A review of exploiting shallow geothermal energy through tunnels: Current status and future prospects

The utilization of ground-embedded structures for harnessing geothermal energy in space heating and cooling has gained significant attention over the past two decades. In this context, this paper reviews the use of tunnels, commonly known as energy tunnels, for exploiting shallow geothermal energy, providing an overview of the current status and future prospects. The different tunnel construction methods and their respective thermal activation processes are described. The factors influencing thermal performance and mechanical responses are analysed, alongside the economic and environmental benefits. This review confirms the viability of energy tunnels as a sustainable and renewable solution, as several studies have demonstrated their potential in providing a substantial amount of thermal energy and reducing natural gas consumption and significantly cutting CO₂ emissions. Energy tunnels can also potentially provide a reliable solution for road de-icing or enable efficient thermal energy storage when groundwater flow is negligible. Finally, current, and future innovations aimed at maximizing shallow geothermal exploitation are discussed, with a call for further research to explore additional thermal retrofitting techniques for the existing tunnels, innovative additions for lining concrete, fluid and pipe materials and integrating energy tunnels with other renewable energy sources in district heating networks.