Freezing strategy for fuel cell cathode auxiliary systems

Abstract

Fuel cell system (10) with a cathode subsystem, wherein the cathode subsystem comprises the following: - a fuel cell stack (12); - a cathode inlet line (22) that supplies cathode air to the fuel cell stack (12); - a cathode exhaust line (26) that discharges cathode exhaust from the fuel cell stack (12); - a backpressure valve (34) in the cathode exhaust line (26), wherein the backpressure valve (34) is connected downstream of a drain rail (82) of the cathode exhaust line (26), the drain rail (82) comprising a projection which prevents the accumulation of condensed water upstream of the backpressure valve (34), the drain rail (82) further comprising a collection chamber (84) which collects droplets of the condensed water from the projection of the drain rail (82); and - a control unit (32) programmed to determine the temperature of the back pressure valve (34) and to estimate the liquid water upstream of the back pressure valve (34), wherein the control unit (32) performs a freeze-cleaning strategy if the temperature and the liquid water upstream of the back pressure valve (34) have reached predetermined limits.

Description

STATE OF THE ART Field of invention

[0001] This invention relates in general to a system for the selective provision of a freezing strategy for a cathode subsystem of a fuel cell stack and in particular to a system for the selective provision of a freezing strategy that prevents the freezing of components in the auxiliary systems of the cathode subsystem. Discussion of related technology

[0002] Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device containing an anode and a cathode with an electrolyte in between. The anode receives hydrogen gas, and the cathode receives oxygen or air. The hydrogen gas dissociates in the anode to produce free protons and electrons. The protons pass through the electrolyte into the cathode. The protons react with the oxygen and electrons in the cathode to produce water. The electrons from the anode of one or more end cells cannot pass through the electrolyte and are therefore conducted through a load to perform their work there before being sent to the cathode.

[0003] Proton exchange membrane fuel cells (PEMFCs) are popular fuel cells for vehicles. A PEMFC generally contains a solid polymer electrolyte, proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically contain finely divided catalytic particles, usually platinum (Pt), supported by carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposite sides of the membrane. The combination of the catalytic anode mixture, the catalytic cathode mixture, and the membrane defines a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require specific conditions for effective operation.

[0004] Typically, multiple fuel cells are combined into a fuel cell stack to generate the desired power output. A typical fuel cell stack for a vehicle might contain, for example, two hundred or more stacked fuel cells. The fuel cell stack receives a cathode-input reactant gas, typically a stream of air, which is forced through the stack by a compressor. Not all of the oxygen is consumed by the stack, and some of the air is discharged as cathode exhaust, which may contain water as a byproduct of the stack. The fuel cell stack also receives an anode-hydrogen reactant gas, which flows into the anode side of the stack. The stack also contains flow channels through which a coolant flows.

[0005] A fuel cell stack comprises a series of bipolar plates positioned between the multiple MEAs in the stack, with the bipolar plates and MEAs positioned between two end plates. The bipolar plates contain an anode side and a cathode side for the 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 corresponding side of the MEA. Cathode gas flow channels are also provided on the cathode side of the bipolar plates, allowing the cathode reactant gas to flow to the corresponding side of the MEA. One end plate contains anode gas flow channels, and the other end plate contains 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 through the stack. The bipolar plates also contain flow channels through which a cooling fluid flows.

[0006] It is understood, according to the prior art, that fuel cell membranes with a controlled hydration level operate such that the ionic resistance across the membrane is low enough to conduct protons effectively. The relative humidity (RH) of the cathode output gas from the fuel cell stack is typically controlled to regulate the membrane hydration level by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry, and the relative humidity of the cathode air entering the stack. According to the prior art, it is known to recover water from the cathode exhaust stream and return it to the stack via the cathode inlet air stream. Various devices could be used to perform this function, such as a water vapor transfer unit (WVT). By maintaining a specific setpoint for the relative humidity of the cathode output, e.g.,80%, the proper stack membrane hydration level can be maintained.

[0007] It is known according to the prior art to provide high-frequency resistance (HFR) measurements of the membranes in a fuel cell stack to accurately measure the water or membrane hydration within the stack. HFR measurement systems provide a high-frequency component at the electrical load of the stack, which operates to generate a high-frequency wave at the current output of the stack. The resistance of this high-frequency component is measured, which is a function of the amount of water in the stack membranes.

