Patent Publication Number: US-8114547-B1

Title: Fuel cell stack flow diversion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 11/839,838, filed Aug. 16, 2007, the entire contents of which are hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The invention was made with Government support under Contract No. DE-FC36-04G014287. The Government has certain rights to the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the use of compressors and compressed gas in fuel cell systems. 
     2. Background Art 
     Fuel cell systems produce electrical energy by combining fuel and an oxidant in a fuel cell stack. In one form of fuel cell system, the fuel is hydrogen and the oxidant is oxygen, which may be mixed with other gases as in air. The oxidant is typically gaseous and is often delivered to the fuel cell stack as a compressed flow. 
     Typically, fuel cell stacks operate more efficiently under certain operating conditions, including fuel cell stack temperature. In particular, it is desirable for the fuel cell stack to operate at or above a particular temperature, which may be above ambient temperature. Therefore, there is a need to heat the fuel cell stack at various times such as, for example, during startup. 
     Different types of compressors may be used to provide oxidant to the fuel cell stack. For example, non-positive displacement compressors are sometimes used for a variety of reasons such as size, weight, efficiency, noise, vibration, and harshness characteristics. However, non-positive displacement compressors may operate in an undesirable condition known as surge. Surge occurs when the compressor is operated at low flow rates in combination with a high ratio of output pressure to input pressure. Under these conditions, surge may result in vibrations which can lead to poor operation, malfunction system damage, and the like. In fuel cell systems that use ambient air to provide the oxidant, variations in air density and pressure can affect compressor performance. This is particularly true at higher elevations, where the onset of surge is more likely. 
     Fuel cell stacks are typically placed in a housing. Unwanted gasses may accumulate in the housing, requiring some mechanisms to vent or purge the unwanted gasses. 
     Fuel cell systems are often part of a larger system such as, for example, an automotive vehicle. These larger systems often require various environmental modification systems that could benefit from synergistic operation with the fuel cell system. 
     Accordingly, a need exists for improved fuel cell system operation which addresses some or all of the above issues without unduly affecting cost, complexity, performance, and the like. 
     SUMMARY OF THE INVENTION 
     The present invention provides a control valve to affect the flow of compressed gas in a fuel cell system. 
     In one embodiment, a compressor supplies compressed gas to the fuel cell stack. The compressor may be used as a load dump for energy produced by the fuel cell stack. In this case, the compressor generates an excess flow of compressed gas which is diverted by a control valve away from the fuel cell stack. In one application, excess work done by the fuel cell stack to power the compressor generates heat which warms the fuel cell stack. 
     Control logic may be used to manage the compressor and the control valve so as to maintain efficient fuel cell stack operating conditions. This control logic may receive as input one or more conditions of the compressed gas, ambient air, compressor, fuel cell stack, control valve, and the like. 
     The control valve may be used to avoid a surge condition in the compressor. The compressor may generate an increased flow to avoid surge. The control valve may then divert the increased flow away from the fuel cell stack. 
     According to an embodiment of the present invention, componentry incorporated for expending electrical energy from the fuel cell stack may be reduced or eliminated by running the compressor at a level above that needed to supply compressed gas to the fuel cell stack and diverting excess flow away from the fuel cell stack. 
     Another embodiment involves utilizing compressed gas diverted away from the fuel cell stack. This diverted gas may be used to modify the environmental conditions of a wide variety of systems such as, for example, a passenger compartment, a radiator, and the like. The diverted gas may also be used to evacuate gases from the fuel cell stack housing or enclosure. 
     In another embodiment, the fuel cell system may be utilized in an automotive vehicle. 
     Other aspects, features, and uses of the disclosed inventions will become apparent to one skilled in the art from a study of the following description and associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a fuel cell system according to an embodiment of the present invention; 
         FIG. 2  is a flow diagram illustrating operation of a fuel cell system according to an embodiment of the present invention; 
         FIG. 3  is a graph illustrating surge avoidance according to an embodiment of the present invention; and 
         FIG. 4  is a schematic diagram illustrating an automotive vehicle according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for the claims and/or as a representative basis for teaching one skilled in the art to utilize the present invention. 
     Referring now to  FIG. 1 , a schematic diagram illustrating a fuel cell system according to an embodiment of the present invention is shown. A simplified oxidant path of a fuel cell system, shown generally by  20 , includes fuel cell stack  22 . Fuel cell stack  22  generates electrical energy, shown as  24 , by combining ionized fuel and oxidant. The oxidant is provided by fuel cell stack inlet  26 . Unused oxidant, and other gases in some systems, exits the fuel cell stack at outlet  28 . A wide variety of fuel cell types are available based on different fuels and oxidants, cell stack configuration, desired power output, fuel cell application, and the like. As will be recognized by one of ordinary skill in the art, the present invention does not depend upon the type or construction of the fuel cell stack used. 
