Review on heat transfer and thermo-mechanical behaviour of energy geostructures

• A comprehensive summary and evaluation of the heat transfer models of energy piles, energy walls, and energy tunnels was conducted, and the factors influencing heat exchange performance of these energy geostructures were summarized. • A summary of thermo-mechanical response characteristics of energy geostructures was carried out. • Comprehensive future perspectives of energy geostructures were presented. Energy geostructures represent a novel building energy-saving technology derived from ground source heat pump technology. Heat transfer and thermo-mechanical response characteristics stand out as pivotal issues in the investigation and design of such energy geostructures. This paper provides an overview of the research on heat transfer models, factors influencing heat exchange performance, and thermo-mechanical behaviour concerning energy piles, energy walls, and energy tunnels. The future perspectives were also presented. Four types consisting of ten basic heat transfer models for energy piles were summarized, and their advantages, limitations, and applicable scenarios were comprehensively discussed from multiple aspects. The heat transfer models for energy walls and energy tunnels are scarce, and only one model was introduced for each of them. The influences of some controllable design parameters on the thermal performance of energy geostructures and the thermal-induced mechanical behaviour were summarized. The key conclusions are that the fluid flow rate should not be too high or too low, which is generally considered sufficient to ensure that the flow state is turbulent; and properly intermittent operation is beneficial to the recovery of geothermy, thereby improving the heat exchange performance. Due to the differing conditions considered, it is not possible to draw a definitive conclusion regarding whether heating can increase or decrease the shaft resistance or bearing capacity of energy piles. Generally, thermal effects within energy walls are unlikely to cause severe damage to structural stability. The issues related to thermal-induced ground deformation are considered more critical than those concerning the energy tunnel structure deformation. This paper highlights the aspects that require further research and the new aspects worth exploring in the future. Energy geostructures are not limited to new construction projects, and combining with other renewable energy utilization methods and integrating into district energy networks are the future development trends.