[0008] When the fuel cell system is idle, it is desirable for the membranes to have a specific hydration level, ensuring they are neither too wet nor too dry. This is typically achieved by cleaning the cathode side of the stack with dry air for a specific period. In one known technique, the anode side is cleaned by blowing air through the membranes from the cathode side. Too much water in the stack can cause problems in low-temperature environments, where freezing could create ice that blocks the flow channels and impairs system restarts. However, excessively long cleaning could cause the membranes to become too dry, resulting in insufficient proton conductivity at the next system restart, which would impair restart performance and reduce stack durability.The actual target amount of water in grams in the stack varies depending on the system and certain system parameters.

[0009] In a fuel cell stack with three hundred fuel cells and an active area of ​​nearly 400 cm² 2Each cell in the stack may contain approximately 200 grams of water when the system is shut down. It is desirable for a stack of this size to contain approximately 23 grams of water after a system shutdown to ensure proper membrane hydration. Twenty-three grams of water corresponds to a stack λ of three, where λ represents the membrane hydration, defined as the number of water molecules for each sulfonic acid molecule in the membrane for each fuel cell. Knowing the actual amount of water present in the fuel cell stack at system shutdown allows for the provision of a desirable air purification flow rate and air purification time to achieve the target value of λ, such as λ = 3. Models can be used to estimate the amount of water in the stack based on the stack operating parameters during fuel cell system operation.

[0010] If a fuel cell stack contains too much water from the last system shutdown, the water generated during a prolonged shutdown can block the gas flow channels. Typically, the colder the stack is at restart, the longer it takes to warm the stack sufficiently, and the more likely it is that the water generated during restart will block the gas flow channels. Therefore, at very cold restart temperatures, such as below -15°C, it takes a long time for the fuel cell stack to warm up to 0°C. Thus, the shutdown process becomes critical for a successful restart of the fuel cell stack, especially when the fuel cell stack temperature is -15°C or lower.

[0011] In addition to preventing excess water from freezing in a fuel cell stack, it is desirable to prevent excess water from freezing in the critical area of ​​the ancillary components of a cathode subsystem. The ancillary components refer to components that are part of the cathode subsystem, excluding the cathode side of the fuel cell stack. Therefore, according to the state of the art, there is a need to provide a cost-effective cathode subsystem and an operating procedure that allows the cathode subsystem to operate reliably at an ambient temperature of -40 °C. BRIEF SUMMARY OF THE INVENTION

[0012] According to the teachings of the present invention, a system for a cathode subsystem in a fuel cell system is disclosed, comprising a fuel cell stack, a cathode inlet line that supplies cathode air to a fuel cell stack, and a cathode exhaust line that discharges cathode exhaust from the fuel cell stack. Also included is a backpressure valve in the cathode exhaust line, which is connected downstream of a drain rail of the cathode exhaust line, the drain rail having a projection that prevents the accumulation of condensed water near the backpressure valve. The drain rail further includes a collection chamber that collects droplets of the condensed water from the projection of the drain rail.

[0013] Additional features of the present invention will become apparent from the following description and the attached claims in conjunction with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic block diagram of a fuel cell system; and Fig. Figure 2 is an isometric view of a bypass valve and a cathode exhaust backpressure valve; Fig. Figure 3 is a schematic view of an area of ​​the cathode subsystem; Fig. Figure 4 is a schematic view of another area of ​​the cathode subsystem; Fig. Figure 5 is a schematic view of another area of ​​the cathode subsystem; Fig. Figure 6 is a schematic view of another area of ​​the cathode subsystem, which contains a backpressure valve; Fig. Figure 7 is a schematic view of the cathode subsystem, which includes a backpressure valve according to a further embodiment; and Fig. Figure 8 is a flowchart showing a procedure for selectively determining whether to perform freeze cleaning with a fuel cell shutdown. DETAILED DESCRIPTION OF THE EXECUTION FORMS

[0014] The following discussion of embodiments of the invention relating to a system for the selective provision of a freezing strategy of a cathode subsystem is merely exemplary and is in no way intended to limit the invention or its applications and uses. The present invention, for example, has a specific application in a fuel cell system in a vehicle. However, those skilled in the art will appreciate that the system of the invention is also applicable to other fuel cell systems and other applications.

