A comparison of packed-bed flow topologies for high-temperature thermal energy storage under constrained conditions

Packed-bed thermal energy storage (PBTES) systems are cost-effective when natural rocks and industrial by-products are used as filler materials. In addition, its combination with gaseous heat transfer fluids (HTF) might increase operating temperatures. Nevertheless, the state-of-the-art design of PBTES containers has several drawbacks related to thermal losses and thermocline control in the flow direction. The present article develops a comparative study of four PBTES flow topologies: a conventional vertical cylinder, a truncated cone shape with two flow directions, and the novel radial-flow packed-bed concept. Considering the operational constraints of a high-temperature Concentrated Solar Power (CSP) plant using atmospheric air as HTF, the cyclic behavior of the system is modeled to determine the thermal and exergy performance. Although radial-flow PBTES is a potential candidate for reducing overall thermal losses by 63% against conventional approaches, the spread of the thermocline through the radial direction causes the reduction of the utilization factor, delivering the required thermal power for a limited time. Also, conventional designs are 54% cheaper in terms of costs per stored energy. A parametric analysis of the design and operational conditions suggests that increasing the cut-off charging and discharging temperatures maximizes the utilization factor, but designing for minimum costs and exergy losses increases the round-trip efficiency, limiting the charge-discharge operation. This work contributes as a guideline to size and select operating conditions of different PBTES topologies according to first and second-law approaches.