Fast starting fuel cell

An electrical power supply system has a fuel cell module and a battery. The fuel cell can be selectively connected to the battery system through a diode. The system preferably also has a current sensor and a controller adapted to close a contactor in a by-pass circuit around the diode after sensing a current flowing from the fuel cell through the diode. The system may also have a resistor and a contactor in another by-pass circuit around the diode. In a start-up method, a first contactor is closed to connect the fuel cell in parallel with the battery through the diode and one or more reactant pumps for the fuel cell are turned on. A current sensor is monitored for a signal indicating current flow through the diode. After a current is indicated, a by-pass circuit is provided around the diode.

FIELD

This specification relates to electrical power systems having a fuel cell.

BACKGROUND

A PEM fuel cell module includes a fuel cell stack. The fuel cell stack contains numerous polymer electrolyte membranes, alternatively called proton exchange membranes, which conduct protons between the electrodes. A PEM fuel cell is powered by a first reactant comprising oxygen, for example air, and a second reactant comprising hydrogen, for example essentially pure hydrogen or methane. Other types of fuel cell modules are also known.

A fuel cell module may be combined with a battery to provide a hybrid electrical power supply system. For example, hybrid power systems can be used to provide a back-up power supply in case of grid failure. In another example, a hybrid power system can be used to power a vehicle. In these cases, and others, it would be desirable to be able to start a fuel cell in a short period of time. In the case of a back-up power supply, the battery is required among other things to provide enough storage to carry anticipated loads until the fuel cell is started.

Currently available fuel cell modules typically start in about 20-60 seconds. The start time is measured from the time of a start command until the module delivers power to a load or reaches a specified percentage, for example 80% or 100%, of its rated power output.

A fuel cell produces voltage according to a polarisation curve. The polarisation curve describes the fuel cell voltage as a function of the fuel cell current or the fuel cell current density. In general, as current supplied by the fuel cell increases from zero, the fuel cell voltage initially drops rapidly through an activation region, then drops nearly linearly through an ohmic region, then drops more rapidly through a mass transport region. A battery typically has a different polarisation curve and so at some times, for example when a fuel cell is starting or disconnected, a battery and fuel cell may have incompatible polarisation curves. In some cases, a battery and fuel cell are connected through a DC to DC voltage converter to help manage differences in their polarisation curves.

INTRODUCTION

The following introduction is intended to introduce the reader to the detailed description to follow and not to limit or define any claimed invention.

In an electrical power supply, a faster starting fuel cell may allow a smaller battery to be used. Optionally no battery, other than a battery used to start the fuel cell, may be required if the start-up sequence is reduced sufficiently. In the case of a vehicle, the fuel cell and battery typically work together to provide maximum acceleration or maximum peak power. In this case, a fast starting fuel cell allows the vehicle to be operated without delay and may enable the fuel cell to be shut down, rather than idle, when the vehicle is stopped at traffic signals. It is desirable, for example for the reasons given above, to be able to start a fuel cell faster, for example in 5 seconds or less or 2 seconds or less. It is also desirable to provide a fuel cell starting method and apparatus that can optionally be used without a voltage converter.

This specification describes an electrical power supply system having a fuel cell module and a battery. The fuel cell module and battery operate in at least partially overlapping voltage ranges. The fuel cell module can be selectively connected to the battery through a diode. At least during a start-up procedure, one or more reactant pumps for the fuel cell optionally are driven by the battery. The system preferably also has a current sensor and a controller. The controller is adapted to close a contactor in a by-pass circuit around the diode after sensing a current flowing from the fuel cell through the diode. The system preferably also has a contactor in series with the diode. The system may also have a resistor and a contactor in a discharge circuit.

This specification also describes a start-up method for a fuel cell module, for example a fuel cell module in an electrical power system as described above. In the method, a first contactor is closed to connect the fuel cell in parallel with the battery through a diode and one or more reactant pumps for the fuel cell are turned on. A current sensor is monitored for a signal indicating current flow through the diode. After a current is indicated, a by-pass circuit is connected around the diode.

Optionally, one or more fuel cell module status checks or pre-start procedures may be performed before starting the fuel cell module. In some cases, the fuel cell module is discharged before starting the fuel cell module. A shut down procedure is also described herein.

DETAILED DESCRIPTION

FIG. 1shows a fuel cell module10. The fuel cell module10has a fuel cell stack12containing a plurality of PEM cells. Flow field plates within the stack12define a coolant path, a fuel side path (also called a hydrogen or anode side path) and an air side path (also called an oxygen or cathode side path). Various balance of plant elements are used to manage the flow of materials through these paths. Some examples of balance of plant elements will be described below as shown inFIG. 1but other balance of plant elements or configurations may also be used.

