Method and apparatus for operating a fuel cell

• US 6,461,751 B1
• Filed: 12/06/1999
• Issued: 10/08/2002
• Est. Priority Date: 12/06/1999
• Status: Expired due to Fees

• Current Assignee: BDF IP Holdings Ltd. (Siemens AG)
• Original Assignee: Ballard Power Systems Corporation (Siemens AG)
• Inventors: Schamm, Reinhold, Knights, Shanna, Wilkinson, David P., Boehm, Gustav, Fletcher, Nicholas J.
• Primary Examiner(s): Ryan, Patrick
• Assistant Examiner(s): Crepeau, Jonathan
• Application Number:
  US09/455,033
• Time in Patent Office: 1,037 Days
• Field of Search: 429/13, 429/22, 429/23, 429/30
• US Class Current: 429/432
• CPC Class Codes: H01M 8/04089 of gaseous reactants
• CPC Class Codes: Y02E 60/50 Fuel cells

Abstract

The present invention relates to improving the overall efficiency of a fuel cell system by reducing parasitic power consumption. A controller is programmed to decrease oxidant stoichiometry until oxidant starvation is detected or until oxidant stoichiometry is about one. When oxidant starvation is detected, the oxidant stoichiometry is increased until oxidant starvation is no longer detected. The fuel cell system employs a sensor for detecting an operational characteristic such as voltage output, or oxygen or hydrogen concentration in the cathode exhaust stream. The controller uses the operational characteristic to calculate oxidant stoichiometry or to determine when there is oxidant starvation at the cathode.

Claims

1. A method of operating a fuel cell, wherein said fuel cell has a cathode supplied with an oxidant stream and an anode supplied with a fuel stream, said method comprising:

(a) monitoring a cathode exhaust stream downstream of said cathode to detect hydrogen gas concentration; and

(b) decreasing oxidant stoichiometry when said hydrogen gas concentration is less than a first threshold concentration.

2. The method of claim 1 wherein said fuel cell is one of a plurality of fuel cells arranged in a fuel cell stack and said cathode exhaust stream is monitored downstream of a plurality of cathodes associated with said plurality of fuel cells.

3. The method of claim 1 further comprising increasing said oxidant stoichiometry when said hydrogen gas concentration is greater than a second threshold concentration.

4. The method of claim 3 wherein said first threshold concentration is the lower detection limit of a hydrogen sensor used to monitor said cathode exhaust stream.

5. The method of claim 3 wherein said second threshold concentration is about 20 ppm of hydrogen.

6. The method of claim 3 wherein said second threshold concentration is greater than said first threshold concentration.

7. The method of claim 3 wherein said oxidant stoichiometry is adjusted by adjusting the oxidant concentration in said oxidant stream supplied to said cathode.

8. The method of claim 3 wherein said oxidant stoichiometry is adjusted by adjusting the electrical power output of said fuel cell.

9. The method of claim 3 wherein said oxidant stoichiometry is adjusted by adjusting the mass flow rate of said oxidant stream supplied to said cathode.

10. The method of claim 9 wherein said oxidant stream mass flow rate is adjusted by adjusting the speed of a mechanical device which supplies said oxidant stream to said cathode.

11. The method of claim 9 wherein said oxidant stream mass flow rate is adjusted by a fixed amount or by a fixed percentage of the instantaneous oxidant stream mass flow rate.

12. The method of claim 9 wherein said oxidant stoichiometry is adjusted by adjusting said oxidant stream mass flow rate by an amount that is dependent upon the magnitude of the detected hydrogen gas concentration.

13. The method of claim 1 further comprising steps for reducing said hydrogen gas concentration within said cathode exhaust stream when said hydrogen gas concentration is greater than a second threshold concentration, wherein said steps comprise comparing said oxidant stream mass flow rate to a maximum desired mass flow rate, and

(a) if said oxidant stream mass flow rate is less than said maximum desired mass flow rate, increasing said oxidant mass flow rate; and

(b) if said oxidant mass flow rate is greater than or equal to said maximum desired mass flow rate, ceasing operation of said fuel cell if said hydrogen gas concentration is greater than a third concentration threshold which is greater than said first and second concentration thresholds; and

generating a warning signal and continuing to operate said fuel cell if said hydrogen gas concentration is less than said third concentration threshold.

