Universität Tübingen
Heat transport modeling in shallow aquifers. The role of thermal dispersion in aquifers and heat conduction into confining layers
Abstract
Heat transfer by conduction through the fluid and solid phase and advection through the moving water mainly determine the heat transport in the subsurface. Additionally, heat transfer by thermal dispersion occurs due to differential advection at the pore-scale and heterogeneities of the permeability field at macroscopic scales. Compare to solute diffusion coefficients, heat diffusion coefficients are normally higher. This difference has led to dissimilar ways of treating the processes and the boundary conditions in heat and contaminant transport simulations. In particular, effects of hydrodynamic dispersion in aquifers and diffusion through the confining layers. Most conceptualizations of heat transport in the underground neglect these processes and the error introduced is not often evaluated. Hence, in the present study, an analytical and numerical modeling study is carried out to evaluate the effect of such simplifications on heat transport simulations. Two practical applications of heat transport in the subsurface are used to tackle these problems: Low-enthalpy geothermal systems and natural temperature variations as hydrogeological tracer. A concise discussion of the effect of thermal dispersion on the simulation of temperature plumes in aquifers that evolve from vertical ground source heat pump systems is presented. Results indicate that the effect of thermal dispersion on the temperature plume distribution around a borehole heat exchanger can be neglected if thermal dispersion depends only on the Peclet number. On the other hand, thermal dispersion has a larger effect on temperature plume distribution when thermal dispersivity is assumed to be scale-dependent for aquifers where medium sands and gravel dominate. In order to evaluate the effects of conduction into confining layers, an analytical solution which considers groundwater flow and axial effects is developed. Comparison with existing analytical solutions which either neglect groundwater velocity or axial effects shows the importance of these two processes when evaluating the temperature response of a borehole heat exchanger. In general, for moderate groundwater flow scenarios, the combined effect of advection and axial effects must be accounted for. The importance of axial effects is also corroborated by analyzing the heat transport process by using heat as a tracer. Comparison of different conceptual simplifications of an infiltrating river carrying a temperature signal into an adjacent aquifer is carried out. Results show that temperature propagation within the aquifer can by highly influenced by conduction through the unsaturated zone and the underlying aquitard. The analytical and numerical modeling approaches presented in the present PhD thesis contribute to advance in the understanding of heat transport in the subsurface, mainly in the regulation, monitoring and design of low-enthalpy geothermal systems and hydrogeological interpretations by means of temperature signals.
Author and committee
dc:creator, dc:contributor.*- Author
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- Molina Giraldo, Nelson Alejandro
Identifiers
dc:identifier.*- Identifier
- hdl:10900/49570