Method for operating a fuel cell system, control unit

Abstract

Method for operating a fuel cell system (1) in which hydrogen from a tank (21) and recirculated hydrogen from a recirculation circuit (50) are supplied as anode gas to at least one fuel cell (101) via a fuel line (20) and in which anode gas is removed from the recirculation circuit (50) by temporarily opening a purge valve (41). The following steps are carried out: - Opening or closing the purge valve (41); - Checking the performance of a gas delivery unit (52) in the recirculation circuit (50); - Verifying the opening or closing of the purge valve (41) by means of a change in the performance of the gas delivery unit (52) in the recirculation circuit (50). The invention further relates to a control unit (70) for carrying out the method or individual method steps.

Description

[0001] The invention relates to a method for operating a fuel cell system. Furthermore, the invention relates to a control unit configured to carry out the steps of the method. State of the art

[0002] A PEM fuel cell has a polymer electrolyte membrane arranged between an anode and a cathode. With the help of the PEM fuel cell, hydrogen, which is supplied to the anode, and oxygen, which is supplied to the cathode in the form of air, can be converted into electrical energy, heat, and water. To increase the generated electrical voltage, several fuel cells are combined in practice to form a fuel cell stack.

[0003] Through diffusion processes through the fuel cell, nitrogen enters the recirculation circuit on the anode side. Another source of nitrogen is the fresh fuel, which is not 100% pure hydrogen. Nitrogen is an inert gas for the fuel cell, reducing the cell voltage and thus the stack voltage, which in turn leads to a loss of efficiency. Therefore, anode gas is repeatedly purged from the recirculation circuit during a driving cycle to reduce the nitrogen content. This purge is achieved using the purge valve.

[0004] The supply of fresh hydrogen is carried out according to the state of the art by means of hydrogen dosing valves which can be designed as proportional valves.

[0005] The present invention is concerned with the object of providing a method for operating a fuel cell system which enables a reliable and at the same time cost-effective checking of the purge valve.

[0006] To achieve this objective, the method with the features of claim 1 is proposed. Advantageous further developments of the invention are set forth in the subclaims. Furthermore, a control unit for executing the method or individual method steps is specified. Disclosure of the invention

[0007] In the proposed method for operating a fuel cell system, hydrogen from a tank and recirculated hydrogen from a recirculation circuit are supplied as anode gas to at least one fuel cell via a fuel line. Since nitrogen, water, and to a lesser extent other gases accumulate in the anode gas of the recirculation circuit over time, the Anode gas can be removed from the system by temporarily opening a purge valve. The following steps are carried out: - Opening or closing the purge valve; - Checking the performance of a gas delivery unit in the recirculation circuit; - Verify the opening or closing of the purge valve by checking the performance of the gas delivery unit in the recirculation circuit.

[0008] When the purge valve is opened, anode gas escapes from the recirculation circuit. The set pressure in the recirculation circuit changes and drops below the desired set pressure. To maintain the set pressure, hydrogen flows into the recirculation circuit via the hydrogen metering valve.

[0009] This additional hydrogen leads to a higher propellant mass flow in the jet pump, so the jet pump contributes more to promoting the flow in the recirculation circuit, and the performance of the gas delivery unit decreases. Similarly, the performance of the gas delivery unit increases when the purge valve is closed, as the propellant mass flow decreases. Therefore, the gas delivery unit's performance can be used to verify whether the purge valve is open or closed.

[0010] During a purge process with the purge valve 41 open, the hydrogen added through the hydrogen metering valve also leads to a drop in the performance of the gas delivery unit 51 due to the change in the density conditions.

[0011] The proposed method provides a reliable and cost-effective way to verify whether an open or closed purge valve is open. No additional components or sensors are required to implement the method according to the invention.

[0012] In addition to opening or closing the purge valve, the method according to the invention can also verify a blockage in the purge line, since in this case the same effect occurs as when the purge valve is opened, which cannot be verified.

[0013] The dependent claims specify advantageous embodiments and further developments of the fuel cell system according to the invention and of the method for operating a fuel cell system.

