WO2008052497A2

WO2008052497A2 - Method for regenerating a reformer

Info

Publication number: WO2008052497A2
Application number: PCT/DE2007/001673
Authority: WO
Inventors: Manfred Pfalzgraf; Markus Bedenbecker; Matthias Boltze; Andreas Engl
Original assignee: Enerday Gmbh
Priority date: 2006-11-02
Filing date: 2007-09-17
Publication date: 2008-05-08
Also published as: DE102006051741A1; DE102006051741B4; WO2008052497A3

Abstract

The invention relates to a method for regenerating a reformer (18) associated with a fuel cell system (14) which is a component of an air conditioner (12) of a motor vehicle (10) driven by a drive unit (94) and supplies electric power to the air conditioner. According to the invention, the reformer (18) is regenerated when at least one parameter that is characteristic of an outside temperature of the motor vehicle (10) lies within a predefined parameter interval. The invention further relates to an air conditioner (12) for a motor vehicle (10).

Description

Process for regenerating a reformer

The invention relates to a method for regenerating a reformer, which is associated with a fuel cell system, wherein the fuel cell system is a component of an air conditioner of a motor vehicle drivable by a drive unit and the air conditioning system supplied with electrical energy.

The invention further relates to an air conditioner for stationary air conditioning of a motor vehicle drivable by a drive unit, comprising a fuel cell system having a reformer for operating a refrigeration circuit and a control unit for controlling the fuel cell system.

From DE 102 23 949 Al is an air conditioner with a

Fuel cell for stationary air conditioning of a vehicle known. In order to supply the hydrogen required for its operation to a fuel cell or a fuel cell stack, a reformer is generally used which generates a hydrogen-rich reformate from fuel, in particular the fuel used for the operation of a motor vehicle. By imperfect reaction of the hydrocarbons by-products, such as residual hydrocarbons or carbon black can form. These then at least partly settle on the reformer. This results in deactivation of the catalyst in the reformer, which can go so far that the catalyst is almost completely mixed with carbon black. The pressure loss occurring in the reformer thereby increases. The reformer is unusable, or he must be regenerated. Such regeneration is carried out by default by burning off the soot deposited in the reformer, by operating the reformer, which is operated for example with an air ratio λ = 0.4, with a typical air ratio, ie λ> 1. While such a regeneration process takes place, the reformer can not provide reformate. Other concepts provide for the regeneration in the continuous operation of the reformer, namely, for example, by repeatedly reducing the fuel feed rate during several consecutive time intervals. Between the time intervals, the system then regulates again to the air ratio typical for the reforming, so that overall the supply of the fuel cell stack with reformate does not break off. Furthermore, it may be provided to supply a regeneration by supplying a NO ₂ -containing gas mixture in the reformer, during continuous continuous production of reformate.

Whatever one of the described regeneration concepts is chosen, it is in any case to be expected that the reforming sector will be influenced, either by a total failure of the reform process or by a reduction of the reformed generation rate. This can ultimately have an impact on other components and processes associated with the fuel cell system, which should be avoided if possible.

The invention has for its object to provide a method for regenerating a reformer and an air conditioning system for a motor vehicle available, so that a Fluxing the mode of operation of the air conditioning is excluded.

This problem is solved by the feature of the independent claim.

Advantageous embodiments of the invention are indicated in the dependent claims.

The invention is based on the generic method in that the reformer is regenerated when at least one characteristic of an outside temperature of the motor vehicle parameter is within a predetermined parameter interval. The regeneration can thus be made in times in which the outside temperature allows a non-operation or a reduced efficiency of the fuel cell system without loss of comfort. This can be done on the basis of the automatic verification of a characteristic of the outside temperature of the motor vehicle parameters. If this is within a predetermined parameter interval, which can be fixed in a control unit and / or can be selected by the user, then there is no obstacle to carry out any necessary regeneration of the reformer.

For example, it can be provided that the parameter characteristic of the outside temperature of the motor vehicle is an outside temperature value detected by an outside temperature sensor and the parameter interval is limited upwards by a maximum temperature. The maximum temperature is a variable that can be readily dictated by the control of the system, and is a size that is easily traceable by the user and thus may be differently adjustable by the user. The Maximum temperature, for example, factory set to 15 ⁰ C. If needed, however, the user of the vehicle may also choose more critical temperature thresholds or allow regeneration at higher temperatures.

