Fuel cell system and method for operating a fuel cell system

Abstract

The invention relates to a method (100) for operating a fuel cell system (200), said method (100) having the steps of: - initiating (101) a flushing process in order to discharge inert gas out of an anode subsystem (203) of the fuel cell system (200), - reducing (103) the volumetric flow rate being recirculated in the anode subsystem (203) in comparison to the volumetric flow rate recirculated in the anode subsystem (203) prior to initiating the flushing process, and - opening (105) an outlet valve (209) in order to discharge inert gas out of the anode subsystem (203).

Description

[0001] Description

[0002] title and operating procedures for a

[0003] The presented invention relates to an operating method for a fuel cell system and a fuel cell system according to the appended claims.

[0004] State of the art

[0005] Polymer electrolyte membrane (PEM) fuel cell systems convert hydrogen using oxygen to electrical energy, generating waste heat and water.

[0006] A PEM fuel cell consists of an anode supplied with hydrogen, a cathode supplied with air, and the polymer electrolyte membrane placed in between, where air and oxygen are converted into electricity, water and heat.

[0007] Several such fuel cells are stacked to form a fuel cell stack to increase the voltage provided.

[0008] A systemic approach has been established to supply an anode subsystem with hydrogen in which hydrogen-rich anode exhaust gas is fed back to the anode inlet together with fresh hydrogen by means of gas feed units, a process known as recirculation.

[0009] A measure of recirculation is the ratio of hydrogen supplied to the fuel cell stack (mH2Stack in) to the hydrogen consumed by an electrochemical reaction (mH2Stack consumed'). This ratio is called lambda (A^), as given in formula (1).

[0010] A sufficiently high lambda ensures that a catalyst in the fuel cell stack is supplied with sufficient hydrogen across the entire flow range.

[0011] Through diffusion processes, nitrogen also reaches the anode side or into the anode subsystem. Nitrogen is an inert gas for the electrochemical reaction taking place in the fuel cell. Recirculation causes nitrogen to accumulate in the anode subsystem, so that less hydrogen can be supplied to the anode subsystem. Accordingly, the lambda decreases, which can lead to a reduction in cell voltage.

[0012] If a fuel cell is no longer supplied with sufficient hydrogen, this can lead to damage to the fuel cell.

[0013] During operation, nitrogen accumulates in the anode subsystem, so the inert gas must be purged from the anode subsystem from time to time. Typically, anode gas, consisting of a mixture of hydrogen, nitrogen, and water, is purged via an outlet valve, a so-called "purge valve," and replaced with fresh hydrogen.

[0014] In addition to efficiency losses due to hydrogen loss, the discharge of the anode gas has the disadvantage that sufficient dilution air must be provided by the cathode system in order to keep the hydrogen concentration below a legal limit of 4 or 8 vol.%.

[0015] Disclosure of the invention

[0016] Within the scope of the invention presented, a fuel cell system and an operating method for operating the fuel cell system are presented. Further features and details of the invention emerge from the respective subclaims, the description, and the drawings. Features and details described in connection with the operating method according to the invention naturally also apply in connection with the fuel cell system according to the invention, and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is always made to each other.

[0017] The invention presented serves in particular to provide a possibility for an energy-efficient fuel cell system.

[0018] Thus, according to a first aspect of the invention presented, an operating method for operating a fuel cell system is presented.

[0019] The presented operating method comprises initiating a purging process for discharging inert gas from an anode subsystem of the fuel cell system, reducing a volume flow recirculated in the anode subsystem compared to a volume flow recirculated in the anode subsystem before initiating the purging process, and opening an outlet valve for discharging inert gas from the anode subsystem.

[0020] In the context of the present invention, initiating a purging process is understood to mean a process in which, depending on a state of the fuel cell system, such as a nitrogen concentration above a predetermined threshold or a hydrogen concentration below a predetermined threshold, a process is triggered that leads to the opening of the outlet valve of the fuel cell system. According to the invention, the initiation of the purging process and the opening of the outlet valve are spaced apart in time, so that the volume flow recirculated in the anode subsystem of the fuel cell system can be adjusted between the initiation of the purging process, e.g., by issuing an initiation signal, and the opening of the outlet valve, e.g., by controlling the outlet valve with a control current.By adjusting the volume flow recirculated in the anode subsystem of the fuel cell system, the amount of hydrogen discharged when the outlet valve is opened is minimized and, as a result, the energy efficiency of the fuel cell system is maximized.

[0021] It can be provided that the fuel cell system comprises a jet pump hydrogen metering valve and a bypass hydrogen metering valve, and in order to reduce the volume flow recirculated in the anode subsystem, a first amount of hydrogen metered into the anode subsystem through the bypass hydrogen metering valve is increased relative to a second amount of hydrogen metered into the anode subsystem through the jet pump hydrogen metering valve.

