Analysis and optimisation of packed-bed thermal reservoirs for electricity storage applications

Several emerging electrical energy storage technologies make use of packed-bed reservoirs to store thermal energy for subsequent conversion back to electricity. The present paper describes analysis and optimisation of such reservoirs under transient and steady-state cyclic operation. The focus is on thermodynamic issues, but a simple costing model is also included in order to determine the influence of cost factors on the main design parameters. A major part of the paper is devoted to segmentation (or layering) of the packed beds, which has previously been proposed as a means of simultaneously attaining high storage efficiency and full utilisation of the reservoirs. As illustrative examples, three different reservoirs are modelled, corresponding to the hot and cold thermal stores of a pumped thermal energy storage system, and a larger thermal store suitable for integration with adiabatic compressed air energy storage.