Methods and systems for detecting leaks in a fuel cell stack

Systems and methods for testing a fuel cell stack include a vacuum source, a test head including at least one isolated vacuum plenum configured to be positioned in fluid communication with a first portion of the fuel cell stack, the isolated vacuum plenum in fluid communication with the vacuum source, and a detector in fluid communication with the at least one isolated vacuum plenum for detecting the presence of a particular constituent of a fluid provided in a second portion of the fuel cell stack, where the second portion of the fuel cell stack is separated from the first portion of the fuel cell stack by at least one of an electrolyte and a fuel cell seal.

BACKGROUND

In a high temperature fuel cell system, such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow can be a hydrocarbon fuel, such as methane, natural gas, pentane, ethanol, or methanol. The fuel cell, operating at a typical temperature between 750° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit. A plurality of fuel cells may be assembled in a fuel cell stack, with electrically conductive interconnects located between each fuel cell of the stack.

SUMMARY

Various embodiments include systems for testing a fuel cell stack that include a vacuum source, a test head including at least one isolated vacuum plenum configured to be positioned in fluid communication with a first portion of the fuel cell stack, the isolated vacuum plenum in fluid communication with the vacuum source, and a detector in fluid communication with the at least one isolated vacuum plenum for detecting the presence of a particular constituent of a fluid provided in a second portion of the fuel cell stack, where the second portion of the fuel cell stack is separated from the first portion of the fuel cell stack by at least one of an electrolyte and a fuel cell seal.

Further embodiments include methods for testing a fuel cell stack that include positioning at least one isolated vacuum plenum in fluid communication with a first portion of the fuel cell stack, providing a fluid including a particular constituent to be detected in a second portion of the fuel cell stack that is separated from the first portion of the fuel cell stack by at least one of an electrolyte and a seal, providing fluid samples obtained by the at least one isolated vacuum plenum to a detector, and detecting for the presence of the particular constituent in the fluid samples using the detector.

Further embodiments include methods of testing a fuel cell stack including a plurality of fuel cells, the fuel cell stack being externally manifolded for a first reactant and having openings of the first reactant flow path on one or more open side surfaces of the stack, where the method includes interrogating individual fuel cells of the fuel cell stack using a test head including at least one plenum positioned within or adjacent to an opening of the first reactant flow path and a detector for detecting a particular constituent of a fluid provided in a reactant flow path of the fuel cell stack and determining the presence or absence of a leak in each fuel cell of the fuel cell stack based on detecting the presence of the particular constituent using the detector.

DETAILED DESCRIPTION

In one aspect, the present invention provides accurate, rapid and non-destructive techniques for detecting defects in a fuel cell stack. Various embodiments include methods of testing an assembled fuel cell stack which may enable particular stack defects, such as leaks due to cracks in an electrolyte and/or defective seals, to be identified and located. Thus, the use of defective fuel cell stacks in a fuel cell system may be avoided. In some embodiments, defective components of the fuel cell stack identified with the embodiment method may be removed and replaced prior to utilizing the stack in a fuel cell system. In various embodiments, the testing method may be performed at a temperature that is significantly lower than the operating temperature of the fuel cell stack, and may be performed at a temperature between 0° C. and 50° C., such as between 20° C. and 30° C. (e.g., at room temperature).

An example of a solid oxide fuel cell (SOFC) stack is illustrated inFIG. 1. Each SOFC1comprises a cathode electrode7, a solid oxide electrolyte5, and an anode electrode3. The solid oxide fuel cell illustrated in this figure is an electrolyte supported cell in which the ceramic electrolyte5material can be a stabilized zirconia, such as scandia stabilized zirconia (SSZ) or yttria stabilized zirconia (YSZ). Alternatively, the electrolyte may comprise another ionically conductive material, such as a doped ceria. The cathode electrode7may comprise a thin layer of electrically conductive perovskite material, such as lanthanum strontium manganate (LSM), while the anode electrode3may comprise a thin layer of cermet material containing metal and ceramic phases, such as a nickel metal phase and a stabilized zirconia (e.g., SSZ or YSZ) or doped ceria ceramic phase (e.g., samaria doped ceria, SDC).

Fuel cell stacks are frequently built from a multiplicity of SOFC's1in the form of planar elements, tubes, or other geometries. Fuel and air has to be provided to the electrochemically active surface, which can be large.

