Method and apparatus for improving the performance of a fuel cell electric power system

• US 20040224192A1
• Filed: 05/06/2003
• Published: 11/11/2004
• Est. Priority Date: 05/06/2003
• Status: Active Grant

• Current Assignee: Ballard Power Systems Corporation (Superior Plus Corporation)
• Original Assignee: Ballard Power Systems Corporation (Superior Plus Corporation)
• Inventors: Pearson, Martin T.
• Granted Patent: US 7,632,583 B2
• Time in Patent Office: Days
• US Class Current: 429/13
• CPC Class Codes: H01M 16/006 of fuel cells with rechargeable batteries
• CPC Class Codes: H01M 8/04228 during shut-down
• CPC Class Codes: H01M 8/0488 of fuel cell stacks
• CPC Class Codes: H01M 8/0491 of fuel cell stacks
• CPC Class Codes: H01M 8/0494 of fuel cell stacks
• CPC Class Codes: H01M 8/04947 of auxiliary devices, e.g. batteries, capacitors
• CPC Class Codes: H01M 8/241 with solid or matrix-supported electrolytes
• CPC Class Codes: H01M 8/2465 Details of groupings of fuel cells
• CPC Class Codes: Y02E 60/10 Energy storage using batteries
• CPC Class Codes: Y02E 60/50 Fuel cells

Abstract

Current pulsing improves the performance of fuel cells in a fuel cell stack based power system. Voltage clamping limits the voltage peaks that occur after a current pulse. In a hybrid power system, an electric storage device supplies the loads during current pulsing. The electric storage device may sink current to achieve the voltage clamping, and/or power system may employ other the voltage clamping circuits.

Claims

1. A method of operating a power system comprising a stack of fuel cells, the method comprising:

from time-to-time,

current pulsing the stack of fuel cells; and

clamping a fuel cell voltage below a maximum fuel cell voltage level during at least a period after current pulsing the stack of fuel cells.

2. The method of claim 1 wherein current pulsing the stack of fuel cells comprises:

providing a short circuit path across the stack of fuel cells; and

removing the short circuit path across the stack of fuel cells.

3. The method of claim 1 wherein current pulsing the stack of fuel cells comprises:

electrically coupling a load across the stack of fuel cells; and

electrically uncoupling the load from across the stack of fuel cells.

4. The method of claim 1 wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to short the stack of fuel cells on a periodic basis.

5. The method of claim 1 wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to provide a short circuit path across the stack of fuel cells approximately once every minute.

6. The method of claim 1 wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to provide a short circuit path across the stack of fuel cells approximately once every minute, and to remove the short circuit path approximately 500 milliseconds after providing the short circuit path across the stack of fuel cells.

7. The method of claim 1, further comprising:

from time-to-time, determining a voltage across the stack of fuel cells, and wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to provide a short circuit path across the stack of fuel cells when the determined voltage across the stack of fuel cells exceeds a threshold voltage.

8. The method of claim 1, further comprising:

from time-to-time, determining an actual charge state of an electrical storage device electrically coupled to the stack of fuel cells; and

determining a frequency of operation of at least one switch electrically coupled across a power bus of the stack of fuel cells to maintain a desired charge state of the electrical storage device based on the determined charge state; and

wherein current pulsing the stack of fuel cells comprises operating the at least one switch electrically coupled across the output of the bus of the stack of fuel cells at the determined frequency to provide and to remove a short circuit path across the stack of fuel cells at the determined frequency.

9. The method of claim 1, further comprising:

determining a load condition; and

determining a frequency of operation of at least one switch electrically coupled across a power bus of the stack of fuel cells based on the determined load condition, and wherein current pulsing the stack of fuel cells comprises operating the at least one switch electrically coupled across the output of the bus of the stack of fuel cells at the determined frequency to provide and to remove a short circuit path across the stack of fuel cells at the determined frequency.

10. The method of claim 1 wherein clamping a fuel cell voltage below a maximum fuel cell voltage level comprises supplying power to the load from the electrical storage device before current pulsing the stack of fuel cells, and recharging the electrical storage device from the fuel cell stack for a period after current pulsing the stack of fuel cells.

11. The method of claim 1, further comprising:

clamping the fuel cell voltage above a minimum fuel cell voltage level.

12. A method of operating a power system to power at least one load, the power system comprising a fuel cell stack and at least one electrical storage device electrically coupled in parallel with the fuel cell stack, the method comprising:

temporarily shorting the fuel cell stack from time-to-time; and

supplying power from the electrical storage device to the load at least while shorting the fuel cell stack.

13. The method of claim 12, further comprising:

clamping a voltage on the fuel cells below a maximum fuel cell stack voltage level.

14. The method of claim 12, further comprising:

supplying current from the electrical storage device to the load for a period prior to shorting the fuel cell stack; and

supplying current from the fuel cell stack to the electrical storage device after the shorting of the fuel cell stack to clamp a fuel cell stack voltage below a maximum fuel cell stack voltage level.

