Electronic by-pass of fuel cell cathode gas to combustor

Details Electronic by-pass of fuel cell cathode gas to combustor Electronic by-pass of fuel cell cathode gas to combustor

2001-05-03

2004-03-16

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S006000

Reexamination Certificate

active

06706430

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a fuel cell and, more particularly, to a system and method for controlling gas flow to a fuel cell and a combustor.
BACKGROUND OF THE INVENTION
Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have also been proposed for use in vehicles as a replacement for internal combustion engines. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H
2
) is supplied to the anode and oxygen (O
2
) is supplied to the cathode. In some systems, the source of hydrogen is reformate and the source of oxygen is air.
In a first half-cell reaction, dissociation of the hydrogen (H
2
) at the anode generates hydrogen protons (H
+
) and electrons (e
−
). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through load that is connected across the membrane. In a second half-cell reaction, oxygen (O
2
) at the cathode reacts with protons (H
+
), and electrons (e
−
) are taken up to form water (H
2
O).
To operate efficiently and to produce a maximum amount of electricity, the fuel cell must be properly humidified. To achieve the proper humidity range, the hydrogen stream and/or the oxygen stream are typically humidified by one of several methods known in the art. Conventional humidity control methods generally fail to sufficiently control the humidity of the hydrogen and oxygen streams to the fuel cell. Providing too much humidity to the fuel cell blocks the reacting gases from accessing the catalyst thereby impeding the electrochemical reaction between the hydrogen and the oxygen and reducing the production of electricity. Providing too little humidity to the fuel cell restricts or limits the proton transportation required for reaction within the fuel cell and can also physically damage the fuel cell.
Fuel cells pass the oxygen stream through a cathode flow line of the fuel cell stack to provide oxygen for fuel cell operation. The hydrogen-rich fuel stream passes through an anode flow line of the fuel cell stack to provide hydrogen for the fuel cell operation. A combustor that is connected to an output of the anode flow line burns excess hydrogen passed by the fuel cell stack to provide heat for the fuel cell system. To control the temperature of the combustor, it is necessary to provide an oxygen stream to cool or heat the combustor. The combustor can be run rich or lean. If run rich, more oxygen increases the combustor's temperature. If run lean, more oxygen cools the combustor because the oxygen acts as a diluent. The oxygen stream is typically provided by the outlet of the cathode flow line of the fuel cell stack.
To operate the fuel cell stack efficiently, it is necessary to control the stoichiometry of the gas streams to the anode and the cathode flow lines. Because the fuel cell stack and the combustor are in series, the supplied oxygen flowrate is the maximum of either the required cathode stoichiometry or the required combustor air. Conventionally, the flowrate of the oxygen stream that is required to meet the cathode stoichiometry is considerably less than the oxygen stream that is required by the combustor. Oftentimes, the quality of the oxygen stream passing through the fuel cell stack is much higher than the optimum stoichiometry, which adversely impacts efficiency. There is also a pressure drop due to passing excess oxygen through the fuel cell stack. The pressure drop increases the load on the compressor, which also reduces overall system efficiency.
Additionally, the cathode flow line of the fuel cell stack requires a certain level of humidification for proper operation. On the other hand, the performance of the combustor is adversely impacted by the oxygen stream with high water content. When the combustor requires more air than the fuel cell stack, the fuel cell stack still requires all the air going through to be humidified to the same relative humidity level. The additional water and heat that is used to humidify the oxygen that is not needed by the cathode requires the use of a larger humidifier than would otherwise be required.
SUMMARY OF THE INVENTION
A gas flow control system for a fuel cell includes a gas supply and a humidifier. An inlet of the humidifier is connected to an outlet of the gas supply. A fuel cell includes a cathode flow line. An inlet of the cathode flow line is connected to an outlet of the humidifier. A combustor includes an inlet that receives gas from the outlet of the cathode flow line. A valve and a bypass line bypass gas around the humidifier and the fuel cell stack to the inlet of the combustor.
In other features of the invention, the valve is preferably a gas restriction valve, a throttle valve, or a directional valve. The valve is preferably located in the bypass line, between the gas supply and the humidifier, or between the fuel cell stack and the combustor.
In still other features of the invention, a gas flow sensor generates a gas flow signal based on gas flowing through at least one of the humidifier, the cathode flow line of the fuel cell stack, and the bypass line. A flow controller that is connected to the gas flow sensor and the valve controls the valve based on the gas flow signal.
Other objects, features, and advantages will be readily apparent from the specification, the claims and the drawings.


REFERENCES:
patent: 5935725 (1999-08-01), Dhar et al.
patent: 5952119 (1999-09-01), Wilson
patent: 6432568 (2002-08-01), Salvador et al.

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