The gas flow separator9(referred to as a gas flow separator plate when part of a planar stack), containing gas flow passages or channels8between ribs10, separates the individual cells in the stack. The ribs10on opposite sides of the plate may be offset from each other, as described in U.S. Published Patent Application Number 2008/0199738 A1 (filed on Feb. 16, 2007 as U.S. application Ser. No. 11/707,070) which is incorporated herein by reference in its entirety. Frequently, the gas flow separator plate9is also used as an interconnect which electrically connects the anode or fuel electrode3of one cell to the cathode or air electrode7of the adjacent cell. In this case, the gas flow separator plate which functions as an interconnect is made of or contains electrically conductive material. The interconnect/gas flow separator9separates fuel, such as a hydrocarbon fuel, flowing to the fuel electrode (i.e. anode3) of one cell in the stack from oxidant, such as air, flowing to the air electrode (i.e. cathode7) of an adjacent cell in the stack.FIG. 1shows that the lower SOFC1is located between two interconnects9.

FIG. 2illustrates a column containing one or more fuel cell stacks14. Each fuel cell stack contains a plurality of the SOFCs1and interconnects9, one air end plate19and one fuel end plate39. As shown inFIG. 2, at either end of the stack14, there may be an air end plate19or fuel end plate39for providing air or fuel, respectively, to the end electrode. The air end plate19faces the final cathode electrode7of the stack at one end of the stack (e.g., top or bottom end of the stack), while the fuel end plate39faces the final anode electrode3of the stack at the opposite end of the stack (e.g., bottom or top end of the stack).

Optionally, two side baffles220are placed on opposite sides of the stack. However, more or less side baffles220may be used for stacks having a cross sectional shape other than rectangular. Further, one or more fuel manifolds204may be provided in the column of fuel cell stacks14. An exemplary fuel manifold is described in the U.S. application Ser. No. 11/656,563 incorporated by reference herein in its entirety. Any number of fuel manifolds204may be provided between adjacent fuel cell stacks14as desired. Further, the number of fuel cell stacks14in a column of fuel cell stacks14may be selected as desired and is not limited to the number of fuel cell stacks14illustrated inFIG. 2.

FIGS. 3A and 3Bshow, respectively, top and bottom views of an interconnect9. The portions of interconnect9shown in side cross-section inFIG. 1are provided along lines A-A inFIGS. 3A and 3B. The interconnect9contains gas flow passages or channels8between ribs10. The interconnect9in this embodiment includes at least one riser channel16afor providing fuel to the anode-side of the SOFC1, as illustrated by arrow29. The riser channel16agenerally comprises a fuel inlet riser opening or hole that extends through at least one layer of the fuel cells and interconnects in the stack. As illustrated inFIG. 3B, the fuel can flow through the inlet riser channel16ato the anode-side of each fuel cell. There, the fuel can collect in an inlet plenum17a(e.g., a groove in the interconnect's surface), then flow over the fuel cell anode3through gas flow channels8formed in the interconnect9to an outlet plenum17band then exit through a separate outlet riser channel16b.

The cathode side, illustrated inFIG. 3A, can include gas flow passages or channels8between ribs10which direct air flow44over the cathode electrode of the fuel cell. The cathode side of the interconnect may include elevated portions surrounding the respective riser channels16a,16band seals15a,15bmay be provided on the elevated portions surrounding the riser channels16a,16band may seal the respective riser channels16a,16bto the flat surface of the adjacent SOFC1in the stack to prevent fuel from reaching the cathode electrode of the fuel cell. The seals15a,15bmay have a donut or hollow cylinder shape as shown so that the riser channels16a,16bextend through the hollow middle part of the respective seals15a,15b.

In the embodiment ofFIG. 3A, the air flow44enters the gas flow passages or channels8on a first side301of the interconnect9, flows over the cathode electrode of the fuel cell, and exits the gas flow passages or channels8on a second side302of the interconnect opposite the first side301. The cathode side of the interconnect9may have elevated portions along the periphery of the third303and fourth304sides of the interconnect, and strip seals15cmay be provided on the elevated portions to seal the cathode side of the interconnect to the surface of the adjacent SOFC1. On the anode side of the interconnect, as shown inFIG. 3B, a flat elevated surface may completely surround the periphery of the fuel flow passages8and a peripheral seal15d(i.e., a window seal) may be located on the surface to seal the anode-side of the interconnect to the adjacent SOFC1and prevent air from reaching the anode electrode of the fuel cell.

The side of the air end plate19which faces the adjacent final cathode electrode7of the stack may have the same flow channel8and rib10geometry as the air sides of the interconnects9shown inFIG. 3A. However, the opposite side of the air end plate19which faces away from its stack does not need to have any flow channels8or ribs10, since this side is used for electrical interconnection with an adjacent stack or with an electrical terminal.

