Separating Conductive and Advective Heat Transport in Thermal Response Tests
The thermal response test (TRT) is a common method to investigate subsurface heat transport parameters for the sustainable design of 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 is 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 in using the Kelvin line source is that the heat transport in the subsurface is conductive. Thus, the derived effective thermal conductivity is only an apparent parameter, which does not consider any possible advective heat transport in the aquifer. In order to overcome this limitation, we therefore developed a novel parameter estimation procedure, which utilizes the moving line source. Similar to the Kelvin line source, the proposed procedure also uses an efficient analytical method, which is able to separate conductive and advective heat transport processes during the TRT. 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 and validated by high resolution numerical simulation and a range of field and laboratory studies, respectively. The results show that (1) there is a distinct correlation between the derived thermal conductivity and Darcy velocity, (2) for a Peclet (Pe) number 0.1, the result is insensitive to the velocity, (3) for moderate velocities, the range of the determined parameter pairs is unequivocal, (4) for Peclet numbers 1, a wide range of correlated parameter couples are suitable. 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