IAP2-22-463

How soil characteristics affect the efficiency of energy geostructures?

The most reliable source of heating and cooling is the earth. Several metres below the surface of the earth, the temperature is more moderate than the air temperature, cooler in the summer and warmer in the winter [1]. In the same way, an air conditioner transfers heat from a building interior to the exterior air, heat can be moved to the cooler earth in the summer and vice versa during the winter. This energy efficiency solution can play a significant role in both reducing the demand for energy and decarbonisation.
Since the 1980’s building foundations, in addition to their traditional structural function, have been developed as ground heat exchanger to serve the overlying buildings for heating and cooling purposes, which account for a major share of final energy consumption in the residential sector [2]. That makes this technology a viable solution for global warming challenge, especially for developing countries with the increasing demand for electricity for economic development and social welfare. Energy foundations are now becoming more common in the developed countries as these have been proved to be a clean, renewable, and environmental friendly source of energy, which is aligned with the global efforts in promoting renewable power and reducing the CO2 footprint [3].

Over the last decade, an increasing amount of research has investigated the impact of temperature cycles on the thermo-mechanical behaviour of energy piles [4]. However, knowledge of the impact of soil characteristics has been limited due to the lack of fundamental studies. The aim of this study is to investigate the effect of surrounding soil properties on the performance of energy piles through well controlled physical and numerical modelling.

The mechanical behaviour of soils originates from force transmission at inter-particle contacts [5]. The use of discrete numerical modelling approaches enables the calculation of forces acting on individual particles, which is a non-trivial task in experimental approaches. Since its introduction by Cundall and Strack [6], Discrete Element Method (DEM) has been widely used for the modelling of particle interactions within an assembly. It enables to investigate the influence of soil physical characteristics such as roughness [7] and shape [8] on bulk behaviour. More fundamentally, it was shown that the roughness [9] and shape [10] of particles largely affect the contact behaviour between two individual particles. In a similar way, chemical and thermal properties of particles can be included in the simulations. Attempts at incorporating thermo-mechanical contact models in DEM are already underway. However, no particular attention has been paid so far on the effects of soil physical and textural characteristics on thermo-mechanical contact behaviour and modelling bulk behaviour around energy geostructures.

To address this challenge, a well instrumented model-scale test in sand provides the heat exchange and thermal conductivity under different (i) temperature ranges, (ii) particle morphology (i.e. size and shape), (iii) void ratio, and (iv) degree of saturation. The outcomes of the physical model are used to validate a DEM model. The validated model delivers a fundamental understanding of micro-mechanisms, which in turn enhance our decision-making ability and the efficiency of energy geostrucutres.

Aim
The aim of this project is to understand how soil physical and mechanical characteristics affect the performance of energy geostructures by means of physical (macro-scale) and numerical (micro-scale) modelling. The outcomes of this study will help us to understand the micro-mechanisms influencing the performance of energy geostructures and predict their efficiency in the local geological settings.

Objectives
The specific objectives of this project are:
O1: To experimentally investigate the effect of soil characteristics on heat exchange and thermal conductivity performing model-scale energy pile experiments
O2: To implement a thermo-mechanical contact constitutive model in DEM and validate the numerical model based on model-scale experiments
O3: To analyse the influence of particle morphology and distribution on thermo-mechanical behaviour using DEM modelling

The project design and methodology are closely structured around four key objectives, which also provide a timeline for the sequence of study. The main methods employed include:

Conducting model-scale experiments: This step compromises the experimental measurement of thermal conductivity and heat transfer within a scale-model at Heriot-Watt (HW) (O1). Soil parameters such as (i) temperature ranges, (ii) soil particle size, shape and distribution (potential assessment by XRT and ESEM), (iii) void ratio, and (iv) degree of saturation will be investigated. The energy piles will be casted as close as possible to reality by using reinforcement and circulation tubes. The circulation tubes will be connected to a thermal bath, which can apply temperatures below and above the room temperature.

Developing a numerical model based on Discrete Element Method (DEM) to better investigate and understand the grain-scale mechanisms: A DEM model will be developed to consider both thermal and mechanical aspects governing the soil behaviour. An essential step is a careful development of contact models sensitive to the impact of temperature on force transmission. The numerical model will be validated against experimental measurements and observations (O2). The numerical model will be employed to parametrically study and assess the key mechanisms that control heat transfer around geostructures (O3).

Year 1

Literature review and attendance of appropriate training workshops (such as Winter School on Geomechanics for Energy and the Environment at the EPFL, Switzerland (https://gete-school.epfl.ch/). Conduction of preliminary tests using model-scale at HW (O1). Introduction to DEM modelling (O2).

Year 2

Development and implementation of thermo-mechanical contact model (O2); Completion of the physical modelling at HW (O1). Development of a numerical model for energy piles and validation of the model based on experimental measurements (O2). Attendance of national conference (annual GM3: Geomechanics from Micro to Macro) and appropriate training.

Year 3

Performing numerical experiments for parametric study of the problem (O3). Investigation of the effects of temperature ranges, soil particle size, shape and distribution, void ratio, and degree of saturation on the thermo-mechanical coupling of the soil-foundation system. Attendance of national and international conferences to widen the research network and present results for the larger audience.

Year 3.5

Finalise data analysis, Write-up the thesis.

The School of Engineering requires each student to collect at least 60 PGRDP credits, corresponding to attendance of in-school delivered workshops, taught modules and other activities. Training is provided through five mechanisms: (i) a programme of taught modules; (ii) internal training ‘workshops’ that focus on key research skills and techniques; (iii) input from supervisors; (iv) School and research group seminars by visiting and internal speakers and presentations by postgraduate students themselves; and (v) external workshops.

In addition to generic training offered by the University, the School provides training through a series of in-house ‘workshops’. Engineering research postgraduates normally take the following Workshops: ‘Scientific Writing’, ‘Research Ethics (Theory)’, ‘Data Management’, ‘Time management’, ‘Document Management’, ‘Introduction to Learning and Teaching’ during their first year.

The student will benefit from the wide range of taught modules associated with MSc courses in ‘Engineering Geology’ and ‘Geotechnical Engineering’. Modules particularly relevant for the project are ‘Soil Modelling and Numerical Methods’, ‘Geohazards and Deformation of the Earth’. Most of these modules are delivered in one intensive week so well suited for PhD students.

Bespoke technical training will also be provided by the research supervisors (numerical modelling and image acquisition and processing) and by attending external workshops (in particular, the Winter School on Geomechanics for Energy and the Environment at the Swiss Federal Institute of Technology Lausanne).