Long-term ground response For borehole heat exchangers in clay

Abstract

The use of ground source heat pumps to transfer heat to and from the ground via borehole heat exchangers is among the most energy efficient techniques for space heating and cooling. Designs for seasonal heat exchange systems are becoming increasingly popular in urban environments to reduce energy costs and contribute in decreasing carbon emissions. Broader applications require more careful evaluation of hydro-mechanical behavior of soil to understand the long-term ground response to seasonal cycles of heating and cooling and the impacts on adjacent structures. The goal of this thesis is to develop reliable methods for studying the coupled thermo-hydro-mechanical (THM) response of clay to long-term seasonal heating and cooling induced by vertical heat exchanger arrays in clay. The research investigates and implements a new class of constitutive models based on the work by Zhang and Cheng (2013). The proposed Tsinghua Thermosoil (TTS) model is capable of simulating the cyclic thermo-mechanical response more realistically than other available constitutive models and can describe the accumulation of volumetric strain due to cyclic heating and cooling. Irreversible deformations in the TTS model are simulated through the conversion of bound to free water, a process that has been studied via laboratory measurements of specific gravity variations with temperature for three clays of differing mineralogy. The thesis describes the calibration of the TTS model in detail, using recently published cyclic thermal tests on Geneva clay (Di Donna and Laloui, 2015). The TTS model is integrated within a finite difference framework to solve coupled THM problems in 1-D and axisymmetric spaces. The FD simulator is applied to a prototype project that uses an array of borehole heat exchangers for seasonal heating and cooling. The ground conditions represent typical stress history profiles found in Geneva, Switzerland. An extensive parametric study is undertaken to study the long-term THM response of clay for a broad range of heat exchanger design parameters. The results show that significant settlements can be induced due to their long-term operation (i.e., over periods from 10-50 years), especially within normally consolidated clay. The parametric study gives useful guidelines for heat exchanger design and identifies a design space of optimum solutions that meet pre-specified foundation settlement criteria. The study provides a systematic framework for analyzing the long-term THM response of clay to seasonal heating and cooling. These analyses highlight the need to incorporate geotechnical characterization in the design of large borehole heat exchanger arrays to ensure their reliable long-term performance and to minimize adverse effects on adjacent structures.