Title of Invention

METHOD FOR IMPROVING FCS RELIABILITY AFTER END CELL HEATER FAILURE

Abstract

A method for improving fuel cell system reliability in the event of end cell heater failure in a fuel cell stack. The method includes detecting that an end cell heater has failed. If an end cell heater failure is detected, then he method performs one or more of setting a cooling fluid pump to a predetermined speed that drives a cooling fluid through cooling fluid flow channels in the fuel cell stack, limiting the output power of the fuel cell stack or the net power of one fuel cell system, limiting the maximum temperature of the cooling fluid flowing out of the stack, turning off stack anti-flooding algorithms that may be used to remove water from reactant gas flow channels in the stack, and turning of cathode stoichiometry adjustments for relative humidity control in response to water accumulating in cathode flow channels in the fuel cell stack.

Full Text

METHOD FOR IMPROVING FCS RELIABILITY AFTER END CELL HEATER FAILURE BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] This invention relates generally to a system and method for improving the reliability of a fuel cell system and, more particularly to a system and method for taking preventative measures in response to end cell heater failure in a fuel cell stack to minimize stack degradation and/or prevent stach failure until such time that repair is possible 2. Discussion of the Related Art [0002] Hydrogen is a very attractive fuel because it is cean and car be used to efficiently produce electricity in a fuel cell. A hydrogen feel cell is are electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte. and thus are directed through a load to perform work before being sent to the cathode. [0003] Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymet electrolyte proton conducting membrane, such as a perfluorosilfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with are ionomer. The catalytic mixture is deposited on opposing sides of the membrane The combination of the anode catalytic mixture the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA) [0004] Several fuel cells are typically combined in a fuel cell stack to generate the desired power The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows. [0005] A fuel cell stack typically includes a series of bipc ar plates positioned between the several MEAs in the stack, where the bipolar pates and the MEAs are positioned between two end plates The bipolar plates clude an anode side and a cathode side for adjacent fuel cells in the stack. A lode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also in lude flow channels through which a cooling fluid flows. [0006] The membrane within a fuel cell needs to have a certain relative humidity so that the ionic resistance across the membrane is Iow enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. The Flow of the reactant gas through the flow channels has a drying effect on the membrane, most notice-ably at an inlet of the flow channels. Also, the accumulation of water droplets within the flow channels from the membrane relative humidity and water by-product could prevent reactant gas from flowing therethrough and cause the cell to fail, thus affecting the stack stability. The accumulation of water in the reactant gas flow channels is particularly troublesome at low stack output loads. [0007] The end cells in a fuel cell stack typically have different performance and sensitivity to operating conditions than the other cells in the stack. Particularly, the end cells are nearest in location to the stacks ambient temperature surroundings and thus have a temperature gradient that causes them to operate at a lower temperature as a result of various heat losses Because the end cells are typically cooler than the rest of the cells in the stack gaseous water more easily condenses into liquid water so that the end cells have a higher relative humidity, which causes water droplets to more read form in the flow channels of the end cells. Further at low stack loads, the amount of reactant gas flow available to push the water out of the flow channels is significantly reduced. Also, at low stack loads the temperature of the cooling fluid is reduced, which reduces the temperature of the stack and typically Increases the relative humidity of the reactant gas flow. [0008] It is known in the art to heat the end cells of a fue cell stack using resistive heaters positioned between the end unit and the unipolar plate so as to compensate for heat losses. However, sometimes these end ceII heaters fail where they remain on, which could result in a larger problem then a stack without end cell heaters. SUMMARY OF THE INVENTION [0009] In accordance with the teacnings with the present invention. a system and method are disclosed for improving fuel cell system reliability in the event of end cell heater failure in a fuel cell stack. The metho includes detecting that an end cell heater has failed to be in a constantly on condition. If an end cell heater failure is detected, then the method performs one or more of setting a cooling fluid pump to a predetermined speed that drives a cooling fluid through cooling fluid flow channels in the fuel cell stack, limiting the output power of the fuel cell stack, limiting the maximum temperature of the cooling fluid flowing out of the stack, turning off stack anti-fooding algorithms that may be used to remove water from reactant gas flow channels in the stack and turning off cathode stoichiometry adjustments for relative humidity control in response to water accumulating in cathode flow channels in the fuel cell stack. [0010] Additional features of the present invention wiII become apparent from the following description and