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Research Papers

Effect of Flow Rates on Operation of a Solar Thermochemical Reactor for Splitting CO2 Via the Isothermal Ceria Redox Cycle

[+] Author and Article Information
Brandon J. Hathaway

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455
e-mail: hatha050@umn.edu

Rohini Bala Chandran

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455
e-mail: bala0137@umn.edu

Stephen Sedler

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455
e-mail: sedl0033@umn.edu

Daniel Thomas

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455
e-mail: dthomas@umn.edu

Adam Gladen

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455
e-mail: glad0092@umn.edu

Thomas Chase

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455
e-mail: trchase@umn.edu

Jane H. Davidson

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455
e-mail: jhd@umn.edu

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received July 29, 2015; final manuscript received October 26, 2015; published online December 8, 2015. Assoc. Editor: Prof. Nesrin Ozalp.

J. Sol. Energy Eng 138(1), 011007 (Dec 08, 2015) (12 pages) Paper No: SOL-15-1238; doi: 10.1115/1.4032019 History: Received July 29, 2015; Revised October 26, 2015

A prototype 4 kW solar thermochemical reactor for the continuous splitting of carbon dioxide via the isothermal ceria redox cycle is demonstrated. These first tests of the new reactor showcase both the innovation of continuous on-sun fuel production in a single reactor and remarkably effective heat recovery of the sensible heat of the reactant and product gases. The impact of selection of gas flow rates is explored with respect to reactor fuel productivity and external energy costs of gas separation and pumping. Thermal impacts of gas flow selection are explored by coupling measured temperatures with a computational fluid dynamics (CFD) model to calculate internal temperature distributions and estimate heat recovery. Optimized gas flows selected for operation provide a 75% increase in fuel productivity and reduction in parasitic energy costs by 10% with respect to the design case.

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References

Figures

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Fig. 1

Cutaway views of the solar reactor and heat exchanger: (a) CAD model of the reactor showing the various functional components and (b) details of the reactive element and heat exchanger assembly (aspect ratio distorted to clarify structure of reactive element)

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Fig. 2

Contours of relative heat capacity rate, Crel, for massspecific oxidizer and sweep gas flow rates from 0.83 to 2.5 std mL s−1 g−1. The dashed line represents the locus of flow combinations that result in a normalized average heat capacity rate of 1.

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Fig. 3

Instantaneous gas yield rates for (a) fuel during the oxidation half-cycle and (b) oxygen during the reduction half-cycle at the equal flows condition

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Fig. 6

Temperatures in RE1 for two cycles of steady-periodic reactor operation with 1.67 mL s−1 g−1 of sweep gas and CO2. Spatially averaged, measured wall surface temperatures, modelpredicted bulk gas temperatures, and measured cavity temperature are shown. Oxidation and reduction half cycles are abbreviated as Ox and Rd.

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Fig. 7

Axial variation of model predicted bulk gas temperatures along its flow path in the central channel and the annulus at the onset of reduction with a sweep gas flow rate of 1.67 mL s−1 g−1. Subplot zooms into the axial temperature variation in the packed bed of ceria.

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Fig. 8

Cycle-averaged power requirements for the various gas flow rates listed in Table 1

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Fig. 9

Cycle-average O2 and CO yield rates per unit mass of ceria during the steady-periodic cycling. Open markers are for RE1, and solid markers are for RE2. Note the stability between cycles and similarity in performance between elements.

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Fig. 10

Instantaneous yield rate of fuel from both reactive elements, demonstrating the ability for the reactor to produce a continuous stream of fuel during cycling with multiple elements

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Fig. 4

Average yield rate of (a) carbon monoxide and (b) oxygen during cycling for varied flow configurations

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Fig. 5

Measured temperature profiles in the outer tube wall of RE1 for gas flow rates listed in Table 2: (a) spatially averaged surface temperatures during redox cycling and (b) axial profile of wall temperatures at the end of oxidation. Oxidation and reduction half cycles are abbreviated as “Ox” and “Rd.”

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