Remedial actions for air flow errors in a fuel cell system

1. A method for operating a fuel cell stack of a fuel cell system in a vehicle, said method comprising:

estimating a cathode air flow rate from cathode inlet air mass flow measurements;

detecting an error in the estimated cathode air flow rate by comparing the estimated cathode air flow rate to a cathode air flow model that is based on output power of the fuel cell stack;

determining whether a predetermined minimum cell voltage threshold value of voltage of fuel cells in the fuel cell stack has been reached;

adjusting the cathode air flow rate to the fuel cell stack to be a cathode air flow set point if the predetermined minimum cell voltage threshold value has been reached; and

utilizing high frequency resistance measurements from a high frequency resistance sensor to control a relative humidity of the fuel cell stack when the error in the estimated cathode air flow rate has been detected.

2. The method according to claim 1 further comprising increasing a minimum load that can be applied to the fuel cell system once the error in the estimated cathode air flow rate has been detected.

3. The method according to claim 2 wherein increasing the minimum load includes charging a battery of the fuel cell system.

4. The method according to claim 3 further comprising placing the fuel cell stack in an idle or shutdown mode and operating the vehicle using battery power if a state of charge of the battery is approximately 100%.

5. The method according to claim 1 wherein the cathode air flow set point is a minimum cathode air flow rate that is needed to draw a predetermined current density.

6. The method according to claim 5 further comprising increasing the minimum cathode air flow rate that is needed to draw a predetermined current density until a minimum cell voltage increases above the predetermined minimum cell voltage threshold value.

7. The method according to claim 6 further comprising maintaining the increased minimum cathode air flow rate until the detected cathode air flow rate estimation error fault condition is cleared.

8. A method for operating a fuel cell stack of a fuel cell system in a vehicle, said method comprising:

estimating a cathode air flow rate from cathode inlet air mass flow measurements;

detecting an error in the estimated cathode air flow rate by comparing the estimated cathode air flow rate to a cathode air flow model that is based on power output of the fuel cell stack; and

utilizing high frequency resistance measurements from a high frequency resistance sensor to control a relative humidity of the fuel cell stack when the error in the estimated cathode air flow rate has been detected.

9. The method according to claim 8 further comprising determining whether a predetermined minimum cell voltage threshold value of voltage of fuel cells in the fuel cell stack has been reached, wherein the cathode air flow rate to the fuel cell stack is adjusted to be a cathode air flow set point if the predetermined minimum cell voltage threshold value has been reached.

10. The method according to claim 8 further comprising increasing a minimum load that can be applied to the fuel cell system once the error in the estimated cathode air flow rate has been detected.

11. The method according to claim 10 wherein increasing the minimum load includes charging a battery of the fuel cell system.

12. The method according to claim 11 further comprising placing the fuel cell stack in an idle or shutdown mode and operating the vehicle using battery power if a state of charge of the battery is approximately 100%.

13. The method according to claim 9 wherein the cathode air flow set point is a minimum cathode air flow rate threshold value that is needed to draw a predetermined current density.

14. The method according to claim 13 further comprising increasing the minimum cathode air flow rate threshold value until a minimum cell voltage increases above the predetermined minimum cell voltage threshold value and maintaining the increased minimum cathode air flow rate until the detected cathode air flow rate estimation error fault condition is cleared.

15. A system for operating a fuel cell stack of a fuel cell system in a vehicle, said system comprising:

a controls unit that includes:

means for estimating a cathode air flow rate from cathode inlet air mass flow measurements;

means for detecting an error in the estimated cathode air flow rate by comparing the estimated cathode air flow rate to a cathode air flow model that is based on power output of the fuel cell stack; and

means for utilizing high frequency resistance measurements from a high frequency resistance sensor to control a relative humidity of the fuel cell stack when the error in the estimated cathode air flow rate has been detected.

16. The system according to claim 15 further comprising means for determining whether a predetermined minimum cell voltage threshold value of voltage of fuel cells in the fuel cell stack has been reached, wherein the cathode air flow rate to the fuel cell stack is adjusted to be a cathode air flow set point if the predetermined minimum cell voltage threshold value has been reached.

17. The system according to claim 15 further comprising increasing a minimum load that can be applied to the fuel cell system once an error in an estimated cathode air flow rate has been detected.

18. The system according to claim 17 wherein increasing the minimum loads includes charging a battery of the fuel cell system.

19. The system according to claim 18 further comprising means for placing the fuel cell stack in an idle or shutdown mode and operating the vehicle using battery power if a state of charge of the battery is near a maximum state of charge.

20. The system according to claim 16 wherein the cathode air flow set point is a minimum cathode air flow rate that is needed to draw a predetermined current density.