Abstract:
This specification describes a system and method for controlling the voltage produced by a fuel cell. The system involves providing a bypass line between an air exhaust from the fuel cell and an air inlet of the fuel cell. At least one controllable device is configured to allow the flow rate through the bypass line to be altered. A controller is provided to control the controllable device. The method involves varying the rate of recirculation of air exhaust to air inlet so as to provide a desired change in fuel cell voltage.

Description:
FIELD 
       [0001]    This specification relates to fuel cell power systems. 
       BACKGROUND 
       [0002]    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. In many cases, it would be desirable to alter the polarisation curve of the fuel cell. 
       INTRODUCTION 
       [0003]    This specification describes a system and method for controlling the voltage produced by a fuel cell. The system involves providing a recirculation line between an air exhaust from the fuel cell and an air inlet of the fuel cell. At least one controllable device is configured to allow the flow rate through the recirculation line to be altered. A controller is provided to control the controllable device. The method involves varying the rate of recirculation of air exhaust to air inlet so as to provide a desired change in fuel cell voltage. 
     
    
     
       FIGURES 
         [0004]      FIG. 1  is a schematic drawing of a fuel cell power system. 
       
    
    
     DETAILED DESCRIPTION 
       [0005]      FIG. 1  shows a fuel cell power system  10 , alternatively call a fuel cell power module. The system  10  includes a fuel cell stack  12 , an air blower  14 , an air inlet line  16 , a blower inlet line  18 , an air outlet line  20 , a recirculation line  22  (alternatively called a bypass line), an exhaust valve  24 , a recirculation valve  26  (alternatively called a bypass valve), a voltage sensor  28 , a controller  30  and a temperature sensor  32 . The system  10  also contains several other conventional elements, such as a hydrogen supply, that are not shown in  FIG. 1  to allow the more material elements of the system  10  to be emphasized. The configuration of the elements in system  10  may be altered. For example, there might be only one of the exhaust valve  24  and recirculation valve  26 . In another example, an equivalent system might be arranged with the air blower  14  attached to the air outlet line  20 . 
         [0006]    In operation, hydrogen and air flow through the fuel cell stack  12 . Some or all of the hydrogen is consumed by reacting with oxygen from the air in the fuel cell stack. An excess of air, relative to the amount of air that would carry a stoichiometric amount of oxygen to react with the hydrogen, flows through the fuel cell stack  12 . The excess air serves to remove moisture from the fuel cell stack and to help ensure that local areas within the fuel cell stack are not starved of oxygen. The amount of excess air, measured at the inlet, is typically in the range of 1.5 to 3 times the amount of air that would carry a stoichiometric amount of oxygen. For brevity, this excess amount of air will be described as “N times the stoichiometric amount”. 
         [0007]    In some cases, the voltage produced by the fuel cell is not particularly important. However, in other cases, the variation of voltage with current according to the polarisation curve causes problems. For example, a fuel cell is often connected in parallel with a battery to provide a hybrid electrical power supply system. As current drawn from the hybrid system changes, the difference in the polarisation curves of the battery and the fuel cell can cause either the battery or the fuel cell to over-contribute to the total power demand. Either the battery or the fuel cell may overheat. Therefore, it would be desirable to be able to cause the fuel cell to operate as if it had a polarisation curve more nearly like that of the battery. In other cases, a device powered by a fuel cell has characteristics that would make a particular polarisation curve desirable. In some cases, it would be desirable to have a more nearly flat polarisation curve. In other cases, it would be desirable for the slope of the polarisation curve in the ohmic region to more nearly constant or to extend further into the activation region. In these cases, creating the desired voltage output is sometimes achieved by using a DC to DC voltage converter. 
         [0008]    In the system  10 , air can be permitted to flow through the recirculation line  22  by opening recirculation valve  26 . With recirculation valve  26  at least partially open, the difference in pressure between the air outlet line  20  and the suction side of air blower  14  causes flow in the recirculation line  22 . The flow through recirculation line  22  can be increased by opening recirculation valve  26  or decreased by closing recirculation valve  26 . When the recirculation valve  26  is at least partially open, closing exhaust valve  24  increases flow in the recirculation line  22  and opening exhaust valve  24  decreases flow in the recirculation line  22 . Accordingly, one or both of exhaust valve  24  and recirculation valve  26  can be modulated to vary the flow rate in the recirculation line  22 . Although modulating valves  24 ,  26  may affect the total head loss incurred by blowing air through the fuel cell stack  12 , and therefore the total flow rate of air through the fuel cell stack  12 , the change in total flow rate is small and not of primary importance to the process. The more important result of modulating one or more of valves  24 ,  26  is that the partial pressure of oxygen in the air side of the fuel cell stack  12  can be altered. This effect will be illustrated in the example below. 
