Particle-wall heat transfer in narrow-channel bubbling fluidized beds for thermal energy storage.

Robust metal oxide particles can provide low-cost and stable thermal energy storage (TES) to temperatures up to 1200 ∘C or higher. The transfer of heat into and out of the particles in cost-effective high-temperature particle heat exchangers remains a principal challenge to implementing particle-based TES. The present work expands on prior studies of particle-wall heat transfer in narrow-channel fluidized beds operated in the bubbling fluidization regime. Batch-mode experiments with various oxide particles over a range of temperatures and airflow rates indicate that particle-wall heat transfer increases with higher bed temperature and decreasing particle size. Measured particle-wall heat transfer coefficients in a 12 mm deep channel are fit to a Nusselt number correlation proportional to a non-monotonic function of excess fluidization velocity. Particle-wall heat transfer coefficients rise rapidly with excess fluidization velocities until reaching a maximum at intermediate air velocities due to a trade-off between enhanced transverse particle mixing and decreasing particle volume fraction with increased fluidization velocity. The heat transfer coefficient reaches a maximum of ≈ 400 % of values without fluidization for oxide particles ranging in diameter d p from 159 to 408 μm. For the smallest particles tested, composed of olivine sand, particle-wall heat transfer coefficients peak above 1100 W m−2 K−1 at 450 ∘C. 100-h tests at 500 ∘C and near-optimal heat transfer fluidization velocities indicated minimal wall wear or oxide scaling. High particle-wall heat transfer coefficients suggest narrow channel fluidized beds as a potential pathway for reducing the required surface area and cost of high-temperature particle heat exchangers, as needed in large-scale thermal energy storage applications. • Particle-wall heat transfer coefficients can exceed 1000 W m−2 K−1 at 450 ∘C in a narrow channel fluidized bed. • A Nusselt number correlation captures the peak in heat transfer coefficients at intermediate fluidization velocities. • Wear testing at 500 ∘C for 100 h indicated minimal wall oxide scaling or particle attrition due to mild fluidization. [ABSTRACT FROM AUTHOR]