Packed Rock Bed Thermal Storage in Power Plants: Design Considerations

Thermal storage in packed beds of rock has been shown to be promising at temperatures up to 600 °C. In order to determine whether packed rock beds might provide thermal storage at a lower cost than existing technologies, it is necessary to predict the cost and to find the optimum design parameters. Experiments below 100 °C suggest that the heat transfer characteristics of packed beds may be predicted with existing correlations. However, the use of irregular, asymmetric rock particles results in pressure drop characteristics which are highly variable and unpredictable. Accurate knowledge of the pressure drop over a range of Reynolds numbers requires an empirical correlation which depends on the specific rock and packing method. A simple cost- optimum method is presented to determine the particle size and bed length of a rock bed, by fixing the Biot number. The electricity produced in a steam power cycle supplied with heat from the packed bed is estimated based on the bed exit temperature profile during discharging. The net income for a range of bed lengths and particle sizes leads to an estimate of optimum particle size and bed length.