Hydrogen, or a fuel containing hydrogen, is released from a fuel source14, enters the stack12at a hydrogen inlet16and exits the stack12from a hydrogen outlet18. Un-reacted hydrogen released from the hydrogen outlet18travels through a re-circulation loop21back to the hydrogen inlet16. Flow in the re-circulation loop21is driven by a hydrogen re-circulation pump20. From time to time, hydrogen, other impurities and water are removed from the fuel side of the stack by opening a purge valve22. The purge valve22may be a solenoid valve or another type of valve that can be operated by a mechanical, electrical or computerized controller.

Air (or oxygen or oxygen enriched air or oxygen depleted air) flows into the stack12through an air inlet24. Air and water vapor leave the stack12through an air outlet26. The flow of air is driven by an air pump28. The air pump28may operate at a constant speed or be driven by a variable frequency drive or other speed controllable motor. The air pump28may be connected to the air inlet24or the air outlet26.

Coolant, such as water or a mixture of water and an alcohol or another anti-freeze agent, enters the stack12through a coolant inlet30and exits the stack12from a coolant outlet32. From the coolant outlet32, the coolant passes through a radiator34or other heat exchanger before returning to the coolant inlet30. Coolant moves through this loop, optionally at a generally constant flow rate, driven by pump35. A coolant temperature sensor38sends a signal indicating the coolant or stack12temperature to a coolant system controller40. The coolant temperature sensor38can be located in the stack12or anywhere in the external part of the coolant loop. The coolant system controller40adjusts the speed of a radiator fan36as required to keep the temperature near, or within a specified range around, a temperature set point. Alternatively, the coolant system controller40could adjust the speed of the coolant pump35, move a baffle controlling the flow of air to the radiator34, alter the flow of another fluid through a heat exchanger or otherwise adjust the temperature of the coolant or the stack12.

The module10also has a master controller42. The master controller42operates the hydrogen recirculation pump20, the purge valve22, the air pump28and other balance of plant elements directly or by sending data to controllers associated with those elements. The master controller42also supplies the temperature set point to the coolant system controller40. The master controller42comprises a computer, such as a general purpose computer or a programmable logic controller, communication ports, and data storage. Optionally, the master controller42and coolant system controller40could be combined into a single controller.

Optionally, a signal associated with the recirculation pump20is sent to the master controller42and considered to determine if a cell is flooded. U.S. Provisional application No. 61/642,846 is incorporated by reference. In an example, the recirculation pump20is a regenerative or centrifugal pump operating at a generally constant voltage or speed. When the humidity in the fuel side of a cell stack increases, or a cell contains liquid water, more energy is required to achieve the same volumetric flow rate in the hydrogen recirculation loop. A signal indicating the current drawn by the recirculation pump20is sent to the master controller42. The master controller42can correct flooding on the hydrogen side of a fuel cell stack12most rapidly by opening the purge valve22.

Optionally, there may be a controllable bypass line between the air outlet26and the air inlet24. The flow of air in the by-pass line, if any, may be altered to control the relative content of oxygen in air flowing through the fuel cell stack12. This allows the polarisation curve of the fuel cell module10to be modified if desired. The master controller42may be connected to one or more valves in a by-pass line between the air outlet26and the air inlet24, and/or in one or more of the air outlet26and the air inlet24. Modulating one or more of these valves alters the partial pressure of oxygen (or oxygen concentration) in the air side of the fuel cell stack12even though the total gas flow rate (in this case oxygen depleted air) remains generally unchanged. The voltage of the fuel stack12varies more with the partial pressure of oxygen than with the total flow of gas thru the stack. A higher oxygen partial pressure produces a higher voltage at a given current output while a lower oxygen partial pressure produces a lower voltage at a given current output. U.S. Provisional application No. 61/827,318 is incorporated by reference.

FIG. 2shows an electrical power supply system50. The system50has a fuel cell module10and a battery52. Optionally, the battery52may be replaced with a different type of electrical storage device, for example a capacitor. The electrical power supply system50is used to provide electrical power to a load54. The load54may be, for example, a motor in a vehicle or other machine. In another example, the load54may be a building or other structure, or electronic equipment such as a cellular telephone tower or computer server, that may require back-up power. A rectifier53is optionally provided in cases where the system50is also connected to an electrical grid. A load may be attached to the system50through the rectifier53. The load54may be selectively connected to a bus56and ground58that are also connected to the electrical power supply system50. At various times, for example when a vehicle is required to accelerate or when an electrical grid fails, the load54draws power from the electrical power supply system50.