[0014] It is advantageous to generate an error message if the opening or closing of the purge valve is not verified, as this allows for steps to be taken to compensate for the purge valve's failure to open. Possible measures could include removing purge gas via a drain valve or operating the fuel cell system in emergency mode.

[0015] It is advantageous to perform a functional test of a hydrogen sensor in the cathode exhaust line after a verified opening or closing of the purge valve, because this functional test is based on an elevated hydrogen concentration in the exhaust line. An elevated hydrogen concentration can only be ensured by a verified opening of the purge line.

[0016] In order to avoid fluctuations in the hydrogen concentration due to a variable mass flow, it is advantageous if the mass flow of the exhaust gas in the exhaust line is kept constant during the functional test of the hydrogen sensor.

[0017] Furthermore, a control unit is proposed that is configured to execute steps of the method according to the invention. In particular, the actuator current required to control the hydrogen metering valve can be detected and evaluated with the aid of the control unit. A corresponding algorithm is preferably stored in the control unit for evaluating the actuator current.

[0018] The invention and its advantages are explained in more detail below with reference to the attached drawings: Fig. 1 shows a schematic representation of a topology of a fuel cell system according to a first embodiment and Fig. 2 shows a schematic flow of the method according to the invention. Detailed description of the drawings

[0019] In the Fig. 1 shows a schematic topology of a fuel cell system 1 according to a first embodiment of the invention with at least one fuel cell stack 101.

[0020] The fuel cell stack 101 has a cathode side 105 and an anode side 103.

[0021] The at least one fuel cell system 1 has an air line 10, an exhaust line 12, and a fuel line 20. The at least one fuel cell stack 101 can be used for mobile applications with high power requirements, e.g., in trucks, or for stationary applications, e.g., in generators.

[0022] The air line 10 serves as an air supply line to supply air from the environment to a cathode 105 of the fuel cell stack 101 via an inlet 16. Components required for the operation of the fuel cell stack 101 are arranged in the air line 10. An air compressor 11 and / or compressor 11 is arranged in the air line 10, which compresses or draws in the air according to the respective operating conditions of the fuel cell stack 101. Downstream of the air compressor 11 and / or compressor 11, a heat exchanger can be located, which heats the air in the air line 10.

[0023] Further components, such as a filter 7 and / or a humidifier and / or valves, can be provided within the air line 10. Oxygen-containing air is supplied to the fuel cell stack 101 via the air line 10.

[0024] Furthermore, the fuel cell system 1 has an exhaust line 12. An exhaust gas containing air components from the air line 10 and water is transported via the exhaust line 12 to the environment via an outlet 18. The exhaust gas from the exhaust line 12 can also contain hydrogen (H2) because portions of the hydrogen can diffuse through the membrane of the fuel cell stack 101. Diffusion of the hydrogen also occurs when the fuel cell system 1 is shut down. In addition to the exhaust gas, an anode gas can be transported into the exhaust line 12 via a purge line 40 with a purge valve 41.

[0025] The anode side 103 of the fuel cell stack 101 is supplied with hydrogen via a fuel line 20. Located in the fuel line 20 are a pressure control valve 22 and a hydrogen metering valve 51 with a jet pump 53.

[0026] Upstream of the pressure control valve 22 is a high-pressure tank 21, which is connected to the pressure control valve 22 via the fuel line 20. Additional components may be arranged in the fuel line 20 to supply the anode side 103 of the fuel cell stack 101 with fuel as needed.

[0027] Excess fuel, as well as certain amounts of water and nitrogen that diffuse through the cell membranes to the anode side 103, are returned to a recirculation circuit 50 and mixed with the added fuel from the fuel line 20.

[0028] A gas delivery unit 52 is installed to drive the flow in the recirculation circuit 50. The hydrogen metering valve 51 with jet pump 53 is located at the transition between the fuel line 20 and the recirculation circuit 50. In addition to the gas delivery unit 52, the jet pump 53 contributes to the recirculation of the anode gas in the recirculation circuit 50.

[0029] 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. Reasons for the replenishment of fresh hydrogen can be the consumption of hydrogen through electrochemical conversion within the fuel cell stack 101 or other losses of gas molecules from the recirculation circuit 50, such as through the opening of the purge valve 41.