14. The method of claim 3 wherein said method further comprises taking no steps to change said oxidant stoichiometry when said hydrogen gas concentration is between said first and second threshold concentrations.

15. The method of claim 9 further comprising, after increasing said oxidant mass flow rate, comparing said oxidant mass flow rate to a desired oxidant mass flow rate for the instantaneous fuel cell electrical power output, and generating a warning signal or ceasing operation of said fuel cell if said oxidant mass flow rate is more than a predetermined amount greater than said desired oxidant mass flow rate.

16. The method of claim 13 wherein said desired oxidant mass flow rate is determined from a look-up table.

17. The method of claim 1 wherein said monitoring comprises continuously monitoring said cathode exhaust stream for said hydrogen gas concentration and determining whether said hydrogen gas concentration is increasing or decreasing, and when said hydrogen gas concentration is greater than a second threshold concentration, said method further comprises:

maintaining a substantially constant oxidant stoichiometry when said hydrogen concentration is decreasing; and

increasing said oxidant stoichiometry when said hydrogen concentration is increasing.

18. The method of claim 17 further comprising:

generating a warning signal if said hydrogen gas concentration is greater than said first threshold concentration, said hydrogen gas concentration within said cathode exhaust stream is increasing, and said oxidant stream is flowing at a maximum desired mass flow rate.

19. The method of claim 18 further comprising controlling said fuel stream to decrease fuel stream pressure when said warning signal is generated.

20. The method of claim 19 further comprising monitoring electrical power output of said fuel cell, and continuing to operate said fuel cell if said electrical power output is not less than electrical power demand, and ceasing operation of said fuel cell if said electrical power output is a predetermined amount less than said electrical power demand.

21. The method of claim 19 further comprising checking electrical power output and,

continuing to operate said fuel cell if said electrical power output is not less than electrical power demand, and

generating a warning signal if said electrical power output is less than said electrical power demand and then selecting between operating at a reduced electrical power output and ceasing operation of said fuel cell.

22. The method of claim 1 wherein said monitoring comprises continuously monitoring said cathode exhaust stream for said hydrogen gas concentration and determining whether said hydrogen gas concentration is increasing or decreasing, and when said hydrogen gas concentration is greater than a second threshold concentration, said method further comprises:

maintaining a constant oxidant stoichiometry when said hydrogen concentration is decreasing; and

measuring fuel cell voltage and comparing said fuel cell voltage to a voltage threshold value, and

if said fuel cell voltage exceeds said voltage threshold value and said hydrogen gas concentration is increasing, decreasing the pressure of said fuel stream;

if said fuel cell voltage is less than said voltage threshold value, said hydrogen gas concentration is increasing, and oxidant mass flow rate is less than a desired maximum, then increasing said oxidant stoichiometry; and

if said fuel cell voltage is less than said voltage threshold value, said hydrogen gas concentration is increasing, and oxidant mass flow rate is greater than or equal to a desired maximum, then decreasing the pressure of said fuel stream.

23. The method of claim 22 further comprising regulating fluid pressure of said oxidant and fuel streams to increase a pressure differential between said oxidant and fuel streams.

24. The method of claim 22 further comprising regulating fluid pressure of said oxidant and fuel streams to reduce a pressure differential between said oxidant and fuel streams.

25. The method of claim 22 wherein said voltage threshold value is about 100 millivolts.

26. The method of claim 22 further comprising ceasing operation of said fuel cell when electrical power output is less than electrical power demand.

27. The method of claim 1 wherein said monitoring is performed periodically.

28. The method of claim 9 wherein said oxidant stream mass flow rate is reduced until said second predetermined threshold concentration of said hydrogen gas is detected and then increasing said oxidant mass flow rate by a predetermined percentage so that said oxidant stoichiometry is a predetermined percentage higher than about one.

29. The method of claim 28 wherein said predetermined percentage is less than 50%.