HK40007496A - Fuel cell system and method for operating a fuel cell system - Google Patents

Description

Fuel cell system and method for operating fuel cell system

Technical Field

The present invention relates to a fuel cell system having a plurality of fuel cell modules and a method for operating such a fuel cell system.

Background

Fuel cells generate electrical energy from hydrogen and oxygen. Oxygen is generally supplied in the form of air, while hydrogen is supplied from a storage vessel or produced on site, for example from methanol. Fuel cells are typically combined into one or more fuel cell stacks and form a fuel cell module with a number of peripheral components that are essential to the operation of the fuel cell, such as lines for supplying fresh working gas and cooling water, for removing and/or recycling used working gas and cooling water, sensors, valves, controls, switches, heaters, etc. Some of these components are equipped with a protective cover, housing or enclosure, and all or at least most of the components are assembled as compactly as possible and mounted in a housing together with the fuel cell.

A plurality of such fuel cell modules may be combined into a fuel cell system in which the fuel cell modules are electrically connected in parallel or in series to supply a supply voltage and a load current to an electrical load connected to the fuel cell system. By providing a plurality of fuel cell modules in the fuel cell system, it is relatively simple to adapt to the specific voltage requirements and/or power requirements of the electrical load. Higher operating voltages and greater power output may be provided, for example, by connecting a number of fuel cell modules in series.

One object in this case is mostly to avoid significantly alternating fuel cell operating states when the load is driven by the fuel cell system, so that it does not require a fast-acting, complex chemical energy carrier supply control and strong load fluctuations do not adversely affect the service life of the fuel cell.

Disclosure of Invention

The object of the present invention is to specify a fuel cell system having a plurality of fuel cell modules and a method for operating such a fuel cell system, wherein the fuel cell can be operated over a long service life.

The present invention relates to a fuel cell system with a plurality of fuel cell modules and to a method for operating such a fuel cell system according to the independent claims. Advantageous embodiments and refinements are specified in the dependent claims.

According to a first aspect, the invention relates to a fuel cell system comprising a plurality of fuel cell modules wired to form a fuel cell stack, the fuel cell stack having a first and a second supply terminal configured to be connected to an electrical load, further comprising measuring means connected to the fuel cell modules and set up to measure a load current of the respective fuel cell module, and comprising control means for learning respective operating states of the fuel cell modules from the load currents of the respective fuel cell modules determined by the measuring means, the control means being connected to the fuel cell modules to control the operation of the fuel cell modules. In this case, the control device is set up to supply the load current required by the load by the operation of all the fuel cell modules in full-load operation and to supply the load current required by the load by the operation of all or some of the fuel cell modules in partial-load operation of the load. In addition, the control device is set up to find out whether the operating state of the fuel cell modules is within a respective partial load range of the respective fuel cell module, the partial load being defined by a lower limit higher than the zero-load current and an upper limit lower than the full-load current. The control device is also designed to supply a load current required by the load by the operation of all fuel cell modules of the fuel cell stack in a first partial-load operation of the load, so that all fuel cell modules of the fuel cell stack are located in a respective partial-load range of the respective fuel cell module.

The invention provides the advantageous effect that the load current or the output power can be distributed to several fuel cell modules of the fuel cell stack depending on the power requirement of the load, so that the service life thereof can be optimized. The power management of the invention can therefore be used in the first place in partial-load operation of the load, where it is not necessary to simultaneously supply the full power (nominal power) of all the fuel cell modules. In contrast, all fuel cell modules of the fuel cell stack are operated in partial load operation in a balanced manner according to the load requirements in the partial load range which is preferably advantageous for their service life. In this case, one of the objects to be achieved by the power management (implemented in the control device) may be: all fuel cell modules located in the fuel cell system or the fuel cell stack are consumed identically or almost identically, while each module is operated as far as possible in the partial load range favorable for it.

