Doctor of Philosophy
Dr. Rob Schincariol
Assessment of the current approach taken by guidelines and design methods of vertical closed loop heat exchangers shows that often groundwater flow is either disregarded or is not methodically incorporated. The state of scientific research in this arena reveals that overlooking the groundwater flow in the design procedure may not always be a correct assumption. The significance of advective heat transport compared to conduction is defined by the groundwater flux or Darcy velocity which heavily depends on the hydraulic conductivity of the ground, followed by the hydraulic gradient which has a relatively limited range. A sensitivity analysis on ground and borehole properties ranks groundwater flux together with the thermal conductivity of the ground and the temperature gradient between the antifreeze and the ground (i.e. inlet and background temperatures) as the key factors defining the heat exchange efficiency. The study confirms that the effect of groundwater advection on an operational borehole heat exchanger (BHE) becomes notable at fluxes ≥10-7 m/s; fluxes ≥10-8 m/s accelerate the returning of ground temperatures to the initial background temperature (i.e. thermal recovery) when the BHE is not operational. Examining the groundwater flow impact on multiple BHEs shows that as increasing the number of boreholes causes larger temperature disturbances, the effect of advective transport becomes more substantial. The thermal interference between BHEs induced by groundwater flow in line arrays can be of higher relevance than square arrays, depending on the flow direction. Although the BHE spacing is a major design parameter, in the long-term groundwater flow may be more critical to improving the thermal performance of the system as it considerably shortens the time to reach steady state. The effect of hydrogeological inhomogeneities, i.e. fractures, depends on their dip angle. Modeling of vertical features up to 10 m away from a BHE with aperture ≥1 mm, which can be recognized through geological investigation techniques but not thermal response testing (TRT), shows long-term impacts. Depending on the openness and distance from the borehole, one major fracture has the most influence on the BHE. For horizontal features, fracture frequency is the key parameter to consider.
Dehkordi, S. Emad, "Hydrogeological and Thermal Sustainability of Geothermal Borehole Heat Exchangers" (2013). Electronic Thesis and Dissertation Repository. 1722.
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