[0030] If the operating point changes, larger or smaller amounts of hydrogen are briefly pumped from the fuel line 20 into the recirculation circuit 50 through the hydrogen metering valve 51.

[0031] The method according to the invention provides for checking the performance of the gas delivery unit 52 in the recirculation circuit 50 after and during the opening of the purge valve 41, or as soon as the purge valve 41 is open. A change in the performance of the gas delivery unit 52 in the recirculation circuit 50 can be used to verify whether the purge valve 41 is open.

[0032] The opening of the purge valve 41 can be verified by a drop in the performance of the gas delivery unit 52.

[0033] The performance of the gas delivery unit 52 drops when the purge valve 41 is open, since more hydrogen is fed through the hydrogen metering valve 51 to maintain the pressure. This fed-in hydrogen leads to a higher propellant mass flow in the jet pump 53, so that the jet pump 53 makes a greater contribution to promoting the flow in the recirculation circuit 50, with the result that the performance of the gas delivery unit 52 decreases. At a fixed operating point, which is equivalent to a fixed flow of the fuel cell stack 101, the gas delivery unit 52 is regulated to a fixed speed. During a purge process with the purge valve 41 open, the hydrogen added through the hydrogen metering valve, which leads to an increase in the proportion of hydrogen in the recirculation circuit 50, also leads to a decrease in the performance of the gas delivery unit 51 due to the change in the density conditions.

[0034] The method according to the invention similarly provides for checking the performance of the gas delivery unit 52 in the recirculation circuit 50 after and during the closing of the purge valve 41, or as soon as the purge valve 41 is closed. A change in the performance of the gas delivery unit 52 in the recirculation circuit 50 can be used to verify whether the purge valve is open.

[0035] The closing of the purge valve 41 can be verified by an increase in the performance of the gas delivery unit 52.

[0036] The performance of the gas delivery unit 52 increases when the purge valve 41 is closed due to the decreasing propellant mass flow in the jet pump 53 and the change in the proportion of hydrogen in the recirculation circuit 50.

[0037] In both cases, i.e., if the opening or closing of the purge valve 41 has not been verified, an error message is generated. Within the scope of the error message, a control unit 70 can initiate measures to implement the operating strategy for emergency operation without a purge process. For example, instead of purging via the purge valve 41, a purge process can be initiated via a water reservoir with a drain valve. An error message can also be sent via a communication interface to a user of a vehicle in which the fuel cell system is installed.

[0038] After a verified opening of the purge valve 41, a functional test of the hydrogen sensor 63 in the exhaust gas line 12 of the cathode 105 can be carried out, since the verified opening of the purge valve 41 ensures that the anode gas with an increased hydrogen concentration flows from the recirculation circuit 50 via the purge line 40 into the exhaust gas line 12.

[0039] The anode gas from the recirculation line 50 mixes with the exhaust gas from the exhaust line 12 and leads to an increased hydrogen concentration at the hydrogen sensor 63, which is reflected by a change in the measurement signal, in particular the measured hydrogen concentration, at the hydrogen sensor 63. Consequently, the functional test of the hydrogen sensor 63 is performed by checking the hydrogen concentration measured by the hydrogen sensor 63. When checking the hydrogen concentration after a verified opening of the purge valve, the hydrogen concentration at the hydrogen sensor 63 should increase.Between the time of opening of the purge valve 41 and the time of measurement with increased hydrogen concentration at the hydrogen sensor 63 there is a short time interval dt, which is determined by the length of the line section consisting of parts of the purge line 40 and the exhaust gas line 12 between the purge valve 41 and the hydrogen sensor 63.

[0040] An error message may occur if the hydrogen sensor 63 does not measure any change in the hydrogen concentration after a verified opening of the purge valve 41.

[0041] In the same way, the functional test of the hydrogen sensor 63 can be performed upon verified closure of the purge valve 41. When checking the hydrogen concentration after verified closure of the purge valve 41, the hydrogen concentration at the hydrogen sensor 63 should decrease. Between the time of closing the purge valve 41 and the time of the measurement with a low hydrogen concentration at the hydrogen sensor 63, there is a short time interval dt, which is determined by the length of the line section consisting of parts of the purge line 40 and the exhaust line 12 between the purge valve 41 and the hydrogen sensor 63.

