Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
1999-12-06
2002-10-08
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000, C429S006000
Reexamination Certificate
active
06461751
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for operating a fuel cell that improves the overall efficiency of the fuel cell system. In particular, efficiency is improved by controlling the supply of oxidant so as to reduce excess oxidant flow.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions.
The fuel fluid stream, which is supplied to the anode, typically comprises hydrogen and may be pure gaseous hydrogen or a dilute hydrogen stream such as a reformate stream. Alternatively, other fuels such as methanol or dimethyl ether may be supplied to the anode where such fuels may be directly oxidized. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, and may be pure gaseous oxygen, or a dilute oxygen stream such as air.
For a fuel cell, reactant stoichiometry is defined herein as the ratio of the reactant supplied over the reactant theoretically required to produce the current produced by the fuel cell. For conventionally operated fuel cells which typically supply a surplus of oxidant to the cathode, since the oxidant is preferentially reduced at the cathode, oxidant stoichiometry is commonly expressed as the ratio of the oxidant supplied over the oxidant consumed. However, at lower stoichiometries, the reduction of oxidant may not be responsible for all of the current produced by the fuel cell. Other reactions, such as, for example, the reduction of protons may also occur at the cathode and contribute to the current output (i.e. with the consequence of reduced output voltage). In this example, while oxidant may still be the main component being reduced at the cathode, the amount of oxidant theoretically required to produce the current output may be higher than the amount of oxidant actually supplied. Therefore, when components other than the oxidant are reduced at the cathode, oxidant stoichiometries less than one may be sustained. If the oxidant is a dilute oxidant stream such as air, only the reactant component, namely oxygen, is considered in the calculation of stoichiometry (that is, oxidant stoichiometry is the ratio of the amount of oxygen supplied over the theoretical amount of oxygen required to produce the fuel cell output current).
Hydrogen and oxygen are reactive in the fuel cell and are particularly reactive with each other. Accordingly, in solid polymer fuel cells, an important function of the membrane electrolyte is to keep the hydrogen supplied to the anode separated from the oxygen supplied to the cathode. In addition, the membrane is proton conductive and functions as an electrolyte.
The overall efficiency of a fuel cell system is a function of the total power output of the fuel cell(s) and the parasitic power consumption. Total parasitic power consumption is defined herein as the sum of all power that is consumed by the fuel cell system in the course of generating electrical power. The net electrical power output is the total power output minus the total parasitic power consumption. Therefore, overall efficiency may be improved by reducing the parasitic power consumption.
One source of parasitic power consumption, for example, is the oxidant delivery subsystem that typically employs a mechanical device such as a compressor, fan, pump, rotary piston blower, or an equivalent mechanical device that consumes power to supply oxidant to the fuel cell. Higher oxidant stoichiometries generally result in higher parasitic power consumption because more power is generally required to deliver more oxidant to the cathode. Conventional fuel cell systems typically operate with an oxidant stoichiometry greater than two (2.0). Since conventional fuel cell systems direct at least twice the amount of oxygen to the cathode than is actually required to satisfy the electrical power demand, a significant amount of the parasitic power consumption is for directing surplus oxygen to the cathode. Further, fuel cell systems commonly employ a dilute oxidant stream. A dilute oxidant stream is defined herein as a fluid stream that comprises less than 100% oxidant. For example, air is a dilute oxidant stream that typically comprises about 20% oxygen, in addition to other components such as nitrogen. Accordingly, when air is employed as the dilute oxidant stream, parasitic power consumption is amplified because in addition to the surplus oxygen, the oxidant delivery subsystem must also supply a proportionate amount of the other non-reactive components.
One reason why the parasitic power consumption associated with high oxidant stoichiometries is tolerated, is that excess oxidant is desired at the cathode to avoid oxidant starvation at the cathode electrocatalyst. Oxidant starvation is defined herein as the status when oxidant stoichiometry is less than one. Oxygen starvation typically results in a condition, in the absence of oxidant at the cathode electrocatalyst, favoring the production of molecular hydrogen from protons and electrons at the cathode. In severe cases of oxidant starvation the fuel cell may generate a negative voltage and this condition is known as cell reversal. Oxidant stream delivery systems are typically designed to provide a generous surplus of oxidant to maintain performance, to reduce the likelihood of oxidant starvation, and to reduce the likelihood of hydrogen production at the cathode, even though this results in the aforementioned amplified parasitic power consumption.
In fuel cells, oxidant starvation is most likely to occur in regions furthest downstream from the cathode inlet where the oxidant stream enters the cell, for example, near the cathode outlet. An oxidant stoichiometry that provides a surplus of oxidant to the fuel cell provides an adequate concentration of oxidant to the electrocatalyst throughout the electrochemically active area of the cathode, including near the cathode outlet.
Another reason why conventional fuel cell systems seek to avoid low oxidant stoichiometries is that the temperature within the fuel cell may rapidly increase when oxidant stoichiometry is too low. It is generally desirable to maintain the temperature of solid polymer fuel cells below 100° C. When the temperature increases within the fuel cell, parasitic power consumption increases because of the higher load on the cooling system, offsetting, to some degree, the reduction in parasitic power consumption associated with operating at a lower oxidant stoichiometry.
Another disadvantage of operating a fuel cell system with a high oxidant stoichiometry is that higher oxidant stoichiometries generally require higher speeds for the mechanical devices used by the oxidant delivery subsystem to supply the oxidant stream to the cathode. Now that fuel cell systems are being developed for commercial use, mechanical considerations over the planned lifetime of commercial fuel cell systems are a factor. A mechanical disadvantage of conventional high stoichiometry methods of operation is that such methods may result in increased wear and more frequent maintenance. If the oxidant is air, there may be additional operational costs because supplying a high surplus of oxidant also results in higher flow rates that may increase air filter maintenance and/or reduce filter efficiency.
SUMMARY OF THE INVENTION
A method of operating a fuel cell system controls oxidant stoichiometry to reduce parasitic power consumption to improve overall efficiency, while avoiding low oxidant stoichiometries that might cause reduced performance, cell reversal, hydrogen production at the cathode, and increased heat generation within the fuel cell. The fuel cell system comprises a fuel cell power generating subsystem having at least one fuel cell, and an oxida
Boehm Gustav
Fletcher Nicholas J.
Knights Shanna
Schamm Reinhold
Wilkinson David P.
Ballard Power Systems Inc.
Crepeau Jonathan
McAndrews Held & Malloy Ltd.
Ryan Patrick
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