Ph.D. Thesis Defense by Zhiwen Ma

(Dr. Said Abdel-Khalik, advisor)

"A Combined Differential and Integral Model for High Temperature Fuel Cells "

Abstract

Fuel cells are electrochemical devices that convert chemical energy in a fuel stream directly into low voltage, direct current, electrical energy. The fuel and oxidant stream flow distributions within a fuel cell stack have a significant impact on the fuel cell performance and efficiency; the uniformity of such distributions is a major issue facing fuel cell designers. The aim of this investigation is to develop a combined differential/integral model which allows fuel cell designers to optimize the design of fuel cell stacks and quantify the impact of various flow configuration on the overall performance. The integral model provides the means for determining the overall flow distribution within the stack. A general network analysis code is developed; the code models the various minor losses by including empirical correlations for different types of piping connections and configurations. For a given stack geometry, the code generates the network equations automatically using a network flow algorithm.

The differential model simulates the local transport phenomena within the fuel cells; a simplified one-dimensional model for multi-channel fuel cells and fuel cell stack is solved numerically. In this model, the heat and mass transfer processes, along with the electrochemical reactions inside the fuel cell system/stack, are considered. The differential model is incorporated into the integral network flow system to analyze the overall performance of the fuel cell stack; a general computer code has been developed for that purpose. The combined differential/integral stack model is validated by comparing its predictions with relevant data.

Parametric results for the fuel cell performance under different flow configurations are presented. The effects of non-uniform fuel and oxidant inlet flow conditions on the cell performance have been quantified. The combined differential/integral model developed in this investigation provides a powerful tool for fuel cell designers, inasmuch as it allows various design options to be examined and optimized.