This range may also be referred to as the "comfort range" of the fuel cell because the inventors have recognized that the fuel cell produces electrical power with a low degradation rate due to the electrochemical process in such a partial load range, which results in relatively weak fuel cell module component wear. On the other hand, fuel cells have operating ranges outside such "comfort ranges" from which the inventors have found that they have a higher degradation rate, which adversely affects fuel cell service life when the fuel cell is operated in these operating modes for longer periods of time. According to the invention, such an advantageous partial load range (the "comfort range" in which the degradation rate is low) is defined by a lower limit higher than the zero load current and an upper limit lower than the full load current.

Overall, a fuel cell system having a plurality of fuel cell modules and a method for operating such a fuel cell system can therefore be provided by the invention, wherein the fuel cells can be operated with a comparatively long service life.

When using the same type of fuel cells or fuel cell modules, the partial load ranges that are advantageous for the respective fuel cell modules of the fuel cell stack may be defined substantially equally for all fuel cell modules of the fuel cell stack, that is to say that the respective partial load ranges of the fuel cell modules of the fuel cell stack have substantially the same lower limit and the same upper limit. The respective partial load ranges may also be defined differently, that is to say with different lower or upper limits from one another, for example when different types of fuel cells having different "comfort ranges" are used in the fuel cell modules of the fuel cell stack. A first part load range may be defined for a portion of the fuel cell modules having the same fuel cell type as one another, and a second part load range may be defined for another portion of the fuel cell modules having another fuel cell type that is the same as one another, and so on.

According to one embodiment, the respective partial load range of the respective fuel cell module is defined by a low load current density above zero load current density and a high load current density below full load current density.

According to one embodiment, the low load current density is approximately 0.35A/cm²High load current density of approximately 0.75A/cm²。

According to one embodiment, the control device is designed to supply the load current required by the load by the operation of all fuel cell modules in a second, lower partial load operation of the load, so that all fuel cell modules lie in the operating state at the lower limit of the respective partial load range of the respective fuel cell module.

In particular, the control device is designed to supply the load current required by the load by operating only a part of the fuel cell modules of the fuel cell stack in a third partial load operation of the load, which is lower than the second partial load operation, so that the operating state of the part of the fuel cell modules operated lies within the respective partial load range of the respective fuel cell module and the part of the fuel cell modules not operated is switched off.

According to one embodiment, the control device is set up to find out whether the operating state of one of the fuel cell modules lies within the partial load range of the relevant fuel cell module, and one or more of the fuel cell modules of the fuel cell stack are switched off by the control device if the operating state of the fuel cell module lies outside the partial load range of the relevant fuel cell module.

According to one embodiment, the respective load current-power relationship for the fuel cell type of the respective fuel cell module is stored in the control device to find the operating state of the respective fuel cell module. Advantageously, a load current/power relationship in the form of a load current/power characteristic curve is stored in the control device.

According to a further embodiment, the control device is set up to record the operating time of one or more of the fuel cell modules in the respective load range, wherein it processes the recorded data to determine which fuel cell module is or is not operating in partial load operation of the load under the current load demand.

Advantageously, the control device is set up to determine which fuel cell module is operating or not operating, so that the operating times of the fuel cell modules in the respective load ranges are equalized.

According to one specific embodiment, the control device is designed to calculate the generated electrical energy of the respective fuel cell module. In this case, the control device can be set up to calculate a priority order relating to the respective generated electrical energy for at least a part of the fuel cell modules.

In particular, the control device can be set up to detect whether the operating state of one of the fuel cell modules lies within the partial load range of the relevant fuel cell module and, in the event of detection of an operating state of the fuel cell module outside the partial load range of the relevant fuel cell module, to switch off one or more of the fuel cell modules of the fuel cell stack which produce the highest electrical energy by means of the control device.