appended claims taken in conjunction with the accompanying drawings BRIEF DESCRIPTION OF THE DRAWINGS [0011] Figure 1 is a schematic plan view of a fuel cell system and [0012] Figure 2 is a flow chart diagram showing an peration fo improving fuel cell system reliability in the event of end cell heater failure according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS [0013] The following discussion of the embodiments of the invention directed to a system and method for improving fuel ceil system reliability in the event of end cell heater failure is merely exemply in nature and is in no way intended to limit the invention or its applications isuses. [0014] Figure 1 is a plan view ot a fuel cell system 1 including fuel cell stack 12. The fuel cell stack 12 includes end cell heates 14 and 16 positioned in the end cells of the stack 12 in one non-limiting embodiment the end cell heaters 14 and 16 are resistive heaters. The type of end ceII heater the positioning of the end cell heaters 14 and 16 and the control of the end heaters 14 and 16 during normal operation of the fuel cell system 10 are a well know parameters to those skilled in the art. [0015] The fuel cell stack 12 receives cathode inp t air from compressor 18 on cathode input line 20, ano outputs cathode ex aust gas cathode output line 22. Likewise, the fuel cell stack 12 receives a hydrogen gas flow on anode input line 24, and outputs anode exhaust gas on anc output line 26. Cooling fluid flow channels are provided n the bipolar platest the fuel cell stack 12. A cooling fluid is pumped through the cooling fluid flow annels by a pump 30 and through a cooling fluid loop 32 outside of the stack 12 The cooling fluid in the loop 32 from the stack 12 is sent to a radiator 34 whese it is reduced in temperature prior to being sent back to the fuel cell stack 12 radiator by pass valve 36 allows a controlled amount of :he cooling fluid to go through the radiator 34 or by-pass the radiator 34 on a by-pass line 38 so as to help control the temperature of the fuel cell stack 12 in a manner that is well understood to those skilled in the art. A controller 40 receives various input signals from the system 10 including temperature measurement signals from the end cell heaters 14 and 16. The controller 40 also controls the various elements a the system 10. including the compressor 18. the pump 30 and the by-pass 36. [0016] The controller 40 provides a particular pulse-widir modulation (PWM) signal to the end cell heaters 14 and 16 having a particular duty cycle that identifies when the heaters 14 and 16 are on and v.hen they are off for a particular system operation. The power that actually drive the end cell heaters 14 and 16 can be provided by the power output of the fuel cell stack 1 Because the end cell heaters 14 and 16 and related circuitry, are is a somewhat severe environment, it is possible that some part of the end cells 14 and 16 or their circuitry can fail causing the end cell heaters 14 and 16 to be stucK on or be stuck off. [0017] If the end cell heaters 14 and 16 are stuck continuously on, a significant amount of heat is generated by the end cell heaters 14 and 16 that could damage the fuel cell stack 12. particularly the membranes therein. It has been proposed in the art to provide certain circuitry that would caus the end cell heaters 14 and 16 to remain continuously off if a failure OCCU red or was detected. However, such a condition causes the problem discusse above it no end cell heaters were provided in the stacK 12 With either of he end cell heaters 14 and 16 stuck in the continuous on position, the end cells of the stack 12 get hot and dry, causing poor cell performance due to high ionic resistance This problem increases if the vehicle operator requests high powe where the stack 12 produces a high current density causing end cell performance to decrease significantly. Therefore, it would be cesirable to have an agorithm that took various remedial steps in response to end cell heater failure in the on position so that the fuel cell system 10 can continue to run without causing damage to the stack 12 until such a time tha: the fuel cell systen 10 can be serviced. [0018] Figure 2 is a flow chart diagram 50 showing a process for mitigating or preventing fuel cell stack damage in response to eno cell heater failure, according to an embodiment of the present invention. The algorithm identifies an end cell heater failure at box 52 This can be accomplished in any suitable manner, such as measuring the temperature of the end ;ell and the temperature of the cooling fluid flowing through the cooling fluid loop 32 to determine whether there is a significant difference between the two. Also, the current applied to the end cell heaters 14 and 16 can be measured determine whether more current is being drawn by the end cell heaters 14 at d 16 than is required for a particular duty cycle of the PWM signal, If the algorithm determines that an end cell heater has failed it can cause a warning to be given to the vehicle operator indicative of the failure such as turning on a service soon light. [0019] If the algorithm determines that one or both the end cell heaters 14 and 16 has failed continuously on. the algorithm will set the speed of the cooling fluid pump 30 to a predetermined maximum speed a box 54 By increasing the flow rate of the cooling fluid through the stack 12 the temperature rise of the fuel cells within the stack 12 is limited because more cooling fluid enters the stack 12 to draw away excess heat. Therefore the er d cells of the stack 12 would not dry out and overheat as severely with the speed of the cooling fluid pump 30 in this position [0020] The algorithm may also set a predetermined maximum output power from the stack 12 at box 56. in