The invention is further developed in a particularly preferred manner in that the parameter characteristic for the outside temperature of the motor vehicle is a time supplied by a real time clock and the parameter interval denotes a recurring period of time, in that one expects a particularly low outside temperature compared to other time periods is. The periods of time that are particularly suitable for regeneration will generally be periods of time during the night when the outside temperatures are naturally lower than during the day. This has particular relevance for hot desert regions, as extreme temperature fluctuations between day and night can be observed here. During the day, with a high probability, a permanent climate control will be desired, so that a regeneration of the

Reformers is undesirable, such is readily possible at night, because then the temperatures fall to values at which an air conditioning is unnecessary.

It may be provided that the presence of the characteristic of the outside temperature of the motor vehicle parameter within the predetermined parameter interval is necessary condition for a regeneration of the reformer. It should be understood that when the system requests regeneration, for example due to a particular elapsed time of operation or due to certain sensed measurements such as pressures or temperatures, it is additionally checked whether the condition bound to the characteristic parameter for a regeneration Regenerating the reformer is filled. Only if this is the case, a regeneration takes place.

It can also be provided that the presence of the characteristic of the outside temperature of the motor vehicle parameter within the predetermined parameter interval is sufficient condition for a regeneration of the reformer. The regeneration therefore always takes place when the condition linked to the characteristic parameter is fulfilled, that is to say in particular without an explicit request for regeneration on the basis of certain other system parameters.

The invention is based on the generic air conditioning system in that the control unit is suitable for controlling a method according to the invention. As a result, the advantages and special features of the method according to the invention are implemented in the context of an air conditioner.

The invention will now be described with reference to the accompanying drawings

Drawings exemplified with reference to preferred embodiments.

Show it:

Figure 1 is a schematic representation of an air conditioner according to the invention;

Figure 2 is a schematic representation of the motor vehicle with the air conditioner according to the invention;

FIG. 3 is a flow chart of an air conditioning operation; and FIG FIG. 4 shows a flowchart for explaining a method according to the invention for regenerating a reformer.

Figure 1 shows a schematic representation of an air conditioner according to the invention. The installed in a motor vehicle 10 air conditioning 12 (installation position, see FIG. 2), which is outlined in Figure 1 with a dashed line, comprises as main elements, a fuel cell system 14 and a refrigerant circuit sixteenth

The fuel cell system 14 comprises a reformer 18, to which fuel can be supplied via a fuel line 20 from a fuel tank, not shown. Further, the reformer 18 at a second Brennstoffzuführstufe by means of a fuel strand 22 also from the fuel tank fuel can be supplied. As fuel types are diesel, gasoline, natural gas and other known from the prior art types of fuel in question. Furthermore, the oxidizer 24 via a Oxidationsmittelstrang 24 Oxidati- onsmittel, ie in particular air, can be fed to the reformer 18. The reformate produced by the reformer 18 can be fed to a fuel cell stack 26. Alternatively, instead of the fuel cell stack 26, only one fuel cell may be provided. The reformate is a hydrogen-containing gas which is reacted in the fuel cell stack 26 with the aid of cathode feed through a cathode feed line 28 to generate electrical energy and heat. The generated electrical energy can be fed via an electrical line 30 to an electric motor 32, a battery 34 and an electric heater 36 of the air conditioning system 12. In the case shown, the anode exhaust gas can be fed via an anode exhaust gas line 38 to a mixing unit 40 of an afterburner 42. Furthermore, the Afterburner 42 via a fuel strands 44 fuel from the fuel tank and a Oxidationsmit- tend strand 46 oxidizing agent fed. In the fuel strands 20, 22 and 44 suitable, not shown conveyors, such as pumps, are arranged. Likewise, in the oxidant strands 24 and 46 corresponding, not shown conveyors, in this case, preferably blower arranged. These conveyors can be powered directly from the fuel cell stack 26 or from the battery 34. In the afterburner 42, a conversion of the depleted anode exhaust gas with the conveyed fuel and oxidizing agent to a combustion exhaust gas, which is mixed in a mixing unit 48 with cathode exhaust air, which is conveyed via a cathode exhaust pipe 50 from the fuel cell stack 26 to the mixing unit 48. The combustion exhaust gas, which contains virtually no pollutants, flows through a heat exchanger 52 for preheating the cathode feed air and finally leaves the fuel cell system 14 via an exhaust gas outlet 54.