The fuel cell and battery can operate in at least partially overlapping voltage ranges. For example, the battery52may have a nominal 48 volt potential. However, the battery52may actually have a voltage ranging from about 40-60 volts depending on its state of charge and current output. The fuel cell module10may also have a nominal 48 volt output. However, the fuel cell stack12may actually have a voltage ranging from about 0 to 100 volts depending on the flow of reactants and the current output of the fuel cell module10.

The electrical power supply system50also has a current sensor60, or another sensor capable of indicating whether a current is present, and optionally three parallel circuits between the fuel cell module10and the bus56. Preferably, there is no voltage converter between the fuel cell stack12and the bus56. Optionally, a voltage converter can be used. A first circuit has a discharge resistor62and a discharge contactor66. A second circuit has a diode64and a first, or start-up, contactor68. A third circuit has a by-pass contactor70. The contactors66,68and70are typically relay switches that can be opened or closed by the master controller42. It is desirable to manage the operation of the contactors66,68and70so that they are not required to make or break connections between large voltage differentials or while carrying large currents to reduce the size of the contactors required and increase the lifetime of the contactor.

Referring back toFIG. 1, the master controller42is also connected to a fuel valve44which can be used to isolate or connect the source of fuel14to the fuel cell stack12. The source of fuel14may be, for example, a pressurized hydrogen or methane container. The fuel cell module10may also have a shut-down reservoir46to store a smaller amount of fuel to be used to consume residual oxygen in the fuel cell stack12and blanket the fuel cell stack12with nitrogen on shut down. Referring toFIG. 2, a small shut down resistor72may optionally be connected across the fuel cell stack12to enable nitrogen blanketing on shut down. As shown inFIG. 1, the shut down resistor72optionally has a shut down contactor73. One form of nitrogen blanketing is described in U.S. Pat. No. 7,425,379 B2, entitled Passive Electrode Blanketing in a Fuel Cell and issued on Sep. 16, 2008, which is incorporated by reference. U.S. Provisional application No. 61/619,073 is incorporated by reference. Although a single master controller42is shown inFIGS. 1 and 2, the functions of the master controller42may alternatively be divided across multiple controllers.

The master controller42receives a start command when power is required from the fuel cell module10in particular or from the electrical power supply system50in general. The start command may be generated in various ways by a human, mechanical or electronic operator. For example, the start command may be generated by a controller associated with the load54. Alternatively, the start command may be generated by a sensor detecting a condition, for example low voltage in the bus56. Upon receiving the start command, the master controller42initiates a starting sequence.

In one option, the starting sequence begins by opening the fuel valve44, turning on air pump28, preferably opening hydrogen purge valve22, turning on hydrogen recirculation pump20and closing the start-up contactor68. These steps preferably happen as quickly as reasonably possible. Optionally, a quantity of compressed air or other oxygen containing gas may be connected to the air inlet24to reduce start up time since the air pump28requires time to reach its full operating speed. The air pump28and hydrogen recirculation pump20are preferably operated at essentially their full power, for example at 80% or more of their full power, so that voltage will build in the fuel cell stack12as quickly as possible. The air pump28and hydrogen recirculation pump20are both preferably powered from the bus56, or directly from the battery52, to enable them to draw power before the fuel cell stack12has significant voltage and without initially requiring current from the fuel cell stack12.

The discharge contactor66and by-pass contactor70are left open such that current can only flow from the fuel cell module10to the bus56though the diode64. However, the diode64prevents any current from flowing until the voltage of the fuel cell stack12exceeds the voltage of the battery52or bus56by a threshold value, for example 0.7 V. When the current sensor60detects current above a selected threshold, for example 10 A, the by-pass contactor70is closed. The start-up contactor68is preferably then opened. In this way, neither the start-up contactor68nor the by-pass contactor70is required to make or break connections that would cause rapid changes in the power flowing through the contactor68,70. As an alternative to using current sensor60, the by-pass contactor70can be closed when another form of instrument indicates that current is flowing from the fuel cell stack12. For example, current flow can be determined by a voltage drop across the diode64or a differential voltage between the fuel cell stack12and the bus56. Although power will not flow through the diode64with by-pass contactor70closed, opening the start-up contactor68protects against damage to the diode64if the by-pass contactor70fails or if the fuel cell stack12voltage spikes when the by-pass contactor70is opened intentionally on shut down. Opening the start-up contactor68also provides certainty that opening by-pass connector70after a shut-down command will truly stop the flow of power from the fuel cell stack12to the bus56.

