02-11-2010, 12:39 PM
Proton Exchange Membrane FuelCell Characterizationf or Electric
Vehicle Applications
ABSTRACT
This paper presents experimental data and an analysis of a proton exchange membrane fuel cell system for electric vehicle applications. The dependence of the fuel cell system’s performance on air stoichiometry, operating temperature, and reactant gas pressure was assessed in terms of the fuel cell’s polarity and power density-efficiency graphs. All the experimen~ were performed by loading the fuel cell with resistive heater coils which could be controlled to provi,de a constant current or constant power load. System parmdtic power requirements and individual cell voltage dia~aS[bution were also determined as a function of the electrical load. It was fotmd that the fuel" cell’s performance improved with increases in temperature, pressure and stoich~ometry within the range in which the fuel cell was operational. Cell voltage imbalances increased with increases in current otaput. The effect of such an imbalance is, however, not detrimental to the fuel cell system, as it is in the case of a battery.
INTRODUCTION
An electr~-hemical fuel ceil is a device that converts chemical energy to direct current electrical energy. By converting an on-board fuel to electricity it could be effetely-ely used to power an electric vehicle. As such, a fuel cell is an energy conversion device like an Latemal combustion engiue. This is in contrast to energy storage systems such as bat*,eries, flywheels and ultra capacitors. Further many of a fuel cells operating characteristics are closer to that of an engine th~n a storage battery. A fuel cell sy~m operation involves startup, fuel and air delivery control as a function of load, and removal of heat mad by prochacts of the reaction. The fuel cell, in other words, is an electrochemical engine. While electrochemistry describes the princspIe of operation of a fuel cell, the engineering challenge of baLancing the many variables over a wide variety of operating conditions remains. The fuel cell system consists ofa complex group of support systems that must operate in balance for efficient performance. Different types of fuel cells are conveniently dassLfied the type of electrolyte they use. Electrolytes that are presently being considered include the proton exchange membrane (a solid polymer material), phosphoric acid (a liquid), alkaline (a liquid), molten cadxmate (a liquid) and solid oxide ceramic). The choice of electrolyte directly affects a fuel cell’s operating characteristics; for example, phosphoric acid is a poor ion conductor at room temperature. As a result the phosphoric acid fuel cell must be heated to 150 to 200°C before it can be used. Today many researchers believe that the proton exchange membrane (PF2eD provides the best characteristics for transportation applications. The data and analysis presented in this paper is for a fuel ceil system manufactured by Ballard Power Systems of Vancouwer, Canada. The fuel ceil system consists of: a 35 cell series connected stack; gas, water and thermal management subsystems; and controls and monitors all assembled in a single enclosure. The area of each cell was 232 can2 and the fuet cell stack itself had a maximumgr oss power output greater than 3000 Watts operating on hydrogen and air. The system was modified by the authors to be able to independently control air stoichiometry, air/hydrogen pressure and stack exit air temperature. Previous papers that have presented experimental data on similar Ballard fuel cells systems are referencesland 2 The paper is organized into sections in the following order; Fuel Cell Operating Principle, Experimental Apparatus, Experimental Results, Results Analysis and Conclusions. The section on fuel cell operating principle is intended to give a brief overview of how a fuel cell works and its operating characteristics. The experimental apparatus section briefly describes the fuel cell system used in the experiments and the associated instrumentation. The experimental results present a series of polarity plots (voltage - current relationship) under a variety of operating conditions. The results analysis section presents the results in terms of power density-efficiency plots implicitly demonstrating theoperating characteristics of the fuel cell ~stem for electric vehicle applications.
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