Method of operating a fuel cell system, control device

Abstract

The invention relates to a method of operating a fuel cell system (1), in which a fuel cell stack (2) is supplied via an anode circuit (3) of an anode subsystem (4) with an anode gas containing hydrogen from a tank (5) and recirculated anode gas, and in which recirculated anode gas is removed from the anode circuit (3) from time to time by opening an electromagnetically actuatable purge valve (6) integrated into the anode circuit (3) and is replaced by hydrogen from the tank (5). According to the invention, the composition of the anode gas is determined indirectly by actuating the purge valve (6), and the composition of the anode gas is inferred from the progression of a current or voltage signal of the actuation. The invention further relates to a control unit for a fuel cell system for executing the steps of a method according to the invention.

Description

[0001] R.414553

[0002] - 1 -

[0003] Description

[0004] Method for operating a fuel cell system, control unit

[0005] The invention relates to a method for operating a fuel cell system according to the preamble of claim 1. Furthermore, a control unit for executing steps of the method is proposed.

[0006] State of the art

[0007] Hydrogen-based fuel cell systems are considered a mobility concept of the future, as they essentially emit only water and allow for rapid refueling. The hydrogen is stored in a tank carried on board the vehicle. The required oxygen is extracted from the ambient air. In the fuel cell, hydrogen and oxygen react to form water or water vapor. Simultaneously, electrical power is generated through electrochemical conversion. To increase power output, numerous individual cells are stacked and connected to form a fuel cell stack.

[0008] Hydrogen is supplied to the anode side of the stack via an anode circuit of an anode subsystem. Anode gas exiting the stack, which still contains hydrogen, is recirculated through the anode circuit. This recirculation is achieved passively using a jet pump and / or actively using a recirculation fan. Due to diffusion processes, the recirculated anode gas becomes enriched with nitrogen over time. This reduces the hydrogen concentration in the anode gas, which, in addition to a reduced voltage and thus a reduced efficiency, can also lead to hydrogen depletion. The latter, in turn, can cause irreversible damage.

[0009] To prevent this, the anode circuit is purged periodically. For this purpose, a valve, the so-called purge valve, is opened, through which anode gas is discharged from the anode circuit. The discharged amount is replaced with fresh R.414553.

[0010] - 2 -

[0011] Hydrogen from the tank is replaced. The opening time and duration of the purge valve are generally based on empirical data and are stored in the system's current-dependent purge strategy. However, this heuristic control of the purge processes can lead to either too infrequent purging, resulting in hydrogen depletion, or too frequent purging, leading to increased hydrogen consumption. This is because hydrogen is removed from the anode circuit along with the anode gas during each purge.

[0012] A feedback-controlled purge strategy can prevent purging that is too infrequent or too frequent. However, this requires knowledge of the current hydrogen concentration in the anode gas. A hydrogen sensor can be used to measure the hydrogen concentration in the anode gas. However, these sensors are expensive and / or do not meet the requirements for compatibility with hydrogen and deionized water.

[0013] The invention addresses the objective of providing an operating strategy that enables the determination of the anode gas composition even without a hydrogen sensor, thus allowing for a feedback-controlled purge strategy. This aims to reduce both the risk of hydrogen depletion and the hydrogen consumption during the operation of a fuel cell system.

[0014] To solve the problem, the method with the features of claim 1 is proposed. Advantageous embodiments of the invention are described in the dependent claims. Furthermore, a control unit for executing process steps is specified.

[0015] Disclosure of the invention

[0016] In the proposed method for operating a fuel cell system, an anode gas containing hydrogen from a tank and recirculated anode gas is supplied to a fuel cell stack via an anode circuit of an anode subsystem. In this method, recirculated anode gas is periodically removed from the anode circuit by opening an electromagnetically controlled purge valve integrated into the anode circuit and replaced with hydrogen from the tank. According to the invention, the composition is indirectly determined using R.414553.

[0017] - 3 - the purge valve is controlled by the anode gas and the composition of the anode gas is inferred from the course of a current or voltage signal of the control.

[0018] The method is based on the understanding that when the purge valve is actuated by the stroke of a valve armature, a counter-voltage is generated in a solenoid coil of the purge valve. This voltage exhibits a characteristic curve depending on the current composition of the anode gas. The curve changes when the valve armature completes its flight phase and reaches a stop. From this change in the curve, conclusions can be drawn about the ballistic behavior of the purge valve and thus—indirectly—about the composition of the anode gas.

[0019] To carry out the process, a constant or a variable current can be applied to the purge valve. For example, a multi-phase current profile can be used to actuate the purge valve. This profile could include, for instance, a pull-in phase at a high current level and a holding phase at a reduced current level.

[0020] The indirect determination of the anode gas composition using the proposed method requires no additional sensor, particularly not an expensive hydrogen sensor, thus saving costs. Instead, the proposed method creates a kind of virtual sensor that takes over the function of a hydrogen sensor. The indirectly determined anode gas composition, in turn, enables a feedback-controlled purge strategy that reduces hydrogen consumption and prevents damage to the fuel cells of the fuel cell stack due to hydrogen depletion. The proposed method therefore also has a positive impact on the service life of the fuel cell stack and thus the fuel cell system as a whole.

