Abstract:
A variable capacity compressor that is operable in a normal mode and either an upward or downward rapid transient mode includes a compressor that compresses a fluid and a motor that drives the compressor. A controller powers the motor from a main power source when operating in the normal mode and powers the motor from a supplemental power source when operating in the upward rapid transient mode.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to fuel cell systems, and more particularly to operating a compressor of a fuel cell system.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cell systems include a fuel cell stack that produces electrical energy based on a reaction between a hydrogen-based feed gas (e.g., pure hydrogen or a hydrogen reformate) and an oxidant feed gas (e.g., pure oxygen or oxygen-containing air). The hydrogen-based feed gas and oxidant feed gas are supplied to the fuel cell stack at appropriate operating conditions (i.e., temperature and pressure) for reacting therein. The proper conditioning of the feed gases is achieved by other components of the fuel cell stack to provide the proper operating conditions.  
         [0003]     The fuel cell system includes a compressor for compressing and pumping the oxidant feed gas to an appropriate operating pressure for reaction in the fuel cell stack. The compressor is required to respond to load change requests. For example, when a higher power output from the fuel cell is required (e.g., during a vehicle acceleration event), the compressor must increase capacity to provide the oxidant to the fuel cell at a quicker rate.  
         [0004]     The upward transient response time is required to be a change from 10% to 90% capacity in approximately 1 second. Such rapid response times can cause short-term power drain several times the power rating of the compressor motor. Further, power is wasted when current is used to brake the motor during a downward transient (e.g., 90% to 10% capacity).  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, the present invention provides a variable capacity compressor system that is operable in a normal mode and a rapid transient mode. The variable capacity compressor system includes a compressor such as an impeller or rotor set that compresses a fluid and a motor that drives the compressor. A controller powers the motor from a main power source when operating in the normal mode and powers the motor from a supplemental power source when operating in the rapid transient mode.  
         [0006]     In one feature, the controller operates the motor to maintain a first capacity when in the normal mode.  
         [0007]     In another feature, the controller operates the motor to transfer from a first capacity to a second capacity when in the rapid transient mode.  
         [0008]     In another feature, the supplemental power source is a capacitor.  
         [0009]     In another feature, the controller implements charging of the supplemental power source during the normal mode.  
         [0010]     In still another feature, said controller implements the use of power from the supplemental power source to increase motor speed when in the rapid transient mode.  
         [0011]     In yet another feature, the controller causes regenerative braking of the motor to convert mechanical energy into power to produce charging current.  
         [0012]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. Compressor as used in this specification includes machines including, but not limited to, the following types: compressor, twin-screw compressor, roots blower, blower, centrifugal or radial compressor, axial compressor and positive displacement compressor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0014]      FIG. 1  is a schematic illustration of a fuel cell system including a variable capacity compressor according to the present invention;  
         [0015]      FIG. 2  is a schematic illustration of the variable capacity compressor; and  
         [0016]      FIG. 3  is a flowchart illustrating a compressor control according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0018]     Referring now to  FIG. 1 , a fuel cell system  10  is shown. The fuel cell system  10  includes a fuel cell stack  12  and a compressor  14 . The compressor  14  supplies the fuel cell stack  12  with a gaseous oxidant for processing in the fuel cell stack  12 . The gaseous oxidant is generally provided as air, however, it is anticipated that other oxidants including, but not limited to, pure oxygen can be implemented. The fuel cell stack  12  processes the oxidant and hydrogen to produce electrical power. The hydrogen is supplied to the fuel cell stack  12  through a hydrogen supply system (not shown).  
         [0019]     The fuel cell system  10  further includes a controller  16  that controls operation of the fuel cell system  10 . An input  18  generates a desired power signal to the controller  16 . The controller  16  generates a compressor control signal based on the desired power signal. The compressor  14  operates at a capacity as dictated by the control signal. The compressor  14  is operable in a normal mode and either upward or downward rapid transient modes. While operating in the normal mode, the compressor capacity remains relatively constant or with capacity changes below a corresponding upward or downward threshold rate. While operating in the upward rapid transient mode, the compressor capacity rapidly changes above the upward threshold rate. While operating in the downward rapid transient mode, the compressor capacity rapidly changes relative to the downward threshold rate.  
