Method for operating a fuel cell system, and fuel cell system

Abstract

The invention relates to a method for operating a fuel-cell system and to a fuel cell system. The method comprises the step of: actuating a proportional valve device in order to control an amount of a recirculation medium which is supplied to a fuel line of a fuel cell stack from a recirculation circuit, wherein the proportional valve device is operated continuously over time or in a pulsed manner depending on a predetermined operating parameter, so that in the pulsed operation at least one pressure pulse is produced in the fuel cell stack. The fuel cell system is equipped with - a fuel cell stack with an anode and a cathode; - a recirculation circuit for recirculating a recirculation medium at the anode; - a fuel line for supplying the fuel cell stack with a fuel, in particular hydrogen; - a proportional valve device, which is connected to the fuel line and to the recirculation circuit; - a control device, which is connected to the fuel line and / or to the recirculation circuit and / or to the proportional valve device and is configured to carry out such a method.

Description

[0001] Description

[0002] title

[0003] Method for operating a fuel cell system and fuel cell system

[0004] The present invention relates to a method for operating a fuel cell system. Furthermore, the present invention relates to a fuel cell system.

[0005] State of the art

[0006] Conventional fuel cell systems require oxygen from the air and hydrogen for the chemical reaction. A fuel cell has an anode and a cathode. During operation, the anode is supplied with hydrogen, and the cathode with the oxidant, i.e., the oxygen from the air. Many PEM fuel cells have a connection between the stack anode outlet and the stack anode inlet. The gas in the anode is recirculated through this connection. This is intended to return unused hydrogen to the system to increase efficiency. Furthermore, a metering valve is integrated into the anode circuit to compensate for the hydrogen consumed in the fuel cell, through which fresh hydrogen is metered. Technical equipment is required to maintain recirculation in the anode circuit. This typically involves pumps or fans.Commonly used pumps include jet pump systems, as well as positive displacement pumps or blowers. Combinations are also often used. Systems with a jet pump only, so-called jet pump-only systems, represent a very cost-effective and easy-to-operate option. However, such systems have some limitations. Typically, the regulation or metering of hydrogen in the anode circuit is done via one or more metering valves. These allow the desired pressure to be set.

[0007] Purge and drain valves in the anode circuit can be used to regulate the water content and gas concentration. Information such as pressure, temperature, flow rate, operating point, humidity, or similar can be used for control. The fuel cell is supplied either by continuously metered valves, also known as proportional valves, or by timed valves. With continuous valves or proportional valves, the cross-section or valve stroke is controlled. This allows the flow rate and thus the pressure to be regulated very evenly. With timed valves, the average anode pressure and thus the supplied quantity is regulated by varying the time between an open and a closed position of the valve. In principle, this results in a periodic increase in pressure during the open phase and a decrease in pressure during the closed phase.Due to the timing of the inflow, there are phases with a very high inflow (full-load flow rate) and phases with no inflow at all. During the open phases, there is a very strong flow, resulting in very good water transport through the cell and a very good H2 supply. On average, the inflow corresponds to the inflow through the proportionally opened valve. Therefore, there are systems in which the pressure is kept constant or regulated on average by periodic opening and closing.

[0008] For example, WO 2022 144 183 A1 discloses a method for recirculating anode gas in an anode circuit of a fuel cell system having at least two jet pumps connected in parallel in order to compensate for a decreasing recirculation performance of the jet pump in the partial load range or in the low load range.

[0009] In principle, pressure pulses can represent a lifespan-influencing stress for the fuel cell system, especially for the fuel cell stack and possibly also for the valve used. The shape of the pulse, the pulse strength or amplitude, and the number of pulses over the entire operating time must be taken into account. For this reason, it is advisable to use pulsed operation or pulsed operation no more than necessary.

[0010] Disclosure of the invention

[0011] The invention provides a method for operating a fuel cell system having the features of claim 1 and a fuel cell system having the features of claim 8.

[0012] According to a first aspect of the invention, a method for operating a fuel cell system is provided. The method comprises a step of controlling a proportional valve device to control a quantity of a recirculation medium supplied to a fuel line of a fuel cell stack from a recirculation circuit. The proportional valve device is operated continuously or in a pulsed or clocked manner depending on a predetermined operating parameter, so that at least one pressure pulse is generated in the fuel cell stack during pulsed operation.