According to another aspect, the invention relates to a method for operating a fuel cell system having a plurality of fuel cells wired to be connected into a fuel cell stack having first and second power supply terminals configured for connection to an electrical load. Here, the method has the following steps:

-measuring the load current of the respective fuel cell module,

-learning the respective operating state of the respective fuel cell module from the measured load current of the respective fuel cell module,

-providing the load current demanded by the load by operation of all fuel cell modules in full load operation and by operation of all or a part of the fuel cell modules in part load operation of the load,

it is to be found here whether the operating state of the fuel cell modules lies within a respective partial load range of the respective fuel cell module, which partial load range is defined by a lower limit which is higher than the zero-load current and an upper limit which is lower than the full-load current,

-wherein in a first part-load operation of the load current demanded by the load is provided by the operation of all fuel cell modules of the fuel cell stack, such that all fuel cell modules of the fuel cell stack are located in respective part-load ranges of the respective fuel cell modules.

The control device functions described above and below can be used in a similar manner also as method steps in this method. All embodiments and examples described in this disclosure may similarly be applied to such a method of operation.

Drawings

The invention will be described in detail below in the form of an embodiment with reference to the only figure.

The figure shows an exemplary embodiment of a fuel cell system according to aspects of the present invention.

Detailed Description

The fuel cell system 1 has a number of fuel cell modules 11, 12. They are wired to form a fuel cell stack 10. Within the fuel cell stack 10, these fuel cell modules may be connected in parallel or in series-parallel. The fuel cell stack 10 has a first power supply terminal 101 and a second power supply terminal 102, which are configured for connection to an electrical load 2. In the connected state, a drive voltage U for supplying the load 2_vTo the power supply terminals 101,102 of the fuel cell stack 10. The load 2 may typically comprise, for example, one or more electrical consumers, rectifiers, and/or other electrical components of the load circuit, and represents the electrical components connected to the fuel cell stack 10 on the power consuming side to consume the load current.

The fuel cell modules 11, 12.. 1n themselves each have two supply terminals, at which a module voltage is present during operation of the respective fuel cell module in order to generate a load current. The fuel cell stack 10 may have two or more fuel cell modules 11-1n, which may be connected in parallel or in series (as shown) in a known manner to supply a load current to the electrical load 2. Partial series-parallel wiring connections are also possible, as is known to the skilled person in the various prior art. For example, the fuel cell module 11 has supply terminals 111, 112, wherein in this embodiment the supply terminal 111 forms the supply terminal 101 of the fuel cell stack 10 connected to the load 2 and the supply terminal 112 is connected to the supply terminal of the fuel cell module 12. Accordingly, the other fuel cell modules of the fuel cell stack 10 have a plurality of power supply terminals, which are connected (in series or in parallel) to one respective power supply terminal of another fuel cell module, depending on the selected wiring. Accordingly, the fuel cell module 1n has supply terminals 1n1 and 1n2, wherein in this embodiment the supply terminal 1n2 forms the other supply terminal 102 of the fuel cell stack 10 connected to the load 2. The drive voltage can be generated across the load 2 by the series connection of the fuel cell modules 11-1nU_vWhich is equal to a multiple of the individual voltage of the fuel cell module. In addition, power can be supplied to the load 2, which corresponds to the sum of the powers of the fuel cell modules 11-1n of the fuel cell stack 10 in full-load operation.

The fuel cell system 1 further includes a control device 20 for the fuel cell stack 10. Which on the one hand is used to know the respective operating state of the fuel cell modules 11-1n on the basis of the measured load currents of the respective fuel cell modules. On the other hand, the control device 20 is connected to the fuel cell modules 11-1n of the fuel cell stack 10 so as to control the operation of the fuel cell modules 11-1 n. To this end, the control device 20 is electrically connected to the fuel cell modules 11-1n by means of electrical lines 50 and can either individually switch them on, off for the operation of the fuel cell system 1 or also individually control or regulate its electrical parameters, such as module output voltage, current and/or power output. To this end, the skilled person can employ control or regulating mechanisms which are well known in the art in the cooperation of the control device 20 and the fuel cell modules 11-1 n. The supply of the chemical energy carrier (not shown in the figures) is individually controlled in each case via a line 50, for example by a control device 20 for controlling the output current of the individual modules and thus the individual operating ranges.

