Matching circuit for a fuel cell system of a vehicle, drive system and method for operation of a or the drive system

The invention relates to a matching circuit for a fuel cell system of a vehicle, having an input interface for connection of an output voltage of a fuel cell arrangement of the fuel cell system and having a drive output interface for supplying a power train for a drive motor with a drive voltage. The invention also relates to a corresponding drive system and to a method for operation of the drive system.

Fuel cell systems use an electrochemical process to react a fuel, generally hydrogen, with an oxidant , generally environmental air, and during the course of the process convert chemical energy to electrical energy which, for example, is made available as drive energy to a vehicle. In addition to supplying the vehicle with drive energy, some of the converted energy is normally used for the vehicle's own supply, so that peripheral components such as compressors, heaters, valves or the like, are likewise supplied with the energy produced by the fuel cell system.

Every fuel cell system has a plurality of fuel cells, generally more than 100, which are normally connected in ser ies such that a fuel cell arrangement formed by the plurality of fuel cells produces an output voltage of more than 100 V.

The individual fuel cells each contribute a cell voltage of between about 0.2 V and 0.7 V to this output voltage, with the cell voltage being heavily dependent on the operating state of the fuel cells. In consequence, the output voltage of the fuel cell arrangement also fluctuates severely as a function of the operating state of the fuel cells. In order to compensate for this fluctuation, matching circuits are known which convert the fluctuating output voltage from the fuel cell arrangement so as to produce a DC voltage which is higher than a minimum required operating voltage for the vehicle drive motor and for a supply bus for the peripheral components .

By way of example, the document US 6,428,918 Bl discloses a fuel cell supply system which has a converter which converts the energy produced by a fuel cell arrangement at a fluctuating output voltage so as to produce the output energy at an essentially constant voltage amplitude.

The document PAJ 20032003663 A (publication number) , which in fact represents the closest prior art, likewise discloses a fuel cell supply apparatus having a DC/DC converter which is driven by a control device and is connected so as to produce a predetermined output value at the output of the DC/DC converter.

The invention is based on the object of proposing a matching circuit for a fuel cell system, a drive system for a vehicle, and a method for operation of a drive system for a vehicle which allows the fuel cell system to be operated economically.

This object is achieved by a matching circuit for a fuel cell system having the features of Claim 1, a drive system for a

vehicle having the features of Claim 10, and by a method for operation of a drive system for a vehicle having a fuel cell system with the features of Claim 11. Preferred or advantageous embodiments of the invention are specified in the dependent claims, the following description and/or the figures .

According to the invention, a matching circuit and an apparatus having this matching circuit are proposed, which are suitable and/or are designed for a fuel cell system of a vehicle .

On the input side, the matching circuit has an input interface which is designed, connected and/or can be connected for connection of a fuel cell arrangement of the fuel cell system, with an output voltage from the fuel cell arrangement being produced at the input interface during operation of the fuel cell system. The fuel cell arrangement preferably comprises one or more fuel cell stacks, each having a plurality of fuel cells which, in particular, are operated with hydrogen from a tank or from a reformer. In particular, the output voltage of the fuel cell arrangement is defined as the common voltage of all the fuel cells in the fuel cell system and/or the fuel cell arrangement.

On the output side, the matching circuit has a drive output interface, which is designed, connected and/or can be connected for supplying a power train for a drive motor with a drive voltage. The power train preferably ends in a DC/AC converter, which converts the drive voltage to an AC voltage for the drive motor.

A supply output interface for the matching circuit is proposed within the scope of the invention, and is designed,

connected and/or can be connected for supplying a supply bus for peripheral components of the fuel cell arrangement with a supply voltage. The supply bus is, in particular, a high- voltage bus, preferably with a supply voltage of more than 150 V, in particular of more than 170 V, and in particular more than 180 V. The peripheral components of the fuel cell arrangement are used to supply the fuel cells with the fuels, for example fuel, oxidant and water, and/or to ensure operating conditions, such as the operating pressure or temperature of the process gases or fuels, and preferably comprise any desired selection of the following devices: compressors, valves, demoisturizers, moisturizers, fans, etc.

According to the invention, the matching circuit can be switched to a special operating mode in which the magnitude of the supply voltage is greater than the drive voltage.

In this special operating mode, the power train and the supply bus are supplied with different voltages, with the voltage difference preferably being maintained over a relatively long time period of at least 5 seconds, preferably at least 30 seconds, and in particular at least 2 minutes, and/or being greater than 5 V, preferably greater than 15 V, and in particular greater than 25 V.

