for a Péclet (Pe) number less than 0.1, the result is insensitive to the velocity, (iii) for moderate velocities, the range of the determined parameter pairs is unequivocal, (iv) for Péclet numbers around 1, a wide range of correlated parameter couples are suitable. Even so, for high groundwater velocity (Pe ~ 1), additional information of the thermal conductivity (e.g. estimation of thermal conductivity values based on drilling cuttings) can be exploited to significantly confine the estimated possible range of the Darcy velocity. The novel analytical method thus widens the application range of the TRT to groundwater-influenced conditions beyond a Darcy velocity of 0.1 m day-1. |