Measuring and testing – With fluid pressure – Leakage
Reexamination Certificate
2001-12-19
2003-12-16
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Leakage
C429S010000
Reexamination Certificate
active
06662633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to locating leaks in fuel cell devices. In particular, the invention provides a method and apparatus for locating internal transfer leaks in a solid polymer fuel cell stack.
2. 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 generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cells typically employ a membrane electrode assembly (“MEA”) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The membrane, in addition to being ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant (i.e. fuel and oxidant) streams from each other.
The MEA is typically interposed between two separator plates, which are substantially impermeable to the reactant fluid streams, to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain flow field channels for supplying reactants to the MEA or circulating coolant. The plates, which include the flow field channels, are typically known as flow field plates. 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. Such plates are commonly referred to as bipolar plates and may have flow channels for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
The fuel stream that is supplied to the anode separator plate typically comprises hydrogen. For example, the fuel stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant stream, which is supplied to the cathode separator plate, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
The electrochemical reactions in a solid polymer fuel cell are generally exothermic. Accordingly, a coolant is typically also used 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, for example, U.S. Pat. No. 5,284,718 and U.S. Pat. No. 5,230,966, which are incorporated herein by reference in their entirety.
Fluid isolation is important for several reasons. One reason for fluidly isolating the fuel and oxidant streams from one another in a fuel cell is that hydrogen and oxygen are particularly reactive with each other. Accordingly, the membrane and plates are, therefore, substantially impermeable to hydrogen and oxygen.
One reason for fluidly isolating the coolant fluid from the reactant fluids is to prevent dilution and contamination of the reactant streams. Indeed, water, which is typically used as a coolant, may cause flooding in the reactant fluid passages that 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 as the leaked reactants are not used to generate electrical power. One reason for preventing leakage of any of the fluids to the surrounding atmosphere is the general negative impact such leakage can have on fuel cell stack safety, performance and longevity.
Locating the source of internal transfer leaks has been found to be problematic. Once an internal transfer leak has been detected within a fuel cell stack (which is typically detected through the constant monitoring of the exhaust streams), locating the source of the leak is typically accomplished by disassembling the fuel cell stack into its constituent parts and testing each fuel cell individually. Such method is time consuming and, consequently, expensive. Furthermore, because the disassembling and individual fuel cell testing process can cause further damage/defects to the stack, such method can result in a worsening of a fuel cell stack fluid integrity.
Accordingly, there is a general need for a method and apparatus for locating internal transfer leaks which does not require a fuel cell stack to be disassembled into its constituent parts and each part being tested individually.
BRIEF SUMMARY OF THE INVENTION
The invention provides a method for locating an internal transfer leak in a fuel cell stack. The method comprises:
a) applying a substantially constant gas pressure difference between a first fluid stream passage and a second fluid stream passage, wherein gas pressure in the second fluid stream passage is higher than gas pressure in the first fluid stream passage;
b) supplying a test gas to the second fluid stream passage;
c) supplying a test liquid to the first fluid stream passage; and
d) measuring a parameter indicative of flow rate of the test gas exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
The method may further comprise the step of ascertaining the position of the test liquid inside the first fluid stream passage. In one embodiment, this step comprises splitting a flow of test liquid so as to supply the test liquid to the first fluid stream passage and to a level indicator. As a result, the position of the test liquid within the level indicator is indicative of the position of the test liquid inside the first fluid stream passage.
The method may further comprise positioning the fuel cell stack so that individual fuel cell assemblies are aligned along substantially vertically successive horizontal planes.
In an embodiment of the method, the fluid stream passages are reactant stream passages. In another embodiment of the invention, one fluid stream passage is a reactant stream passage and the other fluid stream passage is a coolant stream passage.
In an embodiment of the method, the parameter indicative of flow rate is flow rate of the test gas. For example, in an embodiment where the test gas is air, the method may comprise measuring the flow rate of air exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
In an alternative embodiment of the method, the parameter indicative of flow rate of the test gas is concentration of the test gas, or component thereof. For example, in an embodiment where the test gas is hydrogen, the method may comprise measuring hydrogen concentration in the air exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
In an alternative embodiment of the method, the parameter indicative of flow rate of the test gas is flow rate of all gases. For example, in an embodiment where the test gas is hydrogen, the method may comprise measuring flow rate of all gases exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
In an embodiment of the method, the gas pressure inside the second fluid stream passage is kept substantially constant.
In an embodime
Ballard Power Systems Inc.
Politzer Jay L.
Seed IP Law Group PLLC
Williams Hezron
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