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Research Papers

Coupled Concentrating Optics, Heat Transfer, and Thermochemical Modeling of a 100-kWth High-Temperature Solar Reactor for the Thermal Dissociation of ZnO

[+] Author and Article Information
W. Villasmil

Solar Technology Laboratory,
Paul Scherrer Institute,
Villigen 5232, Switzerland;
Department of Mechanical
and Process Engineering,
ETH Zurich,
Zurich 8092, Switzerland

T. Cooper

Department of Mechanical
and Process Engineering,
ETH Zurich,
Zurich 8092, Switzerland

E. Koepf, A. Meier

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

A. Steinfeld

Department of Mechanical
and Process Engineering,
ETH Zurich,
Zurich 8092, Switzerland
e-mail: aldo.steinfeld@ethz.ch

1Corresponding author.

Manuscript received August 7, 2016; final manuscript received November 9, 2016; published online December 22, 2016. Assoc. Editor: Wojciech Lipinski.

J. Sol. Energy Eng 139(2), 021015 (Dec 22, 2016) (13 pages) Paper No: SOL-16-1358; doi: 10.1115/1.4035330 History: Received August 07, 2016; Revised November 09, 2016

This work reports a numerical investigation of the transient operation of a 100-kWth solar reactor for performing the high-temperature step of the Zn/ZnO thermochemical cycle. This two-step redox cycle comprises (1) the endothermal dissociation of ZnO to Zn and O2 above 2000 K using concentrated solar energy, and (2) the subsequent oxidation of Zn with H2O/CO2 to produce H2/CO. The performance of the 100-kWth solar reactor is investigated using a dynamic numerical model consisting of two coupled submodels. The first is a Monte Carlo (MC) ray-tracing model applied to compute the spatial distribution maps of incident solar flux absorbed on the reactor surfaces when subjected to concentrated solar irradiation delivered by the PROMES-CNRS MegaWatt Solar Furnace (MWSF). The second is a heat transfer and thermochemical model that uses the computed maps of absorbed solar flux as radiation boundary condition to simulate the coupled processes of chemical reaction and heat transfer by radiation, convection, and conduction. Experimental validation of the solar reactor model is accomplished by comparing solar radiative power input, temperatures, and ZnO dissociation rates with measured data acquired with the 100-kWth solar reactor at the MWSF. Experimentally obtained solar-to-chemical energy conversion efficiencies are reported and the various energy flows are quantified. The model shows the prominent influence of reaction kinetics on the attainable energy conversion efficiencies, revealing the potential of achieving ηsolar-to-chemical = 16% provided the mass transport limitations on the ZnO reaction interface were overcome.

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Topics: Solar energy
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Figures

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Fig. 3

(a) Sun-tracking orientation of a heliostat, relative to the adopted heliostat-centered Cartesian coordinate system (x = west, y = south, z = zenith). The direction of an incident ray r is determined by the solar azimuth angle γs and altitude angle αs. Heliostat angles γn and αn are analogously defined: (b) Reflection of an incident ray on a differential surface area dS, showing the angular dispersion error θerr relative to the directions of a perfectly specular ray r′ and deviant ray r″.

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Fig. 2 Left

side-elevation view of the MegaWatt Solar Furnace (MWSF) of PROMES-CNRS, Odeillo, France. The heliostats, arranged in eight stepped terraces, track the sun and reflect the direct normal irradiation (DNI) onto a parabolic dish concentrator, which in turn focuses the sunrays into a receiving tower where the experimental platform is located. Right: north-facing front-elevation view of the MWSF showing the 36 heliostats (dark-shaded group, referred to as G36) used during the reactor experimental campaign. Heliostats are numbered according to the MWSF convention. Dimensions in meters.

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Fig. 1

Schematic of the 100-kWth solar thermochemical reactor configuration

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Fig. 4

Left: locations of the 33 heat flux measurements (black dots) conducted at the focal plane of the MWSF for each of the G36 heliostats. The gray-shaded portion represents the 19-cm-diameter aperture area of the solar reactor. Dimensions in cm. Right: distribution map of solar radiative flux incident on the focal plane when irradiated by heliostat #47. The location of the peak flux (256 suns) is shown relative to the nominal focal point of the MWSF. The dashed circle delineates the perimeter of the aperture.

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Fig. 5

Superposition of all 36 experimentally measured (left) and numerically computed (right) distribution maps of solar radiative flux incident on the focal plane of the MWSF

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Fig. 6

(a) Comparison of the experimentally measured (solid lines) and numerically computed (dashed lines) solar radiative power and mean solar radiative flux incident on a circular aperture located at the focal plane of the MWSF. Solar radiative power delivered by G36 heliostats. (b) Computed daily evolution of the mean solar concentration ratio at the solar reactor's aperture Cmean during summer solstice (solid line), plotted along with the corresponding sun's path (dashed line). Simulations were performed for G36 heliostats and the shutter doors fully open.

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Fig. 10

Parity plot of the computed amounts of ZnO dissociated versus the experimental amounts collected during the experimental runs #2–8. No ZnO was fed/dissociated during run#1.

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Fig. 9

Modeled and measured solar reactor temperatures at the locations indicated in Fig. 8, for experimental runs 1–6 (see Table 4), plotted in the same order (a)–(f). Shown in the upper plots are the solar radiative power input Qsolar and the measured DNI. The vertical dotted lines indicate the points in time at which Qsolar ceased to be supplied.

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Fig. 7

Distribution of circumferentially averaged solar fluxabsorbed on the surfaces of the solar reactor for G36 heliostats at solar noon of summer solstice (June 21) and DNI = 1000 Wm−2. The ticks around the reactor profile indicate the flux absorbed at the given positions in kW m−2. Orientation of the y-axis indicated in Fig. 2 (left).

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Fig. 8

Schematic longitudinal half cross section of the 100-kWth solar reactor, showing the temperature measurement locations of the type-K and type-B thermocouples. The circled letters designate the various constituent materials considered in the reactor model: (a) Al2O3 bricks, (b) ALTRA KVS-184/400 insulation, (c) Promat Monalite M1-A insulation, (d) Al shell, (e) PORRATH FL32-12 firebricks, and (f) quartz glass.

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Fig. 11

Circled markers: experimentally obtained mean solar-to-chemical energy conversion efficiencies of the 100-kWth solar reactor for runs #2–8 (including 8peak), as a function of the mean temperature of the ZnO bed [24]. Lines: modeled efficiency curves for the baseline case (dotted) and the optimized design schemes (dashed, dashed–dotted).

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Fig. 12

Simulated cyclic evolution of the ZnO bed temperature, ZnO dissociation rate, and ηsolar-to-chemical, when subjecting the reactor to the indicated power profile Qsolar for four consecutive days. Indicated ηsolar-to-chemical values are daily averages.

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Fig. 13

Numerically computed ηsolar-to-chemical curves for various values of the pre-exponential factor k0 of the Arrhenius kinetic law r‴=k0 exp(−Ea/R¯T)

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