GB2545246A

GB2545246A - Fuel cell ventilation system

Info

Publication number: GB2545246A
Application number: GB1521791.2A
Authority: GB
Inventors: Wardle Phillip; Gunkel Michael
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2015-12-10
Filing date: 2015-12-10
Publication date: 2017-06-14
Also published as: GB201521791D0

Abstract

A fuel cell system 1 comprises a fuel cell 2 having a fluid path therethrough, a duct 6 coupled to the fuel cell for conveying fluid to or from the fluid path and a fluid flow amplifier 14 configured to amplify fluid flow through the duct. The amplifier preferably comprises a fluid multiplication nozzle comprising a surface having a Coanda profile (34, figure 3a). The system may comprise a controller 10 configured to operate amplifier without the fluid movement device upon start-up of the fuel cell and to switch on the fluid movement device after start-up when the fuel cell power output exceeds a predetermined threshold. The fluid path may be for supplying an oxidant or a cooling supply, such as air. Also disclosed is a fuel cell system further comprising a fluid movement device 7, such as a compressor blower or fan, configured to direct fluid through the duct and a method of operating a fuel cell system comprising using a fluid movement device to direct fluid through a fluid path of the fuel cell in a main flow phase and using an auxiliary compressor 8 to direct fluid through the fluid path in an auxiliary flow phase.

Description

FUEL CELL VENTILATION SYSTEM

The present invention relates to fuel cell systems, and in particular to fuel cell systems incorporating fuel cell stacks requiring forced ventilation.

Conventional electrochemical fuel cells convert fuel and oxidant, generally both in the form of gaseous streams, into electrical energy and a reaction product. A common type of electrochemical fuel cell for reacting hydrogen and oxygen comprises a polymeric ion transfer membrane, also known as a proton exchange membrane (PEM), within a membrane-electrode assembly (MEA), with fuel and air being passed over respective sides of the membrane. Protons (i.e. hydrogen ions) are conducted through the membrane, balanced by electrons conducted through a circuit connecting the anode and cathode of the fuel cell. To increase the available voltage, a stack is formed comprising a number of series-connected MEAs arranged with separate anode and cathode fluid flow paths. Such a stack is typically in the form of a block comprising numerous individual fuel cell plates held together by end plates at either end of the stack.

The supply of oxidant to the fuel cell stack may be provided by way of a fan, blower or compressor for delivering air to the cells in the stack. Because the reaction of fuel and oxidant generates heat as well as electrical power, a fuel cell stack also requires cooling once an operating temperature has been reached, to avoid damage to the fuel cells. Cooling may be achieved by forcing air through the fuel cell stack. In an open cathode stack, the oxidant flow path and the coolant flow path are the same, i.e. forcing air through the cathode fluid flow paths both supplies oxidant to the cathodes and cools the stack. The amount of oxidant and or cooling air flow required is dependent on the current output of the fuel cell stack as well as other prevailing conditions.

Thus, oxidant supply and/or cooling air are conventionally provided to a fuel cell stack using an appropriate fan, blower or compressor. Such devices require a supply of electricity which, when the fuel cell stack is fully operational, can be provided by the fuel cell stack itself and forms part of the parasitic load on the fuel cell stack. However, on start up and until the fuel cell stack is generating sufficient electricity to service the parasitic load of an air compressor, the compressor is conventionally powered by a battery or other auxiliary power supply. Particularly for high power fuel cell systems, requiring high power compressors, providing such an auxiliary supply can be difficult and/or add cost and complexity to the system.

It is an object of the present invention to provide an improved design for providing oxidant and/or air cooling supply to a fuel cell stack.

According to one aspect, the present invention provides a fuel cell system comprising: a fuel cell having a fluid path therethrough; a duct coupled to the fuel cell for conveying fluid to and/or from the fluid path; a fluid movement device configured to direct fluid through the duct; and a fluid flow amplifier configured to amplify fluid flow through the duct.

The fluid may be air. The fluid flow amplifier may be an air flow amplifier.

