Method for starting a fuel cell system in a vehicle and correspondingly designed fuel cell system

Abstract

A method for starting a hybrid fuel cell system (10), the method comprising: Providing a high-voltage electric bus (16); Providing a fuel cell stack (12) electrically coupled to the high-voltage electrical bus (16); Providing a high-voltage direct current power storage device (14) electrically coupled to the high-voltage electrical bus (16); Providing a diode (28) in the high voltage electrical bus (16) that prevents access of voltage from the high voltage storage device (14) to the fuel cell stack (12); Providing a bypass line (30) around the diode (28); Providing a fuel cell stack switch (18) between the fuel cell stack (12) and the high voltage electrical bus (16); electrical coupling of system loads to the high-voltage electrical bus; electrically coupling a low voltage / high voltage boost converter (36) to the high voltage electrical bus (16); Closing a bypass switch (32) in the bypass line (30) to allow electrical power from the high voltage direct current storage device (14) to bypass the diode (28) and operate the system loads (24); Starting the fuel cell stack (12) using the system loads (24); Determining whether an output voltage of the fuel cell stack (12) is stable; Opening the bypass switch (32) when the voltage of the fuel cell stack (12) is stable; and Closing the fuel cell stack switch (18) after opening the bypass switch (32).

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThis invention relates to a method for starting a fuel cell system in a vehicle and to a fuel cell system of corresponding design.2. Description of the Prior ArtHydrogen is a very interesting fuel because it is clean and can be used to efficiently generate electric current in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is split in the anode to produce free hydrogen protons and electrons. The hydrogen protons move through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte and are therefore passed through a load to perform work before being sent to the cathode.Proton Exchange Membrane Fuel Cells (PEMFCs. Proton exchange membrane fuel cells) is a common fuel cell for vehicles. The PEMFC generally comprises a proton conducting solid polymer electrolyte membrane, for example a perfluorosulfonic acid membrane. The anode and cathode usually comprise finely divided catalytic particles, usually platinum (Pt), supported by carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposite sides of the membrane. The combination of the catalytic mixture of the anode, the catalytic mixture of the cathode, and the membrane forms a membrane electrode assembly (MEA. Membrane Electrode Assembly). MEAs are relatively expensive to manufacture and require certain conditions for efficient operation.In a fuel cell stack, a plurality of fuel cells are usually combined to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically an air flow forced through the stack by a compressor. Not all of the oxygen is exhausted from the stack, and a portion of the air is exhausted as cathode exhaust gas, which may contain water as stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.The fuel cell stack includes a series of bipolar plates positioned between the plurality of MEAs in the stack, the bipolar plates and the MEAs positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates for flowing the anode reactant gas to the respective MEA. On the cathode side of the bipolar plates, cathode gas flow channels are provided that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels and the other end plate includes cathode gas flow channels. The bipolar plates and the end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates direct the electrical current generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.Most fuel cell vehicles are hybrid vehicles that use a supplemental power source, such as a high voltage DC battery or an ultra capacitor, in addition to the fuel cell stack. For example, US 2006 / 0127704 A1 describes a fuel cell system having an electrical power bus line; a fuel cell stack electrically coupled to the power bus line; a battery electrically coupled to the power bus line; and a capacitor electrically coupled to the power bus line in series with the battery, the capacitor providing voltage adjustment for large voltage fluctuations on the power bus line. The power source provides supplemental power to the various auxiliary loads of the vehicle, for system starts, and