Method for operating a mobile fuel cell system, control unit

Abstract

The invention relates to a method for operating a mobile fuel cell system (1), comprising at least one fuel cell stack (2) with an anode and a cathode and a tank (3), in particular a cryogenic tank or a tank system with at least one compressed gas container, for supplying the anode of the at least one fuel cell stack (2) with hydrogen, wherein the hydrogen taken from the tank (3) is fed to an anode circuit via a supply line (4) and to the anode of the at least one fuel cell stack (2) via the anode circuit.According to the invention, a minimum target tank pressure is defined as a threshold value, and when the threshold value is undershot, a pressure reduction is carried out in the anode circuit, wherein, in order to avoid exceeding a maximum permissible differential pressure between the anode and the cathode, the pressure in an air supply path of the fuel cell system (1) via which the cathode of the at least one fuel cell stack (2) is supplied with air is simultaneously reduced. The invention further relates to a control device for a mobile fuel cell system (1).

Description

[0001] The invention relates to a method for operating a mobile fuel cell system having the features of the preamble of claim 1. Furthermore, the invention relates to a control device for carrying out steps of the method.

[0002] The preferred field of application of the invention is fuel cell vehicles that require hydrogen to generate electrical energy. State of the art

[0003] Polymer electrolyte membrane (PEM) fuel cell systems convert hydrogen into electrical energy using oxygen, generating waste heat and water. The PEM fuel cell consists of an anode supplied with hydrogen, a cathode supplied with air, and a polymer electrolyte membrane placed between them, where oxygen from the air is converted into electricity, water, and heat. In practice, several such fuel cells are stacked to increase the electrical voltage generated. Such a fuel cell stack is also called a stack.

[0004] A systemic approach to supplying the PEM anode with hydrogen has been established in which the anode exhaust gas, which is still rich in hydrogen, is fed back to the anode inlet together with fresh hydrogen using gas delivery units. This process is known as recirculation. A measure of recirculation is the ratio of hydrogen supplied to the stack to that consumed by the electrochemical reaction. This ratio is called lambda. Along with the hydrogen concentration, lambda is an essential parameter for the stack. A sufficiently high lambda and thus a sufficiently high hydrogen concentration at the stack inlet ensures that there is no hydrogen undersupply. This is because diffusion processes also bring nitrogen and water or water vapor to the anode side, which are inert gases for the electrochemical reaction in the fuel cells.

[0005] Typically, the process pressure and thus the hydrogen partial pressure in the stack are increased – especially at high load points – to ensure efficient reaction in the fuel cells. This is because the hydrogen partial pressure influences the cell voltage and thus the efficiency of the stack. Although hydrogen is stored under pressure in special tanks, the pressure difference between the tank and the fuel cell system is usually too low to operate at optimal efficiency – especially under full-load conditions. In addition, a pressure loss of 1-2 bar is to be expected on the way from the tank to the fuel cell system.

[0006] Particularly in tank systems that store hydrogen at extremely low temperatures in so-called cryogenic tanks, the pressure in the tank decreases with the removal of hydrogen, which is due to a change in entropy and enthalpy when mass is removed from a closed container. As the removal increases, i.e., in driving situations with high power requirements and high hydrogen consumption, the pressure drops more quickly than in low-load operation. Reheating can be used to attempt to maintain the desired pressure in the tank. If this fails, for example, due to excessive dynamics or continuous load, it can lead to the system shutting down and thus the vehicle coming to a standstill.

[0007] The present invention addresses the problem of expanding the inlet pressure tolerance when supplying a mobile fuel cell system with hydrogen, so that the availability of the mobile fuel cell system is not compromised when tank pressure drops. The application is intended to extend not only to cryogenic tanks, but also to tank systems with at least one pressurized gas container for storing hydrogen.