15. The method of claim 12, further comprising:

reducing an amount of current supplied from the fuel cell stack to the load for a period prior to shorting the fuel cell stack;

supplying an amount of current from the electrical storage device to the load for a period prior to shorting the fuel cell stack; and

supplying current from the fuel cell stack to the electrical storage device after the shorting of the fuel cell stack to clamp a fuel cell stack voltage below a maximum fuel cell stack voltage level.

16. The method of claim 12, further comprising:

determining a load condition from time-to-time; and

ceasing the temporarily shorting of the fuel cell stack until the determined load condition exceeds a threshold load condition.

17. The method of claim 12, further comprising:

from time-to-time, determining when a stack voltage falls below a preset voltage limit; and

wherein temporarily shorting the fuel cell stack from time-to-time comprises temporarily shorting the fuel cell stack in response to determining that the stack voltage has fallen below the preset voltage limit.

18. The method of claim 12, further comprising:

from time-to-time, determining when a stack voltage falls below a float voltage of the electrical storage device; and

wherein temporarily shorting the fuel cell stack from time-to-time comprises temporarily shorting the fuel cell stack in response to determining that the stack voltage has fallen below the float voltage of the electrical storage device.

19. The method of claim 12, further comprising:

from time-to-time, determining when a stack voltage falls below a preset voltage limit; and

wherein temporarily shorting the fuel cell stack from time-to-time comprises temporarily shorting the fuel cell stack in response to determining that the stack voltage has fallen below the preset voltage limit; and

determining a measure of pollutants based on a frequency of the shorting of the fuel cell stack.

20. A method of operating a power system comprising a stack of fuel cells and an electrical storage device electrically couplable to supply power to a load, the method comprising:

from time-to-time,

operating at least one switch to provide an electrical short circuit across the stack of fuel cells;

operating the at least one switch to remove the electrical short circuit across the stack of fuel cells;

supplying power from the electrical storage device to the load at least while shorting the fuel cell stack; and

clamping a fuel cell voltage below a maximum fuel cell voltage level during at least a period after removing the short circuit path across the stack of fuel cells.

21. The method of claim 20 wherein from time-to-time is a periodic basis.

22. The method of claim 20 wherein operating the at least one switch to remove the electrical short circuit across the stack of fuel cells occurs approximately 500 milliseconds after operating the at least one switch to provide the electrical short circuit across the stack of fuel cells.

23. The method of claim 20, further comprising:

from time-to-time, determining a voltage across the stack of fuel cells, and wherein from time-to-time, operating at least one switch to provide an electrical short circuit across the stack of fuel cells comprises operating the at least one switch to provide the electrical short circuit when the determined voltage across the stack of fuel cells exceeds a threshold voltage.

24. The method of claim 20 wherein clamping a fuel cell voltage below a maximum fuel cell voltage level comprises supplying power to the load from the electrical storage device before providing the short circuit path across the stack of fuel cells, and recharging the electrical storage device from the fuel cell stack for a period after removing the short circuit path across the stack of fuel cells.

25. A method of operating a power system having a fuel cell stack and an energy storage device electrically coupled to the fuel cell stack, to power a load and a balance of system, the method comprising:

measuring a stack current being supplied by the fuel cell stack to the load and the balance of system;

determining an amount of energy required by the load and the balance of system during a short circuit of the fuel cell stack;

determining an amount of energy to be clipped off after the short circuit;

determining an amount of energy to be pre-removed from the energy storage device based oh the determined amount of energy required by the load and the balance of system during the short circuit and based on the determined amount of energy to be clipped off after the short circuit;

determining a time required to pre-remove the determined amount of energy from the electrical storage device at a present load;

disconnecting the load and the balance of system from the fuel cell stack;

the determined time required to pre-remove energy from the electrical storage device after disconnecting the load and the balance of system, shorting the fuel cell stack;

reconnecting the load and the balance of system to the fuel cell stack; and

stopping the shorting of the fuel cell stack after shorting the fuel cell stack for a shorting duration.

26. A power system for providing power to at least one load, the power system comprising:

a fuel cell stack;

means for shorting the fuel cell stack from time-to-time; and

means for clamping a fuel cell stack voltage below of maximum fuel cell stack voltage level at least during a period immediately following the shorting of the fuel cell stack.

27. The power system of claim 26 wherein the means for clamping a fuel cell stack voltage comprises a zener diode electrically coupled in parallel with the fuel cell stack.

28. The power system of claim 26 wherein the means for clamping a fuel cell stack voltage comprises a shunt regulator electrically coupled in parallel with the fuel cell stack.

29. The power system of claim 26 wherein the means for clamping a fuel cell stack voltage comprises an electrical storage device electrically coupled in parallel with the fuel cell stack.

30. The power system of claim 26 wherein the means for clamping a fuel cell stack voltage comprises a controller executing control logic that operates a transistor electrically coupled in parallel with the fuel cell stack.

