Effect of design parameters on the exergy efficiency of a utility-scale packed bed

The optimization of a packed bed for utility-scale applications is presented herein. The paper discusses comprehensively the effects that particle size, aspect ratio and storage mass have on the exergy losses of the store throughout a complete working-cycle and seeks to provide a clear reference of what is an adequate range of aspect ratios to consider for the design of a grid-scale packed bed. A one-dimensional model that accounts for temperature-dependent properties of both, storage medium and heat transfer fluid, and self-discharge losses is used for performing the analyses. The working cycle considered for the modelling work is a 24 h long sinusoidal profile (12 h charge/12 h discharge) with a 10 MW peak power and a total energy storage requirement of 79.4 MWhth. The results show that a substantial improvement in performance can be achieved by adopting a configuration based on an aspect ratio between 0.5 and 0.8 and fine-tuning the particle size for the specific shape of the container. Furthermore, the study reveals that increasing the thermal storage mass leads to a considerable increase in efficiency. It is determined that a design with 50% additional storage mass, an aspect ratio of 0.6 and a particle size of 3.7 mm, is the optimum configuration from a techno-economic perspective, having a roundtrip exergy efficiency of 98.24%. Additionally, the paper presents a brief study on the effect of the frequency of the work-cycle, which serves as an introduction to the concept of optimizing a thermal storage system by splitting the load into multiple frequency components.