Fuel cell system and operating procedures for a fuel cell system

Abstract

The presented invention relates to an operating method (100) for operating a fuel cell system (200), wherein the operating method (100) comprises: - initiating (101) a purging process to discharge 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) to discharge inert gas from the anode subsystem (203).

Description

The present invention relates to an operating method for a fuel cell system and a fuel cell system according to the appended claims.Prior ArtPolymer electrolyte membrane (PEM) fuel cell systems convert hydrogen by means of oxygen to electrical energy with the generation of waste heat and water.A PEM fuel cell consists for this purpose of an anode which is supplied with hydrogen, a cathode which is supplied with air, and the polymer electrolyte membrane placed therebetween being converted in the air and oxygen into current, water and heat.A plurality of such fuel cells are stacked to form a fuel cell stack to increase a supplied voltage.Systemically, for supplying an anode subsystem with hydrogen, an approach has become established in which still hydrogen-rich anode off-gas is fed back to the anode inlet together with fresh hydrogen by means of gas feed units, which is known as recirculation.A measure of the recirculation is the ratio of hydrogen supplied from the fuel cell stack (mH2Stack in) and hydrogen consumed by an electrochemical reaction (mH2Stack consumption), this ratio being called lambda (λ H2) as indicated in formula (1).A sufficiently high lambda ensures that sufficient hydrogen is supplied to a catalytic converter in the fuel cell stack over an entire flow region.By diffusion processes, nitrogen also reaches the anode side or into the anode subsystem. Nitrogen represents an inert gas for the electrochemical reaction taking place in the fuel cell. As a result of the recirculation, nitrogen accumulates 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 the cell voltage.If a fuel cell is no longer supplied sufficiently with hydrogen, this can lead to damage to the fuel cell.During operation, nitrogen accumulates in the anode subsystem, requiring the purging and purging of inert gas from the anode subsystem. Usually, anode gas, which consists of a mixture of hydrogen, nitrogen and water, is discharged via an outlet valve, i.e. a so-called "purge valve", and replaced by fresh hydrogen.The discharge of the anode gas has, in addition to efficiency losses due to hydrogen loss, the disadvantage that sufficient dilution air must be provided by the cathode system in order to keep a concentration of hydrogen below a legal limit value of 4 or 8% by volume.Disclosure of the InventionWithin the scope of the present invention, a fuel cell system and an operating method for operating the fuel cell system are presented.Further features and details of the invention are evident from the respective dependent claims, the description and the drawings. Features and details which are 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 reference is or can always be made reciprocally to the individual aspects of the invention.The present invention serves in particular to provide a possibility for an energy-efficient fuel cell system.According to a first aspect of the present invention, an operating method for operating a fuel cell system is thus provided.The presented operating method comprises the introduction of a purging process for discharging inert gas from an anode subsystem of the fuel cell system, the reduction of a volume flow recirculated in the anode subsystem compared to a volume flow recirculated in the anode subsystem before the introduction of the purging process, and the opening of an outlet valve for discharging inert gas from the anode subsystem.In the context of the present invention, the initiation of a purging process is understood to mean a process in which a process is triggered as a function of a state of the fuel cell system, such as a nitrogen concentration above a predefined threshold value or a hydrogen concentration below a predefined threshold value, which leads to the outlet valve of the fuel cell system opening. According to the invention, the initiation of the flushing process and the opening of the outlet valve are spaced apart in time, so that between the initiation of the flushing process, for example by outputting an initiation signal, and the opening of the outlet valve, for example by actuating the outlet valve with a control current, the volume flow recirculated in the anode subsystem of the fuel cell system can be adapted.By adapting the volume flow recirculated in the anode subsystem of the fuel cell system, an amount of hydrogen discharged when the outlet valve is opened is minimized and, as a result, an energy efficiency of the fuel cell system is maximized.It may be provided that the fuel cell system comprises a jet pump hydrogen metering valve and a bypass hydrogen metering valve, and for reducing the volume flow recirculated in the anode subsystem, a first hydrogen quantity metered into the anode subsystem by the bypass hydrogen metering valve is increased relative to a second hydrogen quantity metered into the anode subsystem by the jet pump hydrogen metering valve.In a fuel cell system with a jet pump hydrogen metering valve and bypass hydrogen metering valve, a fresh hydrogen mass flow necessary for a respective load point is divided between the jet pump hydrogen metering valve and the bypass hydrogen metering valve. In this case, a necessary fresh hydrogen mass flow through the jet pump hydrogen metering valve ensures recirculation in the anode subsystem, since this fresh hydrogen mass flow carries along a recirculated hydrogen mass flow.If, on the other hand, a fresh hydrogen mass flow is metered into the anode subsystem through the bypass hydrogen metering valve, the recirculation takes place independently of a recirculated hydrogen mass flow, so that a recirculation power decreases as a result of increased power of the bypass hydrogen metering valve.This means that by shifting a ratio of fresh hydrogen metered in by the bypass hydrogen metering valve to fresh hydrogen metered in by 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, 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.In order to keep a recirculation performance in the anode subsystem as low as possible, the jet pump hydrogen metering valve can be opened to a lesser extent than in a state in which the recirculation is optimally carried out. At the same time, a hydrogen mass flow required for a respective load point can be metered through the bypass hydrogen metering valve into the anode subsystem, so that a drive of the recirculation mass flow through the fresh hydrogen mass flow metered in by the jet pump hydrogen metering valve is minimized.Alternatively, it can be provided that the fuel cell system comprises a working fan for controlling the volume flow recirculated in the anode subsystem.By means of a working fan which 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 further operating parameters of the fuel cell system, so that, for example, the working fan can be deactivated before opening the outlet