Efficiency based stand-by mode for fuel cell propulsion systems

1. A method for operating a fuel cell system in a system stand-by mode, said method comprising:

determining a power limit value when a system power request falls below the power limit value the system will enter the system stand-by mode;

determining that the system power request has fallen below the power limit value;

entering a dynamic stand-by mode where a fuel cell stack in the fuel cell system is turned off and a compressor providing cathode air to a cathode side of the fuel cell stack is operated at an idle speed;

accumulating a compressor power value identifying how much energy has been consumed by operating the compressor at the idle speed during the dynamic stand-by mode; and

entering a static stand-by mode when the accumulated compressor power value reaches a compressor restart energy value that identifies how much energy it takes to start the compressor, wherein entering the static stand-by mode includes turning the compressor off.

2. The method according to claim 1 wherein determining a power limit value includes determining the power limit value based on an optimization between fuel cell stack power and battery power.

3. The method according to claim 2 wherein the optimization between the fuel cell stack power and battery power includes an optimization for charging the battery using fuel cell stack power until the battery reaches a predetermined battery state-of-charge.

4. The method according to claim 1 wherein entering a dynamic stand-by mode includes delaying the time that the system enters the dynamic stand-by mode from when the system power request falls below the power limit value.

5. The method according to claim 4 wherein delaying the time includes delaying the time the system enters the dynamic stand-by mode from 1-2 seconds.

6. The method according to claim 1 further comprising exiting the system stand-by mode when the system power request rises above the power limit value.

7. The method according to claim 1 wherein the compressor includes air bearings.

8. The method according to claim 1 where the fuel cell system is on a vehicle.

9. A method for operating a fuel cell system in a system stand-by mode, said fuel cell system including a fuel cell stack, a high voltage battery and a compressor that provides cathode air to a cathode side of the fuel cell stack, said method comprising:

determining a power limit value when a system power request falls below the power limit value the system will enter the system stand-by mode, wherein the power limit value is determined based on an optimization between fuel cell stack power and battery power that includes using the fuel cell stack power to charge the battery so that the battery state-of-charge is at a predetermined value;

determining that the system has fallen below the optimization power limit value;

entering a dynamic stand-by mode after a predetermined time period has passed from when the system power request falls below the optimization power limit value where the fuel cell stack is turned off and the compressor is operated at an idle speed, wherein entering the dynamic stand-by mode includes delaying the time that the system enters the dynamic stand-by mode from when the system power request falls below the power limit value;

accumulating a compressor power value identifying how much energy has been consumed by operating the compressor at the idle speed;

entering a static stand-by mode when the accumulated compressor power value reaches a compressor restart value that identifies how much energy it takes to start the compressor, wherein entering the static stand-by mode includes turning the compressor off; and

exiting the stand-by mode when the power request rises above the optimization power limit value.

10. The method according to claim 9 wherein delaying the time includes delaying the time the system enters the dynamic stand-by mode between 1-2 seconds.

11. The method according to claim 9 wherein the compressor includes air bearings.

12. The method according to claim 9 where the fuel cell system is on a vehicle.

13. A control system for operating a fuel cell system in a system stand-by mode, said control system comprising:

means for determining a power limit value when a system power request falls below the power limit value the system will enter the system stand-by mode;

means for determining that the system power request has fallen below the power limit value;

means for entering a dynamic stand-by mode where a fuel cell stack in the fuel cell system is turned off and a compressor providing cathode air to a cathode side of the fuel cell stack is operated at an idle speed;

means for accumulating a compressor power value identifying how much energy has been consumed by operating the compressor at the idle speed during the dynamic stand-by mode; and

means for entering a static stand-by mode when the accumulated compressor power value reaches a compressor restart energy value that identifies how much energy it takes to start the compressor, wherein entering the static stand-by mode includes turning the compressor off.

14. The control system according to claim 13 wherein the means for determining a power limit value determines the power limit value based on an optimization between fuel cell stack power and battery power.

15. The control system according to claim 14 wherein the optimization between the fuel cell stack power and battery power includes an optimization for charging the battery using fuel cell stack power until the battery reaches a predetermined battery state-of-charge.

16. The control system according to claim 13 wherein the means for entering a dynamic stand-by mode delays the time that the system enters the dynamic stand-by mode from when the system power request falls below the power limit value.

17. The control system according to claim 16 wherein the means for entering a dynamic stand-by mode delays the time the system enters the dynamic stand-by mode from 1-2 seconds.

18. The control system according to claim 13 further comprising means for exiting the system stand-by mode when the system power request rises above the power limit value.

19. The control system according to claim 13 wherein the compressor includes air bearings.

20. The control system according to claim 13 where the fuel cell system is on a vehicle.