Heat exchange and thermal interactions of twin energy tunnels in sand

In recent years, energy tunnels have gained significant attention as sustainable solution for heating and cooling demands in urban environments. Meanwhile, with the increasing demand for underground space, twin tunnel configurations are often adopted to optimize space utilization and accommodate dense city infrastructures. Therefore, this paper investigates the thermal performance of twin energy tunnels operating simultaneously. A numerical model, based on an approach validated against field test results of Turin Metro line 1, was developed to study the influence of groundwater flow velocity, groundwater level, and clear distance on heat exchange rates and thermal interactions of twin energy tunnels embedded in sand. The results show that when tunnels are fully submerged under the ground water table, the thermal plumes induced by the upstream tunnel decrease the heat exchange efficiency of the downstream tunnel. This decreasing efficiency continues to increase with increasing groundwater flow velocity up to 0.5 m/d. Beyond this velocity, the growing influence of thermal recharge becomes more significant, reducing the thermal plumes effect of one tunnel to the other. At the same time, a smaller clear distance results in higher thermal interactions and consequently lower efficiency of the downstream tunnel. Additionally, the optimal clear distance was observed to significantly depend on groundwater flow velocity. The influence of thermal plumes of the upstream tunnel become less significant when the groundwater level at or below the tunnel invert. The performance of both tunnels was found to remain consistent over three years of thermal operations, and this is attributed to groundwater flow and sufficient recovery periods.