Thermal Activation of Tunnels of Different Sizes

Today’s world is facing two issues regarding its energy demand. First, it needs secure and accessible energy sources for supporting humans lives to allow their progress. Second, fossil fuels as the main sources of energy are threatening the future of life on earth due to their emissions. Shallow geothermal energy as a renewable energy source is a promising subject of study whose real-world implementations can contribute to overcome these issues. This research focuses on energy tunnels as a main energy geostructure that allow shallow geothermal energy exploitation from the ground. The geothermal performance of energy tunnels is dependent on geometrical and thermophysical parameters of the tunnel and its surroundings, i.e. ground and tunnel air. However, the effect of tunnel size has not been studied on this energy geostructure’s thermal output. Therefore, in this research the diameter of tunnels with segmental lining is chosen as a study parameter that represents the size of an energy tunnel. To investigate the role of the diameter on the geothermal output of an energy tunnel, a finite element software (i.e. COMSOL) is used for simulating the thermo-hydraulic behavior of the tunnel. The numerical model was validated using the results of previous real field test campaigns. Subsequently, the validated model was used to capture the geothermal output of the tunnel under an extensive parametric study that investigated the effect of seasonality, tunnel diameter, groundwater flow velocity and direction, intrados thermal boundary condition, and ground temperature and thermal conductivity. The study concluded by implementing the ratio of groundwater flow velocity over inner diameter as a characteristic parameter that allows plotting design charts for preliminary assessment of energy tunnel projects. These charts showed good agreement with some available numerical and real case studies, indicating a higher reliability compared to more generalized design charts that did not consider the role of diameter in geothermal performance of energy tunnels.