In the refrigerant circuit 16, a compressor 56, a condenser 58, an expansion device 60 and an evaporator 62 are arranged. The compressor 56 can be driven by the electric motor 32, which in turn is preferably supplied with energy by the fuel cell stack 26 of the fuel cell system 14, but can also be supplied with energy by the battery 34 for a short time. The evaporator 62 is associated with a blower 64. An outside air line 66 can be used to suck in ambient air from the outside. The term "from the outside", as used in connection with this invention, means from outside the interior space 78, thus designating the air surrounding the motor vehicle 10. The outside air duct 66 leads to a Actuator 68, which can supply the outside air to the blower 64. The air directed from the actuator 68 to the fan 64 flows past the evaporator 62 as airflow 70. In this way, the air flow 70 through the evaporator 62 heat energy can be withdrawn. The cooled air stream can then be fed via an adjusting device 72 and an air guide 74 via a hat rack 76 a vehicle interior 78. The adjusting device 72 can be realized, for example, by a solenoid valve or check valves, which allow only one flow from the two supply lines to the air guide 74. The cooled air flows through the vehicle interior 78 (as illustrated by arrows in FIG. 2) and exits beneath a seat 80, preferably the rear seat. Subsequently, the air flows via an air guide 82 back to the adjusting device 68, where it is completely or partially discharged to the outside or back to the blower 64 is passed. For the guidance of the air to the outside, a corresponding line is provided, which is not shown for reasons of clarity. The circuit of the adjusting device 68 thus makes it possible to realize either a fresh air concept or a circulating air concept in which air is drawn in from outside via the outside air line 66 or the air is circulated out of the air duct 82. Mixed forms of these modes are possible. Furthermore, by means of the adjusting device 68, the air introduced via the outside air duct 66 can be supplied to an air duct 84 and via this to a blower 86. In this case, this air flows as an air flow 88 on hot parts of the fuel cell system 14 directly past or by (not shown) heat exchanger, which mediate between the air stream 88 and the hot parts. The hot parts of the fuel cell system 14 are preferably the reformer 18, the fuel cell stack 26 and the - S -

Afterburner 42. In this way, 88 heat energy can be supplied to the air stream 88 by the waste heat of the hot parts of the fuel cell system. The heated air stream 88 leads via an air guide 90 to the electric heating device 36, which is supplied directly by an energy generated by the fuel cell stack 26 or stored by the battery 34. Thus, in a heating operation, the already preheated air in the air duct 90 can be further heated and fed via the adjusting device 72 and the air guide 74 to the interior 78. After flowing through the interior 78 of the air flow over the air guide 82 to the adjusting device 68, where it is either discharged to the outside or is passed back to the fan 86. In this case as well, a recirculation concept can be realized via the circuit of the adjusting device 68, optionally in such a heating operation, in which air is drawn in from outside via the outside air line 66 or the air is recirculated out of the air guide 82.

Below, various operating conditions are shown, which can be realized by means of the air conditioning described above:

Cooling operation with circulating air circulation: In this operating state, the adjusting device 68 is switched such that air is guided from the interior space 78 via the air guide 82 to the fan 64. This air flow 70 is cooled and guided via the adjusting device 72 and the air guide 74 into the interior 78, whereby it is cooled. In order to avoid heating of the trunk in which the air conditioning system 12 is arranged in cooling mode, corresponding blowers and lines (not shown) are provided, which reduce the waste heat of the air conditioning system 12 (in particular of the fuel cell system 14, the condenser 58, the grit). pressors 56 and the electric motor 32) to the outside. In the case of the capacitor 58, this could alternatively also be arranged on the outside of the vehicle 10, in order thus to remove the waste heat directly.

Cooling operation with outside air supply: In this operating state, the adjusting device 68 is switched so that outside air is conducted via the outside air line 66 to the blower 64. The air flow 70 is cooled and guided via the adjusting device 72 and the air guide 74 into the interior 78. The over the air guide 82 from the interior 78 leading air flow is discharged from the actuator 68 to the outside. With regard to the removal of the waste heat of the air conditioning system 12, the measures explained in the context of the above-described cooling operation are taken.