     Fuel cell system  20  also includes compressor  30  and control valve  32 . Compressor  30  receives gas at compressor inlet  34  and produces compressed gas at compressor outlet  36 . Compressor  30  runs on electrical energy, shown by  38 , which is at least a portion of fuel cell stack generated electrical energy  24 . The operation of compressor  30  is controlled by compressor control signal  40  provided by control logic  42 . Compressor control signal  40  may be one or more of analog voltage signals, analog current signals, pulse width modulated signals, digital signals, and the like. Compressor electrical supply may be AC or DC and may be controlled by control logic  40  so as to provide compressor control signal  40 . A wide variety of compressors are available for use in the present invention depending upon the type of oxidant used; flow parameters including pressure, temperature, and velocity; amount of compression needed; fuel cell system application; and the like. In one embodiment, compressor  30  is a centrifugal compressor. As will be recognized by one of ordinary skill in the art, a wide variety of compressor types and configurations may be used in the present invention. 
     Control valve  32  is disposed in the flow path between compressor outlet  36  and fuel cell stack inlet  26 . Control valve  32  diverts flow from compressor  30  away from fuel cell stack  22 . Control valve  32  is controlled by valve control signal  44  from control logic  42 . Control signal  44  may be any type of analog or digital signal depending upon the type of control valve  32  chosen, including electrical, magnetic, pneumatic, hydraulic, optical, and the like. In addition, any suitable type of control valve  32  may be used. While a single control valve is illustrated, the term control valve includes one or more control valves controlled by one or more valve control signals. As will be recognized by one of ordinary skill in the art, a wide variety of control valve types and configurations may be used to implement the present invention. Moreover, various other components may be disposed in the flow path, including intercooler, filters, water injectors, humidifiers, and the like. 
     Control logic  42  generates compressor control signal  38  and valve control signal  44  based on one or more control inputs  46 , the specific connections of which are not shown for clarity. Control inputs may include flow parameters including mass flow rate, volume flow rate, velocity, temperature, and the like, at various locations in fuel cell system  20  such as compressor inlet  34 , compressor outlet  36 , fuel cell stack inlet  26 , fuel cell stack outlet  26 , ambient, and the like. Control logic  42  may also monitor various components in fuel cell system  20  including fuel cell stack  22 , compressor  30 , control valve  32 , and the like. Control inputs  46  may include inputs from a user or another controller. In one embodiment, control logic  46  measures a temperature related to the operation of fuel cell stack  22  and uses compressor  30  as an electrical load for warming fuel cell stack  22 . Control logic  42  may be implemented as a computer executing software, as programmable logic, as discrete logic components, as electromechanical, hydraulic, or pneumatic systems, any combination of these, and the like. Control logic  42  may be a single unit or may be distributed between or amongst various units. As will be recognized by one of ordinary skill in the art, the present invention may be implemented in a wide variety of control logic types and configurations. 
     Referring now to  FIG. 2 , a flow diagram illustrating operation of a fuel cell system according to an embodiment of the present invention is shown. As will be appreciated by one of ordinary skill in the art, the operations illustrated are not necessarily sequential operations. The order of steps may be modified within the spirit and scope of the present invention and the order shown here is for logical presentation. Also, methods illustrated may be implemented by any combination of hardware, software, firmware, and the like, at one location or distributed. The present invention transcends any particular implementation and the embodiments are shown in sequential flow chart form for ease of illustration. 
     The fuel cell stack generates electricity to drive the compressor, as in block  60 . The compressor provides a flow of compressed gas to the fuel cell stack at a first flow rate, as in block  62 . This flow rate may be determined by one or more of a variety of techniques, including directly or indirectly measuring the mass flow rate, the volumetric flow rate, and the like. 
     A check is made to determine whether or not to increase the flow rate above the flow needed by the fuel cell stack, as in block  64 . This increase may be triggered, for example, by the need to increase the load on the fuel cell stack. One purpose for increasing the load may be to generate heat for warming the fuel cell stack. Another purpose for increasing the load may be to test the fuel cell stack and/or some other component of the fuel cell system. In addition, or rather than, responding to a need to increase fuel cell stack load, the flow rate may be increased so as to generate excess flow for purposes other than to provide oxidant to the fuel cell stack. This excess flow may be used to modify an environmental condition of an element within or outside of the fuel cell system. 
     If flow is to be increased, the fuel stack electrical output is increased to drive the compressor, as in block  66 . Additional flow is provided from the compressor, as in block  68 . The additional flow is diverted away from the fuel cell stack, as in block  70 . The diverted flow may be directly or indirectly returned to the compressor or vented to the atmosphere. The diverted flow may also be used for a variety of purposes, as disclosed elsewhere herein. 
     Returning again to  FIG. 1 , various embodiments for use of diverted flow according to the present invention are shown. Fuel cell stack  22  is contained in housing  80 . Operation of fuel cell stack may cause the formation of gases within housing  80 , shown generally by  82 . Diverted flow  84  from control valve  32  may be routed into housing  80  to purge housing gases  82  from housing  80 . 
