Method for determining how long a fuel cell system was shut down

Abstract

A method for determining how long a fuel cell system has not delivered hydrogen to an anode side of a fuel cell stack after a system shutdown, the method comprising: determines whether the system has been switched off; a standby time is increased when the system has been switched off and a fuel cell controller is switched on; a shutdown shutdown time is increased when the system has been switched off and the engine controller is switched off; determining whether the ignition has been switched on after a previous ignition shutdown event; and the standby time and the shutdown time are added to provide a shutdown time that can be used to determine the gas composition in the anode side of the stack.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] This invention relates generally to a method for determining the length of time a fuel cell system has been shut down, and more particularly to a method for determining how long a fuel cell system in a vehicle has been shut down, comprising a combination of a standby timer that provides a time count when the vehicle ignition is still on but the fuel cell system is shut down, and a shutdown timer that provides a time count when the vehicle ignition is shut down.

[0002] For example, DE 11 2006 001 778 T5 discloses recording the downtime during which a fuel cell is stopped. If the ignition switch is subsequently activated, the time until the fuel cell is started is measured, with the downtime being added to this time to determine the total downtime. Furthermore, US 2007 / 0 026 277 A1 describes a timer with which the shutdown time of a fuel cell system can be determined. 2. Description of the state of the art

[0003] Hydrogen is a very attractive fuel because it is pure and can be used to efficiently generate electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that has an anode and a cathode with an electrolyte in between. The anode accepts hydrogen gas, and the cathode accepts oxygen or air. The hydrogen gas is split in the anode to produce free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and electrons in the cathode to produce water. The electrons from the anode cannot pass through the electrolyte and are thus passed through a load where they perform work before being delivered to the cathode.

[0004] Proton exchange membrane fuel cells (PEMFCs) are a popular fuel cell for vehicles. The PEMFC generally features a proton-conducting solid polymer electrolyte membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically comprise finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposite sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane defines a membrane electrode assembly (MEA).

[0005] Typically, multiple fuel cells are combined in a fuel cell stack to produce the desired power. For the automotive fuel cell stack mentioned above, the stack may contain two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air, forced through the stack via a compressor. Not all of the oxygen is consumed by the stack, and some of the air is exhausted as a cathode exhaust gas, which may contain water as a stack byproduct. The fuel cell stack also receives an anode hydrogen reactant gas, which flows into the anode side of the stack.

[0006] The fuel cell stack includes a series of bipolar plates positioned between the various MEAs in the stack, with the bipolar plates and MEAs positioned between two endplates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates, allowing the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates, allowing the cathode reactant gas to flow to the respective MEA. One endplate includes anode gas flow channels, and the other endplate includes cathode gas flow channels. The bipolar plates and endplates are made of a conductive material, such as stainless steel or a conductive composite. The endplates conduct the electricity generated by the fuel cells out of the stack.The bipolar plates also have flow channels through which a cooling fluid flows.

[0007] It has been proposed in the art to provide stack order switching or anode flow switching in a fuel cell system employing split stacks. Specifically, valves and piping are provided in the system such that, in a cyclical manner, the anode exhaust gas leaving a first substack is delivered to the anode side of a second substack, and the anode exhaust gas leaving the second substack is delivered to the anode side of the first substack.

[0008] When a fuel cell system is shut down, unreacted hydrogen gas remains in the anode side of the fuel cell stack. This hydrogen gas is able to diffuse through, or cross, the membrane and react with the oxygen in the cathode side. As the hydrogen gas diffuses to the cathode side, the overall pressure on the anode side of the stack is reduced below ambient pressure. This pressure differential draws air from the ambient air into the anode side of the stack. As the air enters the anode side of the stack, it creates a hydrogen / air front that shorts out the anode side, resulting in a cross-flow of hydrogen ions from the hydrogen-flooded section of the anode side to the air-flooded section of the anode side. This high ion current, combined with the high transverse ion resistance of the membrane, creates a significant cross-potential drop (-0.5 V) across the membrane.This creates a locally high potential between the cathode side opposite the air-filled portion of the anode side and adjacent to the electrolyte, which drives rapid carbon corrosion and causes the carbon layer to thin. This reduces the support for the catalyst particles, thus reducing fuel cell performance.

