(304b) Transport Phenomena in a High-Temperature Solar Air Receiver: Experimental and Computational Characterization

Industrial processes account for nearly one-third of the global primary energy consumption, almost 75% of which is required in the form of heat. High-temperature industrial processes that proceed at above 1000°C, such as the extractive metallurgy, cement production, and waste materials recycling are energy-intensive and remain fully dependent on carbon-intensive fossil fuel combustion. Solar concentrating technologies offer a clean source of process heat at temperatures required by such processes, while also enabling the production of solar fuels via thermo-chemical splitting of captured CO₂ and water. Air receivers, employing porous ceramic structures as volumetric absorbers of concentrated solar radiation and air as the heat transfer fluid, have been shown to deliver high-temperature process heat. Morphological properties of the porous ceramic structures, especially mean pore size and porosity, significantly influence the heat transfer within the receiver and thus its thermal performance. In this talk, we report on the experimental and modelling works performed on a lab-scale windowless air receiver comprising a cavity lined with reticulated porous ceramic (RPC) structures as absorbers. RPCs made out of ceria, alumina and silicon-infused silicon carbide (SiSiC), with different mean pore sizes, were tested under high-flux solar simulator at radiative input power levels between 2.5-4.9 kW through a 4 cm diameter aperture, corresponding to mean solar concentrations of 1990-3900 suns. Steady-state air outlet temperatures between 1160-450°C were achieved at air mass flow rates between 2-10 kg/h, resulting in receiver thermal efficiencies of up to 69%. In general, across different materials, the experiments demonstrate that RPCs with larger pores result in improved heat transfer, mainly due to improved radiation propagation across the RPC (owing to lower extinction coefficient). Energy balance over the receiver highlights the important role of optical and thermophysical properties while comparing different materials. To further study the complex, inter-related transport phenomena, a steady-state heat transfer and fluid flow model of the solar receiver has been developed. Radiation is the predominant mode of heat transfer at these high temperatures. In this work, the collision-based Monte Carlo (MC) ray-tracing method was implemented to solve the radiation exchange within the RPC, treated as non-gray scattering-absorbing-emitting participating medium. The MC solver yields the net divergence of radiative flux for each discrete volume of the RPC domain, which is further used in a commercial computational fluid dynamics (CFD) code to solve the Navier-Stokes equations within the RPC and air domains, yielding temperature and fluid flow fields. The net divergence in the MC solution depends on the RPC emission, determined by its temperature field, which in turn is determined by the CFD solution. This iterative cycle between the MC and CFD models is run until energy is conserved and temperatures converge. The coupled MC-CFD model, validated using experimental data, serves as a powerful tool to optimize morphological and optical properties of the RPCs for maximum solar-to-heat conversion efficiency and to perform scale-up studies for large-scale implementation of the receiver.