         [0020]     Exemplary rapid capacity transients include an upward transient from 10% to 90% capacity change in approximately 1 second (i.e., 80%/s rate) or a downward transient from 90% to 10% capacity in approximately 1 second. In general, a rapid transient can be defined as being greater than the threshold rate. An exemplary threshold rate includes a transient of at least 40%/s upward or downward. A range that is considered “rapid” transient, requiring implementation of the supplemental power source is on the order of 40%/sec. (forty percent per second). Such rapid upward transients can result in the compressor motor drawing 2 to 3 times its rated power. In the case of an exemplary 14 kW compressor, such rapid upward transients can result in the compressor drawing 28 kW-42 kW.  
         [0021]     The compressor  14  is powered by a power system  20 . The power system  20  includes a main power source  22  and a supplemental power source  24 . The main power source  22  is preferably provided as a battery including, but not limited to, lead-acid (PbA), nickel-metal hydride (NiMH), lithium-ion (Li) or any one of several battery types known in the art.  
         [0022]     The supplemental power source  24  preferably includes an energy storage device having rapid discharge and charging characteristics. Such energy storage devices include capacitors or super-capacitors. Capacitors and super-capacitors are energy storage devices that include both battery and traditional capacitor characteristics. Both can be charged and discharged quickly, however, the super-capacitor exhibits significantly enhanced capacitance over traditional capacitors. As a result, the super-capacitor can supply more power for powering the compressor  14  during rapid upward transients. The rapid discharge characteristic of energy storage devices such as capacitors and super-capacitors provide the rapid, high energy demand of the compressor  14  during rapid upward transients.  
         [0023]     In accordance with the present invention and as explained in further detail below, the compressor  14  is powered by the main power source  22  during normal operation and the supplemental power source  24  during rapid upward transient operation. The fuel cell stack  12  charges both the main power source  22  and the supplemental power source  24  through a power electronics module (PEM)  26 . The PEM  26  manages power distribution from the fuel cell stack  12  for charging and powering vehicle accessories  28  including a stereo, lights, powered mirrors, doors, locks and the like. During rapid downward transients, however, the compressor motor is regeneratively braked to produce charging current. The charging current is preferably used to charge either the main or supplementary power sources  22 , 24 . Although not preferred, it is within the scope of the invention to charge both the main and supplementary power sources  22 ,  24  simultaneously. In addition, the main power source  22  is useable to power accessories  28 .  
         [0024]     Referring now to  FIG. 2 , the compressor  14  includes a compressor controller  30 , a motor  32  and a compressor unit  34 . The compressor  34  draws an oxygen-rich feed gas in through a suction and discharges the compressed feed gas through a discharge to the fuel cell stack  12 . The compressor controller  30  receives the control signal from the controller  16  and operates the compressor  14  based thereon. Although separate system and compressor controllers  16 , 30  are illustrated, it is anticipated that a single controller can control operation of the fuel cell system  10  including the compressor  14 .  
         [0025]     The compressor controller  30  determines whether the compressor  14  is to be operated in the normal mode or one of the rapid transient modes based on the control signal. For example, if the control signal indicates that the compressor capacity is to remain constant or change capacity at a slow transient, the compressor  14  is to be operated in the normal mode. If the control signal indicates that the compressor capacity is to change at a rapid transient, the compressor  14  is to be operated in the corresponding rapid upward or downward transient mode. The compressor controller  30  determines to draw power from the main power source  22  during operation in the normal mode and draws power from the supplemental power source  24  during operation in the rapid upward transient mode. Although not preferred, it is within the scope of the invention to draw power from both the main and supplemental power sources  22 ,  24  during an upward rapid transient.  
         [0026]     Referring now to  FIG. 3 , the compressor control of the present invention will be described in detail. In step  300 , control determines if a transient is requested. In step  301 , control determines whether the transient is upward. If the transient is upward, control continues in step  302 . If the transient is downward, control regeneratively brakes the compressor motor  32  to produce charging current to charge either the main or supplemental power sources  22 , 24  in step  304  and control ends. In step  302 , control determines whether the transient request is greater than the threshold. If the transient request is greater than the threshold, control operates the fuel cell system  10  in the rapid transient mode, powering the compressor motor  32  using the supplemental power source  24  in step  306  and control ends. If the transient request is not greater than the threshold, control operates the fuel cell system  10  in the normal mode, powering the compressor motor  32  using the main power source  22  in step  308  and control ends.  
         [0027]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.