[0009] At the next system start-up, assuming sufficient time has passed, both the cathode and anode flow channels will generally be filled with air. When hydrogen is introduced into the anode flow channels at system start-up, the hydrogen will force out the air in the anode flow channels, which also creates a hydrogen / air front that passes through the anode flow channels. The hydrogen / air front causes a catalytic reaction along the length of the membrane in each fuel cell as the front moves, which, combined with the reaction across the membrane, creates a high electrical voltage potential. This combined electrical voltage potential is high enough to cause serious damage to the catalyst and the carbon particles on which the catalyst is formed, thereby reducing the lifetime of the MEAs in the fuel cell stack.In particular, the reaction generated by the hydrogen / air front, combined with the normal fuel cell reaction, is orders of magnitude greater than the membrane-based fuel cell reaction alone. For example, it has been shown that, without considering the damaging effects of the hydrogen / air front during system startup, it only takes about 100 shutdown and start-up cycles to destroy the fuel cell stack in this way.

[0010] It has been proposed in the art to reduce the damaging effect of the hydrogen / air front at system start-up by forcing hydrogen through the anode flow channels as quickly as possible to reduce the time period during which damage occurs. It has also been proposed to introduce hydrogen into the anode flow channels at a slow rate to provide active mixing of the air and hydrogen to eliminate the hydrogen / air front. It has also been proposed in the art to cool the fuel cell prior to removing the hydrogen from the anode flow channels. However, all of these solutions have not reduced hydrogen / air damage sufficiently to provide a desired fuel cell stack lifetime.In particular, rapid movement of the hydrogen / air front does not completely eliminate catalyst damage and requires oversized piping and other components to quickly purge air from the anode flow channels. Slow introduction of hydrogen at startup has the disadvantage of requiring a recirculation pump, which takes several minutes to completely remove air from the anode flow channels. Furthermore, the requirement for precise control of the amount of hydrogen into the anode flow channels is difficult to implement.

[0011] It has also been proposed in the art to provide a load across the fuel cell stack, such as a resistor, to reduce the electrical potential generated by the hydrogen / air front. However, an extremely low-resistance load requires electrical components with a high power rating. Also, flow balancing between cells in a fuel cell stack can result in corrosion at the cell anodes. Furthermore, in most embodiments, a resistor alone is typically insufficient to minimize carbon corrosion.

[0012] Based on the above discussion, it is evident that it is desirable to know the gas constituents in the anode and cathode sides of a fuel cell stack at system startup for a more efficient and safer startup sequence. Simulations can be run based on the time since the last system shutdown to show which gas constituents are present in the fuel cell stack. Therefore, it is desirable to have knowledge of how long a fuel cell stack has been shut down. SUMMARY OF THE INVENTION

[0013] According to the teachings of the present invention, a method is disclosed for providing an accurate amount of time that a fuel cell system has been shut down so that knowledge of the gas constituents in the anode and cathode sides of the fuel cell stack can be obtained for an efficient next system start-up sequence. The method utilizes two timers: a standby timer that provides a time count of how long the fuel cell system has been shut down but the vehicle ignition is still on, and a shutdown timer that provides a time count of how long the vehicle ignition has been turned off. The two time counts are added to provide a complete time count of how long the fuel cell stack has been shut down.

[0014] Additional features of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic block diagram of a fuel cell system using split stacks operated by anode flow switching; Fig. 2 is a flowchart showing a start-up sequence of a fuel cell system using a time counting system of the invention; and Fig. 3 is a flowchart illustrating a method for providing a time count of how long a fuel cell system has been shut down, according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] Fig. 1 is a schematic block diagram of a fuel cell system 10 having a first split fuel cell stack 12 and a second split fuel cell stack 14. A compressor 16 supplies cathode input air on a cathode input line 18 to the split stacks 12 and 14 through a normally closed cathode input valve 20. Cathode exhaust gas is output from the split stack on line 24 and cathode exhaust gas is output from the split stack 14 on line 26, with the cathode exhaust gas being combined into a single cathode output line 28. A normally closed cathode backpressure valve 30 controls the flow of the cathode exhaust gas through line 28. A cathode bypass line 32 between the input line 18 and the output line 28 allows the cathode input air to bypass the stacks 12 and 14. A normally closed bypass valve 34 controls whether the cathode air bypasses stacks 12 and 14.When valves 20 and 30 are closed and valve 34 is open, air from compressor 16 bypasses stacks 12 and 14. Typically, a cathode humidification unit (not shown) is provided at a suitable location in cathode inlet line 18.

