Thermal energy storage in a tightly packed bed with sodium as the heat transfer fluid: A numerical study

Liquid sodium is widely recognised as an outstanding heat transfer fluid for thermal power generation systems. In the context of concentrating solar power, liquid sodium is considered an enabler of modularity and an option for applications at a temperature higher than is feasible with existing molten nitrate salt. However, storing energy as sensible heat in sodium is not desirable because it is expensive and potentially hazardous when stored in large quantities. This work investigates, via experimentally validated computational fluid dynamics, the performance of a tightly packed bed thermal energy storage (TES) unit. Here, magnesia is proposed as the bulk storage media in direct contact with the liquid sodium. The proposed tight packing of magnesia bricks reduces liquid sodium inventory by >95% of the TES total volume and avoids ratcheting caused by thermal cycling. The energy recovery rate for a charge/discharge cycle (defined as the ratio of input to output energy) was found to be as high as 85% for the initial cycle, then reaching >99.5% at the fourth cycle, noting that we assume no thermal losses through the tank walls. Despite the narrow fluid channels between bricks, pressure drop is predicted to be low, much lower than in conventional pebble-based packed bed TES systems. The effect of brick geometry, size, and flow rate on the storage performance is analysed. The findings are applicable to the design of safe, inexpensive, and efficient TES technologies compatible with liquid sodium, enabling high-performance thermal energy systems.