[0008] To achieve this objective, the method having the features of claim 1 is proposed. Advantageous developments of the invention are set forth in the subclaims. Furthermore, a control unit for a mobile fuel cell system is specified. Disclosure of the invention

[0009] A method is proposed for operating a mobile fuel cell system comprising at least one fuel cell stack with an anode and a cathode, and a tank, in particular a cryogenic tank or a tank system with at least one compressed gas container, for supplying the anode of the at least one fuel cell stack with hydrogen. The hydrogen taken from the tank is fed to an anode circuit via a supply line and to the anode of the at least one fuel cell stack via the anode circuit. According to the invention, a minimum target tank pressure is defined as a threshold value, and when the pressure falls below the threshold value, the pressure in the anode circuit is reduced. To avoid exceeding a maximum permissible differential pressure between the anode and the cathode, the pressure in an air supply path of the fuel cell system, via which the cathode of the at least one fuel cell stack is supplied with air, is simultaneously reduced.

[0010] The proposed method is particularly suitable for cases where the specified minimum target tank pressure cannot be maintained by reheating the tank alone, resulting in a risk of the system shutting down. Using the proposed method, system shutdown can be avoided or at least delayed, thus maintaining the availability of the fuel cell vehicle at low tank pressure. The proposed method thus leads to an extension of the inlet pressure tolerance when supplying the mobile fuel cell system with hydrogen.

[0011] Reducing the pressure in the anode circuit is accompanied by a reduction in system efficiency, but in return, it improves the availability of the fuel cell vehicle. Furthermore, the pressure reduction is only temporary, so the reduction in system efficiency is also not permanent.

[0012] Cryogenic tanks typically have pressures between 4 bara and 16 bara. Therefore, a tank pressure in the range of 6-4 bara is suitable as a minimum target tank pressure or threshold. The maximum permissible differential pressure can be, for example, 500 mbar.

[0013] In a further development of the invention, it is proposed that if a further threshold value, which is below the minimum target tank pressure, is undershot, the system power is reduced until the hydrogen mass flow in the anode circuit has reached a predetermined target value. This means that, in addition to the first threshold value, a second threshold value is defined that sets a limit below the minimum target tank pressure. The second threshold value can, for example, be a tank pressure at which the system would already have safely shut down without the pressure reduction already carried out on the anode and cathode sides. The proposed power reduction thus leads to a further extension of the inlet pressure tolerance in the hydrogen supply. This measure also ensures that there is no undersupply of hydrogen to the anode of at least one fuel cell stack.

[0014] Furthermore, it is proposed that the reduction in system power be indicated on a display and / or reported via a loudspeaker. This informs the vehicle driver about the power reduction and the resulting power limitation of the fuel cell system. The driver should be informed of this, especially if the power reduction is implemented over a longer period of time.

[0015] Furthermore, at least one other subsystem, for example a control system or control device, is preferably informed of the reduction in system power. Since the electrical energy generated by the fuel cell system is generally also made available to other electrical consumers, their requirements can be adjusted accordingly with the help of the control system or device.

[0016] Advantageously, when determining at least one threshold value, variable effects over the system's lifetime are taken into account, such as stack efficiency and / or valve flow characteristics. Both stack efficiency and valve flow characteristics are subject to aging and the associated changes. This can be taken into account by incorporating these effects from the outset or by adjusting at least one threshold value over time.

[0017] Furthermore, a control unit for a mobile fuel cell system is proposed. The control unit is configured to carry out steps of a method according to the invention. For example, the control unit can be used to reduce the pressure on the anode and cathode sides if the minimum target tank pressure or the first threshold is undershot. The first threshold is stored in the control unit for this purpose. If the second threshold is undershot, the system power can then be reduced with the help of the control unit until the hydrogen mass flow in the anode circuit reaches a predetermined target value. The second threshold and the target value are also stored in the control unit for this purpose.

