Oxide particles offer a potential solution to challenges facing high-temperature thermal energy storage (TES) for concentrating solar power (CSP) \[1, 2\]. Primary heat exchanger designs for integrating particle-based TES subsystems with high efficiency power cycles using supercritical carbon dioxide (sCO~2~) have focused on narrow-channel, moving packed beds coupled to microchannel sCO~2~ plates \[3\]. In such heat exchangers, the dominant thermal resistance is the particle-wall heat transfer resistance at 4 to 5 m^2^ K kW^-1^.  Recent studies have revealed that bubbling fluidization of particle beds reduces the particle-wall thermal resistance to 1.0 m^2^ K kW^-1^ at expected operating temperatures \[4\]. More recent studies of narrow-channel fluidized beds with upward flowing gas and downward net flow of particles provided correlations to predict local particle-wall heat transfer coefficients (*h*~T,w~).  These correlations enable modeling for particle-sCO~2~ heat exchanger designs based on narrow-channel fluidized beds.  Such design studies are pursued in this paper using a quasi-1-D heat exchanger model. 

The MATLAB-based, particle-sCO~2~ heat exchanger model employs a quasi-1-D fluidized bed sub-model coupled to a plug-flow, microchannel sCO~2~ sub-model.  The fluidized bed sub-model utilizes two-phase mass, momentum, and energy transport equations from a previous reference \[5\] and provides very good agreement with experimental trends in solid volumes fraction as a function of downward particle mass fluxes (from 4 to 12 g cm^-2^ s^-1^) and upward gas flow rates which range from (1 to 3% of particle mass flow rates).  Experimentally fitted correlations provide a basis for calculating particle-wall *h*~T,w~, and the plug-flow sCO~2~ microchannel flow model utilizes standard channel-flow correlations based on laminar or turbulent flow.  Temperature-dependent particle properties are based on those for Carbo Accucast ID50 \[2\], and the sCO~2~ model uses a fitted Redlich-Kwong equation-of-state for operating pressures around 200 bar.

The model was run to simulate conditions for a 40-kW~th~ particle-sCO~2~ heat exchanger.  With particle inlet temperatures of 620°C and counterflow sCO~2~ inlet temperatures of 450°C, heat exchanger performance was studied as a function of geometrical parameters (bed depth and sCO~2~microchannel number and size) and operating conditions (particle, gas, and sCO~2~ flows).  With much higher *h*~T,w~ in fluidized beds relative to moving packed beds, the narrow-channel fluidized bed heat exchanger has much more sensitivity to sCO~2~ channel geometries because the wall-sCO~2~ thermal resistance is comparable to the particle-wall thermal resistance.  However, decreasing wall-sCO~2~ thermal resistance with smaller channels comes at a significant sCO~2~pressure drop penalty.  Nonetheless overall heat transfer coefficients per unit area of bed wall can approach 600 W m^-2^ K^-1^ at the design inlet conditions with sCO~2~ microchannel hydraulic diameters around 0.3 mm.  Exploration of the parametric space based on the design of a 40-kW~th~ heat exchanger shows that target effectiveness of 80% and overall particle-sCO~2~ heat transfer coefficient  ≥ 750 W m^-2^ K^-1^ can be readily met for particle diameters around 200 mm and fluidizing gas to particle mass flow ratios \< 2%. These studies show that low particle-wall thermal resistances significantly reduce heat exchanger size per kW~th~ of heat transferred and enable single-pass sCO~2~ flow paths for smaller and more cost-effective particle-sCO~2~ heat exchangers.

References

1.   Jackson, G.S., L. Imponenti, K.J. Albrecht, D.C. Miller, and R.J. Braun*,* *Journal of Solar Energy Engineering - Transactions of the ASME,* 141(2), 2019.

2.   Siegel, N.P., M.D. Gross, and R. Coury*,* *Journal of Solar Energy Engineering - Transactions of the ASME*, 137(4), 2015.

3.   Albrecht, K.J. and C.K. Ho*,* *Solar Energ*y, 178: 331, 2019.

4.   Miller, D.C., C.J. Pfutzner, and G.S. Jackson*,* *International Journal of Heat and Mass Transfer*, 126: 730, 2018.

5.   Trendewicz, A., R. Braun, A. Dutta, and J. Ziegler*,* *Fuel*, 133: 253, 2014.


Modeling Narrow-Channel Fluidized Bed Particle-to-Sco2 Heat Exchangers for High-Temperature Thermal Energy Transport and Storage

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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