one non-limiting embodiment the maximum stack power is set to 10kW. If more than this maximum amount of power were allowed to be drawn from the stack 12, then more current is drawn from the end cells that are already being over-heated, which would reduce the voltage of the end cells, possibly causing them to be unstable other woras as the temperature of the end cell goes up. the relative humidity within the end cell goes down, causing the resistance of the cell to go up. As more current is being drawn from an end cell in this condition, more voltage losses occur causing the voltage across the end cell to go down. If this phenomenon continues, the voltage across the end cell may go negative possibly causing end cell and/or stack failure. [0021] Further, the algorithm may limit the maximum temperature of the cooling fluid out of the stack 12. at box 58 to be below a formal system maximum if the end cell heaters 14 and 16 were operating property in one non- limiting embodiment, the maximum allowable cooling fluid temperatue could be about 70°C. By limiting the temperature of the cooling fluid, the temuerature of the end cells in the stack 12 can be reduced to prevent chemical aggradation and performance problems. The algorithm can reduce the tempers cure of the cooling fluid by causing more of the cooling fluid to flow through the adiator 34 as opposed to by-passing the radiator 34 on the by-pass line 38. Farticularly. the controller 40 controls the by-pass valve 36 to reduce or eliminate ne amount of cooling fluid that flows through the by-pass line 38 so that the temperature of the cooling fluid is further reduced. [0022] The algorithm also can turn off anti-flooding aluorthms at box 60 that may be changing system parameters in a predeterminer manner to control the amount of water accumulation in the various reactant gas flow channels in the bipolar plates in the stack 12 Algorithms for this purpose are well known to those skilled in the art. Because the end cells would be at a higher temperature if the end cell heaters 14 and 16 were continuously on, tne amount of water within the cathode or anode flow channels would be reduced from normal end cell heater operation. Therefore, it may be desirab e to turn off the algorithms that act to limit the amount of water in the flow channels because the fuel cell system may appear to be operating properly, i.e., the end cell heaters are operating properly, from the anti-flooding algorithm's point of view. [0023] The algorithm also can turn off cathode stoichiometry adjustments for stack relative humidity control at box 62. The cathode stoichiometry, i.e., the relationship between the cathode input airfow and the stack output current, has a particular set-point for each stack curent density Cathode stoichiometry control is typically employed during powe transients where the relative humidity control in the cathode flow channels is affected by the water production rate, i.e.. current, of the stack 12 and the flov. of cathode input air to the stack 12. For those times when the system may war to increase the cathode stoichiometry to reduce the cathode flow channel wate accumulation, it may be desirable to stop the operation of this algorimm because the end cells are already hot from the failed end cell heater, which reduces the ability to accumulate water. By increasing the speed of the compoessor 18 tc increase the cathode stoichiometry more drying air is forcec through the reactant gas flow channels, which could even further dry out an aready heated and dry cell. [0024] The algorithm may also periodically check the temperature of the end cell heaters 14 and 16 at box 64 Even with all of the ther previous steps of the algorithm, the end cell heater temperature could too high to prevent performance loss and increased cell degradation. Approprate action a this case could include, but not be limited to further reducing the temperature of the cooling or system shut-down. Therefore, the algorithm may return tc the step of the box 58 to further reduce the temperature of the coolir g to an even lower value. [0025] The algorithm can also maintain the end cell beater control mode for the next system start-up at box 66 After the algorithm has determined that there is an end cell heater failure where the end cell heater is stuck in the on position, and has taken one or more of the measures discussed above to reduce stack degradation, it may be desirable to maintain the end cell eater failure control mode after the vehicle has been shut-down and restarted. By maintaining the end cell heater failure control mode for the next start-up the system will not have to go through the process of again determining that the end cell heater 14 and/or 16 has failed. [0026] The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled the art will readily recognize from such discussion and from the accompanyong drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention is defined in the following claims. CLAIMS What is Claimed is: 1. A method for taking remedial action in response to failure >f an end cell heater in a fuel cell stack, said method comprising: identifying that the end cell heater has failed in a continuously on condition; setting a cooling fluid pump to a predetermined maximum speed that drives a cooling fluid through cooling fluid flow channels in the fuel cell stack. limiting the maximum output power of the fuel cell stack. limiting the maximum temperature of the cooling fluic flowing through the stack; turning off anti-flooding processes that may be used to reduce water accumulation in reactant gas flow channels in the stack; and turning off cathode stoichiometry adjustments for relative umidity control from water accumulating in cathode flow channels in the fuel c—II stack. 