         [0009]    Ambient air has about 20.9% oxygen. To simplify the following discussion, it will be assumed that ambient air is 20% oxygen and 80% nitrogen and all percentages in the following sentences will be proportion to these amounts in ambient air. If ambient air is provided to the fuel cell stack  12  at 3 times the stoichiometric amount, then the incoming air has a total flow rate of 300% consisting of 60% oxygen and 240% nitrogen. One stoichiometric amount of oxygen, or 20%, is consumed by hydrogen as the air passes through the fuel cell stack 12. The exhaust gas produced has a flow rate of 280% made up of 40% oxygen and 240% nitrogen. The ratio of oxygen to total gas in the exhaust gas is now 1:7 rather than 1:5 for the ambient air. If some or all of the exhaust gas passes through the fuel cell stack again, even lower ratios of oxygen to total gas are achieved although partially attenuated by the need to provide make-up air. Systems that operate at lower total air flow rates relative to the stoichiometric amount can achieve even greater variation in oxygen to total gas ratio. 
         [0010]    Modulating one or more of valves  24  and  26  controls the relative amounts of exhaust gas and ambient air that flow through the fuel cell stack  12 . This in turn controls the ratio of oxygen to total gas in the air side of the fuel cell stack. This ratio in turn causes a change in partial pressure of oxygen in the air side of the fuel cell stack. The voltage of the fuel stack  12  varies more with the partial pressure of oxygen than with the total gas flow. 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. Generally speaking, changing the partial pressure of oxygen changes the shape of the polarisation curve. Despite the change in partial pressure, total gas flow rate through the fuel cell stack remains at or above the minimum rate chosen for water removal. Further, provided that the amount of oxygen in the exhaust/ambient air blend does not result in a flow of oxygen less than one stoichiometric amount, the high total gas flow rate helps deliver enough oxygen to all parts of the fuel cell stack  12 . 
         [0011]    To enable real time control, one or both of the valves  24  and  26  are connected to a controller  30 . The controller  30  may be programmed to modulate one or both of the valves  24  and  26  in a pre-programmed manner based on a stored formula or table giving the valve movements predicted to give a voltage desired under different operating conditions. Optionally, the controller  30  is connected to a voltage sensor  28  connected to the fuel cell stack  12  to allow for a feedback loop to allow one or both of the valves  24  and  26  to be trimmed in response to a variance between desired and actual voltage. Further optionally, the controller  32  may be connected to a temperature sensor  32  to provide an emergency avoidance feature. If the fuel cell stack  12  is in danger of overheating, the controller  30  can module one or both of the valves  24  and  26  to reduce the cell stack voltage. 
         [0012]    Alternatively, the controller  30  may be part of, or connected to, a larger system. In this case, a desire for a change in the fuel cell voltage can be determined by any part of the larger system. For example, if the fuel cell stack  12  is connected in parallel with a battery, the battery voltage can be communicated to the controller  30  which then modulates one or more of the valves  24  and  26  to more nearly match the voltage of the battery, to move towards the voltage of the battery, or to be within a range relative to the voltage of the battery. In this case, the voltage sensor  28  can still be used to provide an inner control loop operating inside of the outer control loop of the larger system. 
         [0013]    References to a battery above include a bank or other assembly of batteries connected wired in parallel or in some combination of series and parallel with the fuel cell stack  12 . Similarly, a system  10  may have multiple fuel cell stacks  12 , or multiple systems  10  may be assembled into a bank or other structure and then connected to one or more batteries. In this case, one or more than one of the fuel cell stacks  12  or systems  10  may be controlled to control the voltage of the assembly as a whole to more nearly match the voltage of one or more batteries connected in parallel. The method and apparatus described herein can also be used when connecting a fuel cell power system  10  to a load without a battery, for example directly or through a DC-DC voltage converter. The method and apparatus can help keep the voltage of the fuel cell power system  10  within a range required by the load, for example computer servers or other electronic equipment, for example in a DC grid data center.