[0013] According to a second aspect of the invention, a fuel cell system is provided. The fuel cell system comprises a fuel cell stack with an anode and a cathode, as well as a recirculation circuit for recirculating a recirculation medium at the anode. Furthermore, the fuel cell system comprises a fuel line for supplying the fuel cell stack with hydrogen and a proportional valve device connected to the fuel line and to the recirculation circuit. Furthermore, the fuel cell system comprises a control device connected to the fuel line and / or to the recirculation circuit and / or to the proportional valve device and configured to carry out a method according to the invention.

[0014] One idea underlying the present invention is to provide mixed operation consisting of a clocked control for intermittent or pulsed operation and a continuous operation, whereby a fundamentally continuously operating valve or a proportional valve device is used to implement both operating modes. This means that the operation of the fuel cell system is divided into an operating mode in which pulsed / clocked operation always occurs and an operating mode in which this generally does not occur. This is possible because a quasi-clocked control also allows valves that are actually designed for continuous operation to be operated in intermittent or pulsed operation. The proportional valve device is controlled by a control device, in particular by the fuel cell control unit.The proportional valve device directly controls the amount of fuel supplied, while indirectly influencing the amount of recirculation medium.

[0015] Pressure pulses or pressure oscillations in the anode circuit during cycle operation can be used to periodically generate large pressure differences between the anode inlet and the anode outlet. The pressure at the anode inlet is measured, particularly at a position between the fuel cell stack and the proportional valve device, i.e., downstream of the proportional valve device and upstream of the fuel cell stack. Furthermore, the pressure at the anode outlet is measured, particularly downstream of the fuel cell stack. Pressure pulses or pressure oscillations thus effectively remove water deposits in the fuel cell stack, even at low operating loads, and increase recirculation and the anode lambda.

[0016] One advantage is that the fuel cell system can be operated in an optimized manner, especially when equipped with a jet pump-only system. Through the targeted and demand-driven use of pulsed operation, the load on the fuel cell system can be kept within acceptable limits. In particular, the total number of pulse cycles can be significantly reduced compared to pure intermittent operation.

[0017] In the context of the present invention, pulsed operation refers to a change in the valve position between a slightly open position, for example, at most 10% open, and a significantly larger open position, for example, at least 90% open. This means that in pulsed operation, the proportional valve device is alternately essentially fully open and essentially fully closed, resulting in a nearly maximum flow rate or virtually no flow rate.

[0018] Advantageous embodiments and further developments emerge from the further subclaims and from the description with reference to the figures of the drawing.

[0019] According to a further development of the method, the predetermined operating parameter comprises a model variable, a measured value, and / or an operating load. The predetermined operating parameter of the fuel cell system can vary depending on operating modes, with a change between the operating modes depending on a predetermined limit value of the operating parameter. In this way, pressure pulses, including individual pressure pulses, can be generated as needed. For example, the proportional valve device can be operated in a pulsed manner when cell voltage signals or other measurement signals detect that limit values ​​have been exceeded. Depending on the type of predetermined operating parameter or the associated operating state, a single pressure pulse or a sequence of pressure pulses can be generated.

[0020] According to a further development of the method, if the model variable detects the occurrence of water retention in the fuel cell stack, the proportional valve device is operated in a pulsed mode. For example, the probability of water retention is comparatively high at low operating loads, so that a switch from continuous operation to pulsed operation can be made early upon reaching a low operating load, particularly before the low operating load is reached.

[0021] According to a further development of the method, when the model variable or the measured value reaches a predefined nitrogen concentration or a predefined fill level, the proportional valve device is operated in a pulsed manner. According to a further development of the method, the proportional valve device is variably controlled by a sinusoidal, sawtooth, trapezoidal, rectangular, pulse-width-modulated, or any desired control signal to generate the at least one pressure pulse. "Variably controlled" means, in particular, that the control signal can have a variable amplitude and / or a variable frequency. By controlling the proportional valve device with such optimized control signal waveforms, the load on the fuel cell stack during operation can be further reduced.

