Radiative characterization of heterogeneous media for solar thermochemical applications

Abstract

Transport radiative properties of ceria ceramics as a function of wavelength and morphology are presented. Experimental facilities for measuring transmitted and re- flected radiant energy were developed. A Monte Carlo ray-tracing technique was devel- oped to infer transport scattering coefficient and absorption coefficient from measured transmittance and reflectance. An experimental setup to measure normal-emittance of materials employed in solar thermochemical applications is also designed. This experi- mental setup can achieve temperatures on the order of 1500 K and facilitate maintaining an inert, oxidizing or reducing environment to control composition of materials. Transmittance and reflectance of two types of morphologies were examined. Sin- tered ceria discs with pores and grains of size a few microns and a random grain and pore structure, and a three-dimensionally ordered macro-porous (3DOM) ceria with nano-structured features were considered. The 3DOM samples were also ther- mochemically cycled for 60 cycles consisting of high-temperature reduction and low- temperature oxidation. Two porosities were considered for the sintered ceria discs with porosity, p = 0.08 (“dense” samples), and p = 0.72 (“porous” samples) and porosi- ties of the packed bed before and after themochemical cycling were 0.9 and 0.83 re-spectively. Morphological characterization was performed by Scanning Electron Mi- croscopy (SEM), and porosity was verified by mass and volume measurements of the samples. Sintered ceria discs had extremely high optical thickness, with low and uncertain values of directional-hemispherical transmittance, and bi-normal transmittance on the order of 0.001 %. The transport radiative properties did not show a strong wavelength dependence after a wavelength of 0.5 μm. The transport scattering coefficient was on the order of 30mm−1 for the porous ceramics and 15mm−1 for the dense ceramics after a wavelength of 0.5 μm. The absorption coefficient was on the order of 0.005 mm−1 for the dense ceramics and 0.03 mm−1 for the porous ceramics in the spectral range of weak absorption. Experimental uncertainty resulted in only approximate determination of transport radiative properties, particularly for the dense ceramics. For the 3DOM packed bed, measured transmittance was of higher accuracy lead- ing to lower uncertainty in measurements and identification procedure. The transport scattering coefficient showed a stronger spectral dependence than that of the sintered ceramics especially after a wavelength of 0.5 μm. Thermochemical cycling resulted in changes in the 3DOM morphology due to sintering of walls of the 3DOM struc- ture. The morphology of 3DOM ceria after thermochemical cycling resembled sintered structures and this change also impacted the predicted radiative properties. The trans- port scattering coefficient showed strong dependence on morphology, with a drastic increase by a factor of four for 3DOM packed bed after thermochemical cycling. The absorption coefficient was only dependent on porosity, and did not show any change after thermochemical cycling, because the porosity only decreased weakly as a result of thermochemical cycling. Predicted radiative properties indicate that an approximation applicable in the lim- its of large optical thickness, such as the Rosseland approximation can be employed to model heat and mass transfer rates in the ceria-based redox thermochemical cycles. Approximate models for optically thick media can greatly reduce computational cost of determination of radiative heat flux in such combined heat transfer problems. Mor- phological features that promote effective absorption of solar radiation in the visible spectrum and confinement of infrared radiation in the reactor are suggested.