Pore-level engineering of macroporous media for increased performance of solar-driven thermochemical fuel processing

Keywords: Solar energy Solar fuels Heat and mass transfer Porous media Computed tomography Morphology a b s t r a c t The performance of high-temperature solar reactors incorporating porous ceramic materials that serve as radiative absorbers and chemical reaction sites can be improved significantly by tailoring their pore structure. We investigated the changes in their effective heat and mass transport properties with increasing mass loading of porous ceramics fabricated by the replica method. We applied a methodology consisting of the experimental characterization of the structure via 3D tomographic techniques coupled to pore-level direct numerical simulations for the determination of the effective transport properties. This approach was extended by using digital image processing on the structure data to allow for artificial changes in the morphological characteristics – corresponding to actual variations in the fabrication process. We derived transport correlations of porous ceria foam with varying mass loading, i.e. reticulate to dense foams with porosity from 0.85 to 0.45. We observed that the correlations proposed in literature do not accurately describe the behavior of low-porosity foams. The numerical findings of this study provide guidance for pore-level engineering of materials used in solar reactors and other high-temperature heat and mass transfer applications. Porous ceramic materials exhibit favorable morphological, mechanical, and transport properties when applied as absorbers [1], heat exchangers [2], insulators [3], chemical reaction site, and reactants [4], in a wide variety of high-temperature applications ranging from chemical processing, combustion, and filtering, to solar reactor technology. The effective heat and mass transport properties of these porous materials largely depend on their morphology [5,6]. For example, solar reactors designed for thermo-chemical water and CO 2-splitting using porous, ceria-based redox materials have shown an increase in the efficiency by a factor of four when changing the material's morphology from monolithic-type geometry with lm-range pore size to a foam-type geometry with mm-range pore size [1,4]. Thus, pore-level engineering of materials can significantly improve the performance of solar reactors. Frequently, the effective transport properties of macroporous media are approximated by empirical correlations or semi-empirical and analytical models derived for simplified morphologies or unit-cell structures. To predict the permeability of a porous medium, approximations based on the semi-heuristic packed-bed model of Carman and Kozeny [7] are used with a modified shape factor for e.g. assemblies of parallel cylinders [8] and fibrous beds [9]. Another drag flow approach was analytically derived by Ergun [10] for packed columns. An …