GB2509926A

GB2509926A - Compressor system for fuel cells

Info

Publication number: GB2509926A
Application number: GB1300850.3A
Authority: GB
Inventors: Iain Matheson Fraser
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2013-01-17
Filing date: 2013-01-17
Publication date: 2014-07-23
Also published as: GB201300850D0

Abstract

A fuel cell system comprises a fuel cell stack 2 with a plurality of cells each having a ventilation path therethrough. A main compressor 7 is configured to direct fluid through the ventilation paths of the plurality of cells and an auxiliary compressor 8 is also configured to direct fluid through the ventilation paths of the plurality of cells. The auxiliary compressor is of reduced power capability compared to the main compressor. The auxiliary compressor is operated without the main compressor upon start up of the fuel cell and the main compressor is switched on after start up when the fuel cell stack power output exceeds a predetermined threshold. The auxiliary compressor may be shut down when the main compressor has been switched on.

Description

COMPRESSOR SYSTEM FOR FUEL CELLS

The present invention relates to electrochemical fuel cells disposed in a stack formation, and in particular to cooling and/or air supply systems for such fuel cell stacks.

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,

I

requiring high power compressors, providing such an auxillary 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 andlor air cooling supply to a fuel cell stack.

According to one aspect, the present invention provides a fuel cell system comprising: a fuel cell stack having a plurality of cells each having a ventilation path therethrough; a main compressor configured to direct fluid through the ventilation paths of the plurality of cells; and art auxiliary compressor configured to direct fluid through the ventilation paths of the plurality of cells; wherein the auxiliary compressor is of reduced power capability compared to the main compressor.

The auxiliary compressor may operate from a lower voltage than the main compressor.

The fuel cell system may include a controller configured to operate the auxiliary compressor without the main compressor upon start up of the fuel cell and to switch on the main compressor after start up when the fuel cell stack power output exceeds a predetermined threshold The controller may shut down the auxiliary compressor when the main compressor has been switched on. The controller may be configured to operate the auxiliary compressor without the main compressor upon shut down of the fuel cell. The main compressor and the auxiliary compressor may be in series configuration. The main compressor and the auxiliary compressor may be both disposed upstream of the fuel cell stack. The auxiliary compressor may be configured with a cogging torque or braking resistance sufficiently high that the blades remain substantially stationary when (i) the auxiliary compressor is unpowered and (ii) air flow driven by the main compressor is passing through blades of the auxiliary compressor.

The auxiliary compressor may comprise a brushless controller and motor.

According to another aspect, the present invention provides a method of operating a fuel cell system comprising a fuel cell stack having a plurality of cells each having a ventilation path therethrough, the method comprising: using a main compressor to direct fluid through the ventilation paths of the plurality of cells when the fuel cell is generating electrical power above a first threshold; using an auxiliary compressor to direct fluid through the ventilation paths of the plurality of cells during periods when the fuel cell is generating power below the first threshold, the auxiliary compressor being of reduced power capability compared to the main compressor.

The method may include the steps of: operating the auxiliary compressor without the main compressor upon start up of the fuel cell; and switching on the main compressor after start up when the fuel cell stack power output exceeds a predetermined threshold.

The method may include the step of shutting down the auxiliary compressor when the main compressor has been switched on.

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 a main compressor and an auxiliary compressor The invention 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. 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. ci 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 ft 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 conduit 6, to a main compressor, blower or fan 7 and an auxiliary compressor, blower or fan 8. 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.

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, or a supply from a host system to which the fuel cell system is coupled.

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.

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 compressor 8 is has a lower air or fluid flow capability than the main compressor 7.

As shown in figure 1, the main compressor 7 and the auxiliary compressor 6 are preferably in a series configuration in which they share the same ducted air flow path 9, one after the other. However, other configurations can be envisaged, such as when the main and auxiliary compressors 7, 6 are in a parallel configuration, e.g. supplying the conduit 6 by way of separate, parallel flow paths (not shown). The series configuration of figure 1 may be preferred to avoid the need for separately controllable valves to open and close parallel flow paths. Valves may be preferably avoided as they can be susceptible to blockage in freezing conditions; however, this problem could be avoided by the use of valves adapted to avoid such problems.

As shown in figure 1, the main compressor 7 and the auxiliary compressor 8 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 and auxiliary compressors 7, 8 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 and auxiliary compressors 7, 8 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 embodiment having both main and auxiliary compressors 7, 6 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 compressors disposed downstream of the fuel cell stack. The main compressor can be upstream of the auxiliary compressor, or downstream of the auxiliary compressor.

The controller 10 is configured to control the operation of the main and auxiliary compressors 7, 8 during one or both of fuel cell stack start up and fuel cell stack shut down.

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 it 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 compressorS is driven from the low voltage supply 11 to provide 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 certain 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, it is preferred that the controller 10 should shut down the auxiliary compressor 8 since this may create an undesirable drag on the fluid flows generated by the main compressor 7, e.g. by creating a pressure drop across the auxiliary compressor. This could increase parasitic losses on the fuel cefl system and is preferably avoided.