valve or can be operated at a minimum or minimized rotational speed.It can furthermore be provided that the volume flow recirculated in the anode subsystem is set to a predefined value.As an alternative to minimizing a 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 predefined value, so that, for example, a minimum volume flow required for a function of the fuel cell system is maintained and a complicated, for example hydrogen concentration-dependent regulation of a fresh hydrogen mass flow can be dispensed with.Alternatively, it can be provided that the volume flow recirculated in the anode subsystem is adjusted as a function of a hydrogen concentration at the outlet valve such that the hydrogen concentration at the outlet valve becomes minimum.By means of a regulation or a control, the hydrogen concentration at the outlet valve can be actively adjusted, for example, by dynamically adapting a recirculation power to a currently determined hydrogen concentration at the outlet valve, in order to adjust it to a predefined setpoint value.It can be provided that the hydrogen concentration at the outlet valve is measured by means of a hydrogen concentration sensor.By direct measurement of a current hydrogen concentration at the outlet valve, a particularly precise and rapid adjustment of the recirculation performance can be effected.Alternatively, it can be provided that the hydrogen concentration at the outlet valve is determined by means of a mathematical model.By means of a mathematical model which determines the hydrogen concentration at the outlet valve, for example on the basis of an amount of fresh hydrogen metered into a respective anode subsystem by a hydrogen metering system, it is possible to dispense with a hydrogen concentration sensor for measuring a hydrogen concentration at an outlet valve of the anode subsystem. Therefore, a corresponding fuel cell system can be provided particularly cost-effectively.It can furthermore be provided that, when the purging process is initiated, a control signal is provided which sets the fuel cell system in such a way that the volume flow recirculated in the anode subsystem is reduced compared to the volume flow recirculated in the anode subsystem before the purging process is initiated.In order to prepare a respective fuel cell system for a purging process in such a way that as little hydrogen as possible is discharged from the fuel cell system by the purging process, a control signal can be used which is generated when the purging process is initiated, i.e. is transmitted to respective actuators or control units in order to set the recirculated volume flow or a recirculation power before the outlet valve of the fuel cell system is opened.The control signal can comprise a control pulse, such as a PWM signal and / or a flag signal for reporting 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 metering system of the fuel cell system in response to the flag signal.According to a second aspect, the present invention relates to a fuel cell system for converting energy. Advantages described in detail with respect to the operation method for operating a fuel cell system according to the first aspect of the invention apply equally to the fuel cell system for converting power according to the second aspect of the invention.The fuel cell system presented 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 to perform a possible configuration of the presented operating method.In other words, the arithmetic unit of the presented fuel cell system is configured to carry out a possible configuration of the presented operating method.In the context of the present invention, a computing unit is understood to mean a computer, a processor, a control device or any further programmable circuit.It can be provided that the hydrogen supply system 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.Further advantages, features and details of the invention will become apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description can be essential to the invention individually or in any combination.The following are shown: FIG. 1 shows a possible embodiment of the operating method presented, FIG. 2 shows a possible embodiment of the fuel cell system presented.FIG. 1 illustrates an operating method 100 for operating a fuel cell system.The presented operating method 100 comprises an introduction step 101 in which a purging process for discharging inert gas from an anode subsystem of the fuel cell system is introduced, 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 introduction step 101, and an opening step 105 in which an outlet valve for discharging inert gas from the anode subsystem is opened.FIG. 2 shows a fuel cell system 200 for converting energy.The fuel cell system 200 includes a fuel cell stack 201 including 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 flow rate from the anode subsystem 203, and a computation unit 211.The arithmetic unit 211 is configured to control the hydrogen supply system 207 to perform the operating method 100 according to FIG. 1.The hydrogen supply system 207 comprises a jet pump hydrogen metering valve and a bypass hydrogen metering valve, between which a respective fresh hydrogen volume flow can be divided in order to adjust the recirculation performance indirectly.Alternatively, the hydrogen supply system 207 comprises a working fan and a working fan hydrogen metering valve, by means of which the recirculation power can be adjusted directly.

Claims

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).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) by the bypass hydrogen metering valve is increased relative to a second amount of hydrogen metered into the anode subsystem (203) by the jet pump hydrogen metering valve.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.Operating method (100) according to 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).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 predefined value.Operating method (100) according to one of the preceding claims, characterized in that the volume flow recirculated in the anode subsystem is set as a function of a hydrogen concentration at the outlet valve (209) in such a way that the hydrogen concentration at the outlet valve (209) becomes minimum.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.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.Operating method (100) according to one of the preceding claims, characterized in that, when the purging process is initiated, a control signal is provided which sets the fuel cell system (200) in such a way that the volume flow recirculated in the anode subsystem (203) is reduced in comparison with the volume flow recirculated in the anode subsystem (203) before the purging process is initiated.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), wherein the computing unit (211) is configured to drive the hydrogen supply system (207) to perform an operating method (100) according to any one of claims 1 to 9.The fuel cell system (200) of 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 work blower and a work blower hydrogen metering valve.

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