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The present invention provides a method of operating a fuel cell system and a fuel cell system that can adjust the operating pressure of the fuel cell system to maximize efficiency. This present invention recognizes that under certain operating conditions appropriately matched operating pressures en

The present invention provides a method of operating a fuel cell system and a fuel cell system that can adjust the operating pressure of the fuel cell system to maximize efficiency. This present invention recognizes that under certain operating conditions appropriately matched operating pressures enable a substantially more efficient system operation. The method of the present invention and the fuel cell system of the present invention incorporate the recognition that a higher system efficiency can be achieved when the operating pressure produced by the air compressor is matched to the prevailing operating temperature of the fuel cell system.

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1. A method for operation of a fuel cell system comprising:providing a fuel cell stack defining a plurality of operating points wherein each of said operating points are characterized by an operating temperature, an operating pressure, and a reactant stoichiometry within said fuel cell stack; provid

1. A method for operation of a fuel cell system comprising:providing a fuel cell stack defining a plurality of operating points wherein each of said operating points are characterized by an operating temperature, an operating pressure, and a reactant stoichiometry within said fuel cell stack; providing a cooling system that establishes an actual operating temperature of said fuel cell stack independent of said reactant stoichiometry within said fuel cell stack; determining at least one actual operating temperature of said fuel cell stack; and providing a compressed flow of an oxygen containing gas to a cathode inlet at an operating pressure controlled such that it represents a minimum pressure necessary to ensure operation of said fuel cell system at an actual operating point defined by said actual operating temperature. 2. A method as claimed in claim 1, wherein said fuel stack comprises an anode and said stoichiometry is taken at said anode.3. A method as claimed in claim 1, wherein said fuel stack comprises a cathode and said stoichiometry is taken at said cathode.4. A method as claimed in claim 1, wherein said actual operating point is additionally defined by a stoichiometry within said fuel cell stack.5. A method as claimed in claim 4, wherein said fuel stack comprises an anode and said stoichiometry of said actual operating point is taken at said anode.6. A method as claimed in claim 4, wherein said fuel stack comprises a cathode and said stoichiometry of said actual operating point is taken at said cathode.7. A method as claimed in claim 1, wherein said at least one actual operating temperature is determined from at least one of an actual operating temperature, a value corresponding to an actual operating temperature, and a value proportional to an actual operating temperature.8. A method as claimed in claim 1, wherein said fuel cell stack comprises a plurality of fuel cells that generate electrical energy, an anode inlet for a fuel, a cathode inlet for an oxygen containing gas, and a cathode outlet.9. A method as claimed in claim 8, wherein the operating pressure is effected by controlling a restrictor valve connected to said cathode outlet.10. A method as claimed in claim 1, wherein said cooling system includes a cooling fluid that carries away heat generated in said fuel cell stack.11. A method as claimed in claim 1, wherein said compressed flow of an oxygen containing gas is provided by a compressor, and a compressor component.12. A method as claimed in claim 11, wherein said compressor comprises at least one component having a variable geometry and wherein said method further comprises the step of optimizing the efficiency of said compressor by controlling said variable geometry component.13. A method as claimed in claim 12, wherein said control of said at least one component having a variable geometry component takes into account an operating pressure and mass flow value of said oxygen containing gas provided by said compressor.14. A method as claimed in claim 12, wherein said optimizing step comprises adjustment of adjustable guide vanes associated with said compressor.15. A method as claimed in claim 12, wherein said optimizing step comprises adjustment of adjustable guide nozzles associated with said compressor.16. A method as claimed in claim 1, wherein said operating points are further characterized by a humidity of a gas within said fuel cell stack.17. A method as claimed in claim 1, wherein said actual operating point is additionally defined by a humidity of a gas within said fuel cell stack.18. A method as claimed in claim 1, wherein said operating pressure is further controlled so as to take account of a desired characteristic humidity number (FK) at said cathode outlet.19. A method as claimed in claim 18, wherein said characteristic humidity number (FK) is kept in a predetermined range to ensure damage-free operation of said fuel cell system.20. A method as claimed in claim 1, wherein said operating pressure is further controlled so as to take account of a relative humidity value of said oxygen containing gas entering into said cathode inlet.21. A method as claimed in claim 1, wherein said operating pressure is further controlled so as to take account of a relative humidity value of gaseous fuel entering said anode inlet in the selection of a respective operating pressure value.22. A method as claimed in claim 1, further comprising matching a mass flow value of said fuel supplied to said fuel cell stack to a respective power requirement.23. A method as claimed in claim 1, further comprising matching a mass flow of said oxygen containing gas supplied to said fuel cell stack to a respective power requirement.24. A method as claimed in claim 1, wherein said fuel cell stack comprises an anode and a cathode and said method comprises taking account of a value for stoichiometry at the anode in the selection of said operating pressure.25. A method as claimed in claim 1, wherein said fuel cell stack comprises an anode and a cathode and said method comprises taking account of a value for stoichiometry at the cathode in the selection of said operating pressure.26. A method as claimed in claim 1, wherein said cooling system is operated to lower said actual operating temperature to a lowest possible value that is acceptable for efficient operation of said fuel cell system.27. A method as claimed in claim 1, wherein said cooling system is operated to keep a respective operating pressure value as small as permissible to operate with said actual operating