during high power demands when the fuel cell stack cannot provide the desired power. The fuel cell stack provides power to a traction electric motor for vehicle operation through a DC high voltage electrical bus. The battery supplies supplementary power to the electrical bus during these periods when additional power beyond the value that the stack can supply is required, for example during heavy acceleration. The fuel cell stack may provide a power of 70 kW, for example. However, vehicle acceleration may require a power of 700 kW. The fuel cell stack is used to charge the battery or ultra capacitor at the times that the fuel cell stack can provide the system power requirement. The generator power available from the traction motor during regenerative braking is also used to charge the battery or ultra capacitor.In the above-mentioned hybrid vehicle, a bidirectional DC / DC converter is typically used to boost the DC voltage from the battery, to match the battery voltage to the voltage of the electric bus specified by the output voltage of the burning stack, and to decrease the stack voltage during charging of the battery. Such a DC / DC converter is also used in the fuel cell system described in WO 2007 / 140798 A1. However, DC / DC converters are relatively large, expensive, heavy and unreliable, which entails obvious disadvantages. It is desirable to eliminate the DC / DC converter in a fuel cell vehicle that includes a supplemental power source.Various attempts have been made in the industry to eliminate the DC / DC converter in fuel cell powered vehicles by providing a power source capable of coping with the large voltage variation from the fuel cell stack during the operating conditions of the vehicle. Certain types of batteries have also been used for eliminating the DC / DC converter in the fuel cell systems of vehicles. However, these systems are typically limited by the ability to discharge the battery beyond a certain value. In other words, these types of batteries could be damaged due to large voltage variations on the electrical bus during operation of the system.SUMMARY OF THE INVENTIONAccording to the invention, a method for starting a hybrid fuel cell system having the features of claim 1 or claim 9 and a fuel cell system having the features of claim 13 are presented.In accordance with the teachings of the present invention, a hybrid fuel cell system utilizing a fuel cell stack and an ultra capacitor is disclosed. A diode is provided on a high voltage bus between the fuel cell stack and the ultra capacitor, so that high voltage from the ultra capacitor does not affect the operation of the fuel cell stack. During system startup, a bypass switch is closed to bypass the ultra capacitor, so power from the ultra capacitor can be used to start various system loads, for example, a cathode side air compressor that supplies air to the fuel cell stack. A 12 volt / high voltage converter is used to provide a short duration, low power supply from a low voltage battery to the system loads at start-up when the bypass switch is open but before a fuel cell stack switch is closed.Further features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.Brief Description of the DrawingsFIG. 1 is a schematic block diagram of a hybrid fuel cell system using a fuel cell stack and an ultra capacitor according to an embodiment of the present invention; and FIG. 2 is a flowchart showing a starting process for the fuel cell stack shown in FIG. 1 using a bypass switch for bypassing a diode and a low power 12-volt / high-voltage DC-DC converter according to an embodiment of the present invention.Detailed Description of the EmbodimentsThe following description of the embodiments of the invention directed to a hybrid fuel cell system and method for starting the system is merely exemplary in nature.FIG. 1 is a schematic block diagram of a fuel cell system 10 that includes a fuel cell stack 12 and an ultra capacitor 14, both electrically coupled to a high voltage electrical bus 16. Although an ultra capacitor is used as a supplemental power source in this non-limiting embodiment, other high-voltage DC storage devices may be used instead of the ultra capacitor 14, for example, a high-voltage battery. A fuel cell stack switch 18 selectively connects and disconnects the fuel cell stack 12 and the high voltage electrical bus 16. The high voltage electrical bus 16 has various electrical components electrically coupled thereto, such as a traction electric motor 22 that drives the vehicle. Further, a