In addition, a measuring device is provided which is connected to the fuel cell modules 11-1n of the fuel cell stack 10 and is set up for measuring the load current of the individual fuel cell modules 11-1 n. In the present exemplary embodiment, several measuring modules 21-2n are provided in the control device 20, which can be implemented in hardware or in software or in a combination of hardware and software, which measure the load current of the individual fuel cell modules, on the one hand, and the associated operating time in this example as well. In this way, the following parameters (for example, the load current after a predetermined operation time, measured as Ah) can be calculated, and the generated energy of each fuel cell module 11-1n can be calculated based on the parameters. In the present embodiment, the measurement modules 21-2n are part of the control device 20, for example in the form of one or more calculation modules, by means of which the microprocessor of the control device 20 calculates the respective required parameters. Via the line 50, corresponding measurement signals of the physical measurements, such as the load current and the drive voltage, are supplied to the measurement modules 21-2 n. Measurement module 21 measures or calculates desired parameters associated with fuel cell module 11, while measurement module 22 measures or calculates desired parameters associated with fuel cell module 12, and so on. The measuring modules 21-2n may be connected to or comprise suitable current measuring means, voltage measuring means and/or power measuring means (not explicitly shown in the figures) via lines 50 or other connection means and/or implement or be connected to corresponding time measuring means etc. for measuring and processing the corresponding physical parameters.

Further, the control device 20 stores a load current-power relationship 30 for the fuel cell type of each fuel cell module 11-1n of the fuel cell stack 10. In the case where all fuel cell modules 11-1n have the same fuel cell type, it is sufficient to store only one load current-power relationship 30 for all fuel cell modules. However, if a variety of fuel cell types are used in the fuel cell stack 10, it makes sense to store a respective load current-power relationship 30 for each fuel cell type.

For example, a load current-power relationship in the form of a load current-power characteristic curve 30 is stored, as shown by way of example. In the present embodiment, the load current-power characteristic curve 30 is expressed in terms of a load current density ID (load current in terms of the electrochemical reaction area of the fuel cell, in terms of a/cm)²Measured) represents the relative power output Pr of each fuel cell module (between 0% and 100% in relation to the rated power of the fuel cell module).

The operating state of each fuel cell module 11-1n can be found from the load current-power characteristic curve 30. According to the invention, each fuel cell module has a plurality of operating ranges, such as: a lower part-load range 41, a middle part-load range 42 and an upper part-load range 43. In particular, the respective mid-load range 42 is defined by a lower limit 421 above zero load current and an upper limit 422 below full load current. In the present embodiment of the load current-power characteristic 30, each partial load range 42 is defined by a lower load current density 421 above the zero load current density and an upper load current density 422 below the full load current density.

The part load range 42 is defined as follows: the associated fuel cell of the module generates electrical power at a low degradation rate over the part load range. As described above, this range 42 may also be referred to as a "comfort range" for the fuel cell. The other operating ranges 41 and 43 have a higher degradation rate, wherein it adversely affects the service life of the fuel cell when the fuel cell is operated in these operating modes 41, 43 for a long time.

To define the favorable part load range 42, the low load current density 421 is approximately 0.35A/cm²The high load current density 422 is approximately 0.75A/cm². Driving the fuel cell module with a load current density in the range 42 results in a lower degradation rate, and it is therefore desirable to operate the fuel cell in this range for as long a time as possible. In this example, it is less than 0.35A/cm²Load current density (especially due to excessive cell voltage and condensation of liquid water) and higher than 0.75A/cm²The load range of the load current density (especially because of local temperature maxima and mass transfer problems) is defined as a potentially harmful mode of operation.

The control device 20 now controls the fuel cell system 1 according to the invention in such a way that, in full-load operation, the load current I required by the load 2 is made available by the operation of all fuel cell modules 11-1n of the fuel cell stack 10_L. In contrast, in the partial load operation of the load, the load current I required by the load 2 is provided by the operation of all or only a part of the fuel cell modules 11-1n of the fuel cell stack 10_LDepending on the magnitude of the required load current. One or more parameters characterizing the load demand (and hence the required load current) of the load 2 are transmitted to the control device 20 via line 60.

