Method for operating a fuel cell system

Abstract

Method for operating a fuel cell system (2) with an air conveying device (14) and an exhaust air turbine (15) operatively connected thereto, wherein a system bypass (21) with a bypass valve (22) open in the normal de-energized state connects the pressure sides of the air conveying device (14) and the exhaust air turbine (15), in the event that the bypass valve (22) is defective and remains in its position open in the normal de-energized state, characterized in that the conveying capacity of the air conveying device (14) is increased when a decrease in the air mass flow to the fuel cell (3) is detected due to a defective bypass valve (22).

Description

[0001] The invention relates to a method for operating a fuel cell system with an air conveying device and an exhaust air turbine mechanically connected thereto, as well as a system bypass with a bypass valve open in the normal, de-energized state, which connects the pressure sides of the air conveying device and the exhaust air turbine, in the event that the bypass valve is defective and remains in its position open in the normal, de-energized state.

[0002] It is known from the prior art to provide a system bypass valve in a fuel cell system with a flow compressor as the air supply unit and an exhaust turbine mechanically connected to it. Typically, an electric machine is also mechanically connected to these two components, so the entire assembly is also referred to as an electric turbocharger or motor-assisted turbocharger. The air supplied by the air supply unit to the fuel cell is split, both in steady-state and dynamic operation, into a line to the fuel cell itself and into this bypass line that bypasses the fuel cell. The bypass line connects the pressure sides of the air supply unit and the exhaust turbine, so that the compressed air flowing through the bypass line benefits at least the exhaust turbine for energy recovery.The air volume through this system bypass is regulated by a bypass valve, for example in the form of a control valve. The bypass valve serves to protect the air supply system from damaging operation in the pumping section and to increase the pressure on the cathode side of the fuel cell as needed, depending on the operating range, even though this is not possible in principle due to the characteristics of the air supply system as a turbomachine.

[0003] A failure of the bypass valve in the system bypass typically results in the bypass valve, which is normally open when de-energized, remaining permanently open. This leads to a potential insufficient supply of air to the fuel cell and can ultimately damage the fuel cell.

[0004] In principle, it is conceivable to shut down the fuel cell system if the bypass valve fails. This protects the fuel cell from damage. However, shutting down the fuel cell system always has significant drawbacks, as, for example, if the fuel cell system is used in a vehicle to provide electric drive power, the vehicle will then "break down." This represents a serious disadvantage for the vehicle's user.

[0005] From the prior art, specifically German patent application DE 10 2009 050 934 A1, a method is known that addresses the same problem, in which the system bypass is used in conjunction with an arbitrary compressor, and in which, in this system, an additional pressure control valve for the cathode side of the fuel cell is located upstream of the system bypass opening into the exhaust gas line. The aforementioned German patent application deals with a backup strategy for the event of a failure of the bypass valve in the system bypass. In this case, due to the pressure control valve in the exhaust gas line downstream of the fuel cell's cathode section, the air is practically completely discharged into the exhaust gas line through the system bypass in such a failure scenario.To still enable emergency operation, the pressure control valve on the cathode side of the exhaust pipe is now fully opened to reduce the pressure in the fuel cell stack and thus allow a higher air mass flow through it. This strategy only permits very limited operation, since regardless of the valve position, the pressure loss in the fuel cell itself will always be higher than in the system bypass, and therefore a large proportion of the air mass flow still enters the exhaust pipe directly.

[0006] The object of the present invention is now to provide a method for operating a fuel cell system in the manner described above, which minimizes the disadvantages in the event of a failure of the bypass valve.

[0007] According to the invention, this problem is solved by the method with the features in claim 1. Advantageous embodiments and further developments result from the dependent claims.

[0008] In the method according to the invention, if the bypass valve fails, which then typically returns to its normally open position (as it would in the de-energized state) and thus fully opens the system bypass, the delivery rate of the air supply unit is increased. In contrast to the prior art mentioned above, the method according to the invention makes it possible to continue operating the fuel cell system almost without restriction as long as the air supply unit has not yet reached its performance limit. Therefore, if the bypass valve in the system bypass fails, the air mass flow rate increases in the method according to the invention by increasing the delivery rate of the air supply unit. This results, on the one hand, in a larger air mass flow rate through the system bypass, and on the other hand, the air mass flow rate through the fuel cell is also increased, since the total available air mass flow rate is greater.

[0009] A failure of the bypass valve can be detected, for example, by a sensor on the bypass valve itself. A suitable controller in the control unit can also detect such a problem and an excessive airflow through the system bypass, based on a decrease in the air mass flow through the fuel cell. This can be done using pressure sensors upstream and downstream of the bypass valve, which are typically already implemented in the fuel cell system. This occurs even with a constant air mass flow supplied by the air delivery system. In both cases, the air mass flow through the air delivery system is increased, preferably to a level that does not impair the operation of the fuel cell system, provided this is possible due to the ability to increase the delivery capacity of the air delivery system.This strategy therefore works very well if the air conveying system still has sufficiently large power reserves at the time of the failure, or if it is fundamentally designed to be large enough to completely compensate for a failure of the bypass valve in the system bypass.

