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 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 h w 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  μ 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 α 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 ° C. Increasing U g further above U mf decreased h w due to an associated decrease in α 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 α 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,ext ≈ 1020 ° C. This potential performance motivates further exploration of this fluidized bed configuration for particle receivers for CSP applications.