Further, the control device 20 finds whether the operating state of the fuel cell module 11-1n is in each partial load range 42 (comfort range) of each fuel cell module. If atThe load demand in the first part-load operation of the load is lower than the nominal load (for example 40-60% of the nominal load), the control device 20 controls the fuel cell modules 11-1n (for example the supply of chemical energy carriers) such that the load current I required by the load 2 is lower than the nominal load_LThis is provided by the operation of all fuel cell modules 11-1n of the fuel cell stack 10, specifically such that all fuel cell modules 11-1n of the fuel cell stack 10 are in the respective partial load range 42 (comfort range).

And if the required load current I is found in a second, lower part-load operation of the load, such as in the range of 30-40% of the rated load_LThen load current I_LThis is always also provided by the operation of all the fuel cell modules 11-1n of the fuel cell stack 10 in the following manner: all the fuel cell modules 11-1n of the fuel cell stack 10 are in the lower limit 421 of the partial load range 42 in the operating state. Therefore, even if the part load is low, all the fuel cell modules of the fuel cell stack 10 are still operated within the respective "comfort ranges".

The required load current I is only found in a third partial load operation of the load, which is lower than the second partial load operation (for example, lower than 30% of the rated load)_LThen, the control device 20 controls the fuel cell module as follows: the operating state of a portion of the fuel cell modules 11-1n that are still operating is within the partial load range 42, while the portion of the fuel cell modules 11-1n that are not operating is shut off. It is thus still ensured that as many fuel cell modules as possible are operated in the partial load range 42 which is favorable for them.

Furthermore, the control device 20 can be set up to record the operating time of one or more of the fuel cell modules 11-1n in the respective different load ranges 41,42,43 and to process the recorded data in order to determine which fuel cell module 11-1n is or is not operating under the current load demand in partial load operation of the load 2.

The fuel cell modules 11-1n are operated, for example, as follows: so that the operating times of the fuel cell modules in the various load ranges 41,42,43 are equalized. Thus, a more uniform degradation rate or service life may be achieved for all fuel cell modules.

In addition, the control device 20 may be set up as follows: it calculates a priority order for at least a part of the fuel cell modules 11-1n with respect to the respective generated amounts of electric energy. When, for example, the operating state of one of the fuel cell modules 11-1n lies outside the partial load range 42 of the relevant fuel cell module, the fuel cell module or modules 11-1n which generate the highest electrical energy is switched off by the control device 20. The operating time and the service life of the fuel cell module can thus be further equalized.

The load current can thus be distributed to several fuel cell modules according to the power demand, so that their service life is optimized. That is, the power management of the control device first intervenes when the fuel cell system is operating at partial load. The basic idea is that: at full load, all fuel cell modules installed in the plant are operated at their maximum allowed continuous load (generally referred to as rated power). If the load of the consumer decreases, however, only one fuel cell module can be switched down (or completely switched off) in sequence, the remaining fuel cell modules remaining at the nominal load point at all times. However, the inventors have found that this is disadvantageous in terms of: fuel cell modules that operate at rated load points for long periods of time suffer from a high degradation rate, which adversely affects service life. In contrast, according to the invention, all fuel cell modules are operated equally in the favorable partial load range according to the load demand. The purpose of this power management is: all fuel cell modules located in the installation are consumed identically or almost identically, and at the same time each module is operated as far as possible in its advantageous partial load range (comfort range). Such a partial load range is particularly found at approximately 0.35A/cm²Low load current density and a current density of approximately 0.75A/cm²Between high load current densities.

Under load demands in the "low" part load range, the power management control of the control arrangement controls all fuel cell modules located in the fuel cell system, preferably equally to the lower limit of the "comfort range". When the power demand of the load demands yet less load current, the control begins to shut down individual modules completely, and the remaining fuel cell modules are again adjusted up to the "comfort range" so that the load demand is still met. Preferably, an operating timer is included in the power management of the control device, which records the operating time of each module within the respective load range and then automatically determines which module is to be adjusted downward in such a case on the basis of this data in order to equalize the operating times of all modules.