2. The method according to claim 1 wherein identifying an end cell heater failure includes measuring the temperature of the end cell and the temperature of the cooling fluid flowing through a cooling fluid loop to determine whether there is a significant difference between the two. 3. The method according to claim 1 wherein identifying an end cell heater failure includes determining whether more current is being drawn by the end cell heater than is required for a particular duty cycle of a pulse-width modulated signal controlling the end cell heater 4. The method according to claim 1 further comprising maintaining the end cell heater failure mode from a system shut-down to the next system start- up. 5. The method according to claim 1 wherein limiting the maximum temperature of the cooling fluid includes reducing or eliminating the amount of cooling fluid that by-passes a radiator outside of the stack. 6. The method according to claim 1 further comprising determining the temperature of the end cells and wherein limiting the maximum temperature of the cooling fluid includes further limiting the temperature of the cooling fluid if the temperature of the end cells is determined to be too high. 7. The method according to claim 1 wherein the heaters are resistive heaters. 8. A method for taking remedial action in response to faiure of an end cell heater in a fuel cell stack, said method comprising: identifying that the end cell heater has failed in a continuously on condition; setting a cooling fluid pump to a predetermined maximum speed that drives a cooling fluid through cooling fluid flow channels in the feel cell stack and limiting the maximum output power of the fuel cell stack 9. The method according to claim 8 further comprising limiting the maximum temperature of the cooling fluid flowing through the stac. 10. The method according to claim 9 wherein limiting the maximum temperature of the cooling fluid includes reducing or eliminating the amount of cooling fluid that by-passes a radiator outside of the stack. 11. The method according to claim 8 further comprising tuning off anti- flooding processes that may be used to reduce water accumulation in reactant gas flow channels in the stack. 12. The method according to clairr 8 further comprising urning off cathode stoichiometry adjustments for relative humidity control from water accumulating in cathode flow channels in the fuel cell stack. 13. The method according to claim 8 wherein identifying a end cell heater failure includes measuring the temperature of the end cell and the temperature of the cooling fluid flowing through a cooling fluid loop to determine whether there is a significant difference between the two. 14. The method according to claim 8 wherein identifying an end cell heater failure includes determining whether more current is being drawn by the end cell heater than is required for a particular duty cycle of a pulse-width modulated signal controlling the end cell heater 15. The method according to claim 8 further comprising maintaning the end ceil heater failure mode from a system shut-down to the next system start- up. 16. A method for taking remedial action in response to failure of an end cell heater in a fuel cell stack, said method comprising. identifying that the end cell heater has failed in a continuo sly on condition: setting a cooling fluid pump to a predetermined maximum speed that drives a cooling fluid through cooling fluid flow channels in the fuel cell stack limiting the maximum output power of the fuel cell stack and turning off anti-flooding processes that may be used to reduce water accumulation in reactant gas flow channels in the stack. 17. The method according to claim 16 further comprising limiting the maximum temperature of the cooling fluid flowing through the stack. 18. The method according to claim 17 wherein limiting the maximum temperature of the cooling fluid includes reducing or eliminating the amount of cooling fluid that by-passes a radiator outside of the stack. 19. The method according to claim 16 further comprising turning oft cathode stoichiometry adjustments for relative humidity control from water accumulating in cathode flow channels in the fuel cell stack, 20. The method according to claim 16 wherein identifying an end cell heater failure includes measuring the temperature of the end cell and the temperature of the cooling fluid flowing through a cooling fluid loop fo determine whether there is a significant difference between the two. 21. The method according to claim 16 wherein identifying an end cell heater failure includes determining whether more current is being drawn by the end cell heater than is required for a particular duty cycle of pulse-width modulation signal controlling the end cell heater 22. The method according to claim 16 further comprising maintaining the end cell heater failure mode from a system shut-down to the next system start-up. A method for improving fuel cell system reliability in the event of end cell heater failure in a fuel cell stack. The method includes detecting that an end cell heater has failed. If an end cell heater failure is detected, then he method performs one or more of setting a cooling fluid pump to a predetermined speed that drives a cooling fluid through cooling fluid flow channels in the fuel cell stack, limiting the output power of the fuel cell stack or the net power of one fuel cell system, limiting the maximum temperature of the cooling fluid flowing out of the stack, turning off stack anti-flooding algorithms that may be used to remove water from reactant gas flow channels in the stack, and turning of cathode stoichiometry adjustments for relative humidity control in response to water accumulating in cathode flow channels in the fuel cell stack.