[0015] Fig. Figure 1 is a schematic block diagram of a fuel cell system 10, including a fuel cell stack 12. The fuel cell stack 12 receives hydrogen gas from a hydrogen source 14 at an anode inlet line 16, and an anode exhaust is discharged from the stack 12 at an anode exhaust recirculation line 18 and returned to the stack 12 using, for example, an injector / ejector 40. A compressor 20 provides an airflow to the cathode side of the fuel cell stack 12 at a cathode inlet line 22 through the water vapor transfer unit (WVT) 24, which humidifies the cathode inlet air. A cathode exhaust is discharged from the stack 12 at a cathode exhaust line 26. The cathode exhaust line 26 directs the cathode exhaust to the WVT unit 24 to provide the water for humidifying the cathode inlet air.The cathode exhaust line 26 includes a backpressure valve 34 to control the pressure in the cathode side of the stack 12 in a manner known to those skilled in the art. A bypass line 28 with a bypass valve 30 is provided to allow the cathode inlet air to be directed to the cathode exhaust line 26, as will be described in detail below. A line 36, which includes a valve 54, selectively supplies hydrogen to the cathode inlet line 22 as desired, e.g., during the catalytic heating of the cathode, in a manner known to those skilled in the art.

[0016] A control unit 32 controls the bypass valve 30, the compressor 20, the back pressure valve 34, and the injector 40. The control unit 32 receives inputs from the various sensors that are part of the system 10, such as the pressure sensor 68 and the temperature sensors 52 and 38, as discussed in detail below. A water buffer model in the control unit 32 is used to calculate how much water is produced as a byproduct by the stack 12 in a manner known to those skilled in the art. High-frequency resistance (HFR) measurements can also be used to determine the humidification of the membranes in the stack 12 in a manner known to those skilled in the art. The fuel cell stack temperature sensor 52 provides a temperature measurement of the fuel cell stack 12, and the ambient temperature sensor 38 provides an ambient temperature measurement.

[0017] Bypass line 28 and bypass valve 30 allow cathode inlet air, which is typically dry, to be added to the cathode exhaust at cathode exhaust line 26. There are several reasons for adding cathode inlet air to the cathode exhaust using bypass line 28, for example, to dilute hydrogen in the cathode exhaust. Because the air flowing through bypass line 28 is typically very dry, it was assumed that freezing of valve 30 would not be a problem. However, research has now shown that this valve may freeze due to splashing of water present in cathode exhaust line 26.

[0018] Fig. Figure 2 is an isometric view of the bypass valve 30 in the bypass line 28 and the backpressure valve 34 in the cathode exhaust line 26. Area 62 is where water splashes out of the cathode exhaust line 26, causing the bypass valve 30 to freeze. Therefore, the bypass valve 30 must be positioned away from the wet cathode exhaust stream in the cathode exhaust line 26 to prevent product water from splashing out, which would cause the valve 30 to freeze and close.

[0019] Fig. 3 is a schematic view of the in Fig. 1 of the area shown 44. As shown, the bypass valve 30 is spaced away from the cathode exhaust line 26. The point furthest away is optimal. Additionally, the more turns there are in the bypass line 28, the better the freezing of the valve 30 can be prevented. If, for example, there is only one 90° turn in the bypass line 28, it is desirable to have a distance of 51 cm (20 inches) or more between the bypass valve 30 and the exhaust line 26 to minimize the possibility of water splashing against the bypass valve 30.

[0020] It is important to ensure that the bypass valve 30 is operational when the system 10 restarts, because it may be necessary to open the valve 30 to dilute the hydrogen in the cathode exhaust line 26. As is already known to those skilled in the art, hydrogen can be added to the cathode inlet line 22 via line 36 when the system 10 restarts, in order to heat the fuel cell stack 12 using catalytic cathode heating. This can cause the percentage of hydrogen exiting the system 10 to exceed the specified limits. Therefore, the bypass valve 30 must be able to dilute the hydrogen in the cathode exhaust line 26 with cathode inlet air before it exits the system 10; that is, the bypass valve 30 must be operational when the system restarts.