[0022] For example, the proportional valve device can have a metering valve associated with the delivery device, or a hydrogen metering valve (HGI - Hydrogen Gas Injector) for short. The hydrogen metering valve can be controlled with a variable amplitude and a variable frequency. Furthermore, control modes that vary between intermittent operation and continuous operation are easily applicable. The control signal, particularly the control signal waveforms, influence the short-term effective pressure delta between the anode inlet and anode outlet of the stack. Furthermore, the control signal can influence the supply level of the cells that develops within a period. In this respect, the control can be optimized for the specific operating point by adjusting the operating parameters.

[0023] According to a further development of the method, the proportional valve device, when operated in a pulsed manner, generates regular pressure pulses at least temporarily, particularly for determining the system state. Pressure pulses can advantageously be used to determine the system state if they occur regularly.

[0024] According to a further development, the method further comprises a step of adjusting the amplitude of the pressure pulse in relation to detected water retention in the fuel cell stack or to a risk of water retention in the fuel cell stack, wherein the risk is calculated based on operating conditions. The amplitude correlates, for example, with the cross-sectional opening of the proportional valve device and corresponds to the flow rate provided by the proportional valve device. The amplitude depends on or is limited by a supply pressure upstream of the proportional valve device, a consumption or operating point of the fuel cell, and / or a size of the proportional valve device.By taking the influencing factors into account and adjusting the amplitude level to a sufficient level to eliminate or prevent water retention, the load is further reduced.

[0025] For example, the amplitude level can be adjusted to provide large amplitudes during acute water retention or during operating conditions with an extreme likelihood of water retention. Medium amplitudes can be provided within the normal range, where there is a normal risk of water retention. Furthermore, reduced amplitudes relative to the medium amplitudes can be provided when there is a reduced risk of water retention.

[0026] The control can be implemented situationally or reactively depending on measured values, i.e., without pre-control. Pre-control is also possible. Alternatively or additionally, closed-loop control can be provided.

[0027] According to a further development of the fuel cell system, the recirculation medium from the fuel cell stack contains unused hydrogen and unseparated nitrogen, and the proportional valve device has a hydrogen metering valve.

[0028] Furthermore, in addition to the hydrogen metering valve, the proportional valve device can, for example, have a jet pump. The jet pump can have a first inlet, a second inlet, an intake region, a mixing tube, and a diffuser region. The anode gas flows at least partially in one flow direction through the jet pump, wherein the flow direction runs parallel to a longitudinal axis of the jet pump. The majority of the flow-through regions of the jet pump are, for example, at least approximately tubular and serve to convey and / or guide a fluid, which is in particular hydrogen with proportions of water and nitrogen, in the proportional valve device. A driving medium is supplied to the jet pump via the second inlet, which flow flows through a channel of a nozzle into the intake region or the mixing tube.In addition, a recirculation medium is supplied to the proportional valve device through the first inlet. The recirculation medium is, in particular, the unused hydrogen from an anode region of the fuel cell, in particular a fuel cell stack. The recirculation medium may also comprise water and nitrogen. The propellant medium may come from a tank and be under high pressure, in particular more than 5 bar.

[0029] The propellant is discharged from the nozzle into the intake area and / or the mixing tube. The hydrogen flowing through the nozzle and serving as the propellant exhibits a pressure and / or velocity difference compared to the recirculation medium flowing into the jet pump from the first inlet, with the propellant medium having a higher pressure of at least 5 bar. When a so-called jet pump effect occurs, the recirculation medium is pumped into the central flow area of ​​the jet pump at a low pressure. The propellant flows through the nozzle into the intake area and / or the mixing tube at the described pressure difference and at a high velocity, which can in particular be close to the speed of sound.

