Thermo-hydraulic and sensitivity analyses on the thermal performance of energy tunnels

Abstract

The efficiency of energy tunnels depends on the ground properties and ambient environment. Another factor that noticeably influences the process is the groundwater supply availability. The design strategy and assessment method may require careful consideration or even adjustment when significant groundwater movements are present. This study provides evidence for assessing thermal exploitation based on parameters in relation to different subsoil conditions. A numerical parametric study based on a Metro Torino line 1 section is performed, followed by deterministic and sample-based sensitivity analyses to determine the relative impacts of different hydraulic and thermal variables on the energy tunnel thermal performance. Results demonstrate that the importance of the groundwater flow direction and velocity on thermal efficiency highly depends on the water table position. Maximum performance improvements of respectively 149%, 127% and 49% can be achieved due to variations of the water table position, water flow velocity and water flow direction in this study. For a long thermally activated tunnel, a water flow parallel to the tunnel axis makes the downstream-most ring approximately 50% less efficient than the upstream-most ring. The temperature difference between the undisturbed ground and the pipe inlet is defined as the most sensitive parameter to the thermal performance of an energy tunnel when the water table is lower than the tunnel lining bottom.