Method for controlling nitrogen fraction in a fuel cell

1. A method of operating a fuel cell system having at least one fuel cell stack with an anode flow path having a bleed device downstream of an active area of the fuel cell stack, the method comprising:

(a) operating the fuel cell stack to produce a voltage output;

(b) determining a variance in a voltage output of at least one fuel cell of the fuel cell stack; and

(c) venting anode effluent from the anode flow path based upon a correlation between said variance and nitrogen fouling of said at least one fuel cell of the fuel cell stack,

wherein (c) includes venting said anode effluent for a predetermined period of time based upon said variance.

2. The method of claim 1 , wherein the bleed device is operable between open and closed positions and (c) includes moving the bleed device to said open position for said predetermined period of time.

3. A method of operating a fuel cell system having at least one fuel cell stack with an anode flow path having a bleed device downstream of an active area of the fuel cell stack, the method comprising:

(a) operating the fuel cell stack to produce a voltage output;

(b) determining a variance in a voltage output of at least one fuel cell of the fuel cell stack; and

(c) venting anode effluent from the anode flow path based upon a correlation between said variance and nitrogen fouling of said at least one fuel cell of the fuel cell stack.

4. The method of claim 3 , wherein (c) includes comparing said variance to a predetermined standard indicative of nitrogen fouling in the fuel cell stack and venting said anode effluent based upon said comparison.

5. The method of claim 3 , wherein (c) includes using a proportional controller and said variance to determine a venting requirement of the anode flow path of the fuel cell stack to avoid nitrogen fouling of the fuel cell stack.

6. The method of claim 3 , wherein (c) includes using a PI controller and said variance to determine a venting requirement of the anode flow path of the fuel cell stack to avoid nitrogen fouling of the fuel cell stack.

7. The method of claim 3 , wherein the bleed device is a proportional device and (c) includes opening the bleed device in proportion to said variance to vent said anode effluent from the anode flow path.

8. The method of claim 3 , wherein (c) includes using a controller having at least two set points, venting said anode effluent when said variance exceeds a first of said set points and ceasing to vent said anode effluent when said variance is below a second of said set points.

9. The method of claim 3 , wherein (b) includes determining a variance in a voltage output of a group of fuel cells of the fuel cell stack and (c) includes venting anode effluent from the anode flow path based upon a correlation between said variance and nitrogen fouling of said group of fuel cells of the fuel cell stack.

10. The method of claim 3 , wherein (b) includes determining a quadratic variance in a voltage output of at least one fuel cell of the fuel cell stack.

11. The method of claim 3 , wherein (c) includes continuously venting a portion of said anode effluent and adjusting said venting of said anode effluent based upon said correlation.

12. The method of claim 3 , wherein (b) includes determining a variance in a voltage output of said at least one fuel cell at two or more different points in time.

13. The method of claim 3 , wherein (b) includes determining a variance in a voltage output between at least two fuel cells of the fuel cell stack by comparing the voltage output of a first fuel cell and at least a second fuel cell at substantially a same point in time.

14. The method of claim 3 , wherein (c) includes creating a pressure differential of at least a predetermined magnitude across the bleed device prior to venting anode effluent.

15. A method of operating a fuel cell stack to avoid nitrogen fouling, the method comprising:

(a) operating the fuel cell stack to produce a voltage output;

(b) determining a variance in a voltage output of a plurality of fuel cells of the fuel cell stack during operation of the fuel cell stack;

(c) determining a venting requirement for the fuel cell stack to avoid nitrogen fouling of the fuel cell stack based upon a correlation between said variance and nitrogen fouling of the fuel cell stack; and

(d) selectively venting anode effluent from the fuel cell stack to meet said determined venting requirement.

16. The method of claim 15 , wherein (c) includes comparing said variance to a predetermined standard and determining said venting requirement based upon said comparison.

17. The method of claim 16 , wherein (c) includes determining a time duration to vent said anode effluent and (d) includes venting said anode effluent for said time duration.

18. The method of claim 16 , wherein (c) includes comparing said variance to first and second set points and (d) includes venting said anode effluent when said variance exceeds said first set point and ceasing to vent said anode effluent when said variance is below said second set point.

19. The method of claim 15 , wherein (d) includes continuously venting a portion of said anode effluent and adjusting a rate of venting said anode effluent based upon said determined venting requirement.

20. The method of claim 15 , wherein (b) includes determining a variance in a voltage output of said plurality of fuel cells at two or more different points in time.

21. The method of claim 15 , wherein (b) includes determining a variance in a voltage output of said plurality of fuel cells by comparing the voltage output of a first fuel cell and at least a second fuel cell of said plurality of fuel cells at substantially a same point in time.

22. The method of claim 15 , wherein (d) includes creating a pressure differential of at least a predetermined magnitude across a bleed device prior to venting anode effluent.