The fuel cell module10can be considered to have started as soon as the by-pass contactor70closes or current starts to flow across the diode64. However, the fuel cell module10may continue to increase its power output after this point. Alternatively, the fuel cell module10may be considered to have started when it reaches a certain percentage of its rated power, for example 80% or 100%. The fuel cell module10is preferably started in 5 seconds or less, 3 seconds or less or even 2 seconds or less. Once started, the master controller42switches to a normal operating mode. In the normal operating mode, the air pump28and hydrogen recirculation pump20are not necessarily run at full speed. Instead, the master controller42operates the fuel cell module to provide power as required while maintaining safe and efficient operation which may include modulating the air pump28and hydrogen recirculation pump20among other balance of plant elements.

With some designs of fuel cell stack12, it is possible for one or more cells to become partially flooded with water if the fuel cell module has been off for a long time, for example 12 hours or more, before being started. If the fuel cell module10is started in this condition, the flooded cell may be damaged. To help prevent such damage, the master controller42may determine whether the fuel cell module10is likely to have a flooded cell before initiating the starting sequence described above. This determination can be based, for example, on a timer started when the fuel cell module10was last turned off, or by another parameter such as a temperature in the fuel cell module10. If the fuel cell module10has been off for more than a selected time, or is below a certain temperature, a fault clearing and/or checking sequence is run. If not, then the starting sequence can begin immediately.

If a fault clearing or checking sequence is required, it is possible that the fuel cell stack may become charged, for example to its open cell voltage, before it can be determined that there are no flooded cells or any flooded cells can be cleared. However, a flooded cell will not be damaged if the fuel cell stack12is not delivering current. Accordingly, although the start-up contactor68could be left closed or even omitted, it is preferable to leave the start-up contactor68open during the fault clearing or checking sequence. Once it has been determined that there are no flooded cells, or flooded cells have been cleared, then discharge contactor66is closed to discharge the fuel cell stack12through the discharge resistor62. Optionally, flood clearing can occur wholly or partially while discharging the fuel cell stack12. As soon as the fuel cell stack12voltage is below the sum of the voltage of the bus56and the threshold voltage of the diode64, or can be predicted to reach that voltage in the closing time of the start-up contactor68, then the start-up contactor68can be instructed to close and the starting sequence described above begins.

The fuel cell stack12may also have a voltage higher than the voltage of the bus56after a very brief shutdown. In this or any other case when the fuel cell stack12has a high voltage, it is preferable to discharge the fuel cell stack12through the discharge resistor62before initiating the starting sequence to avoid exceeding the make limitations of a contactor68or70. Discharge contactor66is opened after either the start-up contactor68or by-pass contactor70is closed.

Optionally, if the fuel cell stack12is initially charged, the starting sequence could be begin by closing the by-pass contactor70instead of the start-up contactor68when the voltage of the fuel cell stack12drops to within the make tolerance of the by-pass contactor70relative to the voltage of the bus56. However, since the voltage of the fuel cell stack12preferably falls rapidly with time, and it takes some time for the by-pass contactor70to close, it can be difficult to time the closing of the by-pass contactor70with sufficient accuracy to avoid requiring a by-pass contactor70with a large make tolerance. Accordingly, it may be preferable to use the start-up contactor68as described above. When using the start-up contactor68, the by-pass contactor70is only required to make a connection through a voltage differential equal to the voltage drop through diode64, which may be on the order of one volt.

Optionally, discharge resistor62could alternatively be connected to ground58rather than bus56, but its resistance would need to be higher to meet the same current limitation of the discharge contactor66or fuel cell stack12and the power rating would need to be much higher.

In the fault clearing and/or checking sequence, if any, the air pump28is run at substantially full speed. This provides oxygen to the fuel cell stack12while simultaneously removing water from the air side of fuel cell stack12. The fuel valve44is also opened and hydrogen recirculation pump20turned on. The master controller42checks the stack for faults. For example, the master controller42may check whether the fuel cell stack12as a whole, or individual cells or groups of cells, are capable of reaching their full open cell voltage. Alternatively, or additionally, the master controller12may check whether the power consumption or speed of the hydrogen recirculation pump20indicates excessive humidity or water in the fuel cell stack12. The hydrogen recirculation pump20is the preferred fault indicator since it can provide a reading before the fuel cell stack12reaches open cell voltage. This saves time before a fault is determined and also reduces time required to discharge the fuel cell stack12. If no fault is detected, then the fuel cell stack12is discharged if it has a voltage above the voltage of the bus56and the start-up sequence first described above continues. If a fault is detected, the hydrogen purge valve22is opened, or other recovery methods occur, until flooding has been cleared from the hydrogen side of the fuel cell stack12.

