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Research Papers

Dynamic Modeling of a Solar Reactor for Zinc Oxide Thermal Dissociation and Experimental Validation Using IR Thermography

[+] Author and Article Information
A. Meier

Solar Technology Laboratory,
Paul Scherrer Institute,
Villigen PSI 5232, Switzerland

A. Steinfeld

Solar Technology Laboratory,
Paul Scherrer Institute,
5232 Villigen PSI, Switzerland
Department of Mechanical and
Process Engineering,
ETH Zurich,
Zurich 8092, Switzerland
e-mail: aldo.steinfeld@ethz.ch

In this paper, the ending -ance is used to distinguish the extensive radiative properties, such as the reflectance of a glass layer, from the intensive properties, such as the reflectivity of a surface.

In the forthcoming, the directional or angular dependence of a given quantity will be indicated by a prime superscript.

The solar concentration ratio is defined as the solar radiative flux normalized to 1 kW·m−2. C is often expressed in the units of “suns.”

Ln denotes normal liters; mass flow rates calculated at 273 K and 1 bar.

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received April 29, 2013; final manuscript received August 23, 2013; published online October 24, 2013. Assoc. Editor: Yogi Goswami.

J. Sol. Energy Eng 136(1), 010901 (Oct 24, 2013) (11 pages) Paper No: SOL-13-1128; doi: 10.1115/1.4025511 History: Received April 29, 2013; Revised August 23, 2013

A dynamic numerical model of a solar cavity-type reactor for the thermal dissociation of ZnO is formulated based on a detailed radiative heat transfer analysis combining the Monte Carlo ray-tracing technique and the radiosity enclosure theory. The quartz window is treated as a semitransparent glass layer with spectrally and directionally dependent optical properties. Model validation is accomplished by comparison with experimental results obtained with a 10-kW solar reactor prototype in terms of cavity temperatures, reaction extents, and quartz window temperature distribution measured by IR thermography. The solar-to-fuel energy conversion efficiencies obtained experimentally are reported, and the various energy flows are quantified.

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Figures

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Fig. 1

Schematic of the 10-kWth solar chemical reactor configuration

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Fig. 2

Surface segmentation adopted for the application of the radiosity enclosure theory. The quartz window—not shown here—is segmented in ten concentric annuli of equal width.

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Fig. 3

Spectral radiant intensity of a typical Xe arc (Iλ,Xe, thin solid line) [17], and the calculated spectral hemispherical transmittance (Uλ, thick solid line) and absorptance (Aλ, dashed line) of the 3-mm-thick quartz window. The vertical lines indicate the limits of the three spectral bands adopted to approximate the window's radiative properties.

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Fig. 4

Schematic cross-section of the 10-kW solar reactor showing the measurement locations of the type-K and type-B thermocouples. The circled letters A–C designate the three Al2O3/SiO2 insulating layers used in the reactor corresponding to the commercial materials ALTRA KVS-184/400, KVS-164, and KVS-144 [29], respectively.

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Fig. 5

Numerically calculated and experimentally measured temperatures at the measurement locations indicated in Fig. 4, for experimental runs 1–6 (in the same order of appearance (a)–(f)). Plotted in the right axis is the solar radiative power incident on the window that enters through the aperture Qsolar.

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Fig. 6

Parity plot of the model numerical predictions and the experimentally measured amounts of ZnO dissociated during experimental runs 1–6

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Fig. 7

(a) Numerically calculated and experimentally measured [24] spectral normal emittance of a 0.84-mm-thick quartz plate at 313 K. (b) Numerically calculated (lines) and experimentally measured (markers) [40,41] total hemispherical emittance of three quartz sheets of different thicknesses: 5.09 mm (□), 2.09 mm (○), and 1.02 mm (◇).

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Fig. 8

(a) IR thermogram of the quartz window at 110 min after the first Xe arc lamp ignition—indicated contour levels labeled in K. (b) Computed image of total directional local emittances of the quartz window. The emissive direction of each pixel is given by the vector connecting the pixel on the image to the lens of the IR camera.

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Fig. 9

Numerically calculated (dashed and dotted lines) and experimentally measured temperatures of the 3-mm-thick quartz window for annuli 1, 5, and 9. Measured data are represented by the shaded ribbons, each of them delimited by the ±2% error of the recorded IR thermograms. Plotted in the right axis is the radiative input power Qsolar.

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Fig. 10

Measured (circle markers) and modeled (lines) mean solar-to-fuel energy conversion efficiencies as a function of the mean temperature of the ZnO bed T¯ZnO–bed, for the six experimental runs listed in Table 1

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