Session: 05-10 Particles for Thermal Storage in CSP 3

Paper Number: 116993

116993 - Particle-Wall Heat Transfer Enhancements With Extended Surfaces in Narrow Channel Fluidized Beds 

High-temperature particle-based thermal energy storage (TES) has been identified as a promising solution for providing renewable solar energy around the clock in concentrating solar power plants. Current particle-based TES subsystems face high costs for the particle-fluid heat exchangers due to the limiting particle-wall heat transfer and the associated large surface areas of expensive Ni-based superalloys needed to accomplish the desired heat transfer.  Our team's previous research demonstrated that bubbling fluidized beds can reach high particle-wall heat transfer coefficients h_T,w over 1000 W m^-2 K^-1 with particle diameters less than 250 mm [1].  However, challenges with thermal dispersion flattening particle temperature profiles in fluidized beds reduce effective log-mean temperature differences due to counterflowing gas bubbles carrying some cooled particles back up to the top of the bed [2].  The lowering of log-mean temperature difference reduces the average wall heat flux and puts further demand on improving particle-wall h_T,w in order to reduce required primary particle heat exchanger size and costs in TES subsystems.  To address this challenge, our team is investigating the effectiveness of extended surfaces in narrow-channel fluidized particle flows to enhance wall-particle heat transfer and reduce the dispersion of particles over the height of the fluidized bed.

Increasing wall surface area with extended surfaces can improve heat exchanger wall fluxes from the particles to the power cycle fluid in a primary particle heat exchanger, but it remains uncertain how h_T,w will be impacted by the extended surfaces breaking up fluidized bed bubble structures that enhance particle mixing.  On the other hand, fin extensions have the potential to reduce particle dispersion and its impact on log mean temperature difference. The classical fin heat transfer effectiveness heat transfer rates assumes uniform h_T,w over the entire fin surface, but the nature of fluidization may not support that approximation.  To test the impact of finned surfaces on particle-wall heat transfer, experiments were conducted to compare fluidized bed heat transfer in smooth-walled channels and in beds with walls extended by brazed fins (assembled by Brayton Energy).  The measurements explored the impact of fin structures on heat exchanger performance both in terms of h_T,w and particle dispersion.  Fluidized beds of CARBOBEAD HSP 40/70 and olivine LE120 in channels with a depth of 12 mm and a height of 250 mm were tested for particle-wall heat transfer with smooth and finned wall configurations.  The results indicate that staggered fins that increase wall surface area by 350% can increase effective particle-wall heat transfer coefficient h_T,w,eff (evaluated on base wall area) by factors between 2 and 3 times relative to h_T,w in smooth walls.  The finned plates produced average h_T,w,eff approaching 2000 W m^-2 K^-1 at bed temperatures around 450°C for the larger CARBOBEAD and the smaller olivine sand particles. Additional experiments with finned walls over a wider range of operating temperatures and particle mass fluxes are showing the effectiveness of the fins in suppressing vertical dispersion and thus providing a pathway for improving the log-mean temperature difference in fluidized bed heat exchangers.  These tests are providing a basis for evaluating how extended surfaces provide a pathway for achieving high particle-wall heat fluxes and significantly lowering particle heat exchanger costs for particle-based TES subsystems.

References

[1] J. R. Fosheim, X. Hernandez, J. Abraham, A. Thompson, B. Jesteadt, G. S. Jackson, H. C.K., C. M., G. P., K. P. AIP Conference Proceedings  2445(1) (2022) 160007.

[2] K. Luo, F. Wu, S. Yang, and J. Fan, “CFD-DEM study of mixing and dispersion behaviors of solid phase in a bubbling fluidized bed,” Powder Technology, 274, pp. 482–493, Apr. 2015

Presenting Author: Fuqiong Lei Colorado School of Mines

Presenting Author Biography: Dr. Fuqiong Lei earned her Ph.D. in Chemical Engineering from Oregon State University, where she focused on both high- and low-temperature thermochemical energy storage, particularly in the area of concentrating solar power. She currently serves as a Postdoctoral Research Fellow in Dr. Gregory Jackson's group at Colorado School of Mines. Her current research focuses on using fluidized bed for high-temperature particle-based thermal energy storage and lab-scale reactor development for solar thermal calcination system.