[0021] Fig. 4 is a schematic view of the in Fig. In the area shown in Figure 1, section 46 is shown. The cathode inlet line 22 leads into a charge air cooler 64, which is operated to cool the cathode air from line 22 using a coolant line 66 in a manner known to those skilled in the art. Research has shown that the strategic placement of the pressure sensor 68, which measures the pressure of the cathode air, is important to avoid erroneous pressure sensor readings due to water freezing on the components of the pressure sensor 68, which can lead to inaccurate readings. The values ​​from the pressure sensor 68 are used to estimate the pressure drop across the entire water vapor transfer unit (WVT) 24 and the charge air cooler 64, and also to predict the pressures entering the stack and exiting the compressor. It is known to insert the pressure sensor 68 into the section of the cathode inlet line 22 which is located between the WVT unit 24 and the stack 12.However, since the WVT unit 24 adds water to the cathode inlet line 22, it has been found that water can freeze on the pressure sensor 68 and cause erroneous readings, as discussed above. The pressure sensor 68 is therefore positioned in a section of the cathode inlet line 22 that is closer to the compressor 20 than the section of the charge air cooler 64, which is located in... Fig. Figure 4 also presents a problem because the temperature of the cathode inlet air coming from compressor 20 can be too high for pressure sensor 68 to provide an accurate reading under hot, humid conditions, such as on a hot summer day. For example, the temperature of the cathode air coming from compressor 20 can be 150 °C, which is too high for the temperature rating of a standard pressure sensor that could be used as pressure sensor 68. To avoid using a more expensive pressure sensor with a higher temperature rating and / or a sensor with a heater, it has been found that it is important to position pressure sensor 68 in the cathode inlet line 22 downstream of the charge air cooler 64 and upstream of the WVT unit 24, as shown in Figure 4. Fig. As shown in section 4, to be arranged.

[0022] Fig. 5 is a schematic view of the in Fig. 1 of the area shown 48. As discussed above, the WVT unit 24 is used to humidify the cathode inlet air in the cathode inlet line 22. However, it is desirable to supply only water vapor and not liquid water to the fuel cell stack 12 because, for example, liquid water can block the flow channels located in the fuel cell stack 12, as is known from the prior art. To avoid adding liquid water, a drain 70 can be provided to collect liquid water in a collection chamber area 72 of the drain 70. A valve is not provided in the drain 70 because a valve would entail additional costs and could freeze. Instead, an opening 74 is provided at the bottom of the drain. The pressure from the cathode airflow forces the water collected by the drain 70 through the opening 74 and into the cathode exhaust line 26.It is expected that a small amount of air may escape from the cathode airflow through opening 74, but the amount of lost air is expected to be small enough that it will not adversely affect the fuel efficiency of system 10, because the compressor 20 will not have to work significantly harder to compensate for the loss of air. A filter 76 is provided in the drain 70 near opening 74 to prevent particles from clogging opening 74.

[0023] When the system 10 is shut down, the water from the cathode subsystem is typically purified, as is known from the prior art. However, water can still accumulate and freeze near the opening 74. To remove ice from the drain 70 and the opening 74 after the fuel cell system 70 is restarted, the portion of the drain 70 containing the filter 76 and the opening 74 extends downwards into the cathode exhaust line 26, allowing heat from the cathode exhaust to thaw the opening 74. This is expected to clear, or at least partially clear, the collection tank 72 of ice, allowing the system 10 to be restarted without first having to thaw any ice that may have formed around the opening 74. To rapidly thaw the opening 74 after the system 10 is restarted, a portion 78 of the drain 70 can be made of a thermally conductive material, such as...A thermally conductive metal is used to rapidly transfer the heat from the cathode exhaust gas to defrost the opening 74. It is desirable for this area 78 to be located within the cathode exhaust duct to prevent exposure of the thermally conductive material to cold air. Alternatively, if the area 78 extends beyond the cathode exhaust duct, it can be insulated. When hot water droplets present in the cathode exhaust gas, which have a high heat capacity, strike the thermally conductive area 78, any water that may be frozen within the area 78 is expected to defrost. Furthermore, a delta pressure (dP) in the drain will assist in removing ice when it becomes thin enough to be forced out. As long as the opening 74 is able to drain water before the drain 70 fills with water, the fuel cell system 10 is expected to function properly.A two-phase drain freezing strategy, which involves an initial drain and removal of a major portion of the water, followed by a cleaning of the stack and then a second drain to eliminate additional water from the stack cleaning, can also be employed to ensure that no water remains in the drain during downtime.