[0021] Preferably, to indirectly determine the composition of the anode gas, the current or voltage signal is compared with previous curves, each characteristic of a specific anode gas composition. This comparison allows for a simple conclusion to be drawn about the current anode gas composition. The previous curves can be viewed during the operation of the R.414553.

[0022] - 4 -

[0023] The fuel cell system data may have been recorded, analyzed, and stored. Alternatively or additionally, previous data from the fuel cell system prior to commissioning, for example on a test bench, may have been recorded, analyzed, and stored.

[0024] Due to the nature of the process, the anode gas composition changes during a purge, which also affects the ballistic behavior of the purge valve and thus the current or voltage signal. This increases the complexity of indirectly determining the anode gas composition.

[0025] In a further development of the invention, it is therefore proposed that, for the indirect determination of the anode gas composition, the current or voltage signal is evaluated using a model that has been previously trained, preferably by machine learning, by evaluating various current or voltage signal profiles. Machine learning, in particular, allows for the processing of large amounts of data, which increases the model's accuracy. During operation of the fuel cell system, the model can be further trained through additional evaluations.

[0026] Advantageously, a model is used that has been previously trained on a test bench. Optimal conditions can be created on the test bench to train the model.

[0027] Preferably, the actual composition of the anode gas is measured, for example using a mass spectrometer, and the measured composition is used to verify the model. In this case, the actually measured composition of the anode gas serves as the "ground truth" when training the model.

[0028] The ballistic behavior of the purge valve is generally influenced by other factors, particularly the pressure differential across the purge valve, the gas density, and the gas viscosity. When the purge valve opens, the valve armature moves against the anode gas, and the anode gas flows past the armature and out through a valve seat. This creates opposing forces acting on the armature, which are counteracted by the opening force of the R.414553.

[0029] - 5 -

[0030] The force must be contained by the purge valve. The balance of forces between the opening force and the opposing forces determines the ballistic behavior of the purge valve.

[0031] In a further development of the invention, it is therefore proposed that, in the indirect determination of the composition of the anode gas, the pressure difference across the purge valve and / or the anode gas temperature be taken into account as a further parameter in order to further increase the accuracy of the method.

[0032] The steps for indirectly determining the composition of the anode gas are preferably performed before a purge process. Before a purge, the purge valve can be opened only briefly to indirectly determine the composition of the anode gas. This brief opening has the advantage that the composition of the anode gas changes only minimally, as little anode gas can escape from the anode circuit. The current or voltage signal can thus be assigned to a specific anode gas composition with high accuracy.

[0033] Alternatively or additionally, it is suggested that the steps for indirectly determining the composition of the anode gas be carried out several times in succession. Repeating these steps can further increase the accuracy of the method.

[0034] Furthermore, a control unit for a fuel cell system is proposed, wherein the control unit is configured to execute steps of a method according to the invention. In particular, the purge valve can be controlled with the aid of the control unit in order to open it. For this purpose, a current or a voltage is applied to a solenoid coil of the purge valve. When the purge valve opens, the composition of the anode gas can be indirectly inferred from the course of a current or voltage signal of the control signal using the control unit. For example, a back voltage induced in the solenoid coil when the purge valve opens can be detected with the aid of the control unit and evaluated with regard to the composition of the anode gas. A model can be stored in the control unit for this purpose, which has been previously trained on a test bench, preferably by means of machine learning. R.414553

[0035] - 6 -

[0036] The inventive method and its advantages are explained in more detail below with reference to the accompanying drawings. These show:

[0037] Fig. 1 shows a schematic representation of a fuel cell system that can be operated according to the method according to the invention, and

[0038] Fig. 2 shows a schematic representation of a control unit for executing steps of a method according to the invention.

[0039] Detailed description of the drawings

[0040] Figure 1 shows a fuel cell system 1 comprising a fuel cell stack 2 with an anode 2.1 and a cathode 2.2. During operation of the fuel cell system 1, an anode gas is supplied to the anode 2.1 via an anode circuit 3 of an anode subsystem 4. This gas contains hydrogen from a tank 5, which is fed into the anode circuit 3 via a hydrogen line 7 with an integrated jet pump 8. A pressure regulator 9 is connected upstream of the jet pump 8 for pressure control. Since the anode gas exiting the anode 2.1 still contains hydrogen, the anode gas is recirculated via the anode circuit 3. In this case, the recirculation is achieved passively with the aid of the jet pump 8 and actively with the aid of a pump 10 integrated into the anode circuit 3. The cathode 2.2 of the fuel cell stack 2 is supplied with air via an air supply path 11. The exhaust air exiting the cathode 2.2 is discharged via an exhaust air path 12.The supply air path 11 and the exhaust air path 12 can be separated from the fuel cell stack 2 by means of shut-off valves 13.

