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Title:Using MT3DMS for simulation of field-scale heat transport with groundwater advection for applications to borehole geothermal systems
Author(s):Zong, Yifei
Advisor(s):Valocchi, Albert J
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:M.S.
Genre:Thesis
Subject(s):numerical simulation
MT3DMS
heat transport
groundwater
borehole heat exchanger (BHE)
shallow geothermal system
Abstract:A typical ground source heat pump (GSHP) system contains either a single borehole heat exchanger (BHE) or an array of sequential or parallel connected BHEs installed into the subsurface to harvest or store shallow geothermal energy. Mathematical modeling of GSHP systems is critical in the stage of BHE design and performance assessment. The goal of the mathematical simulation of the whole GSHP system is twofold. On one side, the energy performance of the GSHP requires systematic evaluation. On the other side, the ground thermal perturbation under varying operational conditions needs to be predicted since GSHP performance is heavily dependent on subsurface thermal conditions. In practice, it is usually difficult to couple these two aspects. Previous research focuses either on the field-scale heat transport by utilizing analytical solutions or numerical models or the thermal transfer behavior within the borehole by utilizing detailed localized models of heat transfer and storage within the borehole. Rarely are both aspects coupled together. In addition, many studies neglect the influence of groundwater flow on field-scale heat transport. This may lead to erroneous thermal analyses and inappropriate sizing of the boreholes in the design phase. The objective of this thesis is to address these issues through the development of an integrated model that couples MODFLOW/MT3DMS with new self-developed iterative routines to fulfill the prediction of the whole GSHP system. This includes the BHE-induced subsurface thermal perturbation, calculation of the U-pipe circulating fluid temperature within the borehole, and the evaluation of the GSHP system efficiency based on available dynamic building heat load. MODFLOW is used to generate the groundwater flow field under various boundary conditions. The groundwater velocity file is input to MT3DMS, which performs the simulation of field-scale heat transport due to advection, conduction, and mechanical dispersion. The numerical MT3DMS result of the temperature at the borehole wall is linked to the self-developed routine which implements the heat transfer analytical model inside the borehole based on the conservation of energy. The whole process is solved iteratively at every time step. Thus, the dynamic simulation of the GSHP system is realized. The proposed model is validated against analytical solutions for a borehole field by the principle of superposition. The results indicate that the mean absolute discrepancy is less than 1.5%, which is acceptable in engineering applications. It is observed that the performance of the GSHP system decreases relatively fast in the initial stage of operation due to heat accumulation or heat loss in the ambient soil, but tends to stabilize through the course of the operation. Groundwater advection will alleviate the reduction in GSHP efficiency since it will lead to maintenance of the ambient background temperature near the borehole. Also, the system reaches steady-state more quickly under faster groundwater flow regime. Finally, test problems are simulated to identify the influence of field-scale heterogeneity and pumping events on the operation of borehole geothermal systems.
Issue Date:2020-12-08
Type:Thesis
URI:http://hdl.handle.net/2142/109442
Rights Information:Copyright 2020 Yifei Zong
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12


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