Impact of filler size and shape on performance of thermal energy storage

Stratified thermal energy storages (TES) with a packed bed of filler material offer a high potential for cost reduction compared to the state of the art two tank molten salt storage solution. This paper introduces a new one-dimensional numerical model for such TES systems, which innovatively considers the additional mixing at the inlet and its impact on thermal stratification, which is often neglected in models presented in the literature. The experimental data used for validation were obtained in an experimental setup that was designed using a similarity theory approach, which has not yet been discussed in this context in other publications. In the presented experimental setup, similar conditions between a TES operated with molten salt and a water-operated TES could be demonstrated. Furthermore, the influence of filler configuration in loose bulk on key performance indicators (KPI) and thermal stratification was investigated through an extensive parameter study, in which the filler size, but also, uniquely, the geometry of the filler, was varied. The study found that the choice of filler configuration has a notable effect on the performance of TES for fillers larger than 1 cm in equivalent diameter. However, smaller filler configurations generally exhibit improved storage KPIs. Therefore, this study not only enhances the understanding of the dynamic behavior of TES systems but also provides a validated tool for optimizing design and improving efficiency. The introduced similarity theory approach is to be validated in a further study with results from tests with molten salt.
Core message: Simulations validated with water-based tests reveal that the size of the individual filler granule and the packing density have greater impact on thermocline performance than the shape of the filler. Proper fluid distribution is crucial not only during the initial charge but also throughout cyclic operation.