Numerical and experimental study on thermal performance of ground source heat pump systems
Creators: name: LI, Wenxin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401706
subject: Numerical modelling and mechanical characterisation

description: With the increasing demands on building services and indoor thermal comfort, heating, ventilation and air conditioning (HAVC) systems have claimed almost half of the energy consumed in modern buildings. On the other hand, the increasingly severe global warming and energy crisis have made it urgent to explore appropriate renewable energy for building use, particularly for the HVAC systems. Ground source heat pump (GSHP) systems, which utilise shallow geothermal energy to heat or cool buildings, have been widely applied around the world in recent years due to their high energy-efficiency and environmental friendliness. The efficiency of the GSHP system strongly relies on its operation performance, which generally originates from variations in the system heat load, geological structures and design parameters, etc. Since the performance prediction is usually carried out by analytical or numerical models, its modelling accuracy can also be a potential factor of the system performance evaluation.

Accurate prediction of system operation performance has shown the importance of considering various influential factors for the complicated GSHP system. Among them, some factors including dynamic load variation, system accessory, boundary conditions, ground stratification and groundwater advection worth being further considered and investigated. Meanwhile, the heat transfer mechanism of ground heat exchangers (GHEs) has not been well studied and can hardly be satisfied with the existing experiments and simulation methods. Computational fluid dynamics (CFD) has been proven as a cost-efficient approach to analyse and optimise performance prediction of GSHP systems. However, a holistic optimisation of GSHP system modelling is still absent in existing literature due to the extreme complexity of the impact factors with their relationships unknown. Therefore, this study aims to present comprehensive descriptions of thermal characteristics and operation performance of GSHP systems and further improve the CFD modelling accuracy.

The main body of this thesis consists of nine chapters. The first two chapters introduce the research background and summarise literature with research gaps identified. The research methodology is explained in Chapter 3. Then, the main research contributions are demonstrated from Chapters 4 to 8. Specifically, Chapter 4 and 5 discuss the impact of heat load input on the GSHP system performance. Chapter 4 presents an integrated predictive model for the long-term performance assessment of GSHP systems, emphasising the importance of comprehensively considering the dynamic heat loads of the buildings and heat pump units. Chapter 5 compares the thermal distributions of a GHE predicted by a 3D numerical model with two commonly used thermal boundary configurations. A 2 °C discrepancy found in the prediction result demonstrates the significance of adopting proper thermal boundary conditions in numerical simulations. Meanwhile, Chapter 6 to Chapter 8 introduce a small-scale laboratory apparatus with two U-tubes in a double-layered geological structure to investigate the effects of complex geological structure (ground stratification and groundwater advection). Through a series of experiments and a 3D numerical model, Chapter 6 finds the ground stratification primarily affects the ground temperature distribution, which can become more prominent if affected by more heat loads. Chapter 7 compares predictions from two numerical models with varying and uniform axial heat load distributions along the length of GHEs, and recommends using the variable one to avoid a 76.6% overprediction. Additionally, the ground advection is further considered in Chapter 8, which can redistribute the heat and enhance operational and recovery performances. All the findings and contributions presented in chapters 4 to 8 are concluded and highlighted in Chapter 9.

In summary, this thesis presents a comprehensive analysis of the operating performance of GSHP systems. This research contributes to the following outcomes: (a) A comprehensive understanding of various impacts on the thermal performance prediction of GSHP systems, including system operational characteristics, ground stratification, groundwater advection and modelling boundary conditions; (b) An integrated predictive model for the long-term performance assessment of GSHP systems, considering the dynamic heat loads of the buildings and heat pump units; (c) Experimental investigations to the heat transfer mechanism of GHEs in layered subsurface with/without groundwater advection based on a small-scale laboratory apparatus; (d) Various CFD models and suggestions on boundary conditions to predict the long/short-term operation performance of GSHP systems systematically and accurately. The studies presented in this thesis lay a solid foundation for the prediction, evaluation and optimisation of GSHP systems, which can also be a supplement to the current industry standards.