Session: 07-02: Fluidized Bed Heat Exchangers

Paper Number: 156684

156684 - Troubleshooting a Counterflow, Direct Contact, Solid-Air Heat Exchanger 

Abstract: 

High temperature thermal energy storage (TES) is a much-needed technology for a robust electric grid.  When excess electricity is available on the grid, the TES is charged by adding heat to the storage media.  This avoids curtailment of baseload energy sources that slowly respond to demand changes and can absorb increased renewable energy generation that exceeds grid demand.  The TES can be charged with electric resistance heaters, concentrated solar power, waste process heat, in heat exchanger (HX), or a combination of methods depending on the system architecture.  When demand increases or generation decreases, the TES can discharge its heat to a thermal power cycle or direct heat-to-electricity devices such as thermophotovoltaics or thermoelectrics.  An increasingly popular storage media is solid particles due to their relative inexpensiveness, inertness at elevated temperatures, operation at ambient pressure, and lack of freezing or boiling concerns that otherwise limits the operating temperature range of liquid-based TES systems.  However, solid particle-based TES systems require special consideration.  In pumped TES system, the storage media flows between hot and cold storage silos, passing through a HX where heat may be added or removed.  This distinction from packed bed TES systems better enables partial charging, discretizing elements to maximize performance, and possible simultaneous charging and discharging.  Particle-based systems face challenges in solids conveyance and solid-fluid heat exchangers.  The focus of this work is the modeling, demonstration, and troubleshooting of a counterflow, direct contact, solid-air heat exchanger that is used to heat particles.  Principles and lessons learned may be applicable to HXs that remove heat from particles or to solid-liquid HXs.

This HX is based principally on fluidized bed (FB) technology.  FBs are used extensively in chemical and industrial processes from powder coating to fluidic catalytic cracking to gasification.  FBs are known for having high heat transfer and mass transfer coefficients in three areas: 1) between the fluidizing fluid and solid particles, 2) between the FB and structure walls, and 3) between the FB and immersed structures.  These correlations are dependent on the fluidization regime which is a function of the fluidizing fluid velocity, density difference between fluid and particle, and particle size.  Furthermore, FBs for PTES systems are desirable because of their constant and relatively low pressure drop compared to discharging whole packed bed TES systems.

This study focuses on leveraging the heat transfer between the fluidizing air and solid particles with the intention of achieving high heat exchanger effectiveness, ideally > 0.95 which will minimize parasitic losses in a scaled TES system.  From the traditional HX design perspective, this requires that the approach temperatures between particle and fluid be small at the inflows and outflows.  It is well understood that the temperature difference between particle fluid is small in a FB (with the exception of endo/exothermic reactions and high Biot numbers).  However, the traditional goal of FB is not low approach temperatures which occur at the walls of the, rather to maximize transfer processes within the bulk of the bed.  Thus, the design and operation of this FB HX requires a balance between contradicting processes: inherent FB behavior including mixing and homogeneity, and desirable HX outcomes including stratification and controllability.

This work will explore the principles governing a counterflow, direct contact, solid-air HX inspired by FB.  Tradeoffs and limits are discussed for both the design and operation.  It is shown that particles in Geldart Group B may be unsuitable for use in this FB HX design due to overlapping and conflicting considerations for FB and HX outcomes. 

Presenting Author: Jason Hirschey National Renewable Energy Laboratory

Presenting Author Biography: Jason Hirschey is a postdoctoral researcher in the Thermal Energy Systems Group at the National Renewable Energy Laboratory. Jason graduated with his PhD in mechanical engineering from the Georgia Institute of Technology in December 2022.