The fluid flow amplifier may comprise a fluid multiplication nozzle. The fluid multiplication nozzle may be within the duct. The fluid multiplication nozzle may comprise a surface having a Coanda profile. The Coanda profile may be within the duct. The fluid multiplication nozzle may extend around a portion of, or the entirety of, a cross-section of the duct.

The duct may be unobstructed by fan blades other than those of the fluid movement device. The fluid movement device may comprise a compressor, a blower or a fan.

An auxiliary fluid movement device may be associated with the fluid flow amplifier. The auxiliary fluid movement device may be configured to operate at a lower fluid flow rate than the fluid movement device. The auxiliary fluid movement device may be configured to operate at a lower voltage and/or current than the fluid movement device.

The fluid flow amplifier may comprise an inlet pipe coupled between the auxiliary fluid movement device and the fluid multiplication nozzle. The auxiliary fluid movement device may be configured to force fluid via the inlet pipe to the nozzle and thereby into the duct.

The fluid movement device and the fluid flow amplifier may both be disposed upstream of the fuel cell. The fluid movement device and the fluid flow amplifier may both be disposed downstream of the fuel cell. The duct may extend both upstream and downstream of the fuel cell. The fluid movement device and the fluid flow amplifier may be respectively disposed in portions of the duct on opposing sides of the fuel cell.

The fluid multiplication nozzle may comprise a variable orifice nozzle.

The fuel cell system may further include a controller. The controller may be configured to operate the fluid movement device and/or the fluid flow amplifier in accordance with a state of the system. The controller may be configured to operate the fluid flow amplifier without the fluid movement device upon start up of the fuel cell. The controller may be configured to switch on the fluid movement device after start up when the fuel cell power output exceeds a predetermined threshold. The controller may be configured to shut down the fluid flow amplifier when the fluid movement device has been switched on. The controller may be configured to close a fluid flow path through the fluid flow amplifier when the air movement device has been switched on. The controller may be configured to close the variable orifice nozzle to isolate the fluid flow amplifier from the duct.

The fuel cell system may comprise a fuel cell stack having a plurality of cells. Each cell may have a fluid path therethrough. The fluid path may be a ventilation path and/or oxidant path through the fuel cells. The duct may be a ventilation and/or oxidation duct.

The fuel cell system may comprise a plurality of fluid flow amplifiers. Each amplifier may be configured to amplify fluid flow through the duct. The fluid flow amplifiers may be arranged in parallel or in series. The duct may be furcated and each of the fluid flow amplifiers may be arranged on a different branch of the duct.

According to a further aspect of the invention there is provided a method of operating a fuel cell system comprising a fuel cell having a ventilation path therethrough, the method comprising: using a fluid movement device to direct fluid through the fluid path of the fuel cell in a main flow phase; using an auxiliary compressor to direct fluid through the fluid path of the fuel cell in an auxiliary flow phase.

The main flow phase may occur after the auxiliary flow phase. The method may comprise operating the fluid multiplier without the fluid movement device in the auxiliary flow phase upon start up of the fuel cell system. The method may comprise switching on the fluid movement device in the main flow phase after start up when the fuel cell power output exceeds a predetermined threshold.

The auxiliary flow phase may occur after the main flow phase. The method may comprise switching off the fluid movement device to end the main flow phase upon shut down of the fuel cell system. The method may comprise operating the fluid multiplier without the fluid movement device in the auxiliary flow phase during shut down of the fuel cell system.

The fluid flow amplifier may be shut down when the fluid movement device has been switched on.

Also disclosed is a fuel cell system comprising: a fuel cell having a path therethrough; a duct coupled to the fuel cell for conveying fluid to and/or from the path; a fluid flow amplifier configured to amplify fluid flow through the duct.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawing in which:

Figure 1 shows a schematic diagram of a fuel cell system incorporating an air amplifier connected to a ventilation duct;

Figure 2 shows a schematic diagram of an exterior of an air amplifier connected to a ventilation duct;

Figure 3a shows a schematic diagram of a longitudinal cross section taken through a portion of an air amplifier;

Figure 3b shows a schematic diagram of a transverse cross section taken through a portion of the air amplifier of figure 3a;

Figure 3c shows a schematic diagram of a longitudinal cross section taken adjacent to an air multiplication nozzle of the air amplifier of figure 3a; and

Figure 4 shows a flow diagram for a method of operating a fuel cell system.

