Abstract:
An electrically powered compressor designed for providing high pressure ratio at low flow rate, yet having low power consumption. The compressor is designed for optimizing efficiency in fuel cell systems. The inventive compressors are however not limited to fuel cell system applications.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is concerned with improving the efficiency of operation of a fuel cell system. More specifically, the invention is concerned with an electrically powered compressor designed for providing a high pressure ratio at a low flow rate, and with low power consumption. The inventive compressors are however not limited to fuel cell system applications.  
           [0003]    2. Description of the Related Art  
           [0004]    Fuel cell systems are being developed to generate electricity to power vehicle accessories or as drive systems for propelling vehicles.  
           [0005]    For example, German Patent DE 40 32 993 C1 teaches a fuel cell system wherein a proton-conducting electrolyte membrane (i.e., proton exchange membrane or PEM) is located between two electrodes - a cathode to which oxidizing gas is supplied, and an anode to which fuel gas (e.g., H 2  and CO 2 ) is supplied. The PEM acts like an electrolyte for transport of hydrogen ions obtained at the anode of the fuel cell, towards the cathode, in the form of protons (H + ). Electricity generated by the energy conversion reaction is collected, and excess gas which has not been consumed is exhausted.  
           [0006]    DE 40 21 097 A1 teaches a fuel cell system in which the exhaust air from the fuel cell is conducted to an expansion turbine. The expansion turbine is coupled with a fresh air compressor for boosting the pressure of air supplied to the fuel cell.  
           [0007]    EP 0 629 013 B1 and DE 43 18 818 A1 teach the use of an electric motor driven compressor to boost the fuel cell fresh air intake pressure. Compressing intake air to the usual working pressure of, e.g., 3 bar, consumes approximately 20% of the power developed by the fuel cell. To recover the energy contained in the exhaust air from the fuel cell, the electric motor driven compressor is coupled with an expander mounted on the same shaft as the compressor. When an expander is used for energy recovery, the energy expended in compressing air drops to about 10 to 15%. There remains a need for further improving the efficiency of the system.  
           [0008]    It is also known to provide a catalytic burner to reduce environmental emissions. Fuel is supplied to the catalytic burner in the form of (1) moist anode offgas from the fuel cell, and (2) methanol. In DE 40 32 993 C1 the combustion gases generated in the catalytic burner pass through a gas turbine connected downstream to drive a compressor to compress oxygen containing gas (e.g., air) supplied to the catalytic burner. Thus, the fuel cell offgas and the catalytic burner offgas pass through separate respective expanders.  
           [0009]    U.S. Pat. No. 6,190,791 (Hornburg) teaches that there is a need for higher working pressures on the cathode side (i.e., air side) in order to build a smaller fuel cell having narrower gas channels and to achieve a higher area-related power yield in the fuel cell. The approach taken by Hornburg involves designing the system in such a way as to increase air mass flow, increase temperature, and increase pressure of the exhaust gas going into the expander. This is accomplished by (1) initially supplying the air at the cathode outlet of the PEM fuel cell as the air supply to the catalytic burner before. expansion, and (2) operating the expander with the exhaust air from the catalytic burner. As a result of the additional energy input to the expander in the form of heat and mass flow, its performance is increased to the point where the compressor drive (e.g., electric motor and rectifier) can be made much smaller. Hornburg further teaches that, with an optimum adjustment of the pressure level and/or additional supply of combustion gas in the catalytic burner, the compressor drive can even be completely eliminated entirely.  
           [0010]    Hornburg also teaches an embodiment in which the combustion gas for the PEM fuel cells is generated by a high-pressure (15 to 30 bar) gas-generating system. This pressure is harnessed by means of a second expander/compressor stage upstream from the cathode input of the PEM fuel cell, with the gas pressure dropping from the system pressure of the high-pressure gas-generating system to the working pressure of the catalytic burner (approximately 3 bar). The second compressor stage is in the form of a compressor coupled with an expander, without an electric motor or a turbine.  
           [0011]    While Hornburg does achieve higher working pressures, the inventors considered that there must be a simpler, more reliable and more responsive way to generate these pressures. Further, the Hornburg systems are designed for high flow rate. The inventors considered that, when supplying fresh air to fuel cells, there is a problem in that in fact only small volume flows are required, though at high pressure ratios. Conventional one-step flow compressors require extremely high speeds in combination with high operational energy input to achieve such high pressures at low volume flows, a feat which cannot be accomplished with the electric engines available today.  
           [0012]    Accordingly, a second aspect of the present invention concerns the development of a compressor capable of providing high pressure at low volume flow, which is reliable, economical to construct, highly responsive and easily regulated.  