motor 24 that drives an air compressor to supply air to the cathode side of the fuel cell stack 12 and other system loads 26 are electrically coupled to the electrical bus 16.The fuel cell stack 12 and the ultra capacitor 14 may have different output voltages, where the voltage of the ultra capacitor 14 is typically higher and could damage the fuel cell stack 12. The higher voltage across the ultra capacitor 14 may be a result of regenerative braking energy being absorbed by the traction motor 22. As discussed above, DC / DC converters have typically been provided in the high voltage electrical bus 16 to protect the fuel cell stack from the voltage of the high voltage DC power source. In this embodiment, no DC / DC converter is used. Therefore, another technique for protecting the fuel cell stack 12 is required. To provide this protection, a suitable high voltage diode 28 is provided in the high voltage electrical bus 16 to prevent the fuel cell stack 12 from being exposed to the high voltage of the ultra capacitor 14 in a manner known in the art. However, when the fuel cell stack 12 is not operating at system startup, the energy from the ultracapacitor 14 must operate various system loads, such as the air compressor motor 24.In order for the DC voltage from the ultra capacitor 14 to drive the motor 24, it is necessary to bypass the diode 28. Therefore, a bypass line 30 is provided around the diode 28, and a bypass switch 32 is provided in the bypass line 30 to allow the diode 28 to be selectively bypassed. Therefore, during system startup, switch 18 is opened and switch 32 is closed so that electrical energy from ultra capacitor 14 can go around diode 28 and drive air compressor motor 24 as well as the other system loads 26 without damaging fuel cell stack 12.The fuel cell system 10 also includes a low voltage battery 34, for example, a 12-volt car battery. The battery 34 may provide power to various system and vehicle components that do not require a high voltage. However, during certain periods of time, it is desirable to convert the 12-volt DC potential from the battery 34 to a low power high voltage potential to drive certain system components. To provide this conversion, a 12 volt / high voltage converter 36 is provided. The low high voltage power from the converter 36 may be used to drive the air compressor motor 24 to start the fuel cell stack 12 during periods when the ultra capacitor 14 may be out of function or not have enough power therefor. Thus, this power source serves as a backup system for the starting sequence that uses the ultra capacitor 14.During system start-up, there is a narrow time window in which the ultra capacitor 14 provides power to the air compressor motor 24 through the switch 32 and the fuel cell stack 12 begins to provide stable power at which it can be turned on to the system 10. During this process, the switch 32 is open so that the fuel cell stack 12 is protected from the power of the ultra capacitor 14 by the diode 28, and then the switch 18 is closed so that power from the fuel cell stack 12 can be provided on the high voltage electrical bus 16 for driving the various loads. During this narrow time window, in which both switches 18 and 32 may be open, no power may be supplied to the air compressor motor 24, which may cause a temporary drop in the cathode air supplied to the fuel cell stack 12. During this time, the power from the high voltage converter 36 may keep the air compressor motor 24 operational to provide a seamless transition between opening the switch 32 and closing the switch 18.This starting sequence can be shown by the flow chart 40 in FIG. 2. The starting sequence is initiated at box 42, such as by turning the vehicle ignition key, and at box 44, the starting algorithm closes the bypass switch 32. then the starting algorithm starts the compressor motor at box 46 and starts the boost converter 36 at box 48. then the starting algorithm determines whether the output power of the fuel cell stack 12 is stable at decision diamond 50, and if not, returns to determining stack stability after a certain period of time has elapsed. If the output power of the stack is stable at decision diamond 50, then the start algorithm opens bypass switch 32 at box 52 and closes stack switch 18 at box 54. During the time switch 32 is open and switch 18 is closed, power is provided to compressor motor 24 by boost converter 36. Then, the start algorithm shuts down boost converter 36 at box 56, where system 10 should now operate normally.