The disclosure relates to providing a fluid flow amplifier for providing ventilation to a fuel cell system during periods of low power availability and is applicable to fuel cell systems which use a supply of oxidant fluid to ventilation flow paths through the cathodes of cells in a fuel cell stack and/or a supply of coolant fluid to the stack. Typically, though not exclusively, the oxidant fluid and coolant fluid may be air. A fluid flow amplifier may also be referred to as a “fluid flow multiplier’’, “bladeless fan”, or “thrust jet”, or, in the cases that the fluid is air, an “air amplifier” or “air multiplier”. A fluid flow amplifier may be considered to be bladeless in that no blades are present in a main flow path through the fluid flow amplifier. Typically, though not exclusively, the ventilation flow paths may be common for both oxidant and coolant, such as in open-cathode, air-cooled fuel cell systems.

For high power fuel cell systems, a large volume of air is required and hence a relatively powerful compressor is employed. The power for this compressor can often be taken from a high voltage (HV) battery of a host system (such as a vehicle in which the fuel cell stack is installed) for start up of the fuel cell stack, and then from the fuel cell stack itself during normal running. During start up of such a compressor it is usual to have high peak power demands for a short period (e.g. <1 second). If the host vehicle HV system voltage is compatible with the compressor supply then this may be a simple, low complexity option. However, if there is no host HV supply available, or the host HV supply is incompatible with or insufficient for available fuel cell stack compressors, then another method to supply power to the compressor is required.

If voltage conversion is required for supplying power to a compressor, generating a start up supply via a DC/DC converter can be problematic because the converter will have to be sized to cope with high start up demands of the compressor, if the start up peak demands cannot be moderated. DC/DC converters are relatively expensive components, and more so if they have to be oversized to deliver high peak demands and if the voltage ranges required are non standard. Additionally a DC/DC converter that is only used for a short period during start up and shut down (assuming the compressor can be powered by the fuel cell stack during normal running) is an inefficient use of cost, space and weight. Thus, avoiding this approach can be desirable.

Figure 1 shows a fuel cell system 1 in which a fuel cell stack 2 has a plurality of cells 3 arranged in parallel configuration. Each fuel cell 3 has a fuel flow path (not shown) for delivering fluid fuel to the anode side of the membrane-electrode assembly of the cell, and an oxidant flow path shown schematically by arrows 4 for delivering fluid oxidant to the cathode membrane-electrode assembly of the cell. The flow paths within each cell may be coupled to respective common manifolds for anode and cathode flows, e.g. cathode manifold 5.

The cathode manifold 5 is coupled, via a ventilation duct (or conduit) 6, to a main compressor, blower or fan 7. An air amplifier 14 is provided on the ventilation duct 6 in series with the main compressor 7. An auxiliary compressor, blower or fan 8 is associated with the air amplifier 14. The air amplifier 14 is configured to amplify airflow on the main flow path through the duct 6 using air flow from the auxiliary compressor 8. The structure and operation of the air amplifier is discussed further below with reference to the examples in figures 2 and 3.

For convenience, throughout the present specification, the expression "compressor" will be used to encompass blowers, fans, compressors and other air or fluid movement devices regardless of ratio of discharge pressure over suction pressure and regardless of displacement type.

The main compressor 7 is preferably configured to operate from the fuel cell stack 2 electrical output 12 when the fuel cell stack is generating sufficient power. The auxiliary compressor 8 is preferably configured to operate from the low voltage supply 11 when the fuel cell stack 2 is not generating sufficient power to supply the main compressor 7 or from the low power output available from the stack when under start up conditions. The auxiliary compressor 8 thus has a reduced power capability than the main compressor. The expression "reduced power capability" is intended to encompass any one of more of: (i) the auxiliary compressor 8 is configured to operate on a lower voltage supply than the main compressor 7; (ii) the auxiliary compressor 8 is configured to have a maximum current requirement less than the main compressor 7; and (iii) the auxiliary compressors has a lower air or fluid flow capability than the main compressor 7.

