Fuel cell stack and method of operating a fuel cell system with such a fuel cell stack

1. A method of operating a fuel cell stack comprising a plurality of fuel cells, each said fuel cell having an anode defining an anode side, a cathode defining a cathode side and a membrane arranged between said anode and said cathode, each said fuel cell being arranged between first and second bipolar plates, there being flow fields at said anode sides for a gaseous fuel supplied to said fuel cells and flow fields at said cathode sides for a gaseous oxidizing agent fed into said fuel cells and catalyst material within said fuel cells at which said fuel and said oxidizing agent react for the production of electrical energy with the simultaneous production of water, said fuel cells being connected together and having an upper anode space at a top of said fuel cell stack and a lower anode space at a bottom of said fuel cell stack, said anode side flow fields extending between said upper anode space and said lower anode space, said method comprising the steps of:

collecting water produced during the electrochemical reaction and present in the form of a liquid in or adjacent to said lower anode space,

evaporating at least a portion of said liquid water for the humidification of supplied fuel in or adjacent to said lower anode space,

supplying the fuel humidified in or adjacent to said lower anode space to said upper anode space,

feeding at least a part of said humidified fuel from said upper anode space into said anode side flow fields, and

returning a part of said humidified fuel supplied to said upper anode space via a restriction to said lower anode space.

2. A method in accordance with claim 1 , and comprising the further step of receiving said liquid water in a porous material to provide an enlarged evaporation surface.

3. A method in accordance with claim 2 , and comprising the further step of mixing fresh fuel supplied to said lower anode space by said fuel supply device with non-consumed moist fuel emerging from the anode side flow fields.

4. A method in accordance with claim 3 , and comprising the further step of discharging a part of the gases flowing at said anode sides of said fuel cells one of discontinuously or continuously in order to reduce an amount of nitrogen at said anode sides.

5. A method in accordance with claim 4 , and comprising the step of stopping during said discharging step a return of a part of said fuel supplied to the upper anode space into the lower anode space via said restriction.

6. A method in accordance with claim 5 , said stopping step comprising the step of a closing a restrictor valve forming said restriction.

7. A method in accordance with claim 5 , and comprising the further step of closing a line extending from said lower anode space to said upper anode space for supplying fuel humidified in said lower anode space to said upper anode space during the discharging of said gases flowing at the anode side.

8. A method in accordance with claim 1 , and comprising the further step of one of continuously and discontinuously discharging liquid water from said lower anode space.

9. A method in accordance with claim 1 , and comprising the further step of supplying additional water to said anode side of said fuel cell stack.

10. A method in accordance with claim 9 , said additional water being supplied to said anode sides of said fuel cells from said cathode sides of said fuel cells.

11. A method of operating a fuel cell stack comprising a plurality of fuel cells, each said fuel cell having an anode defining an anode side, a cathode defining a cathode side and a membrane arranged between said anode and said cathode, each said fuel cell being arranged between first and second bipolar plates, there being flow fields at said anode sides for a gaseous fuel supplied to said fuel cells and flow fields at said cathode sides for a gaseous oxidizing agent fed into said fuel cells and catalyst material within said fuel cells at which said fuel and said oxidizing agent react for the production of electrical energy with the simultaneous production of water, said fuel cells being connected together and having an upper anode space at a top of said fuel cell stack and a lower anode space at a bottom of said fuel cell stack, said anode side flow fields extending between said upper anode space and said lower anode space, said method comprising the steps of:

collecting water produced during the electrochemical reaction and present in the form of a liquid in or adjacent to said lower anode space,

evaporating at least a portion of said liquid water for the humidification of supplied fuel in or adjacent to said lower anode space,

supplying the fuel humidified in or adjacent to said lower anode space to said upper anode space,

feeding at least a part of said humidified fuel from said upper anode space into said anode side flow fields, and

heating water collected in a collecting container.

12. A method in accordance with claim 11 , said heating step being effected by feeding a heat transporting medium through a heat exchanger provided in said water in said collecting container.

13. A method in accordance with claim 12 , said heating step being effected using a hot coolant used in a coolant circuit of said fuel cell stack as said heat transporting medium.

14. A method in accordance with claim 12 , said heating step being carried out electrically in addition to heating effected by said heat transporting medium.

15. A method in accordance with claim 11 , said heating step being carried out electrically.

16. A method of operating a fuel cell stack comprising a plurality of fuel cells, each said fuel cell having an anode defining an anode side, a cathode defining a cathode side and a membrane arranged between said anode and said cathode, each said fuel cell being arranged between first and second bipolar plates, there being flow fields at said anode sides for a gaseous fuel supplied to said fuel cells and flow fields at said cathode sides for a gaseous oxidizing agent fed into said fuel cells and catalyst material within said fuel cells at which said fuel and said oxidizing agent react for the production of electrical energy with the simultaneous production of water, said fuel cells being connected together and having an upper anode space at a top of said fuel cell stack and a lower anode space at a bottom of said fuel cell stack, said anode side flow fields extending between said upper anode space and said lower anode space, said method comprising the steps of:

providing fresh fuel to the fuel cell stack,

directly contacting and mixing non-consumed fuel from the anode side flow field with said fresh fuel to provide a mixed feed stream,

collecting water produced during the electrochemical reaction and present in the form of a liquid in or adjacent to said lower anode space,

passing at least a portion of said mixed stream adjacent said collected water to exploit evaporation from said liquid water for the humidification of said mixed stream in or adjacent to said lower anode space,

supplying said mixed stream humidified in or adjacent to said lower anode space to said upper anode space, and

feeding at least a part of said humidified mixed stream from said upper anode space into said anode side flow fields.