Analysis of heat transfer by pile and borehole ground heat exchangers
Student thesis: Doctoral Thesis
Related Research Unit(s)
The need of energy saving in buildings has evoked a great interest in designing and modeling ground heat exchangers (GHEs) used in ground-coupled heat pump systems (GCHPs). This dissertation emerges in response to long-standing difficulties of promotion of GCHPs, which analyzes heat transfer by pile and borehole GHEs using several analytical and numerical techniques, including the Kelvin’s theory of heat sources, the method of images, the method of Laplace transform, and Monte Carlo simulation. Using an analytical heat transfer model of borehole GHEs, two aspects of the parameter estimation problem of short-time in situ thermal response tests are investigated. A close examination of analytical sensitivities yields an ordered list of sensitivities of estimated parameters and testing uncertainties, which is significant for mitigating uncertainties during practical tests. Performance of two optimization algorithms, the Levenberg-Marquardt method and a bounded trust region method, is evaluated by Monte Carlo simulation, indicating that the greatest challenge is the way of estimating thermal diffusivity of soil accurately and precisely. Two real in situ response tests, performed in Mainland China, are used to support these theoretical analyses. To obtain a better alternative to modeling short-time dynamic responses of GHEs, this dissertation develops new composite-medium heat-source solutions. These solutions involve transient effect within bores or piles, as well as property difference between soil and pile or grout, while they remain flexible to model various configurations of pile and borehole GHEs. Moreover, several explicit solutions are derived for heat conduction in infinite and semi-infinite anisotropic media with internal line, spiral-line, or cylindrical-surface sources. Analysis of these solutions shows that anisotropy of soil has little effect on short-term performance of GHEs but it apparently influences their responses on time scales as long as tens of years. The dissertation closes by a thermodynamic optimization of borehole GHEs, using the concept of entropy generation minimization. Two explicit expressions for dimensionless bore length and Re number are developed, firstly illustrating how to incorporate a heat transfer model into a thermodynamic analysis to guide design of GHE loops. The findings and models of the dissertation may contribute to the development of a viable approach to analysis, design, and simulation of GHEs.
- Heat exchangers, Heat, Energy conservation, Transmission