[0016] In this non-limiting embodiment, the split stacks 12 and 14 utilize anode flow switching, in which the anode reactant gas flows back and forth through the split stacks 12 and 14 in a predetermined cycle in a manner well known to those skilled in the art. In an alternating sequence, an injector 38 injects hydrogen gas from a hydrogen gas source 40 through the anode line 42 to the split stack 12, and an injector 44 injects hydrogen gas from a hydrogen source 46 through the anode line 48 to the split stack 14. Normally closed anode flow switching valves 50 and 52 are used to provide the anode flow switching.When valve 50 is closed and valve 52 is open, hydrogen gas flows from source 40 into stack 12 on line 42, through a connector line 54 between split stacks 12 and 14, into split stack 14, and out anode line 48 through valve 52 to mix with the cathode exhaust in cathode exhaust outlet line 28. Similarly, when valve 52 is closed and valve 50 is open, hydrogen gas flows from hydrogen source 46 into split stack 14 on line 48, through connector line 54, into split stack 12, and out through valve 50 to mix with the cathode exhaust in line 28.

[0017] A water separator 60 is coupled to the connector line 54 and collects water in the anode gas flow between the split stacks 12 and 14. A normally closed drain valve 62 is periodically opened to drain the water on line 64 to the cathode exhaust line 28. Further, an anode exhaust purge valve 66 is coupled to the connector line 54 and the line 64 for reasons that will become apparent from the discussion below.

[0018] Fuel cell stacks 12 and 14 generate power. During normal stack operation, the power generated by stacks 12 and 14 is used to operate system loads, such as an electric traction system (ETS) 70 on a vehicle. During a shutdown sequence, the power generated by stacks 12 and 14 may be used to charge a battery 72 or may be dissipated by other system components and then dissipated by a resistor 74.

[0019] For a system shutdown sequence, compressor 16 is stopped and valves 20 and 30 are closed to seal the cathode side of stacks 12 and 14. Hydrogen flow continues so that any remaining oxygen in stacks 12 and 14 is consumed. When stack power drops to a predetermined level, the current generated by stacks 12 and 14 is switched from ETS 70 to battery 72. When stack power decreases to another predetermined level, the stack load is switched to resistor 74. Specifically, once the voltage has degraded to a fixed critical voltage, or cutoff voltage, the stack load is switched to resistor 74. The cutoff voltage may be the lower limit of a DC / DC converter (not shown) or the lower limit of a power device.The role of the battery load is to consume and / or store any energy that would otherwise be unused. It also reduces the power consumption requirements of the resistive load.

[0020] Once the oxygen has been consumed from stacks 12 and 14, the hydrogen flow is shut off, and valves 50, 52, 62, and 66 are closed to seal the anode side of stacks 12 and 14. When system 10 is shut down in this manner, stacks 12 and 14 comprise an N2 / H2 mixture in both the cathode and anode sides. Over time, air leaks into stacks 12 and 14, and the hydrogen in stacks 12 and 14 initially consumes the oxygen. Additionally, hydrogen slowly leaks from stacks 12 and 14. As a result, the composition of the gases in stacks 12 and 14 varies over time from a hydrogen-rich mixture of nitrogen and water to an air mixture.

[0021] According to the invention, a method is used to determine how long the fuel cell system 10 has been shut down or a crash stop has occurred so that an estimate of the gas concentration and composition in the split stacks 12 and 14 can be obtained for a proper start-up sequence. The approximate gas concentration in the cathode side and the anode side can be predicted over time based on gas kinetics and assumptions about the system 10. By knowing the gas concentrations in the system during the time the system 10 is in standby or a key-removed state, certain methods can be used to ensure that an optimized next start-up sequence occurs. Certain requirements exist regarding safety, emissions, reliability, and durability, and these can be achieved by influencing the conditions that define the start-up sequence.