Claims

A method of starting a hybrid fuel cell system (10), the method comprising: providing a high voltage electrical bus (16); providing a fuel cell stack (12) electrically coupled to the high voltage electrical bus (16); providing a high voltage DC power storage device (14) electrically coupled to the high voltage electrical bus (16); providing a diode (28) in the high voltage electrical bus (16) that inhibits access of voltage from the high voltage storage device (14) to the fuel cell stack (12); providing a bypass line (30) around the diode (28); providing a fuel cell stack switch (18) between the fuel cell stack (12) and the high voltage electrical bus (16); electrically coupling system loads to the high voltage electrical bus; electrically coupling a low voltage / high voltage boost converter (36) to the high voltage electrical bus (16); closing a bypass switch (32) in the bypass line (30) to bypass electric power from the high voltage DC storage device (14), the diode (28), and to be able to operate the system loads (24); starting the fuel cell stack (12) using the system loads (24); determining whether an output voltage of the fuel cell stack (12) is stable; opening the bypass switch (32) when the voltage of the fuel cell stack (12) is stable; and closing the fuel cell stack switch (18) after opening the bypass switch (32).The method of claim 1, wherein providing a high voltage DC storage device (14) comprises providing an ultra capacitor (14).The method of claim 1, wherein operating the system loads (24) comprises operating an air compressor motor that provides power to an air compressor (24) that drives air to the fuel cell stack (12).The method of claim 1, further comprising turning on the boost converter (36) prior to opening the bypass switch (32) so that the boost converter (36) may be ready to supply power to the system loads (24) when the bypass switch (32) is open and the stack switch (18) is closed.The method of claim 1, wherein electrically coupling system loads (24) to the high voltage electrical bus (16) comprises electrically coupling a traction electric motor (22) to the high voltage electrical bus (16).The method of claim 5, wherein the electric traction motor (22) drives a vehicle.The method of claim 1, further comprising turning off the boost converter (36) after opening the stack switch (18).The method of claim 1, further comprising electrically coupling a 12-volt battery (34) to the low / high voltage boost converter (36) to provide the low voltage.A method of starting a hybrid fuel cell system (10), the method comprising: electrically disconnecting a fuel cell stack (12) from a high voltage electrical bus (16); supplying electrical power from a high voltage DC power storage device (14) to a compressor motor (24) on the high voltage electrical bus (16) through a bypass switch (32) bypassing a diode (28) in the electrical bus (16), the compressor motor (24) driving a compressor supplying air to a cathode side of the fuel cell stack (12); opening the bypass switch (32) after the stack voltage becomes stable; closing a fuel cell stack switch (18) for connecting the fuel cell stack (12) to the high voltage electrical bus (16) after the bypass switch (32) is opened; and providing electrical power to the compressor motor (24) from a low voltage / high voltage boost converter (36) during the time the bypass switch (32) is open but before the fuel cell stack switch (18) is closed.The method of claim 9, wherein the high voltage DC storage device (14) is an ultra capacitor (14).The method of claim 9, further comprising turning on the boost converter (36) prior to opening the bypass switch (32) so that the boost converter (36) may be ready to supply power to the system loads when the bypass switch (32) is open and the stack switch (18) is closed.The method of claim 9, further comprising electrically coupling a 12-volt battery (34) to the low / high voltage boost converter (36) to provide the low voltage.A fuel cell system (10) comprising: a high voltage electrical bus (16); a compressor motor (24) electrically coupled to the high voltage electrical bus (16); a fuel cell stack (12) electrically coupled to the high voltage electrical bus (16); a fuel cell stack switch (18) for selectively establishing and interrupting connection of the fuel cell stack (12) to the high voltage electrical bus (16); a high voltage DC power storage device (14) electrically coupled to the high voltage electrical bus (16); a diode (28) in the high voltage electrical bus (16) that inhibits access of voltage from the high voltage storage device (14) to the fuel cell stack (12); a bypass line (30) around the diode; a bypass switch (32) in the bypass line (30); and a low-voltage / high-voltage boost converter (36) electrically coupled to the high-voltage electrical bus (16), the boost converter (36) providing power to the compressor motor (24) during system start-up after the bypass switch (32) has been opened but before the fuel cell stack switch (18) is closed.The system of claim 13, wherein the high voltage DC storage device (14) is an ultra capacitor (14).The system of claim 13, wherein the boost converter (36) is turned on prior to opening the bypass switch (32) to allow the boost converter (36) to be ready to supply power to the system loads (24) when the bypass switch (32) is opened and the stack switch (18) is closed.The system of claim 13, further comprising a 12-volt battery (34) electrically coupled to the low / high voltage boost converter (36).The system of claim 13, further comprising an electric traction motor (22) electrically coupled to the high voltage electrical bus (16).

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