Heating mode with circulating air circulation: In this operating state, an air flow 88 is guided from the interior 78 to the fan 86 via the air guide 82, the adjusting device 68 and the air guide 84. The refrigeration circuit 16 is not in operation, d. H. the electric motor 32 is not operated. The blower 86 passes the air flow 88 past the hot parts of the fuel cell system 14. The preheated in this way air is guided by the air guide 90 to the e- lectric heater 36 and on to the adjusting device 72. The electric heater 36 is operated to heat the air in the air duct 90 with electric power. Subsequently, the heated air flows via the adjusting device 72 and the air guide 74 into the interior space 78.

Heating mode with external air supply: In this operating state, outside air is supplied via the outside air line 66 from the adjusting device 68 to the air guide 84. By The waste heat produced by the operation of the fuel cell system 14 heats the air flow 88. This heated air flow is directed into the interior 78 via the air guide 90, the electric heater 36, the actuator 72, and the air guide 74, as in the above-described operation state. Subsequently, this air flow is guided via the air guide 82 to the adjusting device 68, where it is discharged to the outside.

These different operating states are controlled by an electronic control unit, which selects the appropriate operating state depending on the temperature in the interior 78, outside temperature, set target temperatures and desired air conditioning operation. This electronic control unit is not shown in the figures for reasons of clarity, but it is immediately apparent to those skilled in the art that these at least with the corresponding conveyors in the strands 20, 22, 24, 44 and 46 of the power distribution in the electrical line 30, the blowers 64 and 86, the electric heater, the

Electric motor 32, the adjusting devices 68 and 72 and the corresponding temperature sensors is connected.

The above-described flow direction in the interior of the vehicle 78, d. H. Inserting the air over the parcel shelf

76 and discharging the air below the seat 80 may be reversed in the cooling and / or heating mode. For a modification of this kind, the air guide 74 below the seat 80 would have to flow into the vehicle interior 78, and the air guide 82 would have to be routed via the hatrack 76 into the vehicle interior 78

Vehicle interior 78 open.

Figure 2 shows a schematic representation of the motor vehicle 10 with the inventive air conditioner 12. In FIG 2, in particular the installation position of the air conditioner 12 is illustrated. The air conditioner 12 according to the invention can be mounted in the trunk, preferably as a retrofittable unit. It is also possible to install the air conditioning in the passenger compartment of the vehicle. Here it can be used, for example, in addition to cooling a refrigerator. In addition to the described air conditioning system 12, the motor vehicle 10 has a conventional air conditioning system 92, in which a compressor of a conventional refrigeration circuit is mechanically drivable by a drive unit 94, preferably an internal combustion engine. The drive unit is associated with a tailpipe 96 in a known manner. During the drive of the motor vehicle 10 and the associated operation of the drive unit 94, the interior 78 can be cooled by the conventional, on-board air conditioning 92 in a well-known manner or heated by waste heat of the drive unit 94. When the drive unit 94 is at a standstill, the interior 78 can be conditioned via the air conditioning system 12 according to the invention. For detecting the exhaust gas temperature, an exhaust gas temperature sensor 98 electrically connected to the electronic control unit of the air conditioner 12 is provided, which is mounted on the outside of the exhaust pipe 96, or installed in the exhaust pipe 96. Alternatively, or in addition to the exhaust temperature sensor 98, a sensor 100 is provided, which is electrically connected to the electronic control unit of the air conditioner 12. The sensor 100 may be a motion sensor and / or a sound sensor. In the case of a motion sensor, the latter can determine, by detecting an acceleration, that the motor vehicle is being moved as soon as a specific acceleration threshold value is exceeded. From the locomotion of the motor vehicle, it can be concluded that the drive unit is in operation and thus the possibility exists of putting the conventional air conditioning system into operation. men. In the case of the sound sensor, this speaks to a certain frequency range in which the operating noise of the drive unit lie. The sensor 100 is preferably mounted on the housing of the air conditioning system 12, as shown in FIG. Alternatively, the sensor 100 may also be on

Vehicle underbody or any other location of the motor vehicle 10 may be mounted, but this makes it necessary to lay electrical lines.

As an alternative to the sensors 98 and 100, it is also possible to use a signal provided by an in-vehicle on-board computer which indicates whether the drive unit is in operation.