[0030] The nozzle can, for example, have an internal recess in the form of a flow opening through which the fluid can flow. The propellant medium encounters the recirculation medium, which is already located in the intake area and / or the mixing tube. Due to the high velocity and / or pressure difference between the propellant medium and the recirculation medium, internal friction and turbulence are generated between the media. This creates shear stress in the boundary layer between the fast-moving propellant medium and the much slower recirculation medium. This stress causes momentum transfer, accelerating and entraining the recirculation medium. Mixing occurs according to the principle of conservation of momentum.This accelerates the recirculation medium in the direction of flow, creating a pressure drop for the recirculation medium, which creates a suction effect and thus draws additional recirculation medium from the area of ​​the first inlet. This effect can be referred to as the jet pump effect. By controlling the metered addition of the propellant medium using the hydrogen metering valve within the proportional valve device, the flow rate of the recirculation medium can be regulated and adapted to the respective needs of an entire fuel cell system depending on the operating state and requirements.In an exemplary operating state of the proportional valve device in which the hydrogen metering valve is closed, the propellant medium from the second inlet can be prevented from flowing into the central flow region of the proportional valve device, so that the propellant medium can no longer flow in the flow direction to the recirculation medium into the intake region and / or the mixing tube, thus suspending the jet pump effect. After passing through the mixing tube, the mixed medium to be pumped, which consists in particular of the recirculation medium and the propellant medium, flows in the flow direction into the diffuser region, whereby a reduction in the flow velocity can occur in the diffuser region. From there, the medium flows on, for example, into the anode region of the fuel cell.The hydrogen metering valve can be located directly on the proportional valve device and, in particular, form a common assembly with the proportional valve device, wherein the proportional valve device can have an integrated propellant nozzle. Fresh anode gas, in particular the propellant, is supplied to the proportional valve device via the hydrogen metering valve and / or the integrated propellant nozzle.

[0031] Short description of the drawings

[0032] The invention is explained below with reference to the figures of the drawings. The figures show:

[0033] Fig. 1 is a schematic view of a fuel cell system according to an embodiment of the invention; Fig. 2 is a block diagram of method steps of the method for operating a fuel cell system according to an embodiment of the present invention.

[0034] In the figures, the same reference numerals designate identical or functionally equivalent components, unless otherwise stated. The numbering of process steps is for clarity and generally does not imply a specific chronological order. In particular, several process steps can be performed simultaneously.

[0035] Description of the embodiments

[0036] Further advantages, features and details of the invention will become apparent from the following description, in which various embodiments are described in detail with reference to the drawing.

[0037] Fig. 1 shows a schematic view of a fuel cell system 1 according to an embodiment of the invention.

[0038] The fuel cell system 1 here comprises, by way of example, a fuel cell stack 101. The fuel cell stack 101 has a cathode 105 and an anode 103. The anode 103 is supplied with hydrogen via a fuel line 20. A high-pressure tank 21 and a pressure control valve 22 are located at the inlet of the fuel line 20. Additional components can be arranged in the fuel line 20 to supply the anode 103 of the fuel cell stack 101 with fuel as needed. Excess fuel, as well as certain amounts of water and nitrogen that diffuse through the cell membranes to the anode 103, are returned to a recirculation circuit 50 and mixed with the metered fuel from the fuel line 20.

[0039] Various components, such as a pump or a blower 52, can be installed to drive the flow in the recirculation circuit 50. A hydrogen metering valve 51 is located at the transition between the fuel line 20 and the recirculation circuit 50. The hydrogen metering valve 51 ensures the supply of fresh hydrogen to the recirculation circuit 50. The hydrogen metering valve 51 can be designed as a proportional valve, i.e., a proportionally operating valve. The control strategy in the fuel cell system provides for the hydrogen metering valve 51 to regulate the gas pressure within the recirculation circuit 50 to a defined target pressure depending on the system operating point.

[0040] A water separator 2 can also be integrated into the recirculation circuit 50 to separate water from the anode gas present in the recirculation circuit 50. The water separator comprises, for example, a container 3 for collecting the separated water. To empty this container 3, the container is connected to a drain line 40 via a drain valve 41. The drain line 40 typically directs the excess water into an exhaust line connected to the environment.

[0041] A pressure sensor 25 is arranged in the fuel line 20. The pressure sensor 25 is arranged upstream of the hydrogen metering valve 51 and measures the pressure between the pressure regulator 22 and the hydrogen metering valve 51 in the fuel line 20.

[0042] In addition, the fuel cell system 1 comprises a control device, in particular a fuel cell control unit (not shown), which is designed to carry out a method for operating the fuel cell system 1, as described, for example, in the exemplary embodiment according to Fig. 2.

