Numerical CFD Simulation of Geothermal Heat Exchangers

Alexander Steurer

Research output: ThesisMaster's Thesis

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Abstract

This master thesis was carried out in cooperation with the Austrian Institute of Technology (AIT) as part of the GEOFIT project. This research project is embedded in the European funding programme Horizon 2020. The aim of the present work is to record and document the near-field properties of novel, space-saving geothermal heat exchangers (earth baskets and trench collectors) in different soil types. In addition to the experimental test set-ups, simulations are used to create a reproducible physical image and provide data for a dimensioning tool. The data from the near-field simulation are also an important input for further considerations in far-field simulations. These are carried out by the European partner institute EURECAT. An experimental test set-up under controlled conditions in a climate chamber at the AIT serves as basis and comparison for various simulation series. The spatial and temporal resolution of the temperature field is recorded and provided by fibre-optic sensors and PT1000 sensors. Since a sufficiently accurate 3D model representation of the experimental setup turned out to be very resource-intensive, both a simplified 2D cylinder model and a 1D rod model were developed as alternatives. In addition, the correctness of the calculation algorithm could be proven by the 1D rod model. Tests with different time steps result-ed in significant reduction of the calculation effort by choosing optimal simulation parameters. The first series of experiments, carried out with dry sand, showed clear differences in the temperature field compared to the simulation. Various simulation series with temperature dependent material parameters from laboratory tests were able to systematically exclude error causes. It was also possible to gain an understanding of the various adjusting screws of the temperature field, which is mathematically described by the partial DGL of heat conduction. The thermal properties of the soil are unambiguously determined by the thermal diffusivity a. It is important to realise, that the temperatures reached in the steady state of the system depend exclusively on the value of the thermal conductivity ?, while the transient solution depends on the combination of the factor ?*cp. Furthermore, the European research partner EURECAT was provided with temperature distributions and heat fluxes on a cylinder jacket in order to carry out further global field simulations. A final parameter study using different pitches and diameters of the heating helix gives insights into first tendencies regarding the heat flux. This should provide a base line for further international scientific discourse and promote further development of shallow geothermal heat exchangers as part of heat pump systems.
Translated title of the contributionNumerische CFD Simulation von geothermischen Wärmeübertragern
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Sifferlinger, Nikolaus August, Supervisor (internal)
  • Reichl, Christoph, Supervisor (external), External person
Award date22 Oct 2021
Publication statusPublished - 2021

Bibliographical note

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Keywords

  • CFD simulation
  • Shallow geothermal heat exchangers
  • Thermal properties of soil
  • Earth baskets

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