Solar Reactor Design, Materials, and Modeling
Solar thermal systems focus large amounts of sunlight onto a small area, producing extremely high temperatures (up to 2000°C) in the focal area. This thermal energy can be used to drive highly endothermic chemical reactions and produce transportation fuels that are both clean and renewable. However, significant engineering challenges must first be overcome before this technology can be feasible and affordable. One of the paramount challenges is producing robust reactor systems that can efficiently utilize the supplied solar energy and can cope with the rapid temperature changes typical of concentrated solar thermal applications.
Aerosol flow reactors offer the advantage of high heat and mass transfer rates due to the large surface area to volume ratio of small particles entrained in the transport gas. As a result, rapid heating rates (greater than 105 K/s) and kinetically controlled reactions are achievable, allowing for a high throughput of reactants and reduced re-radiation losses by the reactant particles. A novel reflective cavity/receiver design enclosing multiple aerosol flow tubes has been developed to reduce re-radiation losses and enhance efficiency over single tube designs. Highly concentrated solar energy is introduced through a window in the outer reactor cavity and intercepted by the absorbing flow tubes, heating the reactants to extremely high temperatures in short residence times. This design is being tested on-sun at the High Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL) in Golden, Colorado.
Computational fluid dynamics (CFD) modeling can be used to develop an understanding of both transport processes occurring within the reactor and factors limiting reactor efficiency by solving the governing heat, mass, momentum, and radiation transport equations for the solar receiver. Ray trace modeling of the HFSF provides power and flux profile boundary conditions entering the chamber. The complex radiative transfer processes, including absorption and scattering by the particulate phase, can be incorporated into the CFD models using a discrete ordinates method. These models can be experimentally validated with the results of on-sun tests of the prototype reactor, and can then be used to identify parameters influencing conversion, selectivity, and efficiency, and to optimize both design and performance via consideration of reactor geometry and operating conditions.
In addition, usable reactor materials must be capable of withstanding high temperature corrosive environments and rapid temperature variations. The objective of our work in this area is to first, develop a testing and modeling system to characterize thermal stresses indicative of concentrated solar energy systems. The second objective is to use the predicted stress cycles to estimate material lifetimes and optimize material selection. The final purpose of this work is to use ALD to create composites which are optimally suited for high temperature solar thermal applications and to design concentrated solar systems that reduce thermal stresses. From this we hope to be able to create reactor systems that can withstand the harsh conditions indicative of concentrated solar power.
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