Geothermal potential of groundwater in tunnel drainage system: A case study of dublin port tunnel

Underground tunnels offer significant potential for shallow geothermal energy utilization due to their extensive contact area with surrounding geological formations; however, this resource remains largely untapped. This study proposes an Underground Tunnel Open-loop Geothermal System (UTOGS), which harnesses tunnel drainage groundwater as a low-temperature energy source for district heating. The system's principles and applicability are presented, with emphasis on the required hydrothermal conditions and the tunnel's waterproofing and drainage structures. The feasibility of the system is assessed through a case study of the Dublin Port Tunnel (DPT), specifically investigating the potential and technical challenges based on a decade of valuable field data, including long-term monitoring records of drainage flow, groundwater temperature, and water quality. Theoretical analysis indicates that the DPT provides a continuous and thermally stable groundwater source, estimating an annual thermal output of 765 MWh, sufficient to meet the heating demands of approximately 43 apartments in a densely populated area of Dublin. Water quality analysis highlights the necessity of water treatment to mitigate scaling and ensure long-term system performance. This study demonstrates the technical feasibility and energy potential of retrofitting existing tunnel infrastructure for geothermal applications with this system, offering practical insights for integrating energy systems into underground infrastructure.