Cascading pit thermal energy storage: enabling simultaneous heating and cooling in centralized district energy systems

Fifth generation district heating and cooling (5GDHC) networks enable simultaneous heating and cooling and the integration of renewable heat at low temperatures, but rely on distributed, hydraulically complex architectures. Centralized district heating and cooling (DHC) systems are operationally simpler, yet conventional centralized storage in a single large volume spanning a wide temperature range limits thermal stratification and cannot serve heating and cooling simultaneously, which precludes renewable integration in systems without chillers. This study introduces a cascading storage concept in which the total seasonal storage volume is distributed across three units: a high temperature unit, a low temperature unit, and a buffer unit between them. Because the storage is split into narrower temperature ranges, the cascade preserves stratification through its architecture rather than within a single wide volume, enabling simultaneous heating and cooling and the integration of external heat at suitable temperatures from a centralized design. The cascade is evaluated with dynamic system simulations and benchmarked against a centralized single storage base case and against bidirectional and reservoir network configurations under identical boundary conditions. The uninsulated cascade reaches a stratification deviation index of 0.13 after 90 days, outperforming the insulated single storage case (0.23), cuts electricity consumption by 46% versus the base case, and reaches a seasonal performance factor of 5.8 that matches the 5GDHC benchmarks at comparable cost. Beyond new installations, the concept offers a route to decarbonize existing centralized DHC networks by adding cooling and integrating renewable heat at lower supply temperatures, without converting to a distributed architecture.