[0022] In a fuel cell system with a jet pump hydrogen metering valve and a bypass hydrogen metering valve, the fresh hydrogen mass flow required for a given load point is divided between the jet pump hydrogen metering valve and the bypass hydrogen metering valve. The required fresh hydrogen mass flow through the jet pump hydrogen metering valve ensures recirculation in the anode subsystem, as this fresh hydrogen mass flow entrains a recirculated hydrogen mass flow.

[0023] If, however, a fresh hydrogen mass flow is metered into the anode subsystem through the bypass hydrogen metering valve, the recirculation occurs independently of a recirculated hydrogen mass flow, so that the increased power of the bypass hydrogen metering valve reduces the recirculation power.

[0024] This means that by shifting the ratio of fresh hydrogen metered through the bypass hydrogen metering valve to fresh hydrogen metered through the jet pump hydrogen metering valve, a recirculation capacity or a hydrogen mass flow recirculated in an anode subsystem can be adjusted. Accordingly, it can be provided that, to increase the second hydrogen quantity relative to the first hydrogen quantity, the first hydrogen quantity is reduced and / or the second hydrogen quantity is increased.

[0025] To minimize recirculation power in the anode subsystem, the jet pump hydrogen metering valve can, for example, be opened less than in a state of optimal recirculation. At the same time, the hydrogen mass flow required for a particular load point can be metered into the anode subsystem through the bypass hydrogen metering valve, thus minimizing the drive of the recirculation mass flow by the fresh hydrogen mass flow metered in by the jet pump hydrogen metering valve.

[0026] Alternatively, the fuel cell system may include a working fan for controlling the volume flow recirculated in the anode subsystem.

[0027] By means of a working fan that sets a recirculation mass flow, a recirculation mass flow introduced into an anode subsystem of a respective fuel cell system can be set directly, i.e., independently of other operating parameters of the fuel cell system, so that, for example, the working fan can be deactivated before the outlet valve is opened or can be operated at a minimum or minimized speed.

[0028] It may further be provided that the volume flow recirculated in the anode subsystem is set to a predetermined value.

[0029] As an alternative to minimizing the volume flow recirculated in a respective anode subsystem of a respective fuel cell system, the volume flow recirculated in the anode subsystem can be set to a predetermined value so that, for example, a minimum volume flow required for the fuel cell system to function is maintained and complex, e.g., hydrogen concentration-dependent control of a fresh hydrogen mass flow can be eliminated. Alternatively, the volume flow recirculated in the anode subsystem can be adjusted as a function of a hydrogen concentration at the outlet valve such that the hydrogen concentration at the outlet valve is minimized.

[0030] By means of a control or a controller, the hydrogen concentration at the outlet valve can be actively adjusted, for example by dynamically adapting a recirculation output to a currently determined hydrogen concentration at the outlet valve in order to set it to a predetermined target value.

[0031] It may be provided that the hydrogen concentration at the outlet valve is measured by means of a hydrogen concentration sensor.

[0032] By directly measuring the current hydrogen concentration at the outlet valve, the recirculation performance can be adjusted particularly precisely and quickly.

[0033] Alternatively, it may be provided that the hydrogen concentration at the outlet valve is determined by means of a mathematical model.

[0034] By using a mathematical model that determines the hydrogen concentration at the outlet valve, for example, based on the amount of fresh hydrogen metered into a respective anode subsystem by a hydrogen metering system, a hydrogen concentration sensor for measuring the hydrogen concentration at an outlet valve of the anode subsystem can be dispensed with. Therefore, a corresponding fuel cell system can be provided particularly cost-effectively.

[0035] It can further be provided that, upon initiation of the purging process, a control signal is provided which adjusts the fuel cell system such that the volume flow recirculated in the anode subsystem is reduced compared to the volume flow recirculated in the anode subsystem before initiation of the purging process. In order to prepare a respective fuel cell system for a purging process such that as little hydrogen as possible is removed from the fuel cell system by the purging process, a control signal can be used which is generated upon initiation of the purging process, i.e., is transmitted to respective actuators or control units, in order to adjust the recirculated volume flow or a recirculation power before the outlet valve of the fuel cell system is opened.

[0036] The control signal may include a control pulse, such as a PWM signal and / or a flag signal for signaling the initiation of the purging process, so that, for example, respective control units of the fuel cell system generate corresponding control pulses for a hydrogen dosing system of the fuel cell system in response to the flag signal.

[0037] According to a second aspect, the presented invention relates to a fuel cell system for converting energy. Advantages described in detail for the operating method for operating a fuel cell system according to the first aspect of the invention equally apply to the fuel cell system for converting energy according to the second aspect of the invention.

[0038] The presented fuel cell system comprises a fuel cell stack comprising an anode subsystem and a cathode subsystem, a hydrogen supply system configured to introduce hydrogen into the anode subsystem, an outlet valve configured to discharge a volume flow from the anode subsystem, and a computing unit, wherein the computing unit is configured to control the hydrogen supply system in order to carry out a possible embodiment of the presented operating method.

[0039] In other words, the computing unit of the proposed fuel cell system is configured to implement a possible embodiment of the proposed operating method. In the context of the proposed invention, a computing unit is understood to mean a computer, a processor, a control unit, or any other programmable circuit.