31. A power system, comprising:

a power bus;

a fuel cell stack electrically coupled across the power bus;

a pulsing switch electrically coupled across the fuel cell stack and operable to current pulse the fuel cell stack;

a controller coupled to selectively control the pulsing switch to current pulse the fuel cell stack from time-to-time; and

stack voltage clamping means for clamping a stack voltage at least during a period following the current pulsing of the fuel cell stack.

32. The power system of claim 31 wherein the controller comprises:

an oscillator that operates the pulsing switch to periodically short the fuel cell stack.

33. The power system of claim 31 wherein the controller comprises:

an oscillator that operates the pulsing switch to periodically electrically couple a load across the fuel cell stack to current pulse the fuel cell stack.

34. The power system of claim 31 wherein the stack voltage clamping means comprises:

an electrical storage device electrically coupled in parallel with the fuel cell stack.

35. The power system of claim 31 wherein the stack voltage clamping means comprises:

an electrical storage device electrically coupled in parallel, wherein the electrical storage device comprises at least one of a battery and a super-capacitor.

36. The power system of claim 31 wherein the stack voltage clamping means comprises:

a zener diode electrically coupled across the fuel cell stack to clamp a fuel cell stack voltage.

37. The power system of claim 31 wherein the stack voltage clamping means comprises:

a shunt regulator electrically coupled across the fuel cell stack to clamp a fuel cell stack voltage, the shunt regulator comprising a zener diode, a resistor, and a transistor having an input terminal, an output terminal and a control terminal, the zener diode electrically coupled between an output of the fuel cell stack and the control terminal of the transistor, and resistor electrically coupled in series between the zener diode and the control terminal of the transistor.

38. The power system of claim 31 wherein the controller comprises an oscillator that periodically causes the pulsing switch to electrically short the fuel cell stack and wherein the stack voltage clamping means comprises:

a voltage sensor coupled to provide a signal representing a voltage sensed on the power bus; and

shunt regulator logic implemented in the controller that causes the pulsing switch to electrically short the fuel cell stack in response to a fuel cell stack voltage exceeding a threshold voltage.

39. The power system of claim 31 wherein the stack voltage clamping means comprises:

an electrical storage device;

a zener diode electrically coupled across the power bus in parallel with the fuel cell stack;

an fuel cell stack isolation switch electrically coupled in series between the fuel cell stack and the electrical storage device, wherein the controller is further coupled to control the fuel cell stack isolation switch;

a current sensor positioned to sense an output current from the fuel cell stack and provide a resulting value representing the sensed current to the controller; and

a diode electrically coupled between the fuel cell stack and the electrical storage device to protect the electrical storage device from shorting.

40. The power system of claim 31 wherein the stack voltage clamping means comprises:

an electrical storage device;

a first metal oxide semiconductor field effect transistor and a first Schottky diode electrically coupled in series to one another; and

at least a second metal oxide semiconductor field effect transistor and a second Schottky diode electrically coupled in series to one another and electrically coupled in parallel to the first metal oxide semiconductor field effect transistor and the first Schottky diode, the first and the second metal oxide semiconductor field effect transistors and the first and the second Schottky diodes electrically coupled between the fuel cell stack and the electrical storage device.

41. A power system for providing power to at least one load, the power system comprising:

a fuel cell stack;

a shorting switch electrically coupled and operable to electrically short the fuel cell stack;

an electrical storage device electrically coupled in parallel with the fuel cell stack;

a diode electrically coupled between the fuel cell stack and the electrical storage device to protect the electrical storage device from electrical shorts; and

a controller coupled to selectively control the shorting switch to short the fuel cell stack from time-to-time.

42. The power system of claim 41 wherein the controller comprises:

an oscillator coupled to operate the shorting switch to periodically short the fuel cell stack.

43. The power system of claim 41 wherein the controller comprises:

an oscillator coupled to operate the shorting switch to periodically short the fuel cell stack when an electrical load supplied by the fuel cell stack is above a threshold value.

44. The power system of claim 41 wherein the controller executes a control logic that causes the controller to clamp a stack voltage of the fuel cell stack by:

drawing current from the fuel cell stack to the electrical storage device at least during a period following the shorting of the fuel cell stack.

45. The power system of claim 41 wherein the controller executes a control logic that causes the controller to clamp a stack voltage of the fuel cell stack by:

determining a period of time required to drain the electrical storage device to a desired level;

electrically uncoupling the load from the fuel cell stack for the determined period of time, before shorting the fuel cell stack; and

drawing current from the fuel cell stack to the electrical storage device at least during a period following the shorting of the fuel cell stack.

46. The power system of claim 41, further comprising:

a zener diode electrically coupled in parallel with the fuel cell stack, the zener diode having a breakdown voltage approximately equal to a float voltage of the electrical storage device.

47. The power system of claim 41, further comprising:

a shunt regulator comprising a zener diode, a transistor and a resistor coupled between the zener diode and a switching terminal of the transistor, the transistor electrically coupled in parallel with the fuel cell stack.

48. The power system of claim 41, further comprising:

a stack isolation switch operable to electrically uncoupled a balance of system, a load and the electrical storage device, from the fuel cell stack.