Circulating fluidized bed heat recovery/storage and its potential to use coated phase-change-material (PCM) particles

Within the thermal energy capture and/or storage systems currently available or investigated, PCMs are the sole latent heat stores. Despite their low thermal conductivity, that limits charging and discharging times, the higher energy storage capacity per unit weight in comparison with sensible heat stores, makes them increasingly attractive for high temperature applications, resulting in reduced storage volumes and required circulation rates within the heat collector. The present paper introduces these PCMs, and their potential application in high temperature energy capture and storage, using a circulating fluidized bed (CFB) as transfer/storage mode. Thermal considerations determine the optimum size range for the applied particles (<400μm). The heat transfer from the wall of the CFB to the flowing gas–solid suspension is a major design parameter of the collector, and studied for different operating conditions as determined by the gas velocity and solids circulation flux. Measured values of the heat transfer coefficients are discussed, and compared with empirical predictions of Molodtsof–Muzyka, and Gorliz–Grace. Fair agreement is obtained only when the empirical parameters are carefully predicted. The application of a packet renewal mechanism at the wall is also investigated, with a fair prediction of the heat transfer coefficient in terms of the expected solid contact time at the wall.