To shut down the fuel cell module10, the fuel valve44is closed and the air pump28is turned off. Optionally, closing the fuel valve44may be delayed after shutting off the air pump28to allow time to confirm that the shut down will not be very brief. Hydrogen purge valve22is opened. Hydrogen supplied from the reservoir46and residual reactants are consumed in the fuel cell stack causing nitrogen blanketing in the fuel cell stack. When the current sensor60indicates that the fuel cell module10is no longer providing power to the bus56, the by-pass contactor70is opened. The fuel cell stack12continues to discharge through resistor72while hydrogen blanketing continues. Optionally, the air pump28may be mechanically or electrically stopped, or isolated from the fuel cell stack12by a valve, before opening hydrogen purge valve22to reduce the time and hydrogen required for nitrogen blanketing. Alternatively, the air pump28can be operated at essentially full power for a few seconds before closing fuel valve44and before or concurrently with opening the hydrogen purge valve22to remove humidity from the fuel cell stack12and thereby reduce the chance of a flooded cell on the next start up. As a further option, the fuel cell stack12may also be discharged to below the voltage of the bus56through the resistor62if there is a voltage spike in the fuel cell stack12after opening the by-pass contactor70to be ready for the next start up more quickly.

In one particular example of a shut-down procedure, the air pump28is shut off and the fuel valve44remains open. Reactants in the fuel cell stack12are consumed while current delivered by the fuel cell stack12decreases. When the current sensor60indicates that the fuel cell stack12is delivering less than a selected current, for example 10 A, the by-pass contactor70is opened, at which point fuel valve44is closed. Optionally, the air pump28is then operated briefly to remove water from the fuel cell stack12. Whether the air pump28is operated or not, the fuel cell stack12voltage is likely to rise to open circuit voltage when the by-pass contactor70is opened. The fuel cell stack12is discharged through the resistor72, resistor62, or both at the same time or in sequence, to the bus56voltage or less, preferably to essentially no voltage, while nitrogen blanketing continues. If the air pump28is operated to remove water from the fuel cell stack12, either valve44remains open for a longer period of time or the size of reservoir46is increased to facilitate nitrogen blanketing after the air pump28has been operated to remove water from the fuel cell stack12.

Optionally, resistor72may be left connected for only as long, if at all, as required to consume the residual reactants in the fuel cell stack12. The fuel cell stack12may then be left while off in a partially charged stated and either connected to, or disconnected from, the bus56. Optionally, the fuel cell stack12may be left connected to the bus56at all times, or at least for a period of time while the fuel cell module10is off. This is not preferred since residual voltage in the fuel cell stack12can degrade the fuel cell stack over time or may be a hazard. But if the fuel cell stack12is left connected, it must be checked for flooded cells on start-up while not delivering power to avoid damaging a flooded cell. In this case, a flooded cell check is done by monitoring the power consumption or speed at constant power of the hydrogen recirculation pump20before turning the air pump28on and before opening the fuel valve44. The fuel valve44and hydrogen purge valve22are opened as required to remove water if a flooded cell is detected. If no flooding on the hydrogen side is detected, the air pump28is turned on full to remove any flooding on the air side of the fuel cell stack12and to start the fuel cell stack12creating power.

One start up procedure involves first determining if a no load check for flooded cells or other faults is required, and if the fuel cell stack12voltage exceeds the bus56voltage. If a fault check is required, then the fault check is conducted. If a fault is detected, the fault is corrected, primarily by operating the air pump28and opening the hydrogen purge valve22to remove excess water from the fuel cell stack12. If no fault is detected, or if a fault is detected and then cleared, of if the fuel cell stack12was otherwise above the bus56voltage, then the fuel cell stack12is discharged. If no load check was required and the fuel cell stack12voltage was not above the bus56voltage, or the fuel cell stack has been discharged, then the start-up sequence begins by closing the start-up contactor68. When the fuel cell stack is providing a specified current, the by-pass contactor70is closed and the start-up contactor68is opened.