The side of the fuel end plate39which faces the adjacent final anode electrode3of the stack may have the same flow channel8and rib10geometry as the fuel sides of the interconnects9shown inFIG. 3B. However, the opposite side of the fuel end plate39which faces away from its stack does not need to have any flow channels8or ribs10, since this side is used for electrical interconnection with an adjacent stack or with an electrical terminal.

InFIGS. 3A and 3B, the riser channel openings16a,16bare shown as fuel inlet and fuel outlet openings in the interconnect9. This interconnect is configured for a fuel cell stack which is internally manifolded for fuel, in which the fuel travels through the stack through fuel riser channels which are formed by mated openings through the stacked interconnects and fuel cells. However, if desired, the interconnect9may be configured for a stack which is externally manifolded for fuel. In this case, the top and bottom edges of the interconnect9shown inFIG. 3Bwould function as fuel inlet and outlet, respectively, for the fuel which flows externally to the stack. Furthermore, the interconnect9shown inFIGS. 3A and 3Bis configured for a stack which is externally manifolded for air. However, additional openings through the interconnect may be formed, such as on the left and right sides of the interconnect, for the interconnect to be configured for a stack which is internally manifolded for air.

FIG. 4Ais a plan view of a solid oxide electrolyte5. The electrolyte5may comprise a stabilized zirconia, such as scandia stabilized zirconia (SSZ) or yttria stabilized zirconia (YSZ). Alternatively, the electrolyte5may comprise another ionically conductive material, such as a doped ceria. In this embodiment, the electrolyte5has a planar geometry, although it will be understood that other geometries, such as a tubular geometry, could be utilized. Riser channel openings16a,16b, which in this embodiment comprise circular holes, extend through the electrolyte5. The riser channels16a,16bgenerally comprise fuel inlet and outlet openings that extend through at least one layer of the fuel cells. When the fuel cells and interconnects are assembled into a stack, such as shown inFIG. 2, the respective riser channels16a,16bof the fuel cells5and the interconnects9may form a continuous fluid passageway extending through multiple electrolyte layers5and interconnects9. Fuel can flow in a fuel reactant path through the inlet riser channel16ato the anode-side of each fuel cell. There, the fuel flows over the fuel cell anode3via gas flow channels8formed in the gas flow separator/interconnect plate9, and then exits through separate outlet riser channel16b. Air can flow in an air reactant path through inlet openings on a periphery of the stack (i.e., through the open sides301of the interconnects9, as shown inFIG. 3A), over the fuel cell cathode7via gas flow channels8formed in the interconnect9, and exit through outlet openings on a periphery of the stack (i.e., through the open sides302of the interconnects9, as shown inFIG. 3B).

InFIG. 4B, an anode (e.g., fuel) electrode3is shown covering part of a first major surface of the electrolyte5. A cathode (e.g., air) electrode7(not shown) can cover part of the second major surface on the opposite side of the electrolyte5.

The SOFC1in this embodiment is configured for a stack that is internally manifolded for fuel and externally manifolded for air. Thus, the stack is open on the air inlet and outlet sides. Alternatively, the SOFC1may be configured for a stack which is internally manifolded for both air and fuel. In this case, the electrolyte would contain additional air inlet and outlet openings. Alternatively, the SOFC1may be externally manifolded for air and fuel.

In high temperature fuel cell systems, such as SOFC systems, it is difficult to identify certain types of defects, such as small cracks in the electrolyte and defective seals (e.g., seals with cracks or voids between the seal and an adjacent stack component, such as an adjacent fuel cell or interconnect). Such defects may result in leaks which allow mixing of the reactant streams (e.g., air and fuel) in the fuel cell stack and/or leakage of the fuel stream out of an internally-manifolded stack and may result in a shortened useful lifetime of the fuel cell stack. Typically, such defects are not identified until after the stacks are heated to their operating temperatures (e.g., 750° C. and 950° C.) and brought into an operational condition.

Various embodiments include systems methods for accurate, rapid and non-destructive testing of fuel cell stacks that can be performed at temperatures that are significantly lower than the stack operating temperature, including at ambient temperature. In one embodiment, a system for testing a fuel cell stack includes a sniffer device for detecting a leak from one portion of a fuel cell stack to another portion of the fuel cell stack. The sniffer device may include a vacuum source, a test head including at least one vacuum plenum coupled to the vacuum source, where the test head may be positioned within or adjacent to a first portion of the stack, and a detector in fluid communication with the at least one vacuum plenum for detecting the presence of a particular constituent in the first portion of the stack.