The auxiliary compressor 8 is preferably of significantly lower power than the main compressor 7, but of sufficient power to deliver the necessary air flow and pressure for reliably starting and shutting down the fuel cell stack 2 throughout the system life and in an operating envelope within the power available from a low voltage supply 11.

Figure 2 shows an exterior of an air amplifier 14 connected to a ventilation duct 6, such as that described previously with reference to figure 1.

The air amplifier 14 has a main flow path 20 through the duct 6 and an auxiliary flow path 22 for additional airflow in order to provide an air amplification effect. Airflow along the auxiliary flow path 22 is provided through an auxiliary airflow pipe 24 of the air amplifier 14 by the auxiliary compressor.

The air amplifier 14 is configured to “amplify” airflow along the main flow path 20 using airflow from the auxiliary flow path 22. For example, the provision of each unit of airflow through the auxiliary flow path 22 may result in a quantity of airflow through the main flow path 20 that is a factor larger than the airflow through the auxiliary flow path 22. In this way, the air amplifier 14 can be considered to provide a gain in airflow. A specific mechanism by which this gain in airflow may be achieved is discussed further below with regard to figure 3.

One or more air filters (not shown) may be provided in order to filter air passing through the main airflow path 20 or the auxiliary flow path 22. The one or more air filters may be located in the flow path upstream or downstream of each of the main compressor and the auxiliary compressor. In one example, a filter may be provided in the duct 6 downstream of the air amplifier 14 in order to filter air that has been drawn into the main flow path 20 from the auxiliary flow path 22. In this case, only a single air filter may be required for both the main air flow path 20 and the auxiliary flow path 22. Alternatively, a first air filter may be provided upstream of the main compressor and a second air filter may be provided downstream of the auxiliary compressor in order to tailor the air filter arrangement to the pressure drop requirements of the compressor arrangements.

Figures 3a to 3c show schematic cross sections taken through a portion of an air amplifier 14 on a ventilation duct 6. Figure 3a shows a longitudinal cross section in which a plane of the cross section is parallel with an axis of the ventilation duct 6. Figure 3b shows a transverse cross section in which a plane of the cross section is normal to the axis of the ventilation duct 6. Figure 3c shows a longitudinal cross section in the region of the air multiplication nozzle 30. The figures are not necessarily drawn to scale.

The air amplifier 14 comprises an air multiplication nozzle 30 within the ventilation duct 6. Auxiliary air flow 22 is provided to the multiplication nozzle 30 from the auxiliary compressor via the auxiliary airflow pipe 24 and a plenum 26.

The air multiplication nozzle 30 comprises a surface 34 having a Coanda profile within the duct 6. The surface 34 providing the Coanda profile may be provided by an aerofoil over which the auxiliary air flow 22 from the auxiliary airflow pipe 24 passes to provide a boundary layer 32 of air adjacent to the aerofoil. The boundary layer 32 conforms to the shape of the surface 34 due to the viscosity of the air and entrains air from the main flow path 20, thereby drawing further air flow through the duct 6 along the main flow path 20. Air flow from the main flow path 20 may be increased by pressure and/or viscosity effects.

The air multiplication nozzle 30 may be provided by a slit that extends around a transverse cross-section of the duct. The slit is defined by a first lip 35 of a first collar, which forms an edge of the surface 34, and a second lip 36 of a second collar. As shown in figure 3b, the air multiplication nozzle 30 may comprise a discontinuous arrangement of nozzle portions 30a-h provided around the duct 6. In this example, the second lip 36 overlaps the first lip 35 so that the slit has a width when viewed as a transverse cross section through the duct. In the example shown in figure 3b, the first lip 35 of the first collar is outwardly disposed in the duct 6 with respect to the second lip 36 of the second collar.