temperature.28. A method as claimed in claim 1, wherein said cooling system comprises a fan having a fan speed and a pump having a pump speed and wherein cooling performance is reduced by reducing said fan speed, said pump speed, or combinations thereof.29. A method as claimed in claim 1, wherein said cooling system comprises a pump having a pump speed and wherein noise is minimized by reducing said pump speed.30. A method as claimed in claim 1, wherein said cooling system comprises a fan having a fan speed and a pump having a pump speed and wherein said fan speed and said pump speed are reduced when a vehicle powered by said fuel cell system is operating at in an idling condition.31. A method as claimed in claim 1, wherein said cooling system comprises a fan having a fan speed and a pump having a pump speed and wherein said fan speed and said pump speed are reduced when a vehicle powered by said fuel cell system is operating at a low vehicle speed.32. A method as claimed in claim 1, wherein said operating pressure is controlled by taking account of a prevailing ambient temperature and its effect on the cooling system and on said actual operating temperature.33. A method as claimed in claim 1, wherein said operating pressure is used as a regulating parameter having a desired value which is matched to prevailing operating parameters and conditions of said fuel cell system.34. A method as claimed in claim 33, wherein said desired value is determined by an algorithm which takes account of said prevailing operating parameters and operating conditions.35. A method as claimed in claim 33, wherein said desired value is determined from stored characteristic parameters.36. A fuel cell system comprising:a fuel cell stack defining a plurality of operating points wherein each of said operating points are characterized by an operating temperature, an operating pressure, and a reactant stoichiometry within said fuel cell stack; a cooling system adapted to establish an actual operating temperature of said fuel cell stack independent of said reactant stoichiometry within said fuel cell stack; a temperature sensor adapted to determine said actual operating temperature established by said independent cooling system; and pressure components configured to produce a compressed flow of oxygen containing gas for delivery to a cathode inlet of said fuel cell stack at an operating pressure, that represents a minimum pressure necessary to ensure operation of said fuel cell system at an actual operating point defined by said actual operating temperature. 37. A fuel cell system as claimed in claim 36, wherein said fuel cell system further comprises a control system configured to control at least one of said pressure components in accordance with one of said actual operating temperature.38. A fuel cell system as claimed in claim 37, wherein said control system is configured to set said operating pressure to a minimum pressure necessary to ensure operation of said fuel cell system at said actual operating point defined by said actual operating temperature.39. A fuel cell system as claimed in claim 37, wherein said temperature sensor is connected to said control system.40. A fuel cell system as claimed in claim 36, wherein said fuel cell stack comprises a plurality of fuel cells, an anode inlet for a fuel, a cathode inlet for an oxygen containing gas, and a cathode outlet.41. A fuel cell system as claimed in claim 40, wherein said fuel comprises one of hydrogen and a hydrogen rich synthesized gas.42. A fuel cell system as claimed in claim 36, wherein said cooling system directs a cooling fluid through said fuel cell stack to carry away heat produced in said fuel cell stack.43. A fuel cell system, as claimed in claim 36, wherein said temperature sensor determines said actual operating temperature from at least one of an actual operating temperature, a value corresponding to an actual operating temperature, and a value proportional to an actual operating temperature.44. A fuel cell system as claimed in claim 36, wherein said pressure components comprise a compressor, an electric motor, and a compressor component.45. A fuel cell system as claimed in claim 44, wherein said oxygen containing gas comprises air and said compressor is an air compressor.46. A fuel cell system as claimed in claim 44, wherein said compressor component comprises a restrictor valve.47. A fuel cell system as claimed in claim 46, wherein said restrictor valve is connected to said fuel cell system after a cathode outlet of said fuel cell stack.48. A fuel cell system as claimed in claim 44, wherein said compressor comprises at least one compressor component of variable geometry and said control system is adapted to change said geometry of said at least one component.49. A fuel cell system as claimed in claim 44, wherein said compressor is configured to enable the most efficient possible operation of said compressor at each operating point.50. A fuel cell system as claimed in claim 49, wherein said at least one component of variable geometry comprises adjustable guide vanes.51. A fuel cell system as claimed in claim 49, wherein said at least one component of variable geometry comprises adjustable guide nozzles.52. A fuel cell system as claimed in claim 36, further comprising first and second sensors for measuring a relative humidity value of a gas within said fuel cell stack.53. A fuel cell system as claimed in claim 52, wherein said fuel cell stack comprises an anode inlet and a humidity sensor configured to measure said relative humidity value at said anode inlet.54. A fuel cell system as claimed in claim 52, wherein said fuel cell stack comprises a cathode inlet and a humidity sensor configured to measure said relative humidity value at said cathode inlet.55. A fuel cell system as claimed in claim 36, further comprising a memory associated with said control system that stores values for respectively appropriate operating pressures and an algorithm for the calculation of respectively appropriate operating pressures for various actual operating temperatures and various operating parameters of said fuel cell stack.56. A fuel cell system as claimed in claim 36, wherein said fuel cell stack comprises an anode and a cathode and said control system is configured to account for stoichiometry values at said anode and at said cathode for various operating points of said fuel cell stack.57. A fuel cell system as claimed in claim 56, wherein said stoichiometry values are stored in a memory.58. A fuel cell system as claimed in claim 36, wherein said control system comprises an input for a power requirement signal.59. A fuel cell system as claimed in claim 36, wherein said control system is configured such that a valve for determining a mass flow of fuel supplied to said fuel cell stack is controllable by said control system in accordance with a respective power requirement.