The at least one vacuum plenum of the test head may be an isolated vacuum plenum, meaning that the vacuum plenum may obtain gas (e.g., air) samples from a localized portion of the stack (e.g., from a single fuel cell or from a portion of a fuel cell) while minimizing or eliminating contamination from the outside atmosphere. In some embodiments, an isolated vacuum plenum may be a vacuum nozzle that is held against a portion of a fuel cell stack (e.g., a side of the stack) and may optionally be sealed against the stack (e.g., by a gasket or other sealing member). In other embodiments, an isolated vacuum plenum may be a hollow tubular member (e.g., a prong or needle) that extends from the test head and may be inserted within a portion of the stack (e.g., between ribs of the interconnect within an open side surface of a fuel cell stack that is externally manifolded for a reactant) to isolate the plenum from the outside environment. In embodiments, the test head may comprise a plurality of isolated vacuum plenums that may be positioned adjacent to or at least partially within the stack to obtain gas samples from a plurality of localized portions of the stack (e.g., from a plurality of fuel cells and/or from a plurality of locations within a single fuel cell). The gas samples may be analyzed sequentially and/or in parallel by one or more detectors to detect the presence of the particular gas constituent within the samples.

FIG. 5illustrates a fuel cell stack501which may be a SOFC stack comprising a plurality of solid oxide fuel cells separated by conductive interconnects, as described above. The stack501may have a top side503, a bottom side505, and a plurality of side surfaces507,509,511and513. In this embodiment, side surface507may be an open side surface, meaning that the side surface contains a plurality of inlet openings (not visible inFIG. 5) located between the fuel cells and an adjacent interconnect of the stack (between ribs of the interconnect). Side surface509may also be an open side surface that contains a plurality of outlet openings510between the fuel cells and the adjacent interconnects. Side surfaces511and513may be closed side surfaces, meaning that the fuel cells are sealed (e.g., by seals15cand15dshown inFIGS. 3A and 3B) to the adjacent interconnects along the side surfaces511,513(i.e., there are no inlet or outlet openings on side surfaces511and513). The stack501may be assembled by alternately stacking SOFCs1with interconnects9(such as interconnect9shown inFIGS. 3A and 3B) so that the anode facing surfaces of the interconnects9(seeFIG. 3B) face the anode-sides of the fuel cells and the cathode facing surfaces of the interconnects9(seeFIG. 3A) face the cathode sides of the fuel cells. The first sides301of the interconnects9may be located along open side surface507of the stack, the second sides302of the interconnects9may be located along open side surface509of the stack, and the third and fourth sides303,304of the interconnects9may be located along the closed side surfaces511and513of the stack.

The stack501also includes riser channels16a,16bthat extend through the fuel cells and interconnects of the stack501(shown in dashed lines inFIG. 5). In operation, a first reactant515(e.g., fuel) may flow into the stack501through the inlet riser channel16a, portions of the first reactant may flow into spaces between the anode sides of the fuel cells and the adjacent interconnects of the stack, and then out of the stack501via the outlet riser channel16b. A second reactant (e.g., air) may flow into the stack501through the inlet openings on the side surface507of the stack501into the spaces between the cathode sides of the fuel cells and the adjacent interconnects of the stack and then out of the stack501via the outlet openings510. The stack501may include seals, such as seals15a,15b,15cand15dshown inFIGS. 3A and 3B, which prevent the reactants from mixing within the stack501and the fuel from leaking out of the sides507,509,511,513of the stack501.

FIG. 5schematically illustrates an embodiment system for testing the fuel cell stack501for leak defects, which may be the result of electrolyte cracks or defective seals. As shown inFIG. 5, a fluid515(e.g., a gas) comprising a particular constituent to be detected is provided in the first reactant flow path (i.e., the fuel flow path) of the stack. The fluid515may be provided from a fluid source519and flowed through the fuel flow path (i.e., through the inlet riser channel16a, into the spaces between the anode sides of the fuel cells and the adjacent interconnects and out through the outlet riser channel16b) in a first portion of the stack501. The particular constituent of the fluid515may be hydrogen. For example, the fluid515may comprise a gas containing about 3-5 vol. % of hydrogen (e.g., the fluid515may comprise a forming gas comprised of nitrogen, argon or other inert gas with 3-5 vol. % of hydrogen and/or may comprise a mixture of air and hydrogen). Hydrogen gas may be used for the particular constituent for detection due to its relatively high diffusion rate and ability to flow through small areas (i.e., cracks), although it will be understood that other easily-detectable constituents, such as helium gas, natural gas and/or methane could also be utilized.

The flow rate, pressure and/or concentration of the particular constituent in the fluid515may be varied to optimize the testing sensitivity. The flow of the fluid515may be regulated to prevent pressure build up from damaging the stack501. In some cases, it may be desirable to utilize a relatively higher pressure in order to enhance the leak rate.