In some examples, the air multiplication nozzle 30 may be provided as a continuous ring when viewed along the duct 6. The air multiplication nozzle 30 may take a toroidal shape in such examples. In some examples, the first lip 35 may be displaced from the second width along an axis of the duct 6 so that the slit has a width when viewed as a longitudinal cross section through the duct 6, as seen in figure 3a.

The area of the surface 34 having a Coanda profile may be increased by providing a number of air amplifiers in the duct, and so the effectiveness of the air amplifier may be improved for a given supply of air from the auxiliary compressor. For example, a plurality of air amplifiers may be provided in parallel within the duct 6. The duct may be furcated and each of the fluid flow amplifiers may be arranged on a different branch of the duct. In such examples, the cross-section of the duct 6 may be configured to assist packing the plurality of air amplifiers in a confined space defined by the fuel cell system. Further, a plurality of air amplifiers may be provide in series along the duct, in which case each air amplifier may have its own compressor, or alternative a compressor and air flow regulator system may supply air to the plurality of air amplifiers.

It is desirable to isolate the auxiliary flow path 22 from the main flow path 20 when the auxiliary compressor is inactive because otherwise the auxiliary flow path 22 may provide an alternative path for air to escape from the main flow path 20 and thereby a loss of pressure along the main flow path 20. The air multiplication nozzle 30 may comprise a variable orifice nozzle that has one or more open positions and a closed position. As shown in figure 3c, the variable orifice nozzle has valve members provided by the first lip 35 of the first collar and the second lip 36 of the second collar. The second collar is displaceable 38 along the axis of the duct with respect to the first collar in order to vary a width 39 of the slit through which the auxiliary air flow passes to become the boundary layer 32. The variable orifice nozzle may be provided by a valve having a variable opening size that is controlled by, for example, a screw thread arrangement driven by a stepper motor. In examples in which the air multiplication nozzle 30 provides a valve action, the main flow path 20 can be isolated from the auxiliary flow path 22 by way of the valve action. A shut-off valve or multiway valve may be provided at any point along the auxiliary flow path 22, for example between the plenum 26 and the auxiliary air inlet 24, in order to selectively enable or disable air flow through the auxiliary flow path 22.

Returning to figure 1, the air amplifier 14 is in series with the main compressor 7 along the ventilation duct 6, but the auxiliary compressor 8 may be provided remote from the ventilation duct 6 (that is, not in series with the main compressor 8). An advantage of providing the air amplifier 14 in series with the main compressor 8, rather than providing an auxiliary compressor directly in series with the main compressor 8, is that the duct 6 may be unobstructed by fan blades (other than those of the main compressor 8). In this way, the air amplifier 14 provides almost no air flow impedance in the main flow path 20 when the auxiliary compressor 8 is not in use. The air amplifier 14 may also be more power efficient at producing a given volume of air flow than if the auxiliary compressor 8 were provided directly in series with the main compressor 8 on the duct 6. For this reason it may be advantageous to use the air amplifier, rather than an auxiliary fan directly in series with the main compressor, when operating the auxiliary compressor 8 with a reduced power capability. The provision of the air amplifier, rather than the auxiliary fan, directly in series with the main compressor may also reduce the requirement for a large turndown of the main compressor.

As shown in figure 1, the main compressor 7 and the air amplifier 14 are preferably both disposed upstream of the fuel cell stack 2, i.e. providing fluid flow through the fuel cell stack by asserting a positive pressure on an input side of ventilation paths (e.g. oxidant flow paths 4) through the fuel cells. However, other configurations can be envisaged, such as when the main compressor 7 and air amplifier 14 are both disposed downstream of the fuel cell stack, i.e. providing fluid flow through the fuel cell stack by asserting a negative pressure on an output side of ventilation paths through the fuel cells. Another alternative is where the main compressor 7 and air amplifier 14 are disposed on opposing sides of the fuel cell stack, i.e. one of them operates by asserting a positive pressure on an input side of the ventilation paths through the fuel cells and the other one operates by asserting a negative pressure on an output side of the ventilation paths through the fuel cells. The embodiments having the main compressor 7 upstream of the fuel cell stack may be preferred where substantial quantities of water and water vapour exhausted from the fuel cell stack cathode ventilation paths could interfere with operation of main compressor disposed downstream of the fuel cell stack. The main compressor 7 can be upstream of the air amplifier 14, or downstream of the air amplifier 14. A controller 10 is coupled to the electrical output 12 of the fuel cell stack and to a low voltage supply 11. The controller 10 may also be coupled to a high voltage supply 13. The high voltage supply may be that of a host system, such as a vehicle, in which the fuel cell system is installed. The low voltage supply may be a conventional battery forming part of the fuel cell system, a supply from a host system to which the fuel cell system is coupled, or a supply drawn from the fuel cell stack 2 itself. The controller 10 is configured to control the operation of the main and auxiliary compressors 7, 8 in accordance with a state of the system 1, which may be related to a parameter of the system 1 such as a voltage or current level available from the fuel cell stack 2, or a mode of operation of the system 1 such as start up or shut down. The operation of the controller 10 during fuel cell stack start up and fuel cell stack shut down modes is described below.