Documents:

1738-KOL-2008-(17-04-2014)-ABSTRACT.pdf

1738-KOL-2008-(17-04-2014)-ANNEXURE TO FORM 3.pdf

1738-KOL-2008-(17-04-2014)-CLAIMS.pdf

1738-KOL-2008-(17-04-2014)-CORRESPONDENCE.pdf

1738-KOL-2008-(17-04-2014)-DESCRIPTION (COMPLETE).pdf

1738-KOL-2008-(17-04-2014)-DRAWINGS.pdf

1738-KOL-2008-(17-04-2014)-FORM-1.pdf

1738-KOL-2008-(17-04-2014)-FORM-2.pdf

1738-KOL-2008-(17-04-2014)-FORM-5.pdf

1738-KOL-2008-(17-04-2014)-OTHERS.pdf

1738-kol-2008-abstract.pdf

1738-kol-2008-claims.pdf

1738-KOL-2008-CORRESPONDENCE 1.1.pdf

1738-KOL-2008-CORRESPONDENCE 1.2.pdf

1738-kol-2008-correspondence.pdf

1738-kol-2008-description (complete).pdf

1738-kol-2008-drawings.pdf

1738-kol-2008-form 1.pdf

1738-kol-2008-form 18.pdf

1738-kol-2008-form 2.pdf

1738-kol-2008-form 3.pdf

1738-kol-2008-form 5.pdf

1738-kol-2008-gpa.pdf

1738-KOL-2008-OTHERS.pdf

1738-kol-2008-specification.pdf

1738-KOL-2008-TRANSLATED COPY OF PRIORITY DOCUMENT.pdf

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