[0010] According to an advantageous further development of the method, the delivery capacity of the air conveying device is increased in operation with a failed bypass valve.

[0011] This process continues until the required airflow to the fuel cell is achieved, allowing it to operate as desired without further disruption. Only when the air delivery system can no longer provide the necessary flow rate, because its maximum capacity has been reached, does the fuel cell's electrical output decrease. This means the fuel cell will no longer reach its maximum power output at all times and will continue to operate in a kind of "emergency mode," at least during peak loads. However, the disruption is far less severe than if the fuel cell system were to shut down completely or operate in emergency mode at all power levels.

[0012] As already mentioned, according to an advantageous further development of the idea, the air mass flow can be determined using the measured values ​​of pressure sensors before and after the bypass valve in order to determine whether the bypass valve is functioning or not and to estimate the required air mass flow, possibly in combination with an air mass flow meter in the area of ​​the air conveying device or a calculation of the conveyed air mass, for example based on electrical parameters of the air conveying device, pressures and / or temperatures.

[0013] Furthermore, according to an advantageous further development of the idea, the bypass valve can be designed as a flap, in particular a control flap. This allows for continuous control of the air mass flow and thus prevents pressure pulsations in the supply air, which could occur if the bypass valve were also to be controlled by a pulse-width modulated control signal.

[0014] According to a further highly advantageous embodiment of the method according to the invention, the fuel cell system can be used to provide electrical drive power in a vehicle. Particularly in vehicle applications, a complete shutdown of the power supply and thus a vehicle breaking down is a serious disadvantage. Emergency operation can also be extremely inconvenient, for example, during a long-distance journey to the nearest repair shop, and potentially dangerous depending on the traffic situation. The particular advantages of the method according to the invention are therefore especially apparent when the fuel cell system operated in this way is used to provide electrical drive power in a vehicle.

[0015] The only accompanying figure shows a basic representation of a fuel cell system in a vehicle.

[0016] The figure contains a vehicle 1The system is shown in a highly schematic way. The vehicle has a feature for providing the electric drive power. 1 a fuel cell system 2 on, whose core is a fuel cell 3 , preferably a stack of individual cells using PEM technology. The fuel cell is purely an example. 3 a common cathode space 4 as well as a common anode compartment 5 depicted. The anode compartment 5 Hydrogen is taken from a pressurised gas storage tank 6 provided. This flows through a pressure regulating and metering valve. 7 as well as an optional gas jet pump 8 into the anode compartment 5 Unused hydrogen, along with inert gases and product water, is returned via a recirculation line. 9 back to the gas jet pump 8 and is mixed with the fresh hydrogen in the anode compartment 5reintroduced. This setup is also known as an anode circuit and is generally familiar to those skilled in the art. In the recirculation line 9 is typically a water separator 10 arranged, which has a drain pipe 11 with a drain valve 12 with an exhaust pipe 13 from the fuel cell system 2 is connected. Since the anode side is of minor importance for the present invention, it will not be discussed further. However, it is clear to those skilled in the art that the described setup is purely exemplary. They could also omit the anode circuit and / or use a different method for draining water and gas from the water separator. 10 provide.

[0017] The cathode space 4 is supplied with air via an air conveying device 14 supplied. This air conveying system 14It is supposed to be a flow compressor, which, together with an exhaust air turbine, 15 on a common wave 16 is arranged and is thereby in mechanical connection. Furthermore, it can be mounted on the shaft. 16 an electric machine 17 be arranged which, on the one hand, absorb excess energy from the exhaust turbine 15 which is converted into electrical energy in a generator-like manner, and which, on the other hand, powers the exhaust air turbine when needed. 15 this supports the air conveying system 14 to drive. The mechanical connection between 14 and 15 This is not required. Instead of a mechanical connection between the air conveying device 14 and the exhaust turbine 15 as well as the electric machine 17 In principle, a different type of interaction would also be conceivable, for example, dividing the electric machine. 17into a generator in conjunction with the exhaust turbine 15 on the one hand, and in a motor in conjunction with the air conveying system 14 On the other hand, the procedure described below could also be used in that case.

[0018] The compressed supply air flows through a supply air duct. 18 as well as a humidifier 19 to the cathode space 4 the fuel cell 3 Further components, such as an intercooler, could also be provided here. Since this is of minor importance to the invention, it has been omitted from its illustration. However, it is clear to those skilled in the art that such a component could be used. The humidifier could also be... 9 It may be designed differently than shown here and described below.

[0019] Via an exhaust duct 20 The oxygen-depleted exhaust gas flows together with the gas in the fuel cell. 3product water generated from the cathode chamber 4 the fuel cell 3 off. It flows again over the humidifier. 19 , which is preferably designed as a gas / gas humidifier, for example using membranes permeable to water vapor. The exhaust duct 20 then enters the turbine on the pressure side. 19 , so that the residual energy in the exhaust air, heat and pressure, is transferred to the exhaust air turbine. 15 can be used and at least partially recovered.