[0041] Heat is generated during the operation of the fuel cell stack 2. To dissipate this heat, the fuel cell stack 2 is connected to a cooling circuit 14. A coolant pump 15 and a cooler 16 are integrated into the cooling circuit 14.

[0042] In the electrochemical reaction in the fuel cell stack 2, in addition to electrical energy and heat, water, the so-called product water, is also produced. This is separated from the recirculated anode gas by means of a water separator 17 of the anode subsystem 4 and collected in a container 18. When the container 18 is full, it is emptied by opening a drain valve 19. R.414553

[0043] - 7 -

[0044] Over time, the recirculated anode gas becomes enriched with nitrogen, which primarily migrates from the cathode side to the anode side via diffusion processes. To remove the nitrogen, the anode circuit 3 is purged periodically. For this purpose, the anode subsystem 4 has a purge valve 6 through which anode gas is released from the anode circuit 3. The released gas is replaced with hydrogen from tank 5. Since the gas released via purge valve 6 always contains hydrogen, purging should not be performed too frequently, as this would increase hydrogen consumption. However, if purging is performed too infrequently, hydrogen depletion can occur, potentially damaging the fuel cells of the fuel cell stack 2. Therefore, a purge strategy must be found that resolves this conflicting objective.A suitable approach would be a feedback-controlled purge strategy that regulates purge processes depending on the composition of the anode gas in anode circuit 3. However, this requires knowledge of the anode gas composition.

[0045] Knowledge of the composition of the anode gas, particularly its hydrogen content, can be obtained using a hydrogen sensor. However, this is expensive. Therefore, the proposed method according to the invention uses a virtual sensor instead of a hydrogen sensor.

[0046] The virtual sensor can be implemented, for example, using a control unit 20 of the fuel cell system 1. Such a control unit 20 is shown schematically in Figure 2.

[0047] Control unit 20 contains a model that is trained on a test bench using machine learning before the fuel cell system 1 is put into operation. The training data consists of x, y, and z. The x data can, in particular, represent the course of a back EMF induced in a solenoid coil of the purge valve 6 when activated. The back EMF induced in the solenoid coil depends on the ballistic behavior of the purge valve 6 when activated, and this, in turn, depends on the composition of the anode gas in the anode circuit 3. Thus, the composition of the anode gas can be deduced from the induced back EMF. The other data, y and z, can, for example, be R.414553.

[0048] - 8 -

[0049] The pressure difference across purge valve 6 (data y) and the anode gas temperature (data z) are relevant, as these parameters also influence the flight behavior of a valve armature when purge valve 6 opens. The composition of the anode gas determined using the model can then be verified by measuring the actual composition, for example by mass spectrometry, and comparing it with the composition output by the model.

[0050] After training and commissioning of fuel cell system 1, the composition of the anode gas in anode circuit 3 can be indirectly determined using the trained model, which is provided with the same data x, y, and z. During operation of fuel cell system 1, the model can be further trained using the indirectly determined compositions when specific data x, y, and z are available, thus increasing the model's accuracy over time.

Claims

R.414553 - 9 - Claims 1. A method for operating a fuel cell system (1) in which an anode gas containing hydrogen from a tank (5) and recirculated anode gas is supplied to a fuel cell stack (2) via an anode circuit (3) of an anode subsystem (4), and in which recirculated anode gas is removed from the anode circuit (3) from time to time by opening an electromagnetically controlled purge valve (6) integrated into the anode circuit (3) and replaced by hydrogen from the tank (5), characterized in that the purge valve (6) is controlled to indirectly determine the composition of the anode gas and the composition of the anode gas is inferred from the course of a current or voltage signal of the control.

2. Method according to claim 1, characterized in that, for the indirect determination of the composition of the anode gas, the course of the current or voltage signal is compared with previous courses, each of which is characteristic of a specific composition of the anode gas.

3. Method according to claim 1 or 2, characterized in that, for the indirect determination of the composition of the anode gas, the course of the current or voltage signal is evaluated using a model which has been trained in advance, preferably by machine learning, by evaluating various courses of current or voltage signals.

4. Method according to claim 3, characterized in that a model is used which has been previously trained on a test bench. R.414553 - 10 - 5. Method according to claim 3 or 4, characterized in that the actual composition of the anode gas is measured, for example using a mass spectrometer, and the measured composition is used to verify the model.

6. Method according to one of the preceding claims, characterized in that, in the indirect determination of the composition of the anode gas, the pressure difference across the purge valve (6) and / or the anode gas temperature is / are taken into account as a further parameter.

7. Method according to one of the preceding claims, characterized in that the steps for indirectly determining the composition of the anode gas are carried out before a purging process.

8. Method according to one of the preceding claims, characterized in that the steps for indirectly determining the composition of the anode gas are carried out several times in succession.

9. Control unit for a fuel cell system, wherein the control unit is configured to perform steps of a method according to any of the preceding claims.

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