[0040] It may be provided that the hydrogen supply system comprises a jet pump hydrogen dosing valve and a bypass hydrogen dosing valve or that the hydrogen supply system comprises a working blower and a working blower hydrogen dosing valve.

[0041] Further advantages, features, and details of the invention will become apparent from the following description, which describes exemplary embodiments of the invention in detail with reference to the drawings. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination.

[0042] They show:

[0043] Figure 1 shows a possible design of the presented operating procedure,

[0044] Figure 2 shows a possible design of the presented fuel cell system.

[0045] Figure 1 shows an operating method 100 for operating a fuel cell system.

[0046] The presented operating method 100 comprises an initiation step 101 in which a purging process for discharging inert gas from an anode subsystem of the fuel cell system is initiated, a reduction step 103 in which a volume flow recirculated in the anode subsystem is reduced compared to a volume flow recirculated in the anode subsystem before the initiation step 101, and an opening step 105 in which an outlet valve is opened for discharging inert gas from the anode subsystem.

[0047] Figure 2 illustrates a fuel cell system 200 for converting energy. The fuel cell system 200 includes a fuel cell stack 201, which includes an anode subsystem 203 and a cathode subsystem 205, a hydrogen supply system 207 configured to introduce hydrogen into the anode subsystem 203, an outlet valve 209 configured to discharge a volume flow from the anode subsystem 203, and a computing unit 211.

[0048] The computing unit 211 is configured to control the hydrogen supply system 207 in order to carry out the operating method 100 according to Figure 1.

[0049] The hydrogen supply system 207 comprises a jet pump hydrogen dosing valve and a bypass hydrogen dosing valve, between which a respective fresh hydrogen volume flow can be divided in order to indirectly adjust the recirculation capacity.

[0050] Alternatively, the hydrogen supply system 207 comprises a working blower and a working blower hydrogen metering valve by means of which the recirculation capacity can be directly adjusted.

Claims

Claims 1 . Operating method (100) for operating a fuel cell system (200), the operating method (100) comprising: Initiating (101) a purging process for discharging inert gas from an anode subsystem (203) of the fuel cell system (200), reducing (103) a volume flow recirculated in the anode subsystem (203) compared to a volume flow recirculated in the anode subsystem (203) before initiating the purging process, opening (105) an outlet valve (209) for discharging inert gas from the anode subsystem (203).

2. Operating method (100) according to claim 1, characterized in that the fuel cell system (200) comprises a jet pump hydrogen metering valve and a bypass hydrogen metering valve, and in order to reduce the volume flow recirculated in the anode subsystem (203), a first amount of hydrogen metered into the anode subsystem (203) through the bypass hydrogen metering valve is increased relative to a second amount of hydrogen metered into the anode subsystem (203) through the jet pump hydrogen metering valve.

3. Operating method (100) according to claim 2, characterized in that in order to increase the second amount of hydrogen relative to the first amount of hydrogen, the first amount of hydrogen is reduced and / or the second amount of hydrogen is increased.

4. Operating method (100) according to claim one of the preceding Claims, characterized in that the fuel cell system (200) comprises a working fan for controlling the volume flow recirculated in the anode subsystem (203).

5. Operating method (100) according to one of the preceding claims, characterized in that the volume flow recirculated in the anode subsystem (203) is set to a predetermined value.

6. Operating method (100) according to one of the preceding claims, characterized in that the volume flow recirculated in the anode subsystem is adjusted as a function of a hydrogen concentration at the outlet valve (209) such that the hydrogen concentration at the outlet valve (209) becomes minimal.

7. Operating method (100) according to claim 6, characterized in that the hydrogen concentration at the outlet valve (209) is measured by means of a hydrogen concentration sensor.

8. Operating method (100) according to claim 6 or 7, characterized in that the hydrogen concentration at the outlet valve (209) is determined by means of a mathematical model.

9. Operating method (100) according to one of the preceding claims, characterized in that when the purging process is initiated, a control signal is provided that adjusts the fuel cell system (200) such that the volume flow recirculated in the anode subsystem (203) is reduced compared to the volume flow recirculated in the anode subsystem (203) before the purging process is initiated.

10. A fuel cell system (200) for converting energy, the fuel cell system (200) comprising: a fuel cell stack (201) comprising an anode subsystem (203) and a cathode subsystem (205), a hydrogen supply system (207) configured to introduce hydrogen into the anode subsystem (203), an outlet valve (209) configured to discharge a volume flow from the anode subsystem (203), a computing unit (211), the computing unit (211) being configured to control the hydrogen supply system (207) to carry out an operating method (100) according to any one of claims 1 to 9.

11. Fuel cell system (200) according to claim 10, characterized in that the hydrogen supply system (207) comprises a jet pump hydrogen metering valve and a bypass hydrogen metering valve or that the hydrogen supply system comprises a working fan and a working fan hydrogen metering valve.

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