Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
1998-12-23
2002-12-10
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S010000, C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06492043
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of operating a fuel cell and a fuel cell apparatus. More particularly, the invention provides a method and apparatus for detecting a leak by monitoring at least one environment to detect when a tracer has escaped from a fluid passage to the monitored environment, which is otherwise fluidly isolated from the fluid passage in the absence of a leak.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert reactants, namely fuel and oxidant, 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.
Solid polymer fuel cells employ a solid polymer electrolyte, or ion exchange membrane. The membrane is typically interposed between two electrode layers, forming a membrane electrode assembly (“MEA”). The membrane is typically proton conductive, and acts as a barrier, isolating the fuel and oxidant streams from each other on opposite sides of the MEA. The MEA is typically interposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, as well as good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies.
The fuel fluid stream which is supplied to the anode may be a gas such as, for example, substantially pure gaseous hydrogen or a reformate stream comprising hydrogen, or a liquid such as, for example, aqueous methanol. The fuel fluid stream may also contain other fluid components such as, for example, nitrogen, carbon dioxide, carbon monoxide, methane, and water. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen supplied as, for example, substantially pure gaseous oxygen or a dilute oxygen stream, such as for example, air, which may also contain other components such as nitrogen, argon, water vapor, carbon monoxide, and carbon dioxide. Various sealing mechanisms are used to fluidly isolate the fuel and oxidant streams from one another in the fuel cell.
The electrochemical reactions in a solid polymer fuel cell are generally exothermic. Accordingly, a coolant is typically also needed to control the temperature within a fuel cell assembly to prevent overheating. Conventional fuels cells employ a liquid, such as, for example, water to act as a coolant. In conventional fuel cells, the coolant stream is fluidly isolated from the reactant streams.
Thus, conventional fuel cells typically employ three fluid streams, namely fuel, oxidant, and coolant streams, which are fluidly isolated from one another. See U.S. Pat. No. 5,284,718 and
FIGS. 1
,
2
A and
2
B of U.S. Pat. No. 5,230,966, which are incorporated herein by reference in their entirety, for examples of typical fuel cell assemblies configured to fluidly isolate the aforesaid three fluid streams. Fluid isolation is important for several reasons. For example, one reason for fluidly isolating the fuel and oxidant streams is a hydrogen-oxygen fuel cell is that hydrogen and oxygen are particularly reactive with each other. Accordingly, in solid polymer fuel cells, an important function of the membrane and plates is to keep the fuel supplied to the anode separated from the oxidant supplied to the cathode. The membrane and plates are, therefore, substantially impermeable to hydrogen and oxygen. However, since the membrane also functions as an electrolyte, the membrane is generally permeable to protons and water. (Water is generally required for proton transport in membrane electrolytes.)
The coolant fluid is preferably isolated from the reactants fluids to prevent dilution and contamination of the reactant streams. Furthermore, in a conventional fuel cell, it is undesirable to mix a liquid coolant, such as water, with a gaseous reactant such as hydrogen or oxygen. Water may cause flooding in the reactant fluid passages which prevents the reactants from reaching the electrochemically active membrane-electrode interface. It is also undesirable for the reactant streams to leak into the coolant stream because this reduces operating efficiency since the leaked reactants are not used to generate electrical power. Likewise, leakage of any of the fluids to the surrounding atmosphere is generally undesirable.
Accordingly, it is desirable to promptly detect fluid leaks in the fuel cell reactants or coolant fluid passages that may occur, for example, at the membranes, plates, or seals. Once a leak large enough to be an operational concern is detected, prompt corrective action may be taken. However, if the leak is not an immediate operational concern, the leak may be monitored until it does become a concern, or until the next regularly scheduled maintenance service (i.e., whichever occurs first). If action is required before the next regularly scheduled maintenance service, the fuel cell apparatus may be equipped with a mechanism that shuts off the supply of reactants to the defective fuel cell or to the fuel cell stack where the leak is detected. If the fuel cell or fuel cell stack is one unit of an array of fuel cell units, valves may be used to isolate the defective fuel cell unit from the other units so that they may continue to produce power, until it is convenient to repair or replace the isolated defective fuel cell units.
There are several conventional methods of detecting leaks. For example, in a hydrogen-oxygen fuel cell, the oxidant exhaust stream can be monitored to detect the presence of hydrogen. When hydrogen is detected in the oxidant exhaust stream, this may indicate a leak. A problem with this method is that hydrogen may be present in the oxidant exhaust stream for reasons other than a leak. For example, if there is a shortage of oxygen at the cathode, protons arriving at the cathode from the anode may recombine with electrons to form hydrogen. There are many possible causes for such an oxygen shortage. For example, an oxygen shortage may result from a sudden increase in power output demand, a malfunctioning compressor, a blockage in fluid flow field channels caused by an accumulation of product water, or a clogged air filter. An oxygen shortage may result in complete or partial oxygen starvation resulting in a reduction in cell voltage and the production of hydrogen by the combination of protons with electrons.
Unlike a leak where the typical response is to shut down the fuel cell, for an oxygen starvation problem, other corrective measures are available which do not involve shutting down the fuel cell. For example, the oxidant pressure could be raised, or the oxidant flow rate increased, to attempt to remove any accumulated water that may be plugging the flow field channels. Therefore, it is desirable to be able to distinguish between an oxygen starvation problem and a leak. However, without additional information, the detection of hydrogen in the oxidant exhaust is not, by itself, a clear indication that there is a leak. Accordingly, a disadvantage of using hydrogen detection as an indicator of leaks is that other operating parameters must also be monitored and measured to confirm that a leak is indeed the cause of hydrogen being detected in the oxidant stream.
An additional problem with using a constituent such as hydrogen, other reactants, or reaction products, as in indicator of a leak is that these constituents may be reactive within the fuel cell. These constituents may be particularly reactive in the presence of the electrocatalyst at the interfaces between the ele
Knights Shanna D.
Nebelsiek Ruediger
Stumper Jürgen
Wilkinson David P.
Ballard Power Systems Inc.
Crepeau Jonathan
McAndrews Held & Malloy Ltd.
Ryan Patrick
LandOfFree
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