Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Alan W. Weimer

Second Advisor

David Clough

Third Advisor

Richard Noble

Abstract

An extremely important component of concentrated solar energy development is designing robust reactors.

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Many different reactor designs have been proposed in recent literature. Transferring the solar radiation to process reactants can often be difficult. For this study a reactor design was created that uses multiple absorbing tubes. The absorber tubes are housed within a cavity that reflects spilled or emitted radiation to increase solar utilization efficiency. The reactor has been designed to fit the High Flux Solar Furnace at the National Renewable Energy Laboratory. Solar concentrating capabilities of the HFSF facility have been modeled using the ray-trace program SOLTRACE. Absorber tube positions were optimized to intercept a large fraction of the incident radiation. An attempt was also made to more evenly distribute the flux across multiple tubes. The outer cavity was fabricated out of polished, reflective, aluminum to reduce the systems thermal mass and shorten heating and cooling times during testing.

The HFSF facility was qualified through black body calorimetery and flux versus inlet attenuation was mapped. Reactor performance was validated by installing absorber tubes and measuring temperature distributions. Maximum temperature differences between the central and surrounding alumina tubes were less than 350 K, at ~1673 K central tube maximum. This temperature difference decreased with higher absorptivity tube materials. Radial distance from the central tube seemed to have the largest effect on the maximum tube temperature as the tubes were farther from the focal point of the incident radiation. Cavity wall temperatures were kept below 50 K, at maximum absorber temperatures, which indicate excellent heat reflection and dissipation.

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Once thermal stresses were quantified an effort was made to find high temperature materials that are better suited to withstand thermal shock. Graphite is a thermal shock resistant high temperature material that would be well suited for solar thermal applications. However, graphite oxidizes above 773 K. By coating graphite with an oxygen barrier material, thermal shock resistant composites could help in a wide range of applications. Graphite powder was coated with alumina via atomic layer deposition (ALD). The powders had a marginal increase in oxidation resistance but coated powders showed a marked improvement in dispersability. Sedimentation and isoelectric tests showed a change in particle-particle interactions which was also validated by decreased particle size distributions of coated particles. Alumina-graphite composites showed enhanced thermal properties such as thermal diffusivity, thermal conductivity, and thermal expansion when compared to uncoated composites. This research provides a method to enhance bulk material properties of composites specifically using hard to disperse additives such as graphite and potentially carbon nanotubes.

The work in this thesis represents a broad investigation into the feasibility of using concentrated solar thermal technology for renewable fuels production. Research pertaining to materials robustness, reactor design, and fuel production cycles has advanced the state of the art. Rudimentary experimentation with this technology was conducted decades ago. New research is needed to deal with the engineering challenges that are hindering large scale adaptation of concentrated solar energy today.

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