[0022] To achieve this, the present invention proposes a standby timer that counts the amount of time the system 10 is turned off but the key is still in the vehicle ignition and in the ON or accessory (ACC) position, and a shutdown timer that counts the amount of time the ignition is turned off. Therefore, both desired stops and quick stops of the system 10 can be covered to determine how much time has elapsed since the last batch shutdown. When the system 10 is turned off but the key is still in the ignition, the standby timer accumulates the amount of time from the time the system was turned off. When the key is removed from the ignition, the standby time value is saved and the shutdown timer is started.When the key is returned to the ignition and the system 10 is started, the standby time and the shutdown time are added to provide a complete shutdown time value for determining the gas concentration in the anode and cathode sides of the split stacks 12 and 14.

[0023] If the shutdown time value is less than 500 seconds, the hydrogen concentration on the anode side is high and is consumed. The anode pressure decreases rapidly due to a high oxygen consumption rate. If the shutdown time value is between 500 and 1000 seconds, the hydrogen concentration in the anode side has decreased due to continued oxygen consumption. The anode pressure provides a large negative pressure due to a reduced hydrogen concentration. If the shutdown time value is between 1000 and 10,000 seconds, the hydrogen in the anode side has continued to decrease because oxygen in the cathode diffuses back into the anode side. The cathode concentration begins to decrease after a peak. The anode pressure recovers when nitrogen in the air enters the anode side to compensate for the partial pressure difference. If the shutdown time value is greater than 10.000 seconds, the hydrogen concentration in the anode side is low, but diffusion from the anode side to the cathode side continues. The anode pressure reaches ambient pressure when air enters the anode side.

[0024] Fig. 2 is a flowchart showing a fuel cell system startup process using the standby and shutdown time values. While the fuel cell system is in the shutdown state at box 82, the shutdown timer counts at box 84. The fuel cell system 10 receives a start request at box 86, and the system holds the shutdown time value at box 88. The compressor 16 is ramped up at box 90 to provide dilution air at the stack exit for the anode hydrogen gas from the anode side of the split stacks 12 and 14. The system 10 determines at decision diamond 92 whether a false start, aborted start, or rush stop has occurred. If so, the shutdown timer resumes the count that has been held at box 94.If no false start, aborted start, or quick stop has occurred at decision diamond 92, the system 10 determines at decision diamond 96 whether the anode pressure is greater than a predetermined value, such as 150 kPa. If the anode pressure at decision diamond 96 is greater than the predetermined value, the system 10 proceeds to the next step in the start-up sequence, which provides cathode air fill to the cathode side of the split stacks 12 and 14 at box 98 and provides an anode-side differential pressure setpoint. The system 10 then provides a parallel delivery of a hydrogen gas flow to the anode side of the split stacks 12 and 14 at box 100 and provides normal operation at box 102.

[0025] If the anode-side pressure at decision diamond 96 is less than the predetermined value, then the system startup sequence provides a manifold purge for the anode side of split stacks 12 and 14 at box 104. Once the anode pressure reaches a predetermined value, such as 107 kPa, at box 106, the system 10 provides normal anode flow switching at box 108 and stack filling at box 110. System operation then returns to box 98 for normal system operation.

[0026] Fig.3 is a flowchart 120 illustrating an operational sequence for providing the standby time count as well as the shutdown timer count during system shutdown, as discussed above. When the system 10 is running at box 122, the algorithm periodically determines at decision diamond 124 whether a system shutdown is present. If a system shutdown is present at decision diamond 124, the algorithm determines at decision diamond 126 whether a next startup sequence has reached a particular condition where cathode dilution air is provided. If the startup sequence has not reached this condition, then the standby timer continues counting from a previous standby count at box 132. If the startup sequence has reached the cathode dilution condition at decision diamond 126, the algorithm then sets the standby time to zero at box 128.The algorithm then starts an increase of the standby timer at box 130.

[0027] The algorithm then determines at decision diamond 134 whether the ignition has been turned off. If the ignition has been turned off at decision diamond 134, the algorithm then stops incrementing the standby timer at box 136. The standby timer, which provides the system shutdown count while the ignition is on, is stored. The shutdown timer now increments the amount of time the fuel cell system 10 has been shut down.