At commencement of a start up mode of operation, the fuel cell stack 2 will be generating no electrical power and all fuel cell systems may be operated by the low voltage supply 11. In a typical example, the low voltage supply may be a 12 V or 24 V battery either installed in the fuel cell system 1, or part of a host system such as a vehicle LV system. During the start up mode of operation, the auxiliary compressor 8 is driven from the low voltage supply 11 so that the air amplifier 14 provides a relatively low level of the necessary fluid flows to the ventilation paths of the fuel cells. As electrochemical reactions take place within the fuel cell stack, the fuel cells will heat up and the voltage level on output 12 will rise; the available current will rise, and the demand for fluid oxidant will rise, and secondarily, the demand for cooling or temperature regulation of the fuel cells will rise.

When sufficient power is available on the fuel cell electrical output 12, the controller 10 operates to switch on the main compressor 7 to supply the greater quantities of fluid flow now demanded. In a typical example, the fuel cell electrical output 12 may be configured to interface with the high voltage circuits 13 of a vehicle's automotive system, e.g. the 100 - 400 V typically required for motive power units of the vehicle. This is more than adequate to supply the higher capacity main compressor 7. Thus, in a general aspect, the controller is operative to switch on the main compressor 7 once the fuel cell stack power output exceeds a predetermined threshold. The power output may be determined by reference to one or more of, for example: a sensed voltage of fuel cell stack output; performance of some or all cells, by current drawn from the stack by a downstream load; by temperature of the fuel cell stack or individual cells; by a duration of operation since start up; or any other suitable parameters. A typical period of operation of the auxiliary compressor, until the main compressor is switched on, could be of the order of 20 seconds although this could vary widely dependent on the design of fuel cell system and the specification of the main compressor.

Once the main compressor 7 is in operation, the controller 10 may shut down the auxiliary compressor 8. It is preferred that the controller commands a shut-off valve along the auxiliary flow path 22 to be closed in order to isolate the auxiliary compressor 8 from the duct 6 when the auxiliary compressor 8 is off (or when the main compressor 7 is on) since the auxiliary flow path 22 may create an undesirable pressure drop due to fluid escaping through the auxiliary flow path 22. This could increase parasitic losses on the fuel cell system and is preferably avoided.

In one shut down mode of operation, it may be desirable to take as much load off the fuel cell stack 2 as possible prior to stopping fluid flows, e.g. to allow the stack to dry out and thereby prevent problems with freezing of water when the fuel cell is not in use. In this instance, the main compressor 7 can be switched off and the air amplifier 14 relied upon during the shut down procedure. In an alternative shut down mode of operation, the amount of electrical power generated by the fuel cell stack 2 may fall. At some point there will be insufficient power to drive the main compressor, and at or before this point, the controller 10 may switch on the auxiliary compressor 8 associated with the air amplifier 14 and switch off the main compressor 7.

In either case, the auxiliary compressor 8 associated with the air amplifier 14 and other fuel cell systems may be operated by the low voltage supply 11 during a shut down procedure. The auxiliary compressor 8 is driven from the low voltage supply 11 so that the air amplifier 14 provides the relatively low level of necessary fluid flows to the ventilation paths of the fuel cells to effect controlled shut down and possibly also provides additional cooling flows or purging flows to purge excess gases, water and/or water vapour from the ventilation paths.

