Method for operating a fuel cell system

Abstract

The invention relates to a method (100) for operating a fuel cell system (200), wherein the fuel cell system (200) comprises: - a fuel cell stack (201), - a DC-DC converter (203), and - a flyback converter (205), the fuel cell stack (201) being connected to the DC-DC converter (203) via a number of switching contacts (209, 211), and wherein the method (100) comprises: - starting (101) the fuel cell system (200), - activating (103) the flyback converter (205) in pulse-width-modulated operation with a predefined duty cycle and an output voltage (125) which exceeds a voltage (121) present at the fuel cell stack (201), - reducing (105) the output voltage of the flyback converter (205), and - closing (107) a switching contact (209, 211) between the DC-DC converter (203) and the fuel cell stack (201) if a difference between the output voltage (125) of the flyback converter (205) and the voltage (121) present at the fuel cell stack (201) lies in a predefined range.

Description

[0001] Description

[0002] title

[0003] Method for operating a fuel cell system

[0004] The presented invention relates to a method for operating a fuel cell system, a fuel cell system and a program product according to the appended claims.

[0005] State of the art

[0006] Fuel cell systems convert oxygen and hydrogen into water and electrical energy. The electrical energy is typically transferred to a load, such as a battery or a vehicle's powertrain, via a DC / DC converter.

[0007] The DC-DC converter is separated from a fuel cell stack of the fuel cell system by means of switchable contacts, ie, switching contacts, when the fuel cell stack is inactive.

[0008] During a fuel cell system start-up procedure, a check of the electrical contact between the fuel cell stack and the DC-DC converter is typically performed for safety reasons. To detect an open switch contact, a minimum voltage difference is required between the respective contact points of the contact.

[0009] To prevent high currents and the resulting damage to the switching contact when the contact is closed, an input capacitor of the DC-DC converter must be electrically charged, for example, by a flyback converter, to the voltage applied to the fuel cell stack. After the switching contact is closed, the DC-DC converter begins to conduct electrical energy from the fuel cell stack to the load, thereby preventing a damaging overvoltage on the fuel cell stack.

[0010] However, at very low voltage ranges, it is not possible to regulate the voltage of the flyback converter. However, it is necessary to close the switching contact at the lowest possible voltage to prevent a rapid increase in the voltage across the fuel cell stack and to protect the switching contact from damage.

[0011] Disclosure of the invention

[0012] Within the scope of the invention presented, methods for operating a fuel cell system, a fuel cell system, and a program product 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 method according to the invention naturally also apply in connection with the fuel cell system according to the invention or the program product 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.

[0013] The invention presented serves in particular to provide a possibility for quickly and safely starting a fuel cell system.

[0014] Thus, according to a first aspect of the invention presented, a method for operating a fuel cell system is presented.

[0015] The presented fuel cell system comprises a fuel cell stack, a DC-DC converter and a flyback converter, wherein the fuel cell stack is connected to the DC-DC converter via a number of switching contacts and wherein the method comprises starting the fuel cell system, activating the flyback converter in a pulse-width modulated operation with a predetermined duty cycle and an output voltage that exceeds a voltage applied to the fuel cell stack, reducing the output voltage of the flyback converter and closing a switching contact between the DC-DC converter and the fuel cell stack when a difference between the output voltage of the flyback converter and the voltage applied to the fuel cell stack is within a predetermined range.

[0016] In the context of the invention presented, a flyback converter is understood to be a boost / buck converter or a so-called “flyback converter”.

[0017] The presented invention is based on a flyback converter which is operated in pulse width modulated mode at a low initial output voltage, so that a control circuit for regulating the output voltage of the flyback converter can be dispensed with.

[0018] Instead of a time-consuming adjustment process, the flyback converter, which immediately provides an output voltage higher than the voltage applied to the fuel cell stack after or upon activation, is adjusted in such a way that the flyback converter's output voltage decreases again after reaching a technically determined maximum, so that the difference between the flyback converter's output voltage and the voltage applied to the fuel cell stack ultimately lies within a specified range. In particular, the maximum corresponds to the smallest possible voltage technically determined by starting the flyback converter.

[0019] This means that the overshoot of the output voltage of the flyback converter is compensated by the presented method by reducing the output voltage again until it corresponds, for example, to the voltage applied to the fuel cell stack and the switching contact can be closed without load.

[0020] Accordingly, the invention presented here avoids a race between the output voltage of the flyback converter and the voltage applied to the fuel cell stack. Furthermore, the invention presented here enables rapid closing of the switching contact at correspondingly low voltages, thus minimizing the load on the switching contact.

[0021] It can further be provided that the method comprises determining a point in time at which the difference between the output voltage of the flyback converter and the voltage applied to the fuel cell stack lies in the predetermined range, by means of a mathematical model of the fuel cell system, and initiating the closing of the switching contact at a predetermined lead time before the determined point in time.

[0022] By predicting a time at which the difference between the flyback converter's output voltage and the voltage applied to the fuel cell stack falls within a specified range, the latency required by a switching contact to execute a closing command can be compensated. To do this, the closing command is triggered at a specified lead time before the determined time. For example, the specified lead time corresponds to the latency of the switching contact.

