Strategies for multi-basin water-gravel thermal energy storage

Long-term heat storage bridges the mismatch between renewable heat availability and demand. Common storage includes large-scale basins, tanks, or caverns that are heated during summer to supply district heating networks in winter. Integrating not only single but multiple separate storages enables new forms of coordinated operation that have yet to be explored. This study reveals enhanced charging opportunities across multiple basins compared to a single basin. For this, the component-based STORE model and its co-simulation framework are expanded. A parametric study with three sub-basins is simulated in seven scenarios, including a non-subdivided base case, six subdivided variants with three charging strategies (parallel, hybrid, serial), and mid-wall insulation scenarios. Results demonstrate that, relative to a single-basin, serial charging can raise the amount of charged energy by 8.7%, discharged energy by 14.9%, and energy efficiency from 0.69 to 0.75. While hybrid charging achieves two-thirds of this benefit, a parallel charging strategy reduces thermal losses by 8.2% but does not increase discharged energy. Furthermore, mid-wall insulation provides no further performance benefits. Finally, serial charging operates sub-basins at different temperatures, enabling heat supply at multiple temperature levels. The findings highlight choices, challenges, and potential benefits of considering multiple sub-basin operations together with insulation and a tolerable amount of thermal loss. This is particularly relevant for large basins that can be subdivided and expanding networks with the addition of further installations. Moreover, the multi-storage concept enhances operational efficiency, especially for suboptimal facilities, including repurposed idle infrastructure, and in environments with strong subsurface interactions.