Preferably, the auxiliary compressor 8 is configured as a high efficiency unit, for example using a brushless controller and motor to avoid carbon contamination within the duct 6. This means that the motor and control electronics of the auxiliary compressor can be cooled directly by the air flows generated through the compressor without requiring separate containment of the electronic parts.

Figure 4 illustrates a method 40 of operating a fuel cell system comprising a fuel cell having a ventilation path therethrough. The method 40 may be implemented by a controller of a fuel cell system such as that described previously with reference to figure 1. The method 40 comprises using an air movement device such as a main compressor to direct fluid through the ventilation path of the fuel cell during a main flow phase 42 and using an auxiliary compressor to direct fluid through the ventilation path of the fuel cell in an auxiliary flow phase 44.

The method 40 may be used during the start up procedure for the fuel cell system. In this case, the main flow phase 42 occurs after the auxiliary flow phase 44 in chronological order. During the start up procedure, the air amplifier may be operated without the air movement device in the auxiliary flow phase 44 and, subsequently, the air movement device may be switched on in the main flow phase 42 after start up in response to the fuel cell power output exceeding a predetermined threshold. The air amplifier may be off during the main flow phase 42.

The method 40 may be used during the shut down procedure for the fuel cell system. In this case, the main flow phase 42 occurs before the auxiliary flow phase 44 in chronological order. During the shut down procedure, the air movement device may be switched off and so end the main flow phase 42 and, subsequently, the air amplifier may be operated without the air movement device in the auxiliary flow phase 44. The air amplifier may also be off during the main flow phase 42 during the shut down procedure.

The principles of the main compressor and air amplifier described above can be applied to providing any suitable fluid flows to ventilation paths in fuel cells of a fuel cell stack. These ventilation paths may typically comprise the cathode air flow paths for oxidant delivery, but could instead or in addition comprise cooling airflow paths. The oxidant flow paths and the cooling air paths could be the same paths. In typical applications, the fluid being delivered by the main compressor and air amplifier is air but delivery of other oxidant or cooling fluid flows can also be implemented. For example, a main fluid movement device and a fluid flow amplifier may be provided within a liquid coolant loop of a liquid cooled fuel cell system while providing at least some of the benefits described previously.

Although the exemplary arrangement of figure 1 shows a fuel cell system in which a main compressor and an auxiliary compressor provide fluid flows to a single stack, it is possible to use the main and auxiliary compressor configuration to provide fluid flows to plural stacks, e.g. in a parallel arrangement, or to a single fuel cell or a plurality of fuel cells that are not arranged in a stack, e.g. in a planar arrangement.

As indicated above, the expression "compressor" is used to encompass blowers, fans, compressors and other air or fluid movement devices regardless of ratio of discharge pressure over suction pressure and regardless of displacement type. A preferred form of fluid movement device is a centrifugal blower. In one arrangement, the auxiliary compressor could be configured so that it can be powered by hand, e.g. hand cranked directly or supplied electrically by a hand cranked dynamo. Such an arrangement would be useful as an emergency back up or where no power is available until the fuel cell stack is operational.

In another arrangement, the main compressor and the auxiliary compressor could be formed in a unitary housing. The housing could form the shared ducted airflow path.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

1. A fuel cell system comprising: a fuel cell having a fluid path therethrough; a duct coupled to the fuel cell for conveying fluid to and/or from the fluid path; a fluid movement device configured to direct fluid through the duct; and a fluid flow amplifier configured to amplify fluid flow through the duct.

2. The fuel cell system of claim 1 in which the fluid flow amplifier comprises a fluid multiplication nozzle within the duct.

3. The fuel cell system of claim 2 in which the fluid multiplication nozzle comprises a surface having a Coanda profile within the duct.

4. The fuel cell system of claim 2 in which the fluid multiplication nozzle extends around a cross-section of the duct.

5. The fuel cell system of claim 1 in which the duct is unobstructed by fan blades other than those of the fluid movement device.

