Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2000-11-02
2003-05-27
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S010000, C429S010000, C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06569549
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to a method for increasing the operational efficiency of a fuel cell power plant, and deals more particularly with a method of providing a fuel cell power plant with a reactant stream which increases the utilization of the reactant stream, thereby increasing the performance of the fuel cell power plant as a whole.
BACKGROUND OF THE INVENTION
Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a reactant fuel being provided to an anode electrode and a reactant oxidant being provided to a cathode electrode, generating an external current flow therebetween. Such fuel cell assemblies are very useful due to their high efficiency, as compared to internal combustion fuel systems and the like, and may be applied in many fields. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products, typically water, which are produced during their operation. Owing to these characteristics, amongst others, fuel cell assemblies are particularly applicable in those fields requiring highly reliable, stand-alone power generation, such as is required in space vehicles and mobile units including generators and motorized vehicles.
Typically, electrochemical fuel cell assemblies employ a hydrogen-rich gas stream as a fuel and an oxygen-rich gas stream as an oxidant, whereby the resultant reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the anode and cathode electrodes formed of porous, electrically conductive sheet material—typically, carbon fiber paper. One particular type of ion exchange membrane is known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION™ and well known in the art. Catalyst layers are formed between the PEM and each electrode to promote the desired electrochemical reaction. The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. In order to control the temperature within the fuel cell assembly, a water coolant is typically provided to circulate about the fuel cell assembly.
In the typical operation of a PEM fuel cell assembly, a hydrogen rich fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the PEM to the cathode electrode while the electrons flow through an external circuit connected to a load. At the cathode electrode, the oxygen-containing gas supply also permeates through the porous substrate material and reacts with the hydrogen ions and the electrons from the anode electrode at the catalyst layer to form the by-product water. Not only does the PEM facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen rich fuel from the oxygencontaining gas oxidant. The reactions taking place at the anode and cathode catalyst layers may be represented by the following equations:
Anode reaction H
2
→2H
+
+2e
Cathode reaction: 1/2O
2
+2H
+
+2e→H
2
O
In practical applications, a plurality of planar fuel cell assemblies are typically arranged in a stack, commonly referred to as a cell stack assembly. The cell stack assembly may be surrounded by an electrically insulating housing that defines the various manifolds necessary for directing the flow of a hydrogen-rich fuel and an oxygen-rich oxidant to the individual fuel cell assemblies, as well as a coolant stream, in a manner well known in the art. A fuel cell power plant may typically be comprised of the fuel cell stack assembly, reactant storage vessels, reactant control valves, reactant propulsion devices, coolant pumps, heat exchangers, coolant degassifiers or demineralizers, sensors for measuring reactant concentrations, temperatures, pressures, current, voltage, and a microprocessor that controls the operation of the fuel cell power plant.
As will be appreciated by one so skilled in the art, tying these differing components into a cohesive fuel cell power plant operating within specific design parameters results in a complex and oftentimes cumbersome structure.
Specifically, the operating efficiency of a fuel cell power plant is directly related to the utilization of the reactant fuel stream supplied to the fuel cell assemblies making up the fuel cell power plant. This utilization, commonly referred to as ‘hydrogen utilization’, due to the use of a hydrogen-rich fuel stream in PEM fuel cells, is the ratio of reactant fuel consumed at the anode electrode of the fuel cells, divided by the total quantity of reactant fuel supplied to the fuel cells multiplied by 100. While PEM fuel cell power plants are designed to come as close as possible to 100% utilization, this is practically unfeasible.
Current generation PEM fuel cells frequently use thin polymer membranes on the order of approximately 15 microns thick to maximize cell performance of approximately 1000 amps per square foot (ASF). A certain measure of the hydrogen utilized as fuel within these fuel cells will diffuse across this thin membrane from the anode electrode to the cathode electrode to react with the oxygen-rich oxidant to form water. Likewise, oxygen also tends to diffuse across this thin membrane to combine with hydrogen to form water.
The combination of these effects is to reduce the available hydrogen capable of reacting at the anode electrode of the fuel cells and hence, reduces the utilization rate of the fuel cell power plant as a whole.
It is common for PEM fuel cell power plants to be operated by supplying a reactant fuel to the integrated cell stack assembly using a cascade or multiple-pass approach, wherein the individual fuel cells in the cell stack assembly are separated in two or more groups. The reactant fuel is supplied to the first group of fuel cells and then cascades to the next group and so on until exiting the cell stack assembly through a fuel exit manifold. With such an arrangement, the practical hydrogen utilization has been found to be as high as 90% or more for the overall cell stack assembly, with individual cascade groups operating at approximately 60-70% utilization. Many different flow orientations have, however, been utilized in fuel cells. The objective is always to distribute the hydrogen flow such that every section of the fuel cell receives the required quantity of fuel. Flow configurations that have been used within the cell are single pass, two pass, multi-passes, serpentine and interdigitated. Other times an external recycle is used between the cell exit and the cell inlet to improve flow uniformity within the cell stack assembly.
It has been observed, however, that in certain circumstances, for instance during shutdown or startup of the PEM fuel cell power plant, some cascade groups may suffer from fuel starvation to such an extent that the fuel cell power plant is unable to achieve a desired power output. The fuel starvation is typically caused by the latter cascade groups being momentarily deprived of the fuel stream while it is cascading from the first cascade group to the last. In addition, fuel starvation may also occur during ongoing operation of the fuel cell power plant due to nitrogen contamination of the fuel stream, this nitrogen is typically present in the reactant oxidant stream that is also being supplied to the cell stack assembly. In many applications the oxidant stream is comprised of atmospheric oxygen which contains a nitrogen component that diffuses, in part, through the solid polymer PEM to dilute the hydrogen-rich fuel stream. The diluted hydrogen fuel stream may therefore not contain an adequate hydrogen concentration to support the required current density in some sections of the cell stack assembly, thereby
McCormick Paulding & Huber LLP
UTC Fuel Cells LLC
Weiner Laura
LandOfFree
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