One concept of the invention is that special operating modes of the fuel cell system can occur in which it is desirable to keep the operating voltages of the fuel cells and therefore the output voltage of the fuel cell arrangement low. For example, a special operating mode such as this occurs during cold starting or freezing starting of a fuel cell system, during which the fuel cell system is at a temperature considerably below the normal operating temperature. The problem in this special operating mode is to raise the fuel

cell system from the temperature below the normal operating temperature, in particular from below 0°C, to the normal operating temperature of, for example, 80 ⁰ C. This could be achieved by operating the fuel cells with a very low operating voltage, since the emitted heat or heat losses from the fuel cell arrangement are inversely proportional to the operating voltage of the fuel cells and the output voltage of the fuel cell arrangement. A matching circuit is normally used for this procedure, in order to convert the output voltage of the fuel cell arrangement to a voltage whose minimum amplitude is determined by the vehicle requirements.

In contrast, the invention has identified the fact that it is not necessary to convert all of the drive energy to a common voltage, but that it is advantageous to match the supply voltage for the supply bus and the drive voltage for the power train differently or independently of one another to the requirements of the respectively associated loads. Specifically, the drive motor is able to produce adequate torque at a lower drive voltage, in particular at low speeds.

It has therefore been found that - even when the output voltage of the fuel cell arrangement is low - the drive voltage need not necessarily be increased above the output voltage by means of a matching circuit. In consequence, it is possible in the special operating mode to operate the fuel cell arrangement at a low output voltage, for example of less than 50% or 30% of the maximum output voltage, and to match only the supply voltage for the peripheral components while, however, leaving the drive voltage at the output voltage level .

This leads to the advantage that the matching circuit for this special operating mode need be designed only to match

the voltage for a portion of the output power of the fuel cell arrangement. Since the majority of the output power from the fuel cell arrangement is passed to the power train, the matching circuit can be correspondingly small, and therefore cost-effective. For example, the matching circuit can be designed for less than 40%, preferably for less than 25%, and in particular for less than 10%, of the maximum power of the fuel cell arrangement.

In one preferred embodiment of the invention, the matching circuit is designed such that, in the special operating mode, the drive voltage corresponds to the output voltage of the fuel cell arrangement. In particular, the amplitudes averaged over time or smoothed in each case correspond. In this embodiment, no DC/DC converter with a boost functionality is preferably connected between the drive output interface and the input interface or between the power train and the fuel cell arrangement.

In the special operating mode, provision is preferably made for the drive voltage and/or the output voltage to be adequate to supply the drive motor for a driving mode. This condition makes it possible to reduce the voltage produced by a single fuel cell to values of less than 0.3 V, preferably of less than 0.2 V per cell. This means that, even in the special operating mode, it is possible for the vehicle to be moved by the energy in the fuel cell system in the driving mode. This at least allows an emergency driving mode, or operation at a restricted power level.

In one design embodiment, the matching circuit has a converter module which is designed and/or connected to convert the output voltage to the supply voltage. The converter module is preferably a DC/DC converter which, in

particular, is in the form of a DC controller, a DC/DC controller and/or a clocked voltage controller. In particular, the converter module is in the form of a boost converter or has a boost converter.

In one preferred embodiment, the converter module is associated exclusively with the supply bus, and the matching circuit has only the converter module which is associated exclusively with the supply bus.

In order, in particular, to allow switching from the special operating mode to a normal operating mode, the converter module is preferably itself in the form of a bridge and/or has a bypass which can be activated to bridge the converter module. A separate bypass which can be activated is preferably used when the converter module does not itself have a bridging function, as soon as the output voltage that is produced is greater than the required supply voltage for the supply bus.

In one preferred embodiment, the matching circuit has a control apparatus which is designed to activate the special operating mode. In particular, the special operating mode is activated as soon as the output voltage falls below a minimum supply voltage, which in particular is adjustable, for the supply bus. This minimum supply voltage is greater than or equal to the minimum voltage required by the peripheral components. By way of example, at least one or more or all of the peripheral components can be operated only via the minimum voltage requirement.

The control apparatus is preferably designed such that the special operating mode is activated as soon as cold or freezing starting of the fuel cell system is determined or

signalled. Cold or freezing starting occurs in particular when the operating temperature of the fuel cells is below the normal operating temperature, for example 80 ⁰ C, or in particular when the operating temperature is below 0 ⁰ C.

A further subject matter of the invention relates to a drive system for a vehicle having a fuel cell system which, according to the invention, has a matching circuit as has been described above, or according to one of the preceding claims. By definition, the matching circuit is connected to the fuel cell arrangement, to the power train and to the supply bus.

A further subject matter of the invention relates to a method for operation of a drive system for a vehicle having a fuel cell system in a special operating mode, in particular during cold or freezing starting, in which the matching circuit as described above and/or the drive system as described above and/or each of them are preferably used according to one of the preceding claims.