As already detailed above, the following calculation and control tasks are advantageously implemented in the control device 20: handling the power requirements of the overall plant (here by software); calculating the load current (here software) required for this purpose on the basis of all the fuel cell modules comprised by the fuel cell system; checking whether the result satisfies a partial load range (comfort range) (here, software) favorable for each fuel cell module; when the result is negative (below the part-load range), the shutdown of the module or modules (here software) that produce the highest electrical energy is triggered. The control device has one or more microprocessors which can carry out the described functions by means of corresponding memory components, interface components and other hardware components. It is also possible, however, that the functions are implemented only partly in the control device 20 and/or with a distributed control system in which the functions are distributed to a number of entities, such as microprocessors and their memories.

Claims (17)

1. A fuel cell system (1) comprising:

-a plurality of fuel cell modules (11-1n) wired to form a fuel cell stack (10) having first and second supply terminals (101,102) configured for connection to an electrical load (2),

-a measuring means (21-2n) connected to the fuel cell modules (11-1n) and set up for measuring the load current of each fuel cell module (11-1n),

-a control device (20) for learning the respective operating state of the fuel cell modules (11-1n) from the load currents of the fuel cell modules measured by the measuring means (21-2n), the control device (20) being connected to the fuel cell modules (11-1n) for controlling the operation of the fuel cell modules (11-1n),

-wherein the control device (20) is set up to supply a load current (I) required by the load (2) in full-load operation by operation of all fuel cell modules (11-1n)_L) And in partial load operation of the load, the load current (I) required by the load (2) is supplied by operation of all or part of the fuel cell modules (11-1n)_L)，

-wherein the control device (20) is set up for finding whether the operating state of the fuel cell modules (11-1n) is in a respective partial load range (42) of each fuel cell module, which partial load range is defined by a lower limit (421) above zero load current and an upper limit (422) below full load current,

-wherein the control device (20) is set up to supply a load current (I) demanded by the load (2) in a first part-load operation of the load by operation of all fuel cell modules (11-1n) of the fuel cell stack (10) as follows_L): all fuel cell modules (11-1n) of the fuel cell stack (10) are in a respective partial load range (42) of each fuel cell module (11-1 n).

2. A fuel cell system according to claim 1, characterized in that the respective partial load range (42) of each fuel cell module (11-1n) is defined by a lower load current density (421) above zero load current density and an upper load current density (422) below full load current density.

3. The fuel cell system of claim 2, wherein the lower load current density (421) is approximately 0.35A/cm²The upper load current density (422) is approximately 0.75A/cm²。

4. According to claimThe fuel cell system as claimed in one of claims 1 to 3, characterized in that the control device (20) is set up to supply a load current (I) required by the load (2) in a second, lower part-load operation of the load (2) by operation of all fuel cell modules (11-1n) of the fuel cell stack (10) in the following manner_L): all fuel cell modules (11-1n) of the fuel cell stack (10) are in an operating state at a lower limit (421) of a respective partial load range (42) of each fuel cell module.

5. A fuel cell system as claimed in claim 4, characterized in that the control device (20) is set up to supply the load current (I) required by the load (2) by operation of only a part of the fuel cell modules (11-1n) of the fuel cell stack (10) in a third part-load operation of the load, which is also lower than the second part-load operation, in the following manner_L): the operating state of a part of the fuel cell modules (11-1n) operated is within the respective partial load range (42) of the respective fuel cell module and the part of the fuel cell modules (11-1n) not operated is switched off.

6. Fuel cell system according to one of claims 1 to 5, characterized in that the control device (20) is set up to find whether the operating state of one of the fuel cell modules (11-1n) of the fuel cell stack (10) is in the partial load range (42) of the relevant fuel cell module and to shut down one or more of the fuel cell modules (11-1n) of the fuel cell stack (10) by means of the control device (20) in the event that the operating state of the fuel cell module (11-1n) is found to be outside the partial load range (42) of the relevant fuel cell module.

7. The fuel cell system as claimed in one of claims 1 to 6, characterized in that at least one load current-power relationship (30) for the fuel cell type of each fuel cell module (11-1n) is stored in the control device (20) in order to find the operating state of each fuel cell module (11-1 n).