[0023] It may further be provided that the flyback converter is deactivated to reduce the output voltage of the flyback converter.

[0024] Deactivating the flyback converter prevents the flyback converter's output voltage from increasing due to its transformation power and initiates a drop in the output voltage.

[0025] It can be provided that the flyback converter is coupled to an electrical resistor to reduce the output voltage.

[0026] By coupling the flyback converter to an electrical resistor, the outflow of electrical charge from the flyback converter and, accordingly, a drop in the output voltage of the flyback converter is accelerated. The fuel cell system can further be provided with two switching contacts between the fuel cell stack and the DC-DC converter, wherein a first switching contact is closed when a difference between the output voltage of the flyback converter and the voltage applied to the fuel cell stack is within a predetermined range, and a second switching contact is closed at a fixed time after the fuel cell system is started.

[0027] By using two switching contacts, efficient voltage flow from the fuel cell stack to the DC-DC converter can be achieved. A first switching contact can be closed at a predetermined time, independently of the second switching contact.

[0028] It can also be provided that the output voltage of the flyback converter is between 10 volts and 30 volts.

[0029] Due to the method according to the invention, in which the output voltage of the flyback converter is reduced again after reaching a technically determined maximum, the initial output voltage can be kept particularly low.

[0030] Due to the particularly low initial output voltage of the flyback converter, the output voltage of the flyback converter and the voltage applied to the fuel cell stack can be brought particularly quickly into a range in which they correspond to one another or a difference between the output voltage of the flyback converter and the voltage applied to the fuel cell stack is smaller than the specified switching threshold value.

[0031] It may further be provided that the method is carried out as part of a checking procedure of switching contacts between the DC-DC converter and the fuel cell stack during a start-up procedure of the fuel cell system.

[0032] A routinely performed checking procedure of switching contacts between the DC-DC converter and the fuel cell stack during a start-up procedure of the fuel cell system can be accelerated by the presented method, so that the entire start-up procedure of the fuel cell system is accelerated.

[0033] It may further be provided that the flyback converter is supplied with electrical energy by a low-voltage source.

[0034] Since the flyback converter can be deactivated early and at a correspondingly low output voltage due to the presented method, the flyback converter can be supplied with electrical energy by a low-voltage source, such as a 12-volt battery, so that the flyback converter can be activated and deactivated independently of an operating point of the fuel cell stack.

[0035] It can further be provided that an input capacitor of the DC-DC converter is electrically charged by the output voltage of the flyback converter.

[0036] By charging the input capacitor of the DC-DC converter, the DC-DC converter is put into an operational state so that when the flyback converter is deactivated, the DC-DC converter can be activated directly.

[0037] According to a second aspect, the presented invention relates to a fuel cell system for converting energy.

[0038] Advantages described in detail for the 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.

[0039] The proposed fuel cell system comprises a fuel cell stack, a DC-DC converter, a flyback converter, and a computing unit, wherein the computing unit is configured to carry out a possible embodiment of the proposed 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] According to a third aspect, the presented invention relates to a program product, wherein the program product comprises program code means that configure a computing unit to carry out a possible embodiment of the presented method when the program product is executed on the computing unit.

[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 schematically:

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

[0044] Figure 2 shows a detailed representation of the method according to Figure 1, and

[0045] Figure 3 shows a possible design of the presented fuel cell system.

[0046] Fig. 1 shows a method 100 for operating a fuel cell system 200 according to Fig. 3.

[0047] The method 100 includes a start step 101, in which the fuel cell system is started, for example, in response to a start command. Accordingly, a start procedure is initiated.

[0048] Furthermore, the method comprises an activation step 103 in which the flyback converter of the fuel cell system 200 is activated in a pulse-width modulated operation with a predetermined duty cycle, a reduction step 105 in which the output voltage of the flyback converter is reduced after reaching a maximum and a closing step 107 in which a first switching contact of the fuel cell system 200 is closed when a difference between the output voltage of the flyback converter and the voltage applied to the fuel cell stack is within a predetermined range, ie, for example, is less than a predetermined switching threshold value.

[0049] In Fig. 2, a sequence of the method 100 is shown using a diagram 120, which spans a voltage in [V] on its ordinate and a time in [s] on its abscissa.

[0050] A first curve 121 corresponds to a voltage in [V] applied to the fuel cell stack 201.

[0051] A second curve 123 corresponds to an electrical current flow in [A] provided by the fuel cell stack 201.

[0052] A third waveform 125 corresponds to an output voltage of the flyback converter 205, i.e., a so-called “flyback voltage”.

[0053] At a time T0 at 0s, the fuel cell system 200 is started, it heats up and the first curve 121 of the voltage applied to the fuel cell stack 201 begins to rise slowly.

[0054] At a time T1, at approximately 2.9s, a checking procedure for checking a state of the first switching contact 209 and the second switching contact 211 is started.