[0024] Fig. 6 is a schematic view of the one in Fig. 1 shown area 50. The cathode backpressure valve 34 in the cathode exhaust line 26, as in Fig. Figure 6 shows an inverted siphon valve positioned in an elevated section 80 in the cathode exhaust line 26, so that gravity draws water away from the valve 34, thus preventing it from freezing. The elevated section 80 must have a slope steep enough to accommodate inclines that the fuel cell system 10 may experience. For example, in the case of vehicles containing the system 10 parked on sloping terrain, the elevated section 80 can accommodate, for instance, a 17° slope. Unfortunately, not all fuel cell systems are capable of handling the inclines described in Figure 6. Fig. The embodiment shown in Figure 6 is to be used. Therefore, a different technique is needed to prevent the backpressure valve 34 from freezing.

[0025] Fig. Figure 7 is also a schematic view of section 50 and represents a further embodiment that can be used to prevent the backpressure valve 34 in the cathode exhaust line 26 from freezing. According to this embodiment, the backpressure valve 34 is located in the cathode exhaust line 26 in a section downstream of a drain channel 82. The drain channel 82 includes a projection that prevents condensed water from passively, non-electrically, flowing into the backpressure valve 34. The drain channel 82 can be conical in shape. The water droplets fall from the drain channel 82 onto a flat collection container 84. The collection container 84 can hold, for example, approximately 5 ml of water. The optimal amount of water that can be held by the collection container 84 depends on the design of the system.

[0026] The drain rail 82 must be long enough to accommodate changes in the gradient that the fuel cell system 10 may experience, such as in the case of vehicles containing the system 10 that are parked on a slope. For example, a 17° gradient can be accommodated by the drain rail 82. A steeper gradient can be accommodated by increasing the length of the drain rail. For example, Fig. Even if the 7 was rotated 17° clockwise, the water droplets must still drip into the collection container 84. Therefore, the drain rail 82 must be able to accommodate this inclination.

[0027] As discussed in detail below, the control unit 32 can perform a standstill procedure of the fuel cell system 10, which selectively determines whether freeze cleaning should be performed based on various factors. This ensures that freeze cleaning is only carried out when necessary, thus enabling efficient use of system resources, such as hydrogen fuel, and reducing RH cycling of the membranes to increase the stack's durability. As discussed above, freeze cleaning is an extended cleaning process that takes place with the system 10 switched off, resulting in the system 10 continuing to operate for a period of time after system standstill.It is understood that the freeze cleaning discussed herein is more robust during a restart at sub-zero temperatures than the quick cleaning, which removes water from the flow channels of stack 12 and is performed after every shutdown unless freeze cleaning is carried out. Some of the factors to be considered may be determining whether regions 44, 46, 48, and 50 experience freezing of their respective components, as discussed above. A system 10 with one or more components described in regions 44, 46, 48, and 50, as discussed above, can be tested to determine which region is most prone to freezing problems, i.e., which component in the cathode subsystem is the weakest.Once the weakest component has been identified, a temperature sensor is placed near the identified weakest area, or the temperature for this area is modeled using an algorithm of the control unit 32 to determine when freezing of the weakest component is most likely to occur. As described above, for example, the backpressure valve 34 may represent the weakest component in system 10.