A sniffer device520includes a vacuum source523and a test head521having at least one vacuum plenum coupled to the vacuum source523by at least one fluid conduit525. The test head521may be positioned within or adjacent to a second portion of the stack501that is separated from the first portion of the stack which contains the fuel reactant flow path (i.e., the second portion of the stack may be separated from the first portion containing the fuel reactant flow path by one or more electrolytes and/or seals). As shown inFIG. 5, the test head521may be located adjacent to the open side surface509of the stack501containing the outlet openings510of the air flow path. The test head521may be used to collect samples of fluid (e.g., air) from localized areas of the stack501, such as from the openings510of the air flow path and/or from around the side surface(s)507,509,511,513of the stack501. A detector527may be located downstream of the test head521and may be used to detect the presence of the particular constituent in the collected samples. For example, where the particular constituent is hydrogen, the detector527may comprise a hydrogen detector, such as a small proton exchange membrane (PEM) fuel cell. Where the particular constituent is natural gas, the detector527may be a natural gas detector, such as an electrochemical or semiconductor gas sensor. The detection of the particular constituent may be used to determine the existence of a leak in the stack, which may be the result of a crack in a fuel cell electrolyte, a faulty seal, or other defect. The detector (e.g., a PEM fuel cell) may output an electrical signal (e.g., a current or voltage signal) that corresponds to the amount of the particular constituent in the sample gas, which may enable a quantitative measurement of the size of a particular leak. A leak detection method according to the various embodiments may enable corrective action to be taken in response to detecting a leak defect, such as by repairing, replacing or by-passing the defective stack component, which may advantageously increase stack yield and performance.

In embodiments, the test head521may be moved to different locations on the stack501. For example, an actuator531, such as a motorized linear actuator, may be operable to move the test head521to different vertical positions on the stack (i.e., along the direction of the fuel cell stacking, or the z-axis inFIG. 5). One or more actuators may also move the test head towards or away from the surface of the stack501(i.e., along the x-axis dimension inFIG. 5) and/or laterally over the surface of the stack501(i.e., along the y-axis dimension inFIG. 5). Alternately or in addition, the stack501may be provided on a moveable support or stage (not shown inFIG. 5), and the stack501may be moved relative to the test head521to obtain samples from different locations on the stack501. Also, although the sniffer device520ofFIG. 5shows the test head521adjacent to one side surface509of the stack501, it will be understood that a test head may extend adjacent to multiple side surfaces of the stack, including around the entire perimeter of the stack. The test head521may also be configured to be inserted into the stack, such as within the fuel riser channels16a,16b. In addition, various embodiments of a sniffer device520may include multiple test heads521at various positions on the stack501.

FIG. 6Ais a partial cross-section view of a fuel cell stack501with a test head521adjacent to a side surface509of the stack501, such as shown inFIG. 5.FIG. 6Bis a front view of the test head521illustrating a plurality of isolated vacuum plenums601taken along line B-B′ inFIG. 6A.FIG. 6Aillustrates a pair of fuel cells1separated by interconnects9which define the fuel (i.e., anode) and air (i.e., cathode) flow paths. A crack603in the fuel cell electrolyte may allow fuel to leak from the anode side to the cathode side of the cell, as illustrated inFIG. 6A. This can decrease the efficiently as well as the longevity of the fuel cell. In addition, defects in the fuel cell seals, such as in a window seal15daround the periphery of the anode-side of the fuel cells1, or in a seal surrounding the fuel riser openings16a,16b(see seals15a,15binFIG. 3A) may result in similar problems. InFIG. 6A, the fuel riser openings16a,16bmay be located out of the plane of the drawing (e.g., above or below the plane of the drawing) and the seals15a,15bwhich surround the respective fuel riser openings16a,16bon the cathode side of each fuel cell are not shown inFIG. 6Afor clarity to illustrate the path of the fluid.

As discussed above, a system and method for leak detection in a fuel cell stack includes introducing a fluid (e.g., a gas) containing a particular constituent to be detected into a first portion of the fuel cell stack, such as the fuel flow path, and detecting for the presence of the particular constituent in a second portion of the stack that may be separated from the first portion by one or more electrolytes and/or seals. The detected presence of the particular constituent may indicate a leak defect, such as a crack in an electrolyte or seal. This is schematically illustrated inFIG. 6A, which shows a hydrogen-containing gas (indicated by arrows) flowing through the fuel flow path of the stack. A portion of the gas leaks through the crack603in the electrolyte of a fuel cell1and into the air flow path of the fuel cell.