8. A fuel cell system according to claim 7, characterized in that a load current-power relationship in the form of a load current-power characteristic curve (30) is stored in the control device (20).

9. The fuel cell system as claimed in one of claims 1 to 8, characterized in that the control device (20) is set up to record the operating time of one or more of the fuel cell modules (11-1n) in the respective load range (41,42,43) and to process the recorded data to determine which of the fuel cell modules (11-1n) is operating or not operating in partial load operation of the load (2) at the current load requirement.

10. A fuel cell system as claimed in claim 9, characterized in that the control device (20) is set up for determining which of the fuel cell modules (11-1n) is active or inactive, so that the operating times of the fuel cell modules (11-1n) of the fuel cell stack (10) in the respective load ranges (41,42,43) are equalized.

11. The fuel cell system as claimed in one of claims 1 to 10, characterized in that the control device (20) is set up for calculating the electrical energy generated by the individual fuel cell modules (11-1 n).

12. A fuel cell system as claimed in claim 11, characterized in that the control device (20) is set up for calculating a priority order relating to the respective generated electrical energy at least for a part of the fuel cell modules (11-1 n).

13. A fuel cell system as claimed in claim 12, characterized in that the control device (20) is set up to find out whether the operating state of one of the fuel cell modules (11-1n) is in the partial load range (42) of the relevant fuel cell module, and to switch off one or more of the fuel cell modules (11-1n) that generate the highest electrical energy by means of the control device (20) in the event that the operating state of the fuel cell module (11-1n) is found to be outside the partial load range (42) of the relevant fuel cell module.

14. A method for operating a fuel cell system (1) comprising a plurality of fuel cell modules (11-1n) wired to form a fuel cell stack (10) having first and second supply terminals (101,102) configured for connection to an electrical load (2), wherein the method has the steps of:

-measuring the load current of each fuel cell module (11-1n),

-knowing the respective operating state of each fuel cell module (11-1n) on the basis of the measured load current of that fuel cell module,

-providing a load current (I) demanded by the load (2) by operation of all fuel cell modules (11-1n) in full load operation_L) And in partial load operation of the load, the load current (I) required by the load (2) is supplied by the operation of all or a part of the fuel cell modules (11-1n)_L)，

Here, it is to be found that: whether the operating state of one of the fuel cell modules (11-1n) is within a respective partial load range (42) of the respective fuel cell module, which partial load range is defined by a lower limit (421) above zero load current and an upper limit (422) below full load current,

-wherein in a first part-load operation of the load a load current (I) demanded by the load (2) is provided by operation of all fuel cell modules (11-1n) of the fuel cell stack (10) as follows_L): all fuel cell modules (11-1n) of the fuel cell stack (10) are located in a respective partial load range (42) of each fuel cell module.

15. Method according to claim 14, characterized in that in the low second partial load operation of the load current (I) demanded by the load (2) is supplied by the operation of all fuel cell modules (11-1n) in the following manner_L): all fuel cell modules (11-1n) of the fuel cell stack (10) are in an operating state at a lower limit (421) of a respective partial load range (42) of each fuel cell module.

16. A method according to claim 14 or 15, characterized in that the load current (I) demanded by the load (2) is provided by the operation of only a part of the fuel cell modules (11-1n) of the fuel cell stack (10) in a third part-load operation of the load, which is also lower than the second part-load operation, in the following manner_L): the operating state of a part of the fuel cell modules (11-1n) which are operated lies within the respective partial load range (42) of the respective fuel cell module, and the part of the fuel cell modules (11-1n) which are not operated is switched off.

17. The method according to one of claims 14 to 16, characterized by detecting whether the operating state of one of the fuel cell modules (11-1n) of the fuel cell stack (10) is within the respective partial load range (42) of the respective fuel cell module, and in the event that the operating state of one of the fuel cell modules (11-1n) is found to be outside the partial load range (42) of the relevant fuel cell module, one or more of the fuel cell modules (11-1n) of the fuel cell stack (10) is switched off.