6. The fuel cell system of claim 1 in which the fluid movement device comprises a compressor, a blower or a fan.

7. The fuel cell system of claim 1 in which an auxiliary fluid movement device is associated with the fluid flow amplifier, in which the auxiliary fluid movement device is configured to operate at a lower fluid flow, voltage and/or current than the fluid movement device.

8. The fuel cell system of claim 7, in which the fluid flow amplifier comprises a fluid multiplication nozzle within the duct and in which the fluid flow amplifier comprises an inlet pipe coupled between the auxiliary fluid movement device and the fluid multiplication nozzle, the auxiliary fluid movement device configured to force fluid via the inlet pipe to the fluid multiplication nozzle and thereby into the duct.

9. The fuel cell system of claim 1 in which the fluid movement device and the fluid flow amplifier are both disposed in a duct upstream of the fuel cell.

10. The fuel cell system of claim 1 in which the fluid movement device and the fluid flow amplifier are both disposed in a duct downstream of the fuel cell.

11. The fuel cell system of claim 1 in which the duct extends both upstream and downstream of the fuel cell, the fluid movement device and the fluid flow amplifier being respectively disposed in portions of the duct on opposing sides of the fuel cell.

12. The fuel cell system of claim 2 in which the fluid multiplication nozzle comprises a variable orifice nozzle.

13. The fuel cell system of claim 1 further including a controller configured to operate the fluid movement device and the fluid flow amplifier in accordance with a state of the system.

14. The fuel cell system of claim 13 in which the controller is further configured to operate the fluid flow amplifier without the fluid movement device upon start up of the fuel cell and to switch on the fluid movement device after start up when the fuel cell power output exceeds a predetermined threshold.

15. The fuel cell system of claim 14 in which the controller is further configured to shut down the fluid flow amplifier when the fluid movement device has been switched on.

16. The fuel cell system of claim 15 in which the controller is further configured to close a fluid flow path through the fluid flow amplifier when the air movement device has been switched on.

17. The fuel cell system of claim 16 in which the fluid flow amplifier comprises a fluid multiplication nozzle having a variable orifice and in which the controller is configured to close the fluid multiplication nozzle to isolate the fluid flow amplifier from the duct.

18. The fuel cell system of claim 1 comprising a fuel cell stack having a plurality of cells each having a fluid path therethrough, in which the fluid is air and the fluid flow amplifier is an air amplifier, and in which the fluid path is a ventilation path and/or oxidant path.

19. The fuel cell system of claim 1 comprising a plurality of fluid flow amplifiers, each amplifier configured to amplify fluid flow through the duct, in which the fluid flow amplifiers are arranged in parallel.

20. A method of operating a fuel cell system comprising a fuel cell having a ventilation path therethrough, the method comprising: using a fluid movement device to direct fluid through the fluid path of the fuel cell in a main flow phase; using an auxiliary compressor to direct fluid through the fluid path of the fuel cell in an auxiliary flow phase.

21. The method of claim 20 in which the main flow phase occurs after the auxiliary flow phase, the method further comprising the steps of: operating the fluid multiplier without the fluid movement device in the auxiliary flow phase upon start up of the fuel cell system; and switching on the fluid movement device in the main flow phase after start up when the fuel cell power output exceeds a predetermined threshold.

22. The method of claim 20 in which the auxiliary flow phase occurs after the main flow phase, the method further comprising the steps of: switching off the fluid movement device to end the main flow phase upon shut down of the fuel cell system; and operating the fluid multiplier without the fluid movement device in the auxiliary flow phase during shut down of the fuel cell system.

23. The method of claim 20 further comprising the step of shutting down the fluid flow amplifier when the fluid movement device has been switched on.

24. A fuel cell system comprising: a fuel cell having a path therethrough; a duct coupled to the fuel cell for conveying fluid to and/or from the path; a fluid flow amplifier configured to amplify fluid flow through the duct.

25. A fuel cell system or method substantially as described herein with reference to the accompanying drawing.