The test head521may be positioned in contact with the side surface509of the stack501, as shown inFIG. 6A, with at least one vacuum plenum601facing the stack501. In this embodiment, the test head521includes four vacuum plenums601, as shown inFIG. 6B, although it will be understood that the test head521may include a greater or lesser number of vacuum plenums601. In this embodiment, the vacuum plenums601comprise vacuum nozzles that suck in gas from localized regions of the stack501. Each of the nozzles is coupled to a fluid conduit605which provides the gas to a detector527(seeFIG. 5) configured to detect for the particular constituent (e.g., hydrogen or natural gas). Conduit605may comprise part of conduit525shown inFIG. 5. The detector may be sufficiently sensitive to detect small quantities of the particular constituent (e.g., on the order of 10-100 ppm) which may be indicative of a leak through the fuel cell electrolyte and/or seal. The amount of the particular constituent that is detected may be used to determine the size of the leak.

In this embodiment, the vacuum plenums601are isolated from the external environment by one or more sealing members607(e.g., gasket(s)) that may contact the side surface509of the stack501when the test head521is positioned adjacent to the stack501, as shown inFIG. 6A. Individual sealing members607may surround each of the vacuum plenums601. Alternately or in addition, a single sealing member may be located on the mating surface of the test head521and may surround all of the vacuum plenums601of the test head521. The isolation of the vacuum plenum(s)601from the outside environment and optionally from other vacuum plenums of the test head521may improve the sensitivity of the testing. For example, isolated vacuum sensors may prevent the detector from becoming oversaturated by the constituent being detected and may also prevent large leaks in the stack from masking the signals from smaller leaks. Thus, the sniffer device520may be able to identify leak defects from within a particular fuel cell and in embodiments may also be able to determine the location, size and/or type of leak defect within a particular cell.

In one embodiment, the vacuum plenums601may have a nozzle orifice with a dimension in the z-axis direction that is slightly greater than the width of the air outlet openings510of the stack. Openings510may comprise the exposed channels8between ribs10on the air/cathode side of each interconnect9in the stack. For example, the width of the nozzle orifice in the z-axis direction may be greater than the width of the air outlet openings510and no more than twice the width of the air outlet openings510(e.g., the nozzle orifice may be between about 5% and 50% wider than the width of the air outlet openings510.). This may enable the vacuum plenums601to interrogate each fuel cell individually by obtaining localized gas samples from the cell (e.g., from a single air outlet opening510or plural openings510adjacent to a cathode of one fuel cell). In the embodiment ofFIG. 6B, the test head521includes a plurality of vacuum plenums601arranged laterally along the y-axis for obtaining samples from different locations in a single fuel cell. In other embodiments, the test head521may include a matrix of vacuum plenums601in the y- and z-axes that may be used to interrogate multiple fuel cells when the test head521is positioned adjacent to the stack. An example of this configuration is illustrated inFIG. 7.

Each of the vacuum plenums601of the test head521may be coupled to a separate fluid conduit605or channel. Each channel may have a dedicated detector527(seeFIG. 5) to allow simultaneous detection of samples from multiple channels. In other embodiments, a plurality fluid channels may be multiplexed to a single detector. A system of valves (e.g., electronically-actuated valves, such as solenoid valves) may be used to selectively couple each channel to the detector. To clear the fluid conduits from the constituent gas from one channel to the next, the system may provide a wait time between successive readings or may purge the conduits, such as by reversing the air flow.

FIG. 8illustrates an alternative embodiment of a test head521including one or more isolated vacuum plenums, where each of the plenums comprises a hollow tubular member701(e.g., a prong or needle) extending from the surface of the test head521. The tubular member(s)701may be inserted at least partially into the fuel cell stack501, such as into the air flow path of the fuel cell(s), as shown inFIG. 8.FIG. 8is a cross-section view which illustrates a single tubular member701coupled to a fluid conduit605, although it will be understood that a test head521may include a plurality of tubular members701and fluid conduits605in a configuration such as shown inFIGS. 6B and 7. In some embodiments, the tubular member(s)701may be inserted into the openings510which comprise air flow channels8between the ribs10on the air/cathode side of an interconnect9(seeFIG. 1). In embodiments, the tubular members701may be inserted to a depth of up to about 1 cm (e.g., 0-0.5 cm). Sampling the gas from within the channels8may improve the location detection ability of the sniffer device520by eliminating cross-contamination and false rejects from leaks that have polluted the outside atmosphere. In addition, a vacuum plenum in the form of a tubular member may reduce the requirements of the seal and can allow sampling from a stack that does not have a flat surface for sealing. In one embodiment, a test head521may include one tubular member701per air flow channel8per interconnect9, and the tubular members701may be simultaneously inserted into the channels with the gas being sampled in parallel. InFIG. 8, the fuel riser openings16a,16bare shown in dashed lines because they are located out of the plane (i.e., above or below the plane) of the drawing. Thus, member701does not puncture the seals15a,15bwhich surround the fuel riser openings16a,16bon the cathode side of each fuel cell.

