A novel design of a high-temperature pressurized solar air receiver for power generation via combined Brayton-Rankine cycles is proposed. It consists of an annular reticulate porous ceramic (RPC) bounded by two concentric cylinders. The inner cylinder, which serves as the solar absorber, has a cavity-type configuration and a small aperture for the access of concentrated solar radiation. Absorbed heat is transferred by conduction, radiation, and convection to the pressurized air flowing across the RPC. A 2D steady-state energy conservation equation coupling the three modes of heat transfer is formulated and solved by the finite volume technique and by applying the Rosseland diffusion, P1, and Monte Carlo radiation methods. Key results include the temperature distribution and the thermal efficiency as a function of the geometrical and operational parameters. For a solar concentration ratio of 3000 suns, the outlet air temperature reaches 1000°C at 10 bars, yielding a thermal efficiency of 78%.
A combined two-step computational method incorporating (1) transport approximation of the scattering phase function, (2) P1 approximation and the finite element method for computing the radiation source function at the first step, and (3) the Monte Carlo method for computing radiative intensity at the second step, is developed. The accuracy of the combined method is examined for model problems involving two multi-dimensional configurations of an anisotropically scattering medium. A detailed analysis is performed for a medium with scattering phase function described by a family of the Henyey–Greenstein functions. The accuracy of the two-step method is assessed by comparing the distribution of the radiative flux leaving the medium to that obtained by a reference complete Monte Carlo method. This study confirms the main results of previous papers on the errors of the two-step solution method. The combined method leads to a significant reduction in computational time as compared to the reference method, by at least 1 order of magnitude. Finally, possible applications of the combined method are briefly discussed.
Overall transmittance of porous cerium dioxide is measured in the spectral range of 900–1700 nm using dispersive spectroscopy. Dense and porous samples of cerium dioxide with average porosities of 0.08 and 0.72, respectively, are investigated. The transmittance of both sample types increases with decreasing thickness, and this trend is more pronounced for the dense samples. The on-average spectrally increasing transmittance of the dense samples is attributed to the decreasing absorption by bulk cerium dioxide with radiation wavelength. The transmittance of the porous samples, on the other hand, remains a constant over the spectrum. Porous samples attenuate radiation stronger than the dense samples at any wavelength in the considered range, and it is hypothesized that this effect is due to more intense scattering. Sharp local variations of the transmittance are observed for both sample types.
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