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

Abstract

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 hw 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 hw of this flow configuration, a fluidized bed in a 10 cm × 10 cm × 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 μm) in steady-state continuous downward flow and in transient batch mode assessed total hw as functions of particle bed temperatures Tb, bed solids volume fractions αb, and superficial gas velocities Ug. Results showed that the narrow-channel fluidized bed can achieve overall hw as high as 1000 W m−2 K−1. The highest hw were measured at upward Ug between 2 and 4 times the minimum bed fluidization velocities, Umf, which decreased to0.12 m s−1 for the mean particle diameter at Tb=600°C. Increasing Ug further above Umf decreased hw due to an associated decrease in αb. hw increased strongly with Tb in part, because gas-phase conductivity and the radiative heat transfer contribution increased with Tb. The extensive measurements were fit to a modified version of the Nusselt number correlation by Molerus (1992). For αb⩾0.1, the Molerus correlation with adjusted dependence on excess fluidization velocity (Ug-Umf) provided an excellent fit to the measured convective fraction of hw (with <10% error). Adding the radiation component with the Molerus correlation provides an effective tool for calculating hw for this counterflow fluidized bed configuration. A simple analysis explored the impact of such high hw for an indirect receiver design with angled external walls to spread solar aperture fluxes. Results from the analysis indicated that total hw=1000 W m−2 K−1 can enable solar collection efficiencies approaching 90% with external wall temperatures Tw,ext≈1020°C. This potential performance motivates further exploration of this fluidized bed configuration for particle receivers for CSP applications.