A three-dimensional steady state computational fluid dynamics model is developed for a solar receiver consisting of a reflective cavity with a windowed aperture enclosing an array of five tubes. The computational model couples radiative transfer with fluid flow, heat transfer, mass transfer, and chemical reaction kinetics. Radiation heat transfer is included via a combination of ray tracing, Monte Carlo, and finite volume methods. Ray trace modeling of the concentrating system provides the magnitude and direction of the solar energy incident on the receiver window surface. Transport of solar radiation in the cavity is decoupled from all other transport processes occurring in the receiver and profiles of the absorbed solar radiation are determined via a Monte Carlo model using only the receiver geometry, magnitude/direction of radiation incident on the window, and spectral/directional optical properties. A finite volume model is implemented in conjunction with the overall CFD model to account for thermal radiation emitted by heated surfaces in the receiver. Absorption coefficients, scattering coefficients, and the scattering phase function for the cloud of particles in each tube are determined via Mie theory. Transport of the aerosol particles is modeled using a single fluid mixture model with a population balance including transport by convection, Brownian motion, and thermophoretic diffusion. Gasification of 40nm carbon particles is used as a test reaction with Arrhenius kinetic parameters from the literature. Temperature distributions and, correspondingly, reaction conversions among the five tubes are found to be considerably non-uniform, with large temperature gradients developing between the front and back surfaces of each tube. Validation of the base heat transfer and reaction models is accomplished using experimental data taken on-sun at the High Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL) with both inert and reacting materials reaching temperatures up to 1400°C.
This model is used to investigate the impact of factors describing receiver configuration and operating conditions on overall receiver performance. Thirteen individual factors are considered with four factors describing operating conditions (gas flow rate, carbon feed rate, solar power input, and steam to carbon ratio) and nine two-level factors describing receiver geometry. The geometric factors are defined relative to a base design in which all tubes are arranged in a semicircle around the back cavity wall and allow for variations in cavity size, and number, radius, and arrangement of tubes with both staggered and non-staggered arrangements considered along with factors allowing for offset of all or part of the tube semicircle toward the window. Designs are evaluated in the framework of a 213-8 fractional factorial design on the basis of receiver solar to chemical efficiency and reaction conversion for both absorbing and reflective cavity walls.
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