Wave propagation and thermodynamic losses in packed-bed thermal reservoirs for energy storage

Highlights: • Thermal wave propagation in packed beds has been modelled both numerically and analytically. • Non-linear wave 'catch-up' is shown to occur in cold fronts and may result in the formation of shock-like features. • Heat transfer related exergy losses are significant within these thermal shocks, typically 15% of the available energy. • Periodic, cyclic operation of the reservoirs results in lower losses that may be controlled by the particle size. - Abstract: This paper presents a numerical and theoretical analysis of thermal wave propagation in packed bed thermal reservoirs for energy storage applications. In such reservoirs, the range of temperatures encountered is usually such that the solid storage medium will exhibit significant changes in specific heat capacity. This in turn results in non-linear wave propagation and may lead to the formation of shock-like thermal fronts. Such effects have an impact on the exergetic losses due to irreversible heat transfer, and should be taken into account during the design and optimisation of the reservoirs. In the present paper, the emphasis is on thermal losses due to irreversible heat transfer. Frictional (pressure) losses and heat leakage between the storage medium and the environment are also important but are not considered here. The implications of the results for storage material, and particle size are discussed briefly in the context of loss minimisation