Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Alan W. Weimer

Second Advisor

Christine Hrenya

Third Advisor

Robert Sani

Abstract

Concentrated solar energy can be used to provide the heat necessary to drive highly endothermic chemical reactions for renewable fuel production including thermal reduction of metal oxides for water-splitting cycles, and gasification of cellulosic biomass. A computational model coupling radiative transfer with fluid flow, heat transfer, mass transfer, and chemical reaction kinetics is developed for a solar receiver comprised of a specularly reflective cylindrical cavity with a windowed aperture and an array of five tubes. Finite volume techniques for radiative transfer provide accurate depictions of diffuse energy emitted by heated surfaces, but fail to produce viable solutions for solar energy with computationally reasonable mesh sizes. A hybrid Monte Carlo/finite volume strategy is proposed for radiative transfer and coupled with a three-dimensional steady state computational fluid dynamics model describing steam gasification of acetylene black. Maximum predicted temperatures for 6 kW solar power are 1813 K, 1343 K, and 1546 K at the center, front, and back tubes respectively, with corresponding reaction conversions of 40%, 2.5%, and 9.2%. Average discrepancies between temperatures predicted via the computational model and those experimentally measured on-sun up to 1700 K are 21-44 K (2-4%) for both ceramic and metallic tube materials. Predicted solar-to-chemical receiver efficiency is less than 4% with conduction and emission losses accounting for 55-69% and 11-25% of the solar input, respectively. Parameters describing operating conditions and receiver geometry are exploited to optimize the solar-to-chemical efficiency for both cooled reflective and insulated absorbing cavity designs scaled to accept 8 kW solar power. Tubes positioned outside of the solar beam fail to achieve adequate reaction conversion and contribute heavily to conduction losses in reflective cavity designs. Ideal configurations produce up to 13% solar-to-chemical efficiency and contain three moderately sized tubes situated within the solar beam and set back from the aperture such that a portion of the solar energy reflects off of the cavity wall. Insulated absorbing cavity designs are characterized by comparatively greater temperature uniformity, higher reaction conversion, and diminished conduction losses. Ideal configurations produce up to 35% efficiency and contain three large tubes which may be partially located outside of the solar beam.

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