Heat transfer in counterflow fluidized bed of oxide particles for thermal energy storage

The potential for inert oxide particles as a heat transfer and thermal energy storage (TES) media in concentrating solar power (CSP) depends in part on particle receiver designs that provide high wall-to particle heat transfer rates. This paper presents a novel continuous-flow approach to achieve high heat transfer coefficients h(w) for particle receivers by fluidizing net-downward-flowing particles in a narrow vertical channel bounded by an external irradiated/heated wall and a parallel interior wall with a metal mesh opening that allows the upward-flowing fluidizing gas to exit at the top of the channel. To demonstrate the high of this flow configuration, a fluidized bed in a 10 cm x 10 cm x 0.64 cm deep channel was heated through an external aluminosilicate wall with mid-IR quartz lamps that provided external wall heat fluxes up to 20 W cm(-2). Extensive heat transfer measurements with fluidized Carbo Accucast ID50 particles (diameters between 150 and 350 mu m) in steady-state continuous downward flow and in transient batch mode assessed total h(w) as functions of particle bed temperatures T-b, bed solids volume fractions alpha(b), and superficial gas velocities U-g. Results showed that the narrow-channel fluidized bed can achieve overall h(w), as high as 1000 W m(-2) K-1. The highest h(w), were measured at upward U-g between 2 and 4 times the minimum bed fluidization velocities, U-mf, which decreased to 0.12 m s(-1) for the mean particle diameter at T-b = 600 degrees C. Increasing U-g further above U-mf decreased hw due to an associated decrease in alpha(b). h(w) increased strongly with T-b in part, because gas-phase conductivity and the radiative heat transfer contribution increased with T-b. The extensive measurements were fit to a modified version of the Nusselt number correlation by Molerus (1992). For alpha(b) >= 0.1, the Molerus correlation with adjusted dependence on excess fluidization velocity (U-g - U-mf) provided an excellent fit to the measured convective fraction of h(w) (with < 10% error). Adding the radiation component with the Molerus correlation provides an effective tool for calculating h(w) for this counterflow fluidized bed configuration. A simple analysis explored the impact of such high h(w) for an indirect receiver design with angled external walls to spread solar aperture fluxes. Results from the analysis indicated that total h(w) = 1000 W m(-2) K-1 can enable solar collection efficiencies approaching 90% with external wall temperatures T-w,T-ext approximate to 1020 degrees C. This potential performance motivates further exploration of this fluidized bed configuration for particle receivers for CSP applications. (C) 2018 Elsevier Ltd. All rights reserved.