The thermal energy storage potential of underground tunnels used as heat exchangers

This paper presents an unprecedented investigation of the thermal energy storage potential of underground tunnels used as heat exchangers, often called energy tunnels, with a focus on seasonal, medium-temperature thermal energy storage applications. The study is divided into two parts. First, this work defines fundamental physical variables for assessing the thermal energy storage potential of tunnels and other underground structures used as heat exchangers. These are the heat carrier fluid temperature at the inlet and outlet of the pipes, the stored and extracted thermal power, the stored and extracted thermal energy, the storage losses, the storage efficiency, the storage and specific storage capacity, and the thermally induced stresses and displacements. Second, this work numerically investigates the trend of the expanded physical variables for a thoroughly characterized case study under different operating conditions. These are operations characterized by different charging-discharging profiles, charging-discharging temperature differences, ground saturations, and ground uniformities. This study indicates storage efficiencies of energy tunnels of up to about 70%. Therefore, energy tunnels have marked potential to store massive amounts of thermal energy in the shallow subsurface for subsequent reuse. From this perspective, the employment of underground structures and infrastructures as thermal energy storage means appears promising to establish resilient and sustainable energy systems that can serve urban areas from the building to the district scales.