Analysis of heat transport mechanisms in packed-bed thermal energy storage using liquid metal as heat transfer fluid and simulation of a lab-scale experiment

Thermal energy storage (TES) systems are a crucial component of pumped thermal energy storage systems, so-called Carnot Batteries, and are expected to play an important role in the energy transition. As a possible TES configuration, a packed-bed with liquid metal as a heat transfer fluid has been proposed. To optimize and further investigate this concept, simulations of the thermocline propagation in the packed-bed TES are required. For this purpose, one-dimensional heterogeneous continuum models are frequently used. The heat transport mechanisms and the effective transport parameters needed for these models have been extensively studied for conventional fluids. However, due to the high thermal conductivity of the liquid metals used, the governing heat transfer mechanisms differ fundamentally from those of conventional fluids. In this work, the heat transport mechanisms present in the TES were examined for different Péclet numbers. The analysis revealed that axial heat transport across the thermocline region is the dominant mechanism. By contrast, the heat transfer between particles and fluid, as well as the intra-particle heat conduction, contribute only marginally to the overall axial heat dispersion and, consequently, to thermocline propagation. For low Péclet numbers, where molecular heat conduction dominates, axial heat dispersion can be described for liquid metals using established correlations. For these Péclet numbers, the model developed here was successfully validated against experimental data ( Pe 0 ≤ 0 . 32 ) . However, for moderate Péclet numbers, which may occur in industrial-scale TES, the effective transport parameters are subject to high uncertainties. Further research is therefore needed in this area.