FIGS. 9A-9Dillustrate another embodiment test head521comprising a plurality of vacuum plenums601extending parallel in the lateral (i.e., y-axis) direction and spaced in the vertical (i.e., z-axis) direction.FIG. 9Ais a front perspective view of the test head521showing six vacuum plenums601on a front (i.e., mating) surface903of the test head521and six fluid ports905on a top surface907of the test head521.FIG. 9Bis a top view of the test head521showing the configuration of the fluid ports905, andFIGS. 9C and 9Dare cross-section views taken along lines C-C and D-D, respectively, ofFIG. 9Billustrating the fluid conduit configuration within the test head521. In this embodiment, the pitch (i.e., z-axis spacing) between vacuum plenums601is fixed, although in alternative embodiments the pitch of the vacuum plenums may be adjustable.

FIGS. 10A-10Dillustrate another embodiment of a system1001for leak detection in a fuel cell stack. In this embodiment, the system1001includes four sniffer devices520-1,520-2,520-3,520-4that may be located around the side surfaces of a fuel cell stack. Each of the sniffer devices520-1,520-2,520-3,520-4includes a test head521with a vacuum plenum601that is in fluid communication with a vacuum source523and a detector527via a fluid conduit525(schematically illustrated by arrows inFIGS. 10A-10B).

The test heads521in this embodiment each include a single slit-shaped vacuum nozzle601that may extend over at least a portion of a side surface of the stack and may be configured to interrogate each fuel cell individually by obtaining localized gas samples from the cell (e.g., from a single air outlet opening510or plural openings510adjacent to a cathode of one fuel cell and/or from a single seal, such as a window seal15dadjacent to an anode of one fuel cell). The test heads521may also include an air inlet1003in fluid communication with the vacuum nozzle601, as shown inFIG. 10A. The air inlet1003may be off-set or spaced away from the side surface of the fuel cell stack and may be oriented such that is does not directly face the side surface of the stack. The air inlet1003may draw in ambient air under the vacuum pressure provided by the vacuum source523. The ambient air from the inlet1003may mix with the air sample from the stack obtained by the vacuum nozzle601within the test head521, and this mixture may be drawn through the fluid conduit525to the detector527. Because the air inlet1003is offset from the side surface of the stack, the ambient air drawn in by the air inlet1003may not be contaminated by leaks from a neighboring fuel cell of the stack. This may reduce or eliminate the possibility of the vacuum nozzle601sucking in ambient air from adjacent to the surface of the stack that may be contaminated by a leak in a neighboring fuel cell.

As shown inFIGS. 10A and 10B, each of the sniffer devices520may include a separate vacuum source523, which may be a Venturi-type vacuum pump that produces a vacuum by means of the Venturi effect. This may be advantageous for low flow rates and because it does not introduce noise due to cycling action. Other types of vacuum sources523, such as a diaphragm or vane pump, could also be utilized. A filter1005may be located between the test head521and the vacuum source523and detector527to remove dust and other unwanted contaminants from the air samples. The sniffer device520may also include a flow meter1007to measure the volumetric or mass flow rate of the air, which may be used to normalize the measurements of the constituent being detected obtained by the detector527. The flow meter1007may also be used for “real-time” diagnosis of system operating conditions. For example, if the flow rate measured by the flow meter1007is below a predetermined threshold, it may be indicative of a system malfunction, such as a malfunctioning vacuum pump523or a clog somewhere in the system (such as the filter1005). It can be advantageous to know when there is a system malfunction in real-time, as this may indicate that a portion of the test data may be invalid and the stack(s) may need to be re-tested using a properly operating system1001. This may prevent stacks with leak defects from slipping through the testing process.

FIG. 10Cis a cross-section view of the detector527according to one embodiment. The detector527ofFIG. 10Cincludes a housing1008having an inlet1009and an outlet1013and defining a sensor cavity1010including a sensing element1011that is sensitive to the particular constituent being detected. The sensor cavity1010may have an asymmetric configuration in which the distance between the outlet1013and the sensing element1011is less than the distance between the inlet1009and the sensing element1011. This may promote laminar flow over the sensing element1011, as shown inFIG. 10C, which may improve sensitivity of the detector527. In other embodiments, the sensor cavity may have a symmetric configuration in which the inlet1009and outlet1013are generally equidistant from the sensing element1011.

