Method of shut-down and starting of a fuel cell

1. A method of operating a fuel cell system connected to a variable load, said fuel cell system including an anode feed circuit and a cathode feed circuit for feeding of an anode side with an anode gas and for feeding a cathode side with a cathode gas, said method being adapted for shutting down an electricity generating chemical reaction of the fuel cell system, said method comprising:

decreasing the supply of anode gas and cathode gas in response to a shut-down signal so that the electricity generating chemical reaction reduces the anode gas pressure and the cathode gas pressure to well below ambient pressure;

monitoring the anode gas pressure and the cathode gas pressure;

monitoring an output voltage of at least one cell of a fuel cell stack;

after the decreasing the supply of anode and cathode gas in response to the shut-down signal, reducing the load in response to the output voltage falling below a predetermined range so that the output voltage returns to the predefined voltage range;

after the reducing the load, further reducing the load in response to the output voltage falling below the predetermined range;

electrically shunting the at least one cell of the fuel cell stack in response to the output voltage reaching a predefined voltage level;

further reducing the pressure of at least the anode side to a predefined pressure level; and

filling and/or flushing of at least the anode side with an inert gas.

2. The method according to claim 1 , wherein the step of further reducing the pressure of at least the anode side uses a recirculation pump.

3. The method according to claim 2 , wherein the pressure of the anode side and the cathode side of the fuel cell system is reduced to 0.3 bar absolute before using the recirculation pump to further reduce the pressure of at least the anode side.

4. The method according to claim 1 , wherein the difference of the pressure of the anode gas and the pressure of the cathode gas is kept below 0.5 bar.

5. The method according to claim 1 , wherein the pressures of the anode gas and cathode gas are reduced below 0.02 bar before the anode side and/or cathode side are filled and/or flushed with the inert gas.

6. The method according to claim 1 , wherein during reduction of pressure of anode side and cathode side, the cathode gas is released to the environment and/or the anode gas is fed to a storage vessel.

7. The method according to claim 1 , wherein, when the total pressure in the cathode side is in a range between 0.5 to 0.7 bar, the cathode side is temporarily opened to the ambient atmosphere for ingress of ambient air.

8. The method according to claim 1 , wherein nitrogen or nitrogen-rich gas is used as inert gas.

9. The method according to claim 7 , wherein after reduction of pressure of only the anode side, said anode side is filled and/or flushed with inert gas from an inert gas storage vessel.

10. The method according to claim 8 , wherein after reduction of pressure of only the anode side, said anode side is filled and/or flushed with inert gas from an inert gas storage vessel.

11. The method according to claim 7 , wherein nitrogen or nitrogen enriched gas residual of the reaction of the anode gas and cathode gas, is at least partially fed to a storage vessel.

12. The method according to claim 8 , wherein nitrogen or nitrogen enriched gas residual of the reaction of the anode gas and cathode gas, is at least partially fed to a storage vessel.

13. The method according to claim 9 , wherein nitrogen or nitrogen enriched gas residual of the reaction of the anode gas and cathode gas, is at least partially fed to a storage vessel.

14. The method according to claim 10 , wherein nitrogen or nitrogen enriched gas residual of the reaction of the anode gas and cathode gas, is at least partially fed to a storage vessel.

15. The method according to claim 7 , wherein the anode side is filled and/or flushed with nitrogen or nitrogen enriched gas from a storage vessel and/or from the cathode side.

16. A method of operating a fuel cell system including an anode feed circuit and a cathode feed circuit for feeding an anode side with an anode gas and for feeding a cathode side with a cathode gas, said method comprising:

shutting down an electricity generating chemical reaction of the fuel cell system as claimed in claim 1 ; and

after the shutting down, starting an electricity generating operation of the fuel cell system, said starting comprising:

drawing off the inert gas from the anode side and the cathode side for a predefined time interval or until a predefined pressure level has been reached;

filling and/or flushing of the anode side with the anode gas and thereafter;

filling and/or flushing of the cathode side with the cathode gas; and

removing of the electrical shunt,

wherein recirculation pumps of the anode side and/or cathode side are used to reduce pressure of anode and/or cathode sides and/or are used to fill and/or to flush the anode side and/or the cathode side with inert gas.

17. The method according to claim 16 , wherein the pressure difference between the anode side and the cathode side is maintained below 0.5 bar.

18. The method according to claim 16 , wherein the inert gas is fed to an inert gas storage vessel.

19. The method according to claim 17 , wherein the inert gas is fed to an inert gas storage vessel.

20. A fuel cell system, comprising:

an anode feed circuit;

a cathode feed circuit for feeding of an anode side with an anode gas and for feeding a cathode side with a cathode gas,

wherein, for shutting down and for starting up of an electricity-generating operation, the fuel cell system further includes

an inert gas storage vessel, and

at least one recirculation pump to reduce the pressure of at least the anode side to a predefined pressure level, and to fill and/or to flush at least the anode side with the inert gas,

wherein the fuel cell system is configured to

decrease the supply of anode gas and cathode gas in response to a shut-down signal so that the electricity generating chemical reaction reduces the anode gas pressure and the cathode gas pressure to well below ambient pressure;

monitor the anode gas pressure and the cathode gas pressure;

monitor an output voltage of at least one cell of a fuel cell stack;

after decreasing the supply of anode and cathode gas in response to the shut-down signal, reduce the load in response to the output voltage falling below a predetermined range so that the output voltage returns to the predefined voltage range;

after reducing the load, further reduce the load in response to the output voltage falling below the predetermined range;

electrically shunt the at least one cell of the fuel cell stack in response to the output voltage reaching a predefined voltage level;

further reduce the pressure of at least the anode side to a predefined pressure level; and

fill and/or flush at least the anode side with an inert gas.

21. The method according to claim 1 , wherein the reducing the load includes reducing the load a plurality of times so that the output voltage remains in a predefined voltage range until the load is finally disconnected.

22. The method according to claim 1 , wherein the predefined voltage range is between 0.3 V/cell and 0.8 V/cell and the predefined voltage level is 0.3 V/cell.

23. The method according to claim 1 , wherein the electrically shunting the at least one cell of the fuel cell stack includes electrically shunting the at least one cell via a cell-specific resistor.

24. The fuel cell system according to claim 20 ,

wherein the fuel cell system is configured to

draw off the inert gas from the anode side and the cathode side for a predefined time interval or until a predefined pressure level has been reached;

fill and/or flush the anode side with the anode gas and thereafter;

fill and/or flush the cathode side with the cathode gas; and

remove the electrical shunt,

wherein recirculation pumps of the anode side and/or cathode side are used to reduce pressure of anode and/or cathode sides and/or are used to fill and/or to flush the anode side and/or the cathode side with inert gas.