[0043] Fig. 2 shows a block diagram of method steps of the method for operating a fuel cell system 1 according to an embodiment of the present invention.

[0044] The method comprises a step of determining or measuring S1 a predetermined operating parameter. The predetermined operating parameter can comprise a model variable, a measured value, an operating load, and / or comparable parameters. The method further comprises a step of controlling S2 a proportional valve device 51 in order to control a quantity of a recirculation medium that is supplied to a fuel line 20 of a fuel cell stack 101 from a recirculation circuit 50, wherein the proportional valve device 51 is operated continuously or in a pulsed manner depending on the predetermined operating parameter, such that at least one pressure pulse arises in the fuel cell stack 101 during pulsed operation. If, for example, the model variable detects the occurrence of water retention in the fuel cell stack 101, the proportional valve device 51 is operated in a pulsed manner.Alternatively or additionally, the proportional valve device 51 is operated in a pulsed manner when the measured value reaches a predefined nitrogen concentration or a predefined fill level.

[0045] Optionally, the proportional valve device 51 can be variably controlled by a sinusoidal, sawtooth, trapezoidal, rectangular, or pulse-width-modulated control signal to generate the at least one pressure pulse. When operated in a pulsed manner, the proportional valve device 51 can at least temporarily generate regular pressure pulses to determine the system state.

[0046] Furthermore, the method comprises, for example, a step S3 of adjusting an amplitude level of the pressure pulse in relation to detected water retention in the fuel cell stack 101 or to a risk of water retention in the fuel cell stack 101, wherein the risk is calculated based on operating conditions.

[0047] Although the present invention has been explained above using exemplary embodiments, it is not limited thereto but can be modified in a variety of ways. In particular, combinations of the above embodiments are also conceivable.

Claims

Claims 1. A method for operating a fuel cell system (1), comprising the step: Controlling (S2) a proportional valve device (51) in order to control a quantity of a recirculation medium which is supplied to a fuel line (20) of a fuel cell stack (101) from a recirculation circuit (50), wherein the proportional valve device (51) is operated continuously or in a pulsed manner at times as a function of a predetermined operating parameter, so that in the pulsed operation at least one pressure pulse is generated in the fuel cell stack (101).

2. The method according to claim 1, wherein the predetermined operating parameter comprises a model variable, a measured value and / or an operating load.

3. The method according to claim 2, wherein, when the model variable detects an occurrence of water retention in the fuel cell stack (101), the proportional valve device (51) is operated in a pulsed manner.

4. The method according to claim 2 or 3, wherein when the model variable or the measured value reaches a predefined nitrogen concentration or a predefined fill level, the proportional valve device (51) is operated in a pulsed manner.

5. Method according to one of the preceding claims, wherein the proportional valve device (51) is variably controlled by a sinusoidal, a sawtooth-shaped, a trapezoidal, a rectangular or a pulse-width modulated control signal for generating the at least one pressure pulse.

6. Method according to one of the preceding claims, wherein the proportional valve device (51), when operated in a pulsed manner, at least Periodically generates regular pressure pulses, particularly to determine the system status.

7. The method according to any one of the preceding claims, further comprising the step of adjusting (S3) an amplitude level of the pressure pulse in relation to detected water retention in the fuel cell stack (101) or to a risk of water retention in the fuel cell stack (101), wherein the risk is calculated based on operating conditions.

8. Fuel cell system (1) comprising: - a fuel cell stack (101) with an anode (103) and a cathode (105); - a recirculation circuit (50) for recirculating a recirculation medium at the anode (103); - a fuel line (20) for supplying the fuel cell stack (101) with a fuel, in particular hydrogen; - a proportional valve device (51) connected to the fuel line (20) and to the recirculation circuit (50); - a control device which is connected to the fuel line (20) and / or to the recirculation circuit (50) and / or to the proportional valve device (51) and is designed to carry out a method according to one of the preceding claims.

9. Fuel cell system (1) according to claim 8, wherein the recirculation medium from the fuel cell stack contains unconsumed hydrogen and unseparated nitrogen, and wherein the proportional valve device (51) comprises a hydrogen metering valve.

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