FIG. 10Dis a cross-section side view of the system1001that illustrates two sniffer devices520-1and520-3located adjacent to two opposing side surfaces of a fuel cell stack501. In this embodiment, the height of the test heads521and vacuum nozzles601of the respective sniffer devices520-1,520-3is offset relative to one another by a predetermined distance in the z-axis direction (e.g., via an offset spacer1015as shown inFIG. 10D). The offset distance may be equal to a spacing between different (e.g., adjacent) fuel cells of the stack501, such that each of the opposing pairs of test heads521may interrogate a different fuel cell. This configuration may prevent the respective sniffer devices from obtaining samples from the same fuel cell at the same time, thus competing for the same signal.

A sniffer device520such as shown in any ofFIGS. 5-10Dmay use an actuator system (such as the actuator531shown inFIG. 5) to automatically interrogate each fuel cell of a fuel cell stack. In one embodiment, the device520may be configured to determine the spacing (pitch) between fuel cells based on the height of the stack501in order to compensate stacking tolerances of the fuel cell stacks. Alternately or in addition, the device520may use an optical detection system to determine the spacing between fuel cells.

In one embodiment of a stack testing method, a test head521of the sniffer device520may be positioned adjacent to a first fuel cell (or section of fuel cells) of the stack501. An actuator (i.e., x-axis actuator) may move the test head521to seal against the surface of the stack501. The sniffer device520may then obtain gas samples from the stack. For a test head521that includes multiple isolated vacuum nozzles601, the nozzles601may obtain samples simultaneously (i.e., parallel detection) or each nozzle601may be operated sequentially to obtain a sample from a different localized portion of the fuel cell stack. The test head521may then be moved away from the stack501, and an actuator (i.e., z-axis actuator) may then move the test head521to the next fuel cell (or section of fuel cells) to repeat the testing process. This process may be repeated until all of the fuel cells of the stack501have been tested.

In an alternative embodiment of the testing method, the test head521may be spaced a small distance (e.g., 0.5-10 mm) away from the surface of the stack501and may be moved across the surface of the stack while the vacuum nozzle(s)601obtain samples to determine the existence of a potential leak within the stack501. When a potential leak is determined (i.e., the detector detects some quantity of the particular constituent being measured), the test head521may then be moved against the stack521to perform a more thorough test to more accurately determine the location of the leak. This may help improve throughput of the testing process.

A testing method as described above may be used to test fuel cell seals for leaks. For example, the test head521may be used to interrogate the window seals15dsurrounding the anode-side of the fuel cells, and may also be used on the closed side surfaces511,513of the fuel cell stack501(seeFIG. 5).

Furthermore, although the methods and systems for leak detection are illustrated for a fuel cell stack501, the methods and systems may be utilized for testing a column (seeFIG. 2) or any other apparatus that is designed to maintain two separate flow paths. The various embodiments may be used to measure interface leaks between fuel cell stacks and between stacks and termination plates, for example, as well as poor sealing on the cell level. The various embodiments are not limited to any of the other specifics mentions in the above examples, and may use different geometries and applications of the detector and nozzle as required.

In the various embodiments described herein, the fluid (e.g., gas) being tested is provided through the fuel channels (anode side) and the leaking gas is detected in the air channels (cathode side), however it will be understood that the reverse configuration (i.e., providing the fluid in the air-side and testing for leaks in the fuel-side) can also be utilized, depending on the geometry and design of the fuel cell. For example, the vacuum plenum(s)601may be provided in the fuel riser channels16a,16bto detect for leaks from the air flow path of the stack. In this example, the fluid (e.g., gas) containing the particular constituent to be detected may be provided sequentially in the air flow passages (e.g., passages510) of the stack.

For example, instead of a “pull” method where a vacuum pressure is used to collect gas samples from the stack to detect for a particular constituent, a “push” technique could be utilized. For example, a test head521such as described above may be coupled to a source of pressurized fluid (gas) and inject a pressurized fluid containing the particular constituent to be tested into localized portions of the fuel cell stack (e.g., into the air-flow side of a fuel cell). A detector may be provided within or adjacent to a second portion of the fuel cell stack (e.g., within or adjacent to the fuel riser channels) to detect for the presence of the particular constituent. The test head521may interrogate different localized portions of the fuel cell stack with pressurized fluid to determine the presence and location of crack defects within the stack.

While solid oxide fuel cell stacks were described above in various embodiments, embodiments can include any other fuel cell systems, such as molten carbonate or PEM fuel cell systems or stacks.

Further, any step or component of any embodiment described herein can be used in any other embodiment.