Figure 3 shows a flow chart of the air conditioning operation of the air conditioners 12 according to the invention. The routine of Figure 3, which is executed by the electronic control unit starts at step SlOO when the air conditioner 12 is turned on manually. At step S01, it is determined whether a shutdown condition is met. The shutdown condition may be, in the context of this routine, the operation of the power plant or a traveling of the motor vehicle 10. The shutdown condition is therefore met when the drive unit is in operation, z. B. a combustion engine is running, or the motor vehicle 10 is moved. Whether this condition is met can be determined by means of the exhaust gas temperature sensor 98 and / or the sensor 100. Alternatively, the signal supplied by the on-board computer can be evaluated, which indicates whether the drive unit 94 is in operation. Accordingly, if the sensor 100 is used for the determination and this is a sound sensor, the electronic control unit inquires at step S1O1 whether the sound sensor supplies a signal which indicates the presence of a sound frequency. shows that outputs the drive unit in an operated state. The process does not proceed to step S102 until the query in step S101 is negative. In step S102, it is determined whether the user has selected an automatic standby mode via a selector switch or a corresponding programming of the air conditioner 12. If not, the process proceeds to step S103 where it is determined whether the user has manually selected standby air conditioning. If this is not the case then the process starts

Step S104, where it is determined whether the user has manually selected a comfort air conditioning. If this is to be answered with "YES", the process proceeds to step S105, at which a comfort air-conditioning is performed. In this comfort air conditioning of the interior of the motor vehicle 78 ^'10 to a comfortable temperature (z. B. 18 ⁰ C) is conditioned by a choice from the various heating and cooling modes is taken by the electronic control unit. The subsequent step S106 determines that this comfort air-conditioning is automatically stopped when the shut-off condition already explained is satisfied. Accordingly, if it is determined in step S106 that the shut-off condition is not satisfied, it is determined in S107 whether the air conditioner 12 has been turned off manually. For a manual shutdown, the process ends at step S112, otherwise the process returns to step S105. If the user has not selected feel-good conditioning in step S104, the process returns to step S110. If it has been determined in step S102 that an automatic standby air conditioning has been selected, then the process proceeds from there to step S108, where it is determined whether a comfortable air conditioning has been manually selected by the user. If so, then the process proceeds to step S105, where the already described Wohlfühlklimatisierung is performed. If it is determined in step S108 that the user has not selected feel-good air conditioning, then the process proceeds to step S109 where the standby air conditioning according to the present invention is performed. In this standby air conditioning, the temperature in the internal space 78 is controlled to a standby target temperature (eg, 25 ^° C.) that is different from the comfort temperature. This is realized by suitably selecting the electronic control unit from the described heating and cooling modes. If the outside temperature is high, then the ready set temperature is greater than the comfort temperature. If, however, the outside temperature is low, then the ready set temperature is lower than the comfort temperature. Thus, for example, at a high outside temperature, heating of the interior 78 is prevented and, if necessary, a very fast reaching of the comfort temperature is ensured because the interior 78 is already "pre-cooled". After step S109, the process proceeds to step S110 where it is checked if the shut-off condition is satisfied. If so, then the process returns to step S100. Otherwise, the process proceeds to step S11, where it is determined whether the user has manually turned off the air-conditioning - if "YES", then the process ends in step S112 and if "NO", then the process returns to step S108 ,

Between steps S102 and S108, a box R100 is symbolized, symbolizing the regeneration of the reformer. Regenerating the reformer does not necessarily take place at this point. The box merely indicates that regeneration is possible at this point in the process, namely, for example, according to FIG. 4 described below. The preferred operation of the air conditioning system 12 in practice is to select automatic standby air conditioning. If the drive unit 94 is operated, then the interior space 78 can be conditioned via the vehicle-optimized, very effective and specially designed air conditioning system 92. Once the drive unit 94 is turned off (and the occupants may leave the vehicle 10), the air conditioner 12 starts the standby air conditioning, which cools the interior at high outdoor temperature to, for example, 25 ⁰ C. This standby air conditioning operation can be carried out with 12 liters of fuel without any problems for 12 days in continuous operation. The standby air conditioning operation is performed until the user selects a Wohlfühlklimatisierung shortly before departure, which then cools the interior 78 to, for example 18 ⁰ C. The Wohlfühlklimatisierung is then carried out until the drive unit 94 is restarted.

