Abstract
A numerical and experimental analysis is performed on the solar-driven thermochemical reduction of ceria as part of a H2O/CO2-splitting redox cycle. A transient heat and mass transfer model is developed to simulate reticulated porous ceramic (RPC) foam-type structures, made of ceria, exposed to concentrated solar radiation. The RPC features dual-scale porosity in the mm-range and μm-range within its struts for enhanced transport. The numerical model solves the volume-averaged conservation equations for the porous fluid and solid domains using the effective transport properties for conductive, convective and radiative heat transfer. These in turn are determined by direct pore-level simulations and Monte-Carlo ray tracing on the exact 3D digital geometry of the RPC obtained from tomography scans. Experimental validation is accomplished in terms of temporal temperature and oxygen concentration measurements for RPC samples directly irradiated in a high-flux solar simulator with a peak flux of 1200 suns and heated to up to 1940K. Effective volumetric absorption of solar radiation was obtained for moderate optically thick structures, leading to a more uniform temperature distribution and a higher specific oxygen yield. The effect of changing structural parameters such as mean pore diameter and porosity is investigated.
Highlights
Solar-driven thermochemical cycles for splitting H2O and CO2 comprise an endothermic step for the reduction of a metal oxide using concentrated sunlight followed by an exothermic step for the oxidation of the reduced metal oxide with H2O and CO2 to form the basic components of syngas, H2 and CO [1]
Experimental validation is accomplished in terms of temporal temperature and oxygen concentration measurements for reticulated porous ceramic (RPC) samples directly irradiated in a high-flux solar simulator with a peak flux of 1200 suns and heated to up to 1940 K
In a previous paper [14], we proposed the use of reticulated porous ceramic (RPC) foam-type structures having dual-scale porosity: mm-size pores with struts containing micron-size pores
Summary
Solar-driven thermochemical cycles for splitting H2O and CO2 comprise an endothermic step for the reduction of a metal oxide using concentrated sunlight followed by an exothermic step for the oxidation of the reduced metal oxide with H2O and CO2 to form the basic components of syngas, H2 and CO [1]. The mm-size pores enable volumetric absorption of concentrated solar radiation [28] and effective heat transfer during the reduction step, while the micron-size pores within the struts offer increased specific surface area leading to enhanced reaction kinetics during the oxidation step. We follow this methodology to develop a heat and mass transfer model of the ceria RPC with dual-scale porosity and investigate its transient behaviour during the reduction step. To guide the optimization of the RPC structure, virtual samples with a wide range of porosities and mean pore diameters are numerically engineered based on the CT scan of a real RPC sample and their performance is studied by applying the heat and mass transfer model
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