Stability of packed bed thermoclines

Packed bed thermoclines have attracted considerable interest as an economical method for storing large amounts of thermal energy. They are a constituent part of a range of proposed thermo-mechanical energy storage systems, such as Adiabatic Compressed Air Energy Storage (ACAES) and Pumped Thermal Energy Storage (PTES). The low cost of the thermal storage media (crushed rock or gravel) means that even with the cost of the required compression and expansion equipment, these systems potentially have a lower Levelised Cost of Storage than batteries, especially for grid scale storage. Packed bed thermoclines rely on a stratified temperature gradient (thermal front) between heated material at the top and cooler material at the bottom. The stability of this thermal front can affect the exergetic efficiency of the store. We present a simple criterion for the stability of a thermal front and show that during discharge of a hot store, a small cold perturbation in the thermal front can develop into a cold tunnel that propagates ahead of the main thermal front. By contrast, the presence of a small hot perturbation at the thermal front prior to charging with hot gas is shown to be quickly dissipated. We also calculate a theoretical critical perturbation size required for a cold tunnel to develop ahead of the thermal front. Below this size transverse thermal diffusion is able to dissipate perturbations before they can develop. Three dimensional Computational Fluid Dynamics simulations are used to accurately visualise thermal front instabilities and also to quantify their effect on the exergetic efficiency of a cycling thermal store. Adding a small high void fraction region near the bottom of the thermal store caused a significant disruption of the thermal front on each discharge cycle and resulted in a 4.5% increase in the exergy loss rate. Low void fraction adjacent to the walls of the thermal store, which typically occurs during packing, caused a more significant 63% increase in the exergy loss rate relative to a uniformly packed thermal store.