In the special operating mode, the magnitude of the operating voltage of a or the fuel cell arrangement is less than the minimum predetermined supply voltage for a or the supply bus for peripheral components. A power train for a or the drive motor is supplied with the output voltage from the fuel cell arrangement as the drive voltage and the supply bus is supplied with a converted output voltage as the supply voltage, with the magnitude of the converted output voltage being greater than the minimum supply voltage and/or than the drive voltage.

Further features, advantages and effects of the inventions will become evident from the following description of one

exemplary embodiment of the invention, and from the attached figures, in which:

Figure 1 shows a block diagram of a drive system in a vehicle, as one exemplary embodiment of the invention; and

Figure 2 shows the block diagram from Figure 1 with further, optional details.

Mutually corresponding components are provided with the same reference symbols in each of the two figures.

Figure 1 shows a highly schematic illustration of a vehicle 1 with a fuel cell system 2 and a power train 3. The power train 3 comprises a DC/AC converter 4, which converts a drive voltage UA in the form of a DC voltage to an AC voltage, and therefore supplies a drive motor 5 such that it can drive the vehicle 1 in the driving mode.

The fuel cell system 2 has a fuel cell arrangement 6 with a plurality of fuel cells 7 in one or more fuel cell stacks. The fuel cell arrangement 6 produces a voltage US at its power output.

A matching circuit 8 for the fuel cell system 2 is electrically conductively connected via input interfaces 9 to the fuel cell arrangement 6, in particular to the power output, such that the output voltage US from the fuel cell arrangement 6 is applied as an input voltage US to the matching circuit 8.

On the output side, the matching circuit 8 has supply output interfaces 10 where a supply bus 11 for peripheral components 12 is arranged such that this supply bus 11 is supplied with

a supply voltage UV. The supply bus 11 is, for example, in the form of a high-voltage bus at a voltage UV of more than 170 V. The peripheral components are also referred to by the expression BOP (Balance-Of-Plant ) and comprise components which are required during operation and supply the fuel cell arrangement 6, such as compressors, fans, heaters and the like.

The drive voltage UA is produced at a drive output interface 13 of the matching circuit, with the drive output interface 13 being conductively connected to the power train 3.

In a normal operating mode, the fuel cell system 2 is operated and controlled such that the same or essentially the same voltage (UA = UV) is in each case produced at the supply output interface 10 and at the drive output interface 13.

In special operating modes, for example during cold or freezing starting of the fuel cell arrangement 6, however, the output voltage US of the fuel cell arrangement 6 is less than the minimum required supply voltage UV. The minimum supply voltage UV is defined by the lowest voltage with which some, all or all the necessary peripheral components 12 can still be operated.

In order to ensure the operability of the peripheral components 12, the output voltage US is converted via a DC/DC converter such that the supply voltage UV is greater than US and at the same time is greater than the minimum required supply voltage UV. In contrast, the drive voltage UA is not converted and corresponds, or corresponds essentially, to the output voltage US. Nevertheless, this drive voltage UA, which is less than that during the normal operating mode of the fuel cell system 2, can still be used to provide an adequate

supply to the drive motor 5 and to achieve reduced-power operation, in particular reversionary or emergency operation of the vehicle 1.

The matching circuit 8 is in this case designed such that only a maximum of 10% or a maximum of 20% of the total power of the fuel cell arrangement 6 can be converted in the normal operating mode or in the special operating mode. This allows the matching circuit 8 to be designed with comparatively low- power conversion components, while at the same time ensuring very good characteristics of the fuel cell system 2 even during cold or freezing starting.

Figure 2 shows the vehicle 1 with the fuel cell system 2, illustrated in the same way as in Figure 1, but showing an optional circuitry refinement of the matching circuit 8. As can be seen from the illustration in Figure 2, the output voltage US is passed through the matching circuit 8 such that the output voltage US is produced as the drive voltage UA at the drive output interface 13. In contrast, however, a DC/DC booster 14 is connected between the input interface 9 and the supply output interface 10, and converts the output voltage US to the higher supply voltage UV. In the situation in which the output voltage UA is higher than the minimum required supply voltage UV, the matching circuit 8 has a switchable bypass 15, whose circuitry is arranged in parallel with the DC/DC booster 14 and bridges it on activation. Frequently, however, the DC/DC booster 14 already has a self-bridging function, so that there is no need for the bypass 15.

List of reference symbols

1 Vehicle

2 Fuel cell system

3 Power train

4 DC/AC converter

5 Drive motor

6 Fuel cell arrangement

7 Fuel cells

8 Matching circuit

9 Input interfaces

10 Supply output interfaces

11 Supply bus

12 Peripheral components

13 Drive output interface

14 DC/DC booster

15 Bypass