It is also preferable that, when shut down, the vanes or other air displacement elements of the auxiliary compressor 8 should not rotate under the air flows generated by the main compressor 7. This avoids or reduces parasitic losses caused by dissipation of energy by the auxiliary compressor 8. In a preferred configuration, the auxiliary compressor 8 air displacement elements are driven by a motor having sufficient cogging torque or "detent" that the air flow generated by the main compressor 7 which passes through the vanes or other displacement elements of the auxiliary compressor 8 is insufficient to rotate the displacement elements against the detent force provided by the inactive motor. In an alternative arrangement, the auxiliary compressor 8 may be provided by an electromechanical, electromagnetic or other brake which is operative to lock the rotor in position when the auxiliary compressor 8 is inactive. A similar principle can apply in braking the main compressor 7 when it is inactive and when the auxiliary compressorS is operational.

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 auxiliary compressor 8 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 B and switch off the main compressor 7.

In either case, the auxiliary compressor 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 to provide the relatively low level of necessary fluid flows to the ventilation paths of the fuel cells to effect controlled shut down and possibly also to provide additional cooling flows or purging flows to purge excess water and/or water vapour from the ventilation paths.

As discussed above, the main compressor 7 and the auxiliary compressor 8 are preferably disposed in a series configuration in a common air flow duct 9. This may avoid the use of valves required to switch in and out different flow paths or direct air flows to appropriate channels. Reducing the number of valves required may be desirable to avoid increasing design complexity, reducing pressure drops during normal running thereby improving system efficiency, improving mechanical reliability, and reducing components at risk of freezing in position at low temperatures.

If the main compressor 7 and the auxiliary compressor 8 are not in a series configuration, but are disposed in parallel air flow paths, then it may be desirable to incorporate one or more shut-off valves or multi-way valves to isolate the flow path incorporating the compressor that is inactive. This will avoid a loss of pressure caused by a reverse flow path through the inactive compressor.

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.

Preferably, the auxiliary compressor 8 is configured as a high efficiency unit, for example using a brushless controller and motor to avoid ignition sources and carbon contamination within the air flow path 9. 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.

The principles of the main and auxiliary compressor 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 air flow 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 and auxiliary compressors is air but delivery of other oxidant or cooling fluid flows can also be implemented.

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.

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 air flow path. In another arrangement, the main compressor and the auxiliary compressor could share some or all of the vanes or other air displacement elements, the main compressor having a higher power motor driving and the auxiliary compressor having a lower power motor driving. The two motors could be provided on the same rotor.

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

Claims

CLAIMS1. A fuel cell system comprising: a fuel cell stack having a plurality of cells each having a ventilation path therethrough; a main compressor configured to direct fluid through the ventilation paths of the plurality of cells; and an auxiliary compressor configured to direct fluid through the ventilation paths of the plurality of cells; wherein the auxiliary compressor is of reduced power capability compared to the main compressor.
2. The fuel cell system of claim 1 in which the auxiliary compressor operates from a lower voltage than the main compressor.
3. The fuel cell system of claim 1 further including a controller configured to operate the auxiliary compressor without the main compressor upon start up of the fuel cell and to switch on the main compressor after start up when the fuel cell stack power output exceeds a predetermined threshold.
4. The fuel cell system of claim 1 in which the controller is further configured to shut down the auxiliary compressor when the main compressor has been switched on.
5. The fuel cell system of claim 1 further including a controller configured to operate the auxiliary compressor without the main compressor upon shut down of the fuel cell.
6. The fuel cell system of claim 1 in which the main compressor and the auxiliary compressor are in series configuration.
7. The fuel cell system of claim 1 in which the main compressor and the auxiliary compressor are both disposed upstream of the fuel cell stack.
8. The fuel cell system of claim 6 in which the auxiliary compressor is configured with a cogging torque or braking resistance sufficiently high that the blades remain substantially stationary when (i) the auxiliary compressor is unpowered and (ii) air flow driven by the main compressor is passing through blades of the auxiliary compressor.
9. The fuel cell system of claim 1 in which the auxiliary compressor comprises a brushless controller and motor.
10. A method of operating a fuel cell system comprising a fuel cell stack having a plurality of cells each having a ventilation path therethrough, the method comprising: using a main compressor to direct fluid through the ventilation paths of the plurality of cells when the fuel cell is generating electrical power above a first threshold; using an auxiliary compressor to direct fluid through the ventilation paths of the plurality of cells during periods when the fuel cell is generating power below the first threshold, the auxiliary compressor being of reduced power capability compared to the main compressor.
11. The method of claim 10 further comprising the steps of: operating the auxiliary compressor without the main compressor upon start up of the fuel cell; and switching on the main compressor after start up when the fuel cell stack power output exceeds a predetermined threshold.
12. The method of claim 11 further comprising the step of shutting down the auxiliary compressor when the main compressor has been switched on.
13. Apparatus substantially as described herein with reference to the accompanying drawing.