     Diverted flow  84  may also be routed to radiator  86 , heat exchanger  88 , direct application  90 , and the like for modifying one or more environmental parameters. Radiator  86  may use diverted flow  84  to heat or cool a liquid such as, for example, coolant used to regulate the temperature of an internal combustion engine or an electronic circuit. Radiator  86  may also function as a heat sink for electrical componentry cooled by diverted flow  84 . Heat exchanger  88  may provide heat from diverted flow  84  to a surrounding environment such as, for example, a passenger compartment (not shown for clarity). Direct application  90  provides diverted flow  84  directly into an environment to be modified. The path of diverted flow  84  may include various other components such as diffusers, expanders, intercoolers, humidifiers, and the like for regulating properties of diverted flow  84  prior to use by radiator  86 , heat exchanger  88 , or direct application  90 . 
     Output flow, shown generally by  92 , can include one or more of the flow from fuel stack outlet  28  and the diverted flow  84  uses such as purging housing  80 , radiator  86 , heat exchanger  88 , direct application  90 , and the like. Some or all of output flow  92  may be returned to compressor inlet  34 , may be vented to ambient, may be routed for other uses, and the like. 
     Referring now to  FIG. 3 , a graph illustrating surge avoidance according to an embodiment of the present invention is shown. A compressor operating map, shown generally by  100 , plots mass flow rate through an exemplary compressor as a function of compressor pressure ratio. The compressor pressure ratio refers to the ratio of air pressure exiting the exemplary compressor outlet to air pressure entering the exemplary compressor inlet. 
     Compressor pressure ratio, being a function of mass flow, is also dependent upon the velocity with which the compressor rotates its impellers. Six exemplary compressor operating lines with corresponding angular velocities, indicated on compressor operating map  100 , are marked by respective 30, 45, 60, 75, 90, and 97 thousands of revolutions per minute (krpm). Compressor operating maps, such as  100 , are typically created by setting a compressor at a constant angular velocity and subsequently varying mass flow through the compressor. 
     Compressor surge typically occurs when a compressor operates at low flow rates in combination with relatively high compressor pressure ratios. For the example provided, operating the compressor to the left of surge line  102  will more than likely result in compressor  30  experiencing surge. This surge condition may cause unsteady aerodynamic loading, observed in flow and pressure oscillation, which may result in damage to equipment or otherwise affect operation. 
     The present invention may be used to avoid a surge condition in the compressor. For example, various components in the fuel cell system may be monitored, such as the compressor inlet and compressor outlet. Preexisting compressor performance data may be stored such as, for example, that shown on compressor operating map  100 . Surge line  102  represents the pressure at which surge can be expected to occur for a given mass flow rate. When the compressor operates in an intervention region, shown generally by shaded region  104 , the compressor may be controlled to increase output flow, thereby avoiding or removing surge. As previously described, this increased flow may be diverted from the fuel cell stack. For example, if the compressor is operating at surge condition  106 , mass flow can be increased through the compressor. As flow is increased, operation moves along operating line  108  until stable flow and pressure condition  110  is reached beyond surge line  102 . 
     Control of flow rate may also prevent the compressor from initially reaching surge. For this reason, intervention region  104  may encompass surge line  102  and overlap a portion of conditions in which compressor  30  may be operating satisfactorily. For example, if the compressor is operating at 75 krpm and its mass flow rate drops to 120 kg/h, the compressor flow may be increased before the compressor ever reaches a surge condition. 
     Referring now to  FIG. 4 , a schematic diagram illustrating an automotive vehicle utilizing a fuel cell system according to an embodiment of the present invention is shown. An automotive vehicle, shown generally by  120 , is driven by electric motor  122  receiving fuel cell stack generated electrical energy  24 . Electric motor  122  may drive axle  124  extending between wheels  126 . While electric motor  122  is shown to propel vehicle body  120 , one skilled in the art will realize that there are countless alternative applications onboard a motor vehicle which involve an electric motor driving a component. These applications may include operating, for example, power windows, an automatic vehicle closure, power steering, or a plow attached to a vehicle body. 
     Air filter  128  may be included to purify ambient air  130  prior to its use in fuel cell system  20 . Compressor  30 , also driven by fuel cell stack generated electrical energy  24 , compresses ambient air  130 . Air exiting compressor  30  may have an excessive temperature unsuitable for further usage. Air intercooler  132  may be included to modify the temperature of air flow exiting compressor  30  back within a usable range. 
     In the embodiment shown in  FIG. 4 , fuel cell system  20  includes control valves  32   a  and  32   b . Control logic  42  operates control valves  32   a  and  32   b  for a variety of purposes, such as to provide fuel cell stack  22  with substantially constant absolute pressure compressed air flow for particular operating conditions. 
     In one application, fuel cell stack  22  is operated at a higher output power solely for the purpose of generating more waste heat to warm itself and any system coolant volume faster. A portion of the output power of fuel cell stack is dumped into compressor  30  for operation at a higher speed. For example, a 50% efficient fuel cell stack generates 1 kW of heat for every 1 kW of output power. 
     Vehicle  120  includes passenger compartment  134 . Passenger compartment  134  may utilize diverted flow  84  passing through heat exchanger  88  to change an environmental condition of passenger compartment  134 . Alternatively, heat exchanger  88  may use air flow prior to, or instead of, passing through air intercooler  132 . 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.