Energy tunnels: A review of the state of the art and knowledge gaps to harness renewable energy from underground infrastructure.

• Review paper on the application of energy tunnels to harness renewable energy. • The analytical, experimental and numerical studies are summarised. • Thermal and thermo-mechanical behaviour and influential factors are discussed. • Challenges and recommendations for future research are identified. The thermal activation of underground tunnels, also known as energy tunnels, has shown significant potential to harness geothermal and aerothermal energy as a low-carbon and economical solution for space heating and cooling, water heating and road de-icing. The heat and load transfer mechanisms in energy tunnels are, however, far less understood compared to other energy geostructures, due to their complex interaction with the ground and tunnel environment. This paper aims to provide a comprehensive overview of the current state of knowledge on the thermal and thermo-mechanical performance of energy tunnels based on recent analytical models, field tests, model-scale experiments, and numerical studies. Results suggest that a thermal yield ranging from 5 to 6 W/m2 is attainable for each degree Celsius difference between the fluid inlet temperature and the ground temperature in the presence of groundwater flow and tunnel airflow. In the reviewed studies, the thermally induced tunnel structural responses appear to be insignificant compared to those caused by tunnel excavation. However, the long-term thermally induced ground deformation in clay deposits can reach up to 15 mm, which may become a crucial issue and therefore needs to be examined in geotechnical design. As a new area of research, there is a lack of long-term experimental investigations to solidify the current knowledge of the thermal, structural and geotechnical performance of the system. Further research is also required to develop an integrated analysis and design method considering the techno-economic aspects and explore innovative business partnership models. This work can serve as a useful resource for researchers, engineers, and policymakers seeking to further develop and promote the adoption of this technology in infrastructure projects. [ABSTRACT FROM AUTHOR]