Narrow-channel fluidized-bed heat exchangers for particle-based thermal energy storage in concentrating solar power applications
Arthur-Arhin, Winfred John
Arthur-Arhin, Winfred John
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2024
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2025-05-26
Abstract
Concentrating solar power (CSP) with thermal energy storage (TES) promises clean dispatchable electricity at high solar conversion efficiencies when coupled with emerging recompression closed Brayton (RCBC) supercritical carbon dioxide (sCO$_2$) power cycles. Inert-oxide particles provide a cost-effective storage and heat transfer media at adequate firing temperatures ($>$~700~\(^\circ\)C) for RCBC sCO$_2$ power cycles with thermal efficiencies $\geq~50\%$. To achieve cost-effective power at high conversion efficiencies, primary particle-sCO$_2$ heat exchangers (HXs) need high overall heat transfer coefficients to reduce HX area and costs of manufacturing with high-temperature alloys. In this study, mild bubbling fluidization with downward-falling particles and upward-flowing gas in narrow channels is shown to enhance the limiting bed-wall heat transfer and offer a pathway to meet HX cost targets of $\leq~150$ per kW\textsubscript{th}).
Extensive laboratory-scale heat transfer tests characterized narrow-channel fluidization and demonstrated bed-wall heat transfer coefficients $\geq~800$ W~m$^{-2}$~K$^{-1}$ at bed temperatures $\approx~500~\,^\circ$C. Correlations derived from experiments indicate the potential for bed-wall heat transfer coefficients $> 1000$ W m$^{-2}$ K$^{-1}$ at expected HX operating conditions for $\leq~300~\,\mu$m particles. Incorporating correlations into reduced-order models of particle-sCO$_2$ HXs identifies preferred HX geometry and operating conditions for a 40-kW\textsubscript{th} prototype HX. The resulting particle-sCO$_2$ HX design is fabricated and assembled with a unique particle feed system with 12 $\approx~0.5$ m-high parallel fluidized bed channels and counter-flowing sCO$_2$ microchannels embedded in the HX walls.
Prototype HX tests at Sandia National Laboratories yield lower-than-expected overall heat transfer coefficients with values reaching only 200 W~m$^{-2}$~K$^{-1}$ at mild fluidization velocities. High axial dispersion of particles due to fluidizing bubbles limits local driving bed-wall temperature differences and overall HX performance. Nonetheless, stable fluidized bed HX operation achieved 34~kW\textsubscript{th} of heat transfer to the sCO$_2$. Insights from these tests and lab-scale measurements inform updated models, incorporating axial dispersion correlations to identify necessary improvements for meeting performance targets. A system-level analysis reveals minimal balance-of-plant penalties associated with the power required and heat lost in the fluidization gas flows. Furthermore, models indicate that reducing dispersion can significantly improve fluidized bed HX performance by providing adequately high overall heat transfer coefficients that support cost-effective, particle-based TES subsystems for CSP plants.
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