[0028] The algorithm then determines at decision diamond 138 whether the ignition has been turned on after the previous shutdown of the fuel cell system 10. If the ignition has not been turned on, the shutdown timer then continues to increment at box 136. If the ignition has been turned on at decision diamond 138, the algorithm determines at decision diamond 140 whether the shutdown time count is set to zero to determine if any battery power loss occurred during the period the fuel cell system 10 was shut down.If the shutdown time at decision diamond 140 is not zero, meaning that the timer had suitable power during the shutdown period, the algorithm adds the stored standby time at box 142 to the stored shutdown time to yield the total shutdown time that the split stacks 12 and 14 have been shut down and locked out, so that the algorithm can then determine the correct start sequence for hydrogen flow to the anode side of the split stacks 12 and 14. The shutdown time algorithm then returns to decision diamond 134 to determine if the ignition has been turned off. If the shutdown time at decision diamond 140 is zero, then the algorithm has knowledge that battery power has been lost and sets the shutdown period to zero at box 144.

[0029] If no ignition shutdown event has occurred at decision diamond 134, the algorithm continuously determines at decision diamond 146 whether the system 10 has been started and, if so, returns to the system operation box 122 to await the next system shutdown at decision diamond 124. If the ignition is not turned off at decision diamond 134 and no start event has occurred at decision diamond 146, the algorithm then determines at decision diamond 148 whether battery power has been lost and, if not, increments the standby timer at box 130. Thus, if the ignition has not been turned off, no start event has occurred, and no battery power loss has occurred, the fuel cell controller is running, but the system 10 has not started. Thus, the standby timer continues to increment.If a battery power loss has occurred at decision diamond 148, then the algorithm resets the shutdown timer to zero at box 150 and returns to decision diamond 146 to await a start event.

[0030] The foregoing discussion discloses and describes merely exemplary embodiments of the invention. Those skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications, and variations may be made therein without departing from the spirit and scope of the invention as defined in the following claims.

Claims

[1] A method for determining how long a fuel cell system has not supplied hydrogen to an anode side of a fuel cell stack after a system shutdown, the method comprising: determines whether the system has been switched off; a standby time is increased when the system has been switched off and a fuel cell controller is switched on; a shutdown shutdown time is increased when the system has been switched off and the engine controller is switched off; determining whether the ignition has been switched on after a previous ignition shutdown event; and the standby time and the shutdown time are added to provide a shutdown time that can be used to determine the gas composition in the anode side of the stack. [2] The method of claim 1, further comprising determining whether a fuel cell system battery lost power when the ignition was off. [3] The method of claim 2, further comprising setting the standby time to zero when the fuel cell system battery has lost power. [4] The method of claim 1, further comprising determining whether dilution air is provided from a compressor to dilute anode exhaust gas. [5] The method of claim 4, further comprising setting the standby time to zero when the dilution air is supplied. [6] The method of claim 1, wherein the fuel cell stack comprises a first split stack and a second split stack operated with anode flow switching. [7] The method of claim 1, wherein the system shutdown comprises shutting off the cathode and anode sides of the fuel cell stack. [8] The method of claim 1, wherein the shutdown time is used to determine a correct start-up sequence of the fuel cell system. [9] A method for determining how long a fuel cell system in a vehicle has been shut down, the method comprising: a standby time is increased if the system has been switched off but a key is in the vehicle ignition in an on or accessory position; a shutdown time is increased if the system has been switched off and the key is not in the vehicle ignition, and the standby time and the shutdown time are added together to provide a total system shutdown time. [10] The method of claim 9, further comprising determining whether a fuel cell system battery lost power when the ignition was off. [11] The method of claim 10, further comprising setting the standby time to zero when the fuel cell system battery has lost power. [12] The method of claim 9, further comprising determining whether dilution air is supplied from a compressor to dilute anode exhaust gas. [13] The method of claim 12, further comprising setting the standby time to zero when the dilution air is supplied. [14] The method of claim 9, wherein the fuel cell stack comprises a first split stack and a second split stack operated with anode flow switching. [15] The method of claim 9, wherein the system shutdown comprises shutting off the cathode and anode sides of the fuel cell stack. [16] The method of claim 9, wherein the shutdown time is used to determine a correct start-up sequence of the fuel cell system.

Images