[0028] Fig.Figure 8 is a flowchart 90 that shows the operation of an algorithm for selectively determining whether freeze-cleaning of the fuel cell stack 12 and the cathode subsystem should be performed when the fuel cell system is shut down. The algorithm detects that the vehicle operator has switched off system 10 at box 92 and then determines whether the moisture value λ of the membrane is less than a predetermined λ value, as contained in US 8 900 766 B2. As discussed above, λ is a representation of the water molecules in the membranes of the fuel cells in stack 12, where the higher the value of λ, the more water molecules are present. The determination of the λ value is performed because there may be certain times when no significant amount of water has been generated in stack 12, such as during periods of low water levels.This is the case when system 10 is only switched on for a short period, in which case the λ-value indicates that freeze cleaning is not necessary. This can occur when the vehicle containing system 10 is started and operated only briefly, such as when the vehicle is moved from one location to another, for example, to make way for snow removal. For instance, if a λ-value of 4 or less indicates a sufficiently dry membrane, frozen water in the stack 12 would not pose a problem upon the next system restart. The λ-value can be determined by any suitable method known to those skilled in the art. It is also known to monitor the relative inlet humidity of the cathode air to the fuel cell stack 12, which can then be used to determine the λ-value.A model can be used based on the relative humidity (RH) of the cathode inlet air and the amount of water that the fuel cells would produce based on the current density of the stack. From the determined λ value or the modeled value, the amount of water in regions 44, 46, 48, and 50 of the cathode subsystem can then be estimated.

[0029] If the λ value at decision rhombus 94 is greater than 4, indicating a significant amount of water in the cell membranes, the algorithm proceeds to decision rhombus 96 to determine if freeze-cleaning is necessary. Specifically, if the algorithm determines that the ambient temperature is below a certain very low temperature, such as -15 °C, a freeze-cleaning shutdown sequence would be required because stack 12 and / or one or more components of the cathode subsystem are likely to be frozen upon the next system restart. The temperature of -15 °C serves as a non-limiting example and is a calibrated temperature based on various system parameters and test techniques for a specific fuel cell system. Therefore, other temperatures may be more appropriate for other systems.As discussed in detail below, the algorithm regularly activates the control unit 32 if no freeze cleaning is performed during system shutdown, in order to determine whether freeze cleaning has become necessary since the last system shutdown. This regular determination of whether freeze cleaning is necessary requires warming the system prior to freeze cleaning, which necessitates a significant amount of hydrogen fuel to perform the warm-up process and then the freeze cleaning. The temperature of -15 °C is chosen as an optimization temperature in a non-restrictive embodiment, such that if the ambient temperature is lower than the optimization temperature, freeze cleaning would be performed immediately if the stack 12 is already warm enough, thus saving the fuel required for the warm-up process prior to freeze cleaning, as discussed in detail below.

[0030] If the ambient temperature at decision rhombus 96 is warmer than -15°C, meaning a freeze-restart procedure is unlikely, the algorithm proceeds to box 98 to determine whether a shutdown is required for a warm-up procedure of the cathode auxiliary systems to raise their temperature to a predetermined value, such as 10°C, which is also discussed in detail below. This step is normally unnecessary and the algorithm can therefore bypass it, but it is required for a cold, but not frozen, shutdown, such as a start at -10°C, an auxiliary system temperature of 5°C, and a shutdown with a 15-second run.

[0031] The algorithm then instructs system 10 to perform a normal, non-freeze standstill on box 100, which does not include freeze cleaning. The standstill procedure does not perform a rapid standstill cleaning of stack 12, such as for 2 seconds at a cleaning flow rate of 30 g / s, where the flow rate and time would be based on the temperature of stack 12 and the cathode ancillary components. This non-freeze cleaning removes water from the various channels to prevent damage to the stack caused by water freezing from the cathode ancillary components, as detailed above.

[0032] The algorithm then sets an activation time T at box 102. wakefixed, which causes the control unit 32 to be activated in order to determine at box 100 whether a freezer cleaning has become necessary since the last non-frozen standstill. In particular, a calibratable temperature function T is used. wake = f(T amb , T bop ) used to determine the next activation time of the control unit, where T bop the temperature of the weakest component of the auxiliary systems, such as the back pressure valve 34, and where T ambThe ambient temperature is the temperature at which the procedure on box 98 is carried out so that if the stack 12 has only been running for a short period of time, during which the cathode auxiliary systems are relatively cold, with an ambient temperature of more than -15 °C at decision point 96, the temperature of the weakest component of the auxiliary systems, such as the back pressure valve 34, is increased to a predetermined temperature, such as 10 °C, so that the temperature of the auxiliary systems at the time of determining the activation time T wake The device used, which is located at box 102, is high enough to allow for a very short T wake To avoid time. If T wake If the time T is very short, this leads to an additional freeze cleaning cycle, which may not be necessary if the driver restarts the vehicle a short time later or if the ambient temperature increases. wakeOnce this has been determined, control unit 32 on box 104 will be deactivated again until the time T wake Expired. In one embodiment, a lookup table is provided which specifies the activation time T. wake for the possible combined values ​​of the ambient temperature T amb and the temperature of the auxiliary systems T bop specifies a temperature function.

