Numerical Modelling of Geothermal Energy Piles: Thermo-mechanical Behaviour and Thermal Performance

Abstract

In recent decades, geothermal energy piles have been widely used as a sustainable resource for heating and cooling buildings. This relatively new technology aims to utilise the fairly constant underground temperature to address environmental issues by reducing carbon emissions and other harmful chemicals, such as Freon, from entering the atmosphere, by providing alternatives to widely used air conditioning. The application of geothermal energy pile also provides beneficial long-term financial returns because of its low electricity consumption. During building construction, it provides advantages for the vendor and developer by saving land, compared to conventional Geothermal Ground Heat Exchangers (GHEs). The piles provide structural and other functional purposes and therefore offer huge potential economic and environmental motivation for this technology to be further developed. This thesis is based on a numerical approach, the Finite Element Method (FEM), using commercial software ABAQUS to study the performances of geothermal energy piles. Settlement response and thermal performance are two major aspects in the design of geothermal energy piles. Firstly, thermal cyclic loading conditions (heating and cooling the piles) lead to further settlement compared with conventional pile foundations, and its additional settlement may cause structural damage if not considered properly. Secondly, determining the output power of geothermal energy piles is important because grouping the piles may cause spacing effects that would reduce the overall thermal performance. In Chapter 4, numerical modelling of small-scale geothermal energy pile settlement under cyclic loading was conducted and compared with experimental data. Thirty cooling and heating cycles were applied under constant mechanical loading, corresponding to 0%, 20%, 40% and 60% ultimate pile resistance, representing a real situation for 30 years. The results obtained show an increase of irreversible pile head settlement with the thermal cycles. This highlights the importance of pile thermal contraction/expansion in pile/soil interaction under thermo-mechanical loading. Moreover, energy efficiency of energy pile grouping was studied in Chapter 5. Single pile thermal performance was investigated and compared with the analytical solution of finite line source. Then, the study was extended to the pile group for further investigation. Different pile centre-to-centre spacings of 1 m and 2 m were studied to determine the influence of spacing. The study of pile groups enables investigation of the best operational techniques to maximise power output for geothermal energy piles.