Thermodynamic modelling of a thermal energy storage packed bed tank: Exploring the influence of different particle sizes on overall performance

Abstract

Concentrated solar power combined with thermal energy storage (TES) technology is now widely employed in power generation. To enhance heat transfer efficiency during thermal charging and discharging processes, a packed bed TES system has been developed due to its high heat transfer rate and large heat transfer area. To unveil the overall performance of the packed bed, thermodynamic models are introduced to avoid problems like large size and complex structure. However, current developed models are too vague to study the size effect and pressure drop induced by the particle diameter. In this work, a two-solid-phase model is introduced to evaluate the size effect in packed bed TES systems during charging and discharging, utilizing molten salt and natural volcanic ash as the heat transfer fluid and main solid filler, respectively. Compared to single-phase packed bed, introducing small particles to occupy the void between the large particles yields a low void fraction, and thus the energy storage density is improved by 36.4 %. In the meantime, the thermal charging efficiency is enhanced from 54.37 % to 75.64 %. However, the pressure drop is inevitably increased because of the very low void fraction and the increased surface in contact with the fluid. The pressure drop follows an exponential trend with the changes in particle size. Furthermore, the thermocline in the packed bed requires careful consideration, as it corresponds to the location where the maximum pressure gradient occurs. This work provides insights into the effects of the packed bed induced by the particle size, offering valuable information for the design of next-generation TES packed beds.