[0042] An error message may occur if the hydrogen sensor 63 does not measure any change in the hydrogen concentration after a verified closing of the purge valve 41.

[0043] During the functional test of the hydrogen sensor 63, the mass flow of the exhaust gas in the exhaust line 12 should be kept constant so that a change in the hydrogen concentration occurs solely due to the purging process. The mass flow of the exhaust gas in the exhaust line 12 can be achieved by appropriately controlling the air compressor 11 and / or compressor 11.

[0044] In the Fig. Figure 2 shows a schematic flow of the method according to the invention based on measurement results. Four diagrams are shown, each showing a progression over the same time interval.

[0045] Diagram A describes a desired state of the purge valve 41. State 0 describes a closed purge valve 41, while state 1 describes an open purge valve 41.

[0046] Diagram B describes the actual state of the purge valve 41. State 0 describes a closed purge valve 41, while state 1 describes an open purge valve 41.

[0047] Diagram C shows the change in the performance of the gas delivery unit 52 over time in dimensionless form.

[0048] Diagram D shows the signal of a hydrogen sensor 63, which represents the change in the hydrogen concentration in the exhaust air line 12 in dimensionless form.

[0049] The first event depicted along the timeline t involves the successful opening of the purge valve 41. At time t0, which corresponds to the time at which the purge valve 41 opens, the power of the gas delivery unit 53 drops. Thus, by observing the power of the gas delivery unit, it can be verified whether the purge valve 41 has been opened. The hydrogen concentration at the hydrogen sensor 63 rises at time t1, as shown in diagram D. There is a short time delay between the drop in power and the rise in the hydrogen concentration, which is caused by the length of the purge line 41 and exhaust air line 12 between the purge valve 41 and the hydrogen sensor 63.

[0050] At time t2, which corresponds to the time of opening of the purge valve 41, the power of the gas delivery unit 52 increases.

[0051] In the second event shown along the timeline t, the purge valve 41 does not open, although this was intended, which can be seen from the desired state and actual state of the purge valve 41 in diagrams A and B.

[0052] At time t3, which corresponds to the time of the deliberate opening of the purge valve 41, the power of the gas delivery unit increases further, since the density of the anode gas continues to increase, since no anode gas is discharged into the exhaust gas line 12 through the purge line 40.

Claims

[1] Method for operating a fuel cell system (1), in which at least one fuel cell (101) is supplied with hydrogen from a tank (21) and recirculated hydrogen from a recirculation circuit (50) as anode gas via a fuel line (20), and in which anode gas is removed from the recirculation circuit (50) by temporarily opening a purge valve (41), characterized by that the following steps are carried out: - Opening or closing the purge valve (41); - checking the performance of a gas delivery unit (52) in the recirculation circuit (50); - Verifying the opening or closing of the purge valve (41) by means of a change in the performance of the gas delivery unit (52) in the recirculation circuit (50). [2] Method according to claim 1, characterized by that the opening of the purge valve (41) is verified by a drop in the performance of the gas delivery unit (52). [3] Method according to claim 1, characterized by that the closing of the purge valve (41) is verified by an increase in the performance of the gas delivery unit (52). [4] Method according to one of the preceding claims, characterized by that an error message is generated if the opening or closing of the purge valve (41) is not verified. [5] Method according to one of claims 1 to 3, characterized by that after a verified opening or a verified closing of the purge valve (41), a functional test of a hydrogen sensor (63) in an exhaust line (12) of the cathode (105) is carried out. [6] Method according to claim 5, characterized by that the functional test of the hydrogen sensor (63) is carried out by checking the hydrogen concentration measured by the hydrogen sensor (63). [7] Method according to claim 6, wherein an error message is issued if the hydrogen sensor (63) does not measure a change in the hydrogen concentration after a verified opening or a verified closing of the purge valve (41). [8] Method according to one of claims 5 to 7, characterized by that during the functional test of the hydrogen sensor (63) a mass flow of the exhaust gas in the exhaust line (12) is kept constant. [9] Control device (70) which is arranged to carry out steps of the method according to one of the preceding claims.

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