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Research Papers

A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production

[+] Author and Article Information
Ivan Ermanoski

Sandia National Laboratories,
Albuquerque, NM 87185-1415
e-mail: iermano@sandia.gov

Nathan P. Siegel

Mechanical Engineering Department,
Bucknell University,
Lewisburg, PA 17837
e-mail: nate.siegel@bucknell.edu

Ellen B. Stechel

LightWorks,
Arizona State University,
Tempe, AZ 85287
e-mail: Ellen.Stechel@asu.edu

Contributed by the Solar Energy Division of ASME for publication in the Journal of Solar Energy Engineering. Manuscript received March 16, 2012; final manuscript received December 23, 2012; published online February 8, 2013. Assoc. Editor: Wojciech Lipinski.

J. Sol. Energy Eng. 135(3), 031002 (Feb 08, 2013) (10 pages) Paper No: SOL-12-1073; doi: 10.1115/1.4023356 History: Received March 16, 2012; Revised December 23, 2012

We describe and analyze the efficiency of a new solar-thermochemical reactor concept, which employs a moving packed bed of reactive particles produce of H2 or CO from solar energy and H2O or CO2. The packed bed reactor incorporates several features essential to achieving high efficiency: spatial separation of pressures, temperature, and reaction products in the reactor; solid–solid sensible heat recovery between reaction steps; continuous on-sun operation; and direct solar illumination of the working material. Our efficiency analysis includes material thermodynamics and a detailed accounting of energy losses, and demonstrates that vacuum pumping, made possible by the innovative pressure separation approach in our reactor, has a decisive efficiency advantage over inert gas sweeping. We show that in a fully developed system, using CeO2 as a reactive material, the conversion efficiency of solar energy into H2 and CO at the design point can exceed 30%. The reactor operational flexibility makes it suitable for a wide range of operating conditions, allowing for high efficiency on an annual average basis. The mixture of H2 and CO, known as synthesis gas, is not only usable as a fuel but is also a universal starting point for the production of synthetic fuels compatible with the existing energy infrastructure. This would make it possible to replace petroleum derivatives used in transportation in the U.S., by using less than 0.7% of the U.S. land area, a roughly two orders of magnitude improvement over mature biofuel approaches. In addition, the packed bed reactor design is flexible and can be adapted to new, better performing reactive materials.

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Figures

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Fig. 1

Schematic drawing of the moving packed particle bed reactor. TR—thermal reduction chamber, FP—fuel production chamber. The packed bed of particles fills the entire reactor, but is only shown selectively, to preserve the clarity of the schematics.

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Fig. 2

Isothermal log δ versus log pO2 plot for nonstoichiometric CeO2. Reprinted from Ref. [33] with permission from Elsevier.

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Fig. 3

Solar efficiency (η) of the packed bed reactor based on the CeO2 water splitting cycle. The four solid curves represent solar efficiencies (left Y-axis) for four values of recuperator effectiveness. The dashed curve (right Y-axis) shows the reduction extent (δ) of CeO2-δ at the design temperature.

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Fig. 4

Thermochemical efficiency (ηTC) of the packed bed reactor based on the CeO2 water splitting cycle. The legend is as in Fig. 3.

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Fig. 5

Heat loads for the packed bed reactor for εR = 0. The largest heat loads, (a) and (b), require direct solar input, whereas (c) and (d) are met by use of waste heat. The HHV of the output H2 stream is also shown (H). Inset: normalized contributions to waste heat as function of pO2.

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Fig. 6

Required O2 pumping speeds as a function of pO2 in the thermal reduction chamber of a vacuum-pumped packed bed reactor, for four values of recuperator effectiveness

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Fig. 7

Thermochemical efficiency (ηTC) of a packed bed reactor based on the CeO2 water splitting cycle, with N2 sweeping of the thermal reduction chamber

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