Research Papers

Heat Transfer Model of a 50 kW Solar Receiver–Reactor for Thermochemical Redox Cycling Using Cerium Dioxide

[+] Author and Article Information
S. Zoller, P. Roos, A. Steinfeld

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

E. Koepf

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

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 September 18, 2018; final manuscript received November 9, 2018; published online January 8, 2019. Guest Editors: Tatsuya Kodama, Christian Sattler, Nathan Siegel, Ellen Stechel.

J. Sol. Energy Eng 141(2), 021014 (Jan 08, 2019) (11 pages) Paper No: SOL-18-1436; doi: 10.1115/1.4042059 History: Received September 18, 2018; Revised November 09, 2018

This work reports on the development of a transient heat transfer model of a solar receiver–reactor designed for thermochemical redox cycling by temperature and pressure swing of pure cerium dioxide in the form of a reticulated porous ceramic (RPC). In the first, endothermal step, the cerium dioxide RPC is directly heated with concentrated solar radiation to 1500 °C while under vacuum pressure of less than 10 mbar, thereby releasing oxygen from its crystal lattice. In the subsequent, exothermic step, the reactor is repressurized with carbon dioxide as it cools, and at temperatures below 1000 °C, the partially reduced cerium dioxide is re-oxidized with a flow of carbon dioxide. To analyze the performance of the solar reactor and to gain insight into improved design and operational conditions, a transient heat transfer model of the solar reactor for a solar radiative input power of 50 kW during the reduction step was developed and implemented in ANSYS cfx. The numerical model couples the incoming concentrated solar radiation using Monte Carlo ray tracing, incorporates the reduction chemistry by assuming thermodynamic equilibrium, and accounts for internal radiation heat transfer inside the porous ceria by applying effective heat transfer properties. The model was experimentally validated using data acquired in a high-flux solar simulator (HFSS), where temperature evolution and oxygen production results from model and experiment agreed well. The numerical results indicate the prominent influence of solar radiative input power, where increasing it substantially reduces reduction time of the cerium dioxide structure. Consequently, the model predicts a solar-to-fuel energy conversion efficiency of >6% at a solar radiative power input of 50 kW; efficiency >10% can be obtained provided the RPC macroporosity is substantially increased, and better volumetric absorption and uniform heating is achieved. Managing the ceria surface temperature during reduction to avoid sublimation is a critical design consideration for direct absorption solar receiver–reactors.

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Grahic Jump Location
Fig. 1

Schematic of the solar reactor configuration relevant to the modeling domains: (1) aluminum front, (2) steel shell, (3) insulating jacket, (4) Al2O3–SiO2 insulation, (5) ceria RPC with thickness tRPC, (6) receiver cavity, and (7) fluid region behind RPC. Also indicated are the boundary conditions and source terms for the heat transfer model, and the transmissivity τ of the quartz window. The measurement locations of the type-B and type-K thermocouples are indicated: TB,1TB,3 are located at the outer surface of the ceria RPC; TK,1TK,3 are located at different depths within the insulation; and TK,4 and TK,5 are located at the outer surfaces of the reactor shell and the insulating jacket, respectively.

Grahic Jump Location
Fig. 2

Schematic of the experimental setup. During the endothermic reduction step (Eq. (1)), Ar flow is used to protect the quartz window and is pumped out together with the released O2, while the reactor is under vacuum. During the exothermic oxidation step (Eq. (3)), the ceria is fully reoxidized with CO2, thereby producing CO.

Grahic Jump Location
Fig. 6

Nominal RPC temperature TRPC,nom and O2 release rate as a function of time (left column) as well as efficiency ηsolar-to-fuel, reduction time tred, and maximum RPC temperature TRPC,max (right column) for varying ((a) and (b)) RPC thickness tRPC, ((c) and (d)) RPC porosity ϕdual, and ((e) and (f)) input power Psolar

Grahic Jump Location
Fig. 3

(a) Numerically calculated (solid lines) and experimentally measured (dashed lines) temperatures at the locations indicated in Fig. 1, during the reduction step and the subsequent natural cooling phase. (b) Average of the three thermocouple locations measuring the temperature of the RPC at the back surface (TRPC,nom) and O2 evolution as a function of time.

Grahic Jump Location
Fig. 7

(a) Absorbed solar radiation Ssolar and RPC temperature as a function of x, the depth within the RPC, where zero indicates the front, directly irradiated surface. The position of evaluation within the RPC is indicated in Fig. 4. Values are extracted for a simulation time of 298 s, corresponding to tred of the case with nppi = 10. (b) Nominal RPC temperature TRPC,nom and rate of released O2 as a function of time. Solid lines represent the advanced RPC design with nppi = 3 and dashed lines represent the case with nppi = 10. For both cases, Psolar was set to 60 kW.

Grahic Jump Location
Fig. 4

(a) Distribution of Ssolar within the ceria RPC during the reduction step. (b) Temperature distribution of the solid domains and of the RPC domain (also enlarged) at the end of the reduction step. Also indicated is the variable x, which defines the cross section position through the RPC for the evaluation of absorbed solar radiation and temperature (analyzed in Fig. 7).

Grahic Jump Location
Fig. 5

Instantaneous energy balance for the duration of a reduction step. Other heat losses include convection and radiation at the outer reactor surfaces, reflection of incoming solar radiation inside the reactor cavity, and absorption and reflection at the quartz window.



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