Numerical Study of Single-Chamber Solid Oxide Fuel Cells

Citation

Hao, Yong (2007) Numerical Study of Single-Chamber Solid Oxide Fuel Cells. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/H3E7-TY92. https://resolver.caltech.edu/CaltechETD:etd-05252007-110313

Abstract

Single-chamber solid oxide fuel cells (SCFC) are ones in which the fuel and oxidizer are premixed, and selective electrode catalysts are used to generate the oxygen partial pressure gradient that in a conventional dual-chamber design is produced by physical separation of the fuel and oxidizer streams. The SCFC concept is a novel simplification of a conventional solid oxide fuel cell (SOFC), and SCFCs have been shown capable of generating power densities high enough to make them potentially useful in many applications where the simplicity of a single gas chamber and absence of seals offsets the expected lower efficiency of SCFCs compared to dual-chamber SOFCs.

SCFC performance is found to depend sensitively on cell microstructure, geometry, and flow conditions, and optimization of SCFC stacks requires considering complex, coupled chemical and transport processes. However, research activity in this area is far from sufficient and insights about SCFC systems are very limited. The understanding of many fundamental physical and chemical processes required for improving SCFC designs is often beyond the capability of modern experimental techniques, and efficient experimental studies are often held back by the lack of guidance from theoretical models due to the fact that modeling study about SCFC is very rare to date, and existing models about conventional SOFCs are not suitable for simulating SCFCs because of the inherent differences of single-chamber SOFCs from conventional ones. In order to systematically investigate these problems and optimize the electrical performance of SCFC systems, a 2D numerical model of a single-chamber solid oxide fuel cell (SCFC) operating on hydrocarbon fuels is developed and presented in this work.

The model accounts for the coupled effects of gas channel fluid flow, heat transfer, porous media transport, catalytic reforming/shifting chemistry, electrochemistry, and mixed ionic-electronic conductivity. It solves for the velocity, temperature, and species distributions in the gas, profiles of gaseous species and coverages of surface species within the porous electrodes, and the current density profile in an SCFC stack for a specified electrical bias. The model is general, and can be used to simulate any electrode processes for which kinetics are known or may be estimated. A detailed elementary mechanism is used to describe the reactions over the anode catalyst surface. Different design alternatives including flow rates, flow geometry, temperature, optimal fuel to air ratio, anode thickness, YSZ vs. SDC electrolytes, and fuel cell efficiency and fuel utilization are explored. The reaction zones in the anode of an SOFC with hydrocarbon fuel and oxygen addition is also investigated and much deeper insights are obtained compared to the existing literature. Numerical techniques needed for such investigations are also introduced.

The model is also expanded to simulate fuel cells in the commonly seen dual chamber configuration, including ones with either oxygen-ion conducting electrolytes (SOFCs) or proton conducting electrolytes (solid acid fuel cells). Good agreement with literature results and experimental measurements is obtained.

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