Effects of extended surfaces in narrow channel fluidized bed heat exchangers

Abstract

Concentrating solar power (CSP) coupled to high-temperature thermal energy storage (TES) and efficient power cycles offers a promising solution for dispatchable electricity from the sun. Current state-of-the-art CSP plants that use molten nitrate salts for TES do not allow energy storage at high enough temperatures to drive high-efficiency supercritical CO2 or ultra-critical steam cycles. On the other hand, oxide particles provide a TES media that can store energy at very high temperatures adequate for driving those cycles, but primary particle heat exchangers for coupling TES to those power cycles require expensive alloys that can be costly due to poor particle-wall heat transfer rates. Particle heat exchangers can benefit from fluidization of particle beds in narrow channels, but even with fluidization, the particle-wall heat transfer remains the limiting thermal resistance to overall heat transfer to the power cycle fluid. This study explores how the inclusion of fins in the narrow-channel particle beds can further improve particle-wall heat transfer coefficients while reducing axial dispersion that otherwise suppresses the log mean temperature difference across the heat exchanger. The improvements in effective particle-wall heat transfer coefficients and reduction in axial dispersion offer the potential to reduce the size and cost of the particle heat exchanger and the overall TES subsystem. This study assesses under what fluidizing bed conditions that fins can be most effective at enhancing particle-wall heat transfer in narrow-channel fluidized beds.