Narrow-channel fluidized beds for particle-sCO₂ heat exchangers in next generation CPS plants

Transferring heat into and out of inert oxide particles in cost-effective, volumetrically efficient heat exchangers remains a principal challenge to implementing particle-based thermal energy storage (TES) in concentrating solar power (CSP) plants with efficient high-temperature supercritical-CO₂ (sCO₂) Brayton cycles. Engineered alumina-silica particles offer a robust storage media for temperatures well above 700°C. To date, primary particle-sCO₂ heat exchangers rely on moving packed beds, which require large surface areas and volumes of expensive Ni-based alloys due to low overall heat transfer coefficients typically less than 300 W m^-2 K⁻¹. The limiting thermal resistance at the particle-wall interface can be reduced greatly with bubbling fluidization such that the overall heat transfer coefficients increase above 600 W m⁻² K⁻¹. Bubbling fluidized beds of net-downward flowing particles in narrow channels provides an effective method of adopting moving packed bed plate designs while greatly reducing the required surface area and thereby the size and costs of particle-sCO₂ heat exchangers. The present work explores experimentally in a 0.25 x 0.10 x 0.018 m narrow-channel fluidized bed the influence of operating conditions on particle-wall heat transfer. Heat transfer measurements with alumina-silica particles (CARBOBEAD CP 40/100 with Sauter mean diameter of 260 µm) demonstrate that low superficial gas velocities as low as 125% of the minimum fluidization velocity greatly enhance particle-wall heat transfer while maintaining bed solid volume fractions above 40%. Particle-wall heat transfer increases with bed temperature due to increases in gas thermal conductivity and increased radiative exchange at higher temperatures. In agreement with past studies, a shallow peak in particle-wall heat transfer occurs at intermediate gas velocities due to the tradeoff of greater particle mixing and reduced solid volume fraction with increasing gas velocities. A new correlation for particle-wall heat transfer coefficients fitted to the test data provides a basis for identifying operating conditions for particle-sCO₂ heat exchangers to achieve particle-wall heat transfer coefficients above 1000 W m⁻² K⁻¹ and overall heat transfer coefficients above 700 W m⁻² K⁻¹ with gas-to-particle mass flow ratios below 2%. The high heat transfer coefficients in narrow-channel fluidized beds can enable smaller, cost-effective particle heat exchangers for full-scale particle-based TES in next-generation CSP plants.