Techno-economic optimization of a packed-bed for utility-scale energy storage

The optimization of a packed bed for utility-scale applications is presented in this paper. The effects that particle size, aspect ratio and storage mass have on the roundtrip exergy efficiency of the store are thoroughly analysed. The paper seeks to provide a clear insight of what ranges of values for the aforementioned design parameters are adequate to consider when designing a grid-scale packed bed. Simulations were carried out using a one-dimensional model that accounts for temperature-dependent properties and self-discharge losses. The assumed operating temperature range for the packed bed is 290-823 K, which is typical of CSP plants and CAES systems. A 24-h work cycle (12 h charge/12 h discharge) with variable power (10 MW peak) and a total energy storage requirement of 79.4 MWh(th) has been considered for the study. It has been found that exergy losses are minimized if a configuration based on an aspect ratio between 0.5 and 0.8 is adopted and the size of the rocks is finely tuned for the specific shape of container. In this work-unlike similar studies-a cost-benefit analysis has been carried out, which indicates that increasing the thermal storage mass leads to a considerable increase in efficiency. A mass overrating of 50% yields the lowest levelized cost of storage for the economic scenario considered. The optimum design obtained from the optimization process has an aspect ratio of 0.6, a particle size of 4 mm and a mass overrating factor of 1.5. This packed bed attained a roundtrip exergy efficiency of 98.24%.