[0018] The invention and its advantages are explained below with reference to the accompanying drawing. This shows a schematic representation of a mobile fuel cell system suitable for carrying out the method according to the invention or operable according to such a method. Detailed description of the drawing

[0019] The figure shows a mobile fuel cell system 1 or a fuel cell vehicle. This can be a commercial vehicle, in particular. The illustrated mobile fuel cell system 1 or fuel cell vehicle comprises a plurality of fuel cell stacks 2. These can be combined to form a multi-stack system so that they are supplied with the required reaction gases hydrogen and air via common subsystems. Alternatively, each fuel cell stack 2 can have its own subsystems so that each fuel cell stack 2, including the associated subsystems, represents an independent fuel cell system 1. The subsystems, which typically comprise a hydrogen system with an anode circuit for supplying an anode of a fuel cell stack 2 with hydrogen and an air system with an air supply path for supplying a cathode of a fuel cell stack 2 with air, are not shown in detail in the figure.

[0020] The hydrogen required by the fuel cell stacks 2 is stored in a tank 3, which is designed here as a cryogenic tank. This is connected to the anode circuits (not shown) of the fuel cell stacks 2 via a supply line 4. For this purpose, a distribution device 5 is integrated into the supply line 4. The distribution device 5 divides the supply line 4 into two supply paths, which lead to the respective fuel cell stacks 2. A gas conditioner 6 and a shut-off valve 7 are integrated into each of the two supply paths. By closing a shut-off valve 7, the hydrogen supply to a fuel cell stack 2 can be stopped. Furthermore, a temperature sensor 8 and a pressure sensor 9 are integrated into each of the two supply paths.

[0021] As hydrogen is withdrawn from tank 3, the tank pressure drops. The higher the load requirements, the faster the tank pressure drops. Typically, reheating in tank 3 is used to try to maintain a minimum target tank pressure. However, if this measure is insufficient and the tank pressure falls below the minimum target tank pressure, the system shuts down and the fuel cell vehicle stops.

[0022] To prevent this, the method according to the invention first reduces the pressure in the anode circuit of the fuel cell stack 2. At the same time, the pressure in the cathode-side supply air path is reduced to prevent the maximum permissible pressure difference between the anode and cathode from being exceeded. If this measure is also insufficient and a further threshold value below the minimum target tank pressure is reached, the system's power is additionally reduced until the hydrogen mass flow in the anode circuit reaches a specified target value. Compliance with the target value is important, as otherwise there is a risk of an insufficient supply of hydrogen to the anode.

[0023] If the supply situation improves again, for example after a refueling process, the power reduction can be lifted and the pressure in the anode circuit and in the supply air path can be raised again to a process pressure that ensures efficient operation.

Claims

[1] Method for operating a mobile fuel cell system (1), comprising - at least one fuel cell stack (2) with an anode and a cathode and - a tank (3), in particular a cryogenic tank or a tank system with at least one compressed gas container, for supplying the anode of the at least one fuel cell stack (2) with hydrogen, wherein the hydrogen taken from the tank (3) is supplied via a supply line (4) to an anode circuit and via the anode circuit to the anode of the at least one fuel cell stack (2), characterized bythat a minimum target tank pressure is defined as a threshold value and, when the threshold value is undershot, a pressure reduction is carried out in the anode circuit, wherein, in order to avoid exceeding a maximum permissible differential pressure between the anode and the cathode, the pressure in an air supply path of the fuel cell system (1) via which the cathode of the at least one fuel cell stack (2) is supplied with air is simultaneously reduced. [2] Method according to claim 1, characterized by that if a further threshold value which is below the minimum target tank pressure is undershot, the system output is reduced until the hydrogen mass flow in the anode circuit has reached a predetermined target value. [3] Method according to claim 2, characterized by that the reduction in system performance is indicated on a display and / or reported via a loudspeaker. [4] Method according to claim 2 or 3, characterized bythat at least one other subsystem, such as a control system or control device, is informed of the reduction in system performance. [5] Method according to one of the preceding claims, characterized by that when setting at least one threshold value, changing effects over the system lifetime are taken into account, such as stack efficiency and / or valve flow characteristics. [6] Control device for a mobile fuel cell system (1) which is designed to carry out steps of a method according to one of the preceding claims.

Images