FIG. 4 shows a flowchart for explaining a method according to the invention for regenerating a reformer. The regeneration according to the invention can in principle always take place when the characteristic of the outside temperature of the motor vehicle parameter is within the predetermined parameter interval. This may be the case, in particular, if the switch-off condition according to step S1O1 in FIG. 3 is fulfilled. Its special advantages unfolds the method but when the automatic stand-ready air conditioning is set according to step S102 in Figure 3, because then a continuous operation of the stationary air conditioner can only be interrupted under the condition that this is harmless for the climate comfort. The flow illustrating the invention The diagram therefore starts from step S102, and it is explained in the case that the automatic standby air conditioning is set. In step R110, it is then checked whether a regeneration is requested, in particular due to some system parameters, such as for example

Press, temperatures or elapsed operating times. If this is not the case, no regeneration is carried out, which leads to a return to the normal air conditioning procedure. If, on the other hand, a regeneration is requested, it is checked in step R102 whether the outside temperature is lower than a temperature threshold value Ts. If this is not the case, the system returns to the normal air conditioning procedure. If the temperature is sufficiently low, on the other hand, it is checked in step R103 whether there is a predetermined time of day. If this is not the case then normal air conditioning is continued. However, if there is a certain time of day, the regeneration is performed according to step R104.

The method sequence according to FIG. 4 therefore assumes that a regeneration must be requested so that then further necessary conditions (R102, R103) are checked with regard to the execution of the regeneration. It is also possible to check these conditions (R102, R103) without regeneration requested beforehand and to perform a regeneration on this basis. Likewise, there is no requirement for both conditions (R102, R103) to be cumulative. Rather, it may be sufficient that either the outside temperature is low enough or a predetermined time of day exists.

The features of the invention disclosed in the foregoing description, in the drawings and in the claims may be essential both individually and in any combination for the realization of the invention.

List of reference numbers:

10 motor vehicle

12 air conditioning

14 fuel cell system

16 refrigerant circuit 18 reformer

20 fuel strands

22 fuel strands

24 oxidizer strand

26 fuel cell stack 28 Kathodenzuluftsträng

30 Electric line

32 electric motor

34 battery

36 Electric heater 38 Anode exhaust gas line

40 mixing unit 42 afterburner

44 fuel strands

46 Oxidizing agent strand 48 mixing unit

50 cathode exhaust stream

52 heat exchangers

54 exhaust outlet

56 compressor 58 condenser

60 expansion organ

62 evaporator

64 blowers

66 outside air line 68 adjusting device

70 airflow

72 adjusting device

74 air duct

76 hat rack

78 vehicle interior

80 seat

82 air duct

84 air duct

86 blowers

88 airflow

90 air duct

92 Conventional air conditioning

94 drive unit

96 exhaust

98 exhaust temperature sensor

100 sensor

Claims

A method of regenerating a reformer (18) associated with a fuel cell system (14), wherein the fuel cell system is a component of an air conditioner (12) of a motor vehicle (10) drivable by a drive power unit (94) and supplies the air conditioner with electrical energy , characterized in that the reformer (18) is regenerated when at least one parameter which is characteristic of an external temperature of the motor vehicle (10) is within a predetermined parameter interval.

2. The method according to claim 1, characterized in that the for the outside temperature of the motor vehicle (10) characteristic parameter is detected by an outside temperature sensor outside temperature value and the parameter interval is limited upwards by a maximum temperature.

3. The method according to claim 1 or 2, characterized in that the for the outside temperature of the motor vehicle (10) characteristic parameter is supplied by a real time clock time and the parameter interval indicates a recurring period in which, compared to other periods, especially low

Outdoor temperature is expected.

4. The method according to any one of the preceding claims, characterized in that the presence of the for the Außenentempe- is the necessary condition for regeneration of the reformer of the motor vehicle (10) characteristic parameter within the predetermined parameter interval.

5. The method according to any one of the preceding claims, characterized in that the presence of the for the outside temperature of the motor vehicle (10) characteristic parameter within the predetermined parameter interval sufficient condition for a regeneration of the reformer.

6. Air conditioning system (12) for stationary air conditioning of a drive unit (94) drivable motor vehicle (10), with a reformer (18) having fuel cell lens system (26) for operating a refrigeration circuit (16) and a control unit for controlling the fuel cell system, characterized in that the controller is adapted to control a method according to any one of the preceding claims.