Impact areas

As advancements in seasonal Thermal Energy Storage (TES) have progressed through generations, from the pioneering pilot projects to the current fourth-generation focus on effective storage integration, numerous improvements have been made. New waterproofing and thermal insulation materials have already been applied in the second generation. Efficiencies were thus increased (e.g. through optimised loading and unloading systems) and the first problems (especially leakages) were solved at the same time. In the third generation, the priority shifted to cost reductions, for instance by using prefabricated elements for thermal insulations or structural elements of TTES. The new insulation techniques especially bottom thermal insulation improved the efficiencies, while composite foils with vapour diffusion layers and testable waterproofing techniques further reduced water losses. Today, at the fourth-generation level, most attention focuses on effective storage integration and operation in larger networks. This is complemented by tuning temperature levels and combining different energy sources. Nevertheless, several unresolved critical issues remain that require further attention. Issues are attributed to the next, fifth generation. Technically, improvements and new developments of suitable materials are needed. Achieving long-term robustness is a widespread challenge of existing sites, e.g. due to structural fatigue of waterproofings. TES need to keep energy losses at a minimum over a lifetime of several decades, not only for the sake of storage efficiency but also to minimise environmental risks.  However, in practice, suitable monitoring and control systems are required to save the ambient ground and groundwater environment. Regarding TES operation, the vast opportunities to integrate new and diverse energy sources are still not exploited. Most TES rely on solar energy, but smart integration in heating and cooling networks may also facilitate industrial excess energy, geothermal energy, and waste heat from office buildings and data centres. Aside from this, the optimal use of TES requires attuned control engineering.

Finally, a future approach for more flexibility is the consideration of combined storage systems that represent multi-storage solutions of different sizes and different temperature levels. Such solutions offer not only more flexibility but also can be upgraded more easily in case of network expansion or innovative materials. Such modular implementation of TES can also reduce the high initial costs of construction, which are often decisive for the realisation of a project.