[0033] When control unit 32 is activated after the T wake Once the time has expired, the algorithm determines whether the function f(T) amb ,T bop ) for the combination of ambient temperature T amb and the temperature of the auxiliary systems T bop The temperature may fall below a predetermined temperature, such as 5 °C, in which case there is a risk of the stack freezing at decision rhombus 106. If the temperature function is not below the predetermined temperature at decision rhombus 106, then the algorithm sets the activation time T.wake based on the new ambient temperature T amb and the temperature of the auxiliary systems T bop Using the lookup table at box 102, the algorithm continues with this loop until a drive command, as discussed below, is received, or until the temperature function has dropped below the predetermined temperature at decision diamond 106. The algorithm uses the combination of the ambient temperature T amb and the temperature of the auxiliary systems T bop , because different combinations of these temperatures can cause the control unit to execute different operations. If the temperature of the auxiliary systems T bop e.g. 2 °C, the ambient temperature T ambHowever, if the ambient temperature is 6 °C, the algorithm may not take the temperature of the cathode subsystem auxiliary components into account, because the ambient temperature rises and the temperature of the auxiliary components follows accordingly. One possible function would be to always reset control unit 32 to deactivation mode with a recalculated activation time if the ambient temperature T amb is higher than the temperature of the auxiliary systems T bop .

[0034] If the temperature function at decision rhombus 106 falls below the predetermined temperature, indicating a possible frozen state, the algorithm initiates an automatic start of system 10 at box 108 to raise the temperature of the cathode subsystem auxiliary equipment. The automatic start is a minimum operation of the fuel cell stack 12, with various auxiliary sources, such as lights, wipers, radio, air conditioning, etc., kept switched off. After the automatic start, system 10 is warmed up at box 110 in anticipation of the freeze cleaning process and then switched off again at box 112. The warming process can be performed at any suitable current density of the stack to any suitable temperature.In a non-restrictive example, compressor 20 will be operated at 30 kW, supplying 12% hydrogen gas to the cathode. Heating will be performed to a stack temperature of 70 °C and held at this temperature for 30 seconds, where 30 seconds is a non-restrictive, calibratable time based on testing and experimentation. The heating process will be maintained for the calibrated period so that all system components, including the end cells and cathode subsystem ancillary components such as valves and piping, reach the desired temperature, in this case 70 °C. Freeze cleaning will then be performed on box 112, and control unit 32 will be deactivated on box 114 without further activation considerations to determine if freeze cleaning is necessary.

[0035] If the algorithm determines that the ambient temperature T ambIf the ambient temperature T is below the calibrated temperature of -15°C at decision diamond 96, the algorithm proceeds directly to the standstill warm-up procedure at box 110 in preparation for freeze cleaning at box 112. As discussed above, if the ambient temperature T amb If the temperature at decision rhombus 96 is below the calibrated temperature, then stack 12 and the cathode subsystem components will almost certainly be frozen upon the next system restart, requiring a warm-up procedure at box 110 if the normal shutdown was performed at box 100. Since stack 12 and the cathode subsystem components are likely at or near the temperature of 70°C required for warm-up at box 110 for the shutdown, the algorithm proceeds directly to box 110 to perform the frozen shutdown in this situation at box 112.

[0036] If the λ value at decision rhombus 94 is less than 4, the algorithm determines whether the last system standstill was a frozen standstill at decision rhombus 116. If not, it proceeds directly to box 102 to determine the next activation time of control unit 32 and thus whether an automatic start at box 108 is necessary. If the last standstill was a frozen standstill at decision rhombus 116, the algorithm proceeds directly to box 114 until a drive command is received.