[0020] The fuel cell system 2 It also features a so-called system bypass. 21 with a bypass valve 22 open. This bypass valve 22 This can preferably be designed as a valve or control valve. The system bypass 21 connects the pressure sides of the air conveying device 14 on the one hand, and the exhaust turbine 15on the other hand, they are connected. It serves in particular to circulate air around the fuel cell when needed. 3 or their cathode space 4 to redirect the airflow. This may be necessary in certain operating situations to provide the desired mass flow and pressure ratios without disrupting the air conveying system. 14 exceeds its pumping limit, which is detrimental to the operation of the air conveying system. 14 , which is designed as a flow compressor, would be extremely unfavorable and critical.

[0021] The bypass valve 22As indicated by the letters in the diagram, it is designed as a normally open (NO) valve or normally open control valve. The term "normally open" here refers not to the normal state of the system bypass, but to the electrical normal state of the valve. It is therefore open when de-energized, which is why it is also said that the valve is open in its de-energized normal state. If a fault occurs, the bypass valve 22 so it returns to its normal, de-energized state, which is open. Such a failure of the bypass valve 22 in the system bypass 21 This then leads to the air mass flow through the system bypass. 21 Increased and, to the same extent, the air mass flow through the humidifier. 19 to the cathode space 4 the fuel cell 3decreases. Such behavior can be observed particularly through the presence of pressure sensors. 23 in the supply air duct 18 and the exhaust duct 23 , so ultimately in the direction of flow before and after the bypass valve 22 in the system bypass 21 to be determined.

[0022] In this case, a control unit is now suggested. 24 the conveying capacity of the air conveying system 14 increased to prevent the drop in the level to the fuel cell 3 to compensate for the flowing air mass flow. With a sufficiently generously dimensioned air conveying system. 14 or at the time of the bypass valve failure 22 present partial load operation of the fuel cell system 2 , which is the case at the vast majority of times when the fuel cell system 2 in the vehicle 1If the operation is successful, such an increase in the conveying capacity of the air conveying system can be achieved. 14 the faulty bypass valve 22 The problem was completely solved and had no impact on the vehicle. 1 be balanced. The vehicle 1 can be used with the power from the fuel cell system 2 so it can continue operating without any noticeable impairment.

[0023] Furthermore, in this case, the higher conveying capacity of the air conveying system 14 with the bypass valve open 22 also more air via the exhaust turbine. 15 outflows, which reduces the power needed to drive the air conveying system 14 This is further increased. This results in an overall increase in the pressure of the supplied air. This increased pressure creates a [condition] in the fuel cell. 3 or in the cathode space 4A state is reached in which the voltage of the fuel cell increases due to the higher pressure. The electrical power required by the fuel cell can therefore be achieved at lower currents. This advantageously raises the limit beyond which only limited operation is possible.

[0024] Only if the fuel cell 3 The required performance is so high that the maximum possible air mass flow is achieved at the maximum delivery capacity of the air conveying device. 14 and higher voltage of the fuel cell 3 If this is insufficient to meet the requirements, it will be handled via the control unit. 24 into the power control between fuel cell systems 2 and vehicle 1 in such a way that the fuel cell system 2Maximum available electrical power corresponding to the maximum possible air mass flow at maximum delivery capacity of the air conveying device 14 is limited. This method thus enables the fuel cell system to continue operating in the vast majority of operating conditions. 2 without any significant impairment, with regard to the availability of the fuel cell system 2 and thus ultimately the vehicle 1 This is a crucial advantage for the user. QUOTES INCLUDED IN THE DESCRIPTION

[0025] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0026] DE 102009050934 A1

[0005]

Claims

[1] Method for operating a fuel cell system ( 2 ) with an air conveying system ( 14 ) and an exhaust air turbine connected to it ( 15 ), where a system bypass ( 21 ) with a bypass valve that is open in the normal, de-energized state ( 22 ) the pressure sides of the air conveying device ( 14 ) and the exhaust turbine ( 15 ) connects, in case the bypass valve ( 21 ) is defective and remains in its normally open position when de-energized, characterized by that the conveying capacity of the air conveying system ( 14 ) is increased when there is a decrease in the air mass flow to the fuel cell ( 3 ) due to a defective bypass valve ( 22 ) is recognized. [2] Method according to claim 1, characterized by that the conveying capacity of the air conveying system ( 14) is increased until the required air mass flow to the fuel cell ( 3 ) is reached, or the maximum delivery capacity of the air conveying system ( 14 ) is present. [3] Method according to claim 2, characterized by that in the event of the maximum conveying capacity of the air conveying device ( 14 ) the electrical output of the fuel cell ( 3 ) is limited according to the available air mass flow. [4] Method according to claim 1, 2 or 3, characterized by that the air mass flow is determined based on the measured values ​​from pressure sensors ( 23 ) before and after the bypass valve ( 22 ) is determined. [5] Method according to any one of claims 1 to 4, characterized by that as a bypass valve ( 22 ) a flap, in particular a control flap, is used. [6] Method according to any one of claims 1 to 5, characterized by that the fuel cell system ( 2) to provide electric drive power in a vehicle ( 1 ) is used.

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