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Research Papers

Considerations for the Design of Solar-Thermal Chemical Processes

[+] Author and Article Information
Janna Martinek, Melinda Channel

Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309

Allan Lewandowski

 National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401

Alan W. Weimer1

Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309alan.weimer@colorado.edu

1

Corresponding author.

J. Sol. Energy Eng 132(3), 031013 (Jun 21, 2010) (6 pages) doi:10.1115/1.4001474 History: Received December 14, 2009; Revised January 07, 2010; Published June 21, 2010; Online June 21, 2010

A methodology is presented for the design of solar thermal chemical processes. The solar receiver efficiency for the high temperature step, defined herein as the ratio of the enthalpy change resulting from the process occurring in the receiver to the solar energy input, is limited by the solar energy absorption efficiency. When using this definition of receiver efficiency, both the optimal reactor temperature for a given solar concentration ratio and the solar concentration required to achieve a given temperature and efficiency shift to lower values than those dictated by the Carnot limitation on the system efficiency for the conversion of heat to work. Process and solar field design considerations were investigated for ZnO and NiFe2O4 “ferrite” spinel water splitting cycles with concentration ratios of roughly 2000, 4000, and 8000 suns to assess the implications of using reduced solar concentration. Solar field design and determination of field efficiency were accomplished using ray trace modeling of the optical components. Annual solar efficiency increased while heliostat area decreased with increasing concentration due to shading and blocking effects. The heliostat fields designed using system efficiency for the conversion of heat to work were found to be overdesigned by up to 21% compared with those designed using the receiver efficiency alone. Overall efficiencies of 13–20% were determined for a “ferrite” based water splitting process with thermal reduction conversions in the range of 35–100%.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

System boundaries for the solar receiver (dot-dashed line) and for the overall conversion of heat to work (dashed line) via a metal oxide water splitting cycle

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Figure 2

Definition of efficiencies

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Figure 3

Overall receiver efficiency ηabs (solid lines) and system efficiency (dashed lines)

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Figure 4

Schematic multiple field/tower layout (not to scale)

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Figure 5

Daggett, CA TMY2 data (average hourly values)

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Figure 6

Design point efficiency ηs to process for north field. Side field efficiencies are slightly less on a daily basis.

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Figure 7

Annual energy delivered to reaction including all three fields per tower

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Figure 8

Equilibrium composition for (a) ZnO with 3 mol Ar per mol solid reactant and total p=1 bar and (b) NiFe2O4 with 104 mol Ar per mol solid reactant and total p=1 bar

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Figure 9

ZnO process flow diagram

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Figure 10

NiFe2O4 process flow diagram

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