[0055] At a third time T2, at approximately 3 s, the flyback converter 205 is activated in pulse-width modulated operation with a constant duty cycle, resulting in an output voltage of approximately 24 volts, while the voltage 121 applied to the fuel cell stack at time T2 is lower than the initial output voltage and is approximately 21 volts. For example, the second switching contact 211 can be closed at time T2, since 3 seconds have passed since the start at T0.

[0056] At a fourth time T3, at approximately 3.1 s, the output voltage of flyback converter 125 reaches its maximum 127 and flyback converter 205 is deactivated. The output voltage of flyback converter 125 begins to decrease, which is accelerated by coupling flyback converter 125 to resistor 213.

[0057] Meanwhile, the voltage 121 applied to the fuel cell stack continues to rise.

[0058] By means of a mathematical model of the fuel cell system 200, for example based on the gradients of the curves 121 and 125, a fifth time T4 can be determined at which a difference between the output voltage of the flyback converter 125 and the voltage 121 applied to the fuel cell stack lies in a predetermined range, in particular the output voltage of the flyback converter 125 and the voltage 121 applied to the fuel cell stack correspond to one another.

[0059] Accordingly, the closing of the first switching contact 209 can already be initiated at a closing time that lies before the time determined by means of the mathematical model, so that the first switching contact 209 provides an electrical contact between the fuel cell stack 201 and the DC-DC converter 203 precisely when the output voltage of the flyback converter 125 and the voltage 121 applied to the fuel cell stack correspond to one another.

[0060] At the time determined by the mathematical model, the flyback converter 205 is already deactivated and electrical current flows from the fuel cell stack 201 to the DC-DC converter 203.

[0061] Fig. 3 shows a fuel cell system 200. The fuel cell system 200 comprises a fuel cell stack 201, a DC-DC converter 203, a flyback converter 205, and a processing unit 207. The DC-DC converter 203 and the fuel cell stack 201 can be reversibly electrically coupled via a first switching contact 209 and a second switching contact 211.

[0062] The computing unit 207 is configured to execute the method 100 to close the first switching contact 209 and the second switching contact 211 during a startup procedure of the fuel cell system 200. The flyback converter 205 is supplied with input voltage by a low-voltage source 213, such as a 12-volt battery.

Claims

Claims 1. A method (100) for operating a fuel cell system (200), the fuel cell system (200) comprising: - a fuel cell stack (201), - a DC-DC converter (203), - a flyback converter (205), wherein the fuel cell stack (201) is connected to the DC-DC converter (203) via a number of switching contacts (209, 211), the method (100) comprising: - Starting (101) the fuel cell system (200), - activating (103) the flyback converter (205) in a pulse-width modulated operation with a predetermined duty cycle and an output voltage (125) that exceeds a voltage (121) applied to the fuel cell stack (201), - Reducing (105) the output voltage of the flyback converter (205), - closing (107) a switching contact (209, 211) between the DC-DC converter (203) and the fuel cell stack (201) when a difference between the output voltage (125) of the flyback converter (205) and the voltage (121) applied to the fuel cell stack (201) is within a predetermined range.

2. Method (100) according to claim 1, characterized in that the method (100) further comprises: - determining a point in time at which the difference between the output voltage (125) of the flyback converter (205) and the voltage applied to the fuel cell stack (201) lies in the predetermined range, by means of a mathematical model of the fuel cell system (200), - initiating the closing of the switching contact (209, 211) at a predetermined lead time before the determined time.

3. Method (100) according to claim 1 or 2, characterized in that the flyback converter (205) is deactivated to reduce the output voltage (125) of the flyback converter (205).

4. Method (100) according to one of the preceding claims, characterized in that the flyback converter (205) is coupled to an electrical resistor for reducing the output voltage (125).

5. Method (100) according to one of the preceding claims, characterized in that the fuel cell system (200) comprises two switching contacts (109, 211) between the fuel cell stack (201) and the DC-DC converter (203), wherein a first switching contact (205) is closed when a difference between the output voltage (125) of the flyback converter (205) and the voltage (121) applied to the fuel cell stack (201) is within a predetermined range and a second switching contact (211) is closed at a fixed time after starting (101) of the fuel cell system (200).

6. Method (100) according to one of the preceding claims, characterized in that the method (100) is carried out as part of a checking procedure of switching contacts (209, 211) between the DC-DC converter (203) and the fuel cell stack (201) during a start-up procedure of the fuel cell system (200).

7. Method (100) according to one of the preceding claims, characterized in that the flyback converter (205) is supplied with electrical energy by a low-voltage source.

8. Method (100) according to one of the preceding claims, characterized in that an input capacitor of the DC-DC converter (203) is electrically charged by the output voltage (125) of the flyback converter (205).

9. Fuel cell system (200) for converting energy, the fuel cell system (200) comprising: - a fuel cell stack (201), - a DC-DC converter (203), - a flyback converter (205), - a computing unit (207), wherein the computing unit (207) is configured to carry out a method (100) according to one of claims 1 to 8.

10. A program product, wherein the program product comprises program code means that configure a computing unit to execute a method (100) according to any one of claims 1 to 8 when the program product is executed on the computing unit.

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