[0037] The standstill sequence and shutdown discussed above can be interrupted at any time by a drive command for a vehicle start-up sequence. The present invention recognizes this and allows the standstill sequence to be interrupted at any time in order to react to the drive command. In particular, the algorithm proceeds from each of boxes 98, 100, 104, 110, 112, and 114 to box 118 if a drive command has been received. At box 118, the algorithm then stops the shutdown step it is currently in and then proceeds directly to a normal restart sequence at box 120.

[0038] Those skilled in the art will appreciate that several and different steps and methods discussed herein to describe the invention relate to operations performed by a computer or processor or other electronic computing device that manipulates and / or transforms data using an electrical phenomenon. These computers and electronic devices may employ various volatile or non-volatile storage media, including a non-transient, computer-readable medium containing an executable program, including various codes or executable instructions that can be carried out by the computer or processor. The storage media and / or computer-readable medium may comprise all forms and types of storage media and other computer-readable media.

[0039] The foregoing discussion discloses and describes only exemplary embodiments of the present invention. A person skilled in the art will readily recognize from such a discussion, as well as from the accompanying drawings and claims, that various changes, modifications, and variations can be made to it without deviating from the purpose and scope of the invention as defined in the following claims.

Claims

[1] Fuel cell system (10) with a cathode subsystem, wherein the cathode subsystem comprises: - a fuel cell stack (12); - a cathode inlet line (22) that supplies cathode air to the fuel cell stack (12); - a cathode exhaust line (26) that discharges cathode exhaust from the fuel cell stack (12); - a backpressure valve (34) in the cathode exhaust line (26), wherein the backpressure valve (34) is connected downstream of a drain rail (82) of the cathode exhaust line (26), the drain rail (82) comprising a projection which prevents the accumulation of condensed water upstream of the backpressure valve (34), the drain rail (82) further comprising a collection chamber (84) which collects droplets of the condensed water from the projection of the drain rail (82); and - a control unit (32) programmed to determine the temperature of the back pressure valve (34) and to estimate the liquid water upstream of the back pressure valve (34), wherein the control unit (32) performs a freeze-cleaning strategy if the temperature and the liquid water upstream of the back pressure valve (34) have reached predetermined limits. [2] System (10) according to claim 1, further comprising a bypass line (28) which directs the cathode inlet air to the cathode exhaust line (26) using a bypass valve (30), wherein the bypass valve (30) is located downstream of the cathode inlet line (22) in the bypass line (28), wherein the bypass line (28) contains at least one turn downstream of the bypass valve (30) which prevents water from splashing in the cathode exhaust line (26) against the bypass valve (30). [3] System (10) according to claim 1, further comprising an intercooler (64) and a water vapor transfer unit (24) in the cathode inlet line (22), wherein the intercooler (64) is located in the cathode inlet line (22) and is fed into the water vapor transfer unit (24) in the cathode inlet line (22), wherein the intercooler (64) is cooled by a coolant loop, wherein a pressure sensor (68) is positioned on the intercooler (64) such that the pressure of the cathode inlet air (22) is measured after the air has been cooled by the intercooler (64) and the pressure of the cathode inlet air (22) is measured before the water vapor transfer unit (24) adds water to the cathode inlet air. [4] System (10) according to claim 3, further comprising a drain (70) located directly below the steam transfer unit (24), wherein the drain (70) includes a collection container (84) which discharges liquid water from the steam transfer unit (24) into the cathode exhaust line (26) through an opening (74). [5] System (10) according to claim 4, further comprising a grid that sits in the drain (70) directly above the opening (74), wherein a section of the drain (70) containing the opening (74) and the grid extends into the cathode exhaust line (26) such that the heat from the cathode exhaust line (26) is able to melt ice that may accumulate at the opening (74) of the drain (70). [6] System (10) according to claim 5, wherein the section of the drain (70) located near the grid and the opening (74) is made of a thermally conductive material. [7] System (10) according to claim 1, wherein the back pressure valve (34) is an inverted siphon valve located in an elevated area of ​​the cathode exhaust line (26).

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