Impact of groundwater flows on borehole heat exchangers: from in-situ observations to ground parameter identification

Valériane Gigot

Onderzoeksoutput: PhD Thesis


Ground source heat pump (GSHP) system combined with boreholes heat exchangers (BHEs) is increasing worldwide. This technology consists in circulating a fluid within a U-shaped pipe to generate heat exchanges between a BHE and the ground surrounding it, thanks to the temperature difference between the fluid and the ground. BHE can be used for extracting heat (for heating purpose) or injecting heat (for cooling purpose) from/into the ground. In this framework, the presence of groundwater enhances the heat exchanges, as the intrinsic thermal conductivity increases when the pore space is filled by water. Additionally, groundwater fluxes induce advective and dispersive heat exchanges. However, the huge majority of existing norms to dimension multi-BHEs fields only consider conductive heat exchanges, using an apparent ground thermal conductivity inferred from a thermal response test and that generally includes the combined effect of conductive, advective and dispersive heat transfers. The real heat transfers are thus not properly considered in most of the design approaches. Besides, the determination of both the intrinsic ground thermal conductivity and groundwater fluxes (magnitude and direction) requires the characterization of the temperature evolution, not only along an activated BHE, but also around it. However, there is nowadays a lack of experimental studies measuring the heat plume propagation around an activated BHE.

Therefore, the first contribution of this thesis is to develop an experimental set-up able to monitor the temperature field around an activated BHE. The set-up is implemented on a previously-developed experimental platform made up of four BHEs, that cross an unconfined aquifer. The thermal activation of one BHE and the monitoring of the temperature through PT100 sensors installed at different depths in the activated and non-activated BHEs provide an appropriate characterization of the heat plume propagation in both saturated and
unsaturated domains of the unconfined aquifer.

Based on the temperature field measurement, the second contribution of the thesis is devoted to the development of a methodology able to identify the ground hydro-geothermal parameters, including intrinsic thermal conductivity, volumetric heat capacity and groundwater fluxes (velocity and direction). From an analytical solution based on the Finite Line Source model and providing the temperature field around an activated BHE under discontinuous heat loads and considering conductive, advective and dispersive heat transfers, the hydro-geothermal parameters of the ground are calibrated through a stepwise approach based on the measured temperatures evolution. The methodology requires a preliminary knowledge of the geological context of the area (lithology, aquifer, etc.) in order to constraint the ground parameters identification problem through the definition of acceptable range of values for each parameter. The results highlight first the relevance of the identified ground parameters. Also, it is demonstrated that the ground parameters deduced from a conventional Thermal Response Test approach would not be able to properly reproduce the thermal plume around the activated BHE, as the present methodology can do, because the different kind of heat transfer are much better considered. In particular, in addition to thermal conduction and advection, the thermal dispersion play a significant role. Finally, it is shown that the calibration of ground parameters with the present methodology is sensitive to the relative location of measuring sensors with respect to the activated BHE. As a consequence, the possible inclination of BHEs could have a significant impact on the identified parameters.
Originele taal-2English
Toekennende instantie
  • Vrije Universiteit Brussel
  • Université libre de Bruxelles
  • Gerard, Pierre, Promotor
  • Huysmans, Marijke, Promotor
  • Francois, Bertrand, Promotor
Datum van toekenning23 nov 2022
StatusPublished - 2022


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