           [0013]    The inventors first contemplated various electric motor driven compressor assemblies. U.S. Pat. No. 6,193,473 (Mruk, et al) teach that a major drawback associated with the use of electric motors to drive rotating impeller compressors has been the linkage between the electric motor and the compressor impeller. A given compressor will have a specific speed of rotation of the impeller in order to achieve the compression duty required of it. At the same time, an induction electric motor will have an optimum speed of rotation, at which the torque output is at a maximum (and which speed of rotation is generally far lower than the operating speed of high speed centrifugal compressors). Heretofore, in order to link the compressor with a suitable electric drive motor, it has been necessary to employ an arrangement of one or more expensive gear assemblies in the compressor drive. In this way the different optimum speeds of rotation of the compressor and the electric motor can be accommodated.  
           [0014]    Mruk et al have the objective of providing an electric motor driven compressor with no gearing and wherein the electric motor and compressor are directly linked. Mruk et al accomplish this by using a switched reluctance motor to drive the rotating centrifugal impeller(s). The Mruk et al compressor assembly preferably comprises first and second compressors housed in separate compressor casings, mounted on opposite ends of a common drive shaft assembly and rotatable therewith. The first and second compressors may be driven by the same switched reluctance motor. The fluid outlet of the first compressor casing may communicate with the fluid inlet of the second compressor casing, forming a two-stage compressor assembly. In such an arrangement, the switched reluctance motor is most conveniently disposed between the first and second compressor casings, with the rotor of the switched reluctance motor being mounted on the drive shaft assembly between the first and second impellers.  
           [0015]    Considering however the task of the present invention, such an arrangement would be expensive, difficult to regulate particularly at lower pressures, difficult to integrate into a vehicle propulsion system, and difficult to repair.  
           [0016]    Further, with both compressors being driven by the same shaft, response and output may not be optimal in the case of rapidly changing traffic conditions.  
         SUMMARY OF THE INVENTION  
         [0017]    It has now been discovered that the first task of the invention can be accomplished by connecting two electrically powered flow compressors in series (see FIG. 1). These compressors can be optimally coordinated with each other corresponding to the operating requirements of the fuel cell. Each individual flow compressor in the system operates at clearly reduced speeds and reduced electrical power consumption as compared to single stage compressors.  
           [0018]    The second task of the invention has been achieved by providing a two stage or sequential compressor driven by a single electric motor, and preferably also connected to an internal combustion engine of a hybrid combustion/electric vehicle via a belt or pulley system (see FIGS. 2 and 3). The compressor electric motor is preferably constructed using magnetically loaded composite (MLC) rotor technology. In a preferred embodiment, the pulley is located centrally on the rotor shaft, first and second MLC motors are provided on opposite sides of the pulley, and first and second compressor wheels are provided outboard of the MLC motors, on the first and second ends of the rotor shaft. This specially designed two stage or sequential compressor is particularly suited for use with fuel cell systems, but has numerous other applications.  
           [0019]    The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood, and so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other electrically powered compressors for carrying out the same purposes of the present invention. It should also be realized by those skilled in. the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    For a fuller understanding of the nature and objects of the present invention reference should be made by the following detailed description taken in with the accompanying drawings in which:  
         [0021]    [0021]FIG. 1 is a schematic showing two electric motor driven compressors connected in series for providing regulated high pressure low volume air flow to a fuel cell;  
         [0022]    [0022]FIG. 2 shows a partial sectional view of a preferred two-stage compressor particularly suited for use with a fuel cell system; and  
         [0023]    [0023]FIG. 3 shows the two stage compressor of FIG. 2 in diagrammatic form showing a belt attached to the pulley.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    In the first aspect of the invention, the problem of supplying air to the fuel cell at high pressure yet low flow rate and low compressor power consumption is solved by connecting two separate compressors in series, as shown in FIG. 1, wherein each compressor is independently driven an electric motor. This arrangement makes it possible to operate each individual flow compressor in the system at clearly reduced speeds and reduced electrical power consumption. Further, these compressors can be optimally coordinated with each other corresponding to the operating requirements of the fuel cell.  
         [0025]    A control valve is provided between the first and second compressor. This makes it possible to bypass the first compressor, operating only the second compressor, as desired.  
         [0026]    The design of a low power consumption compressor system for provision of high pressure, low flow rate for a fuel cell according to the present invention is different from prior art systems such as disclosed in, e.g., U.S. Pat. No. 6,079,211, wherein a supercharging system is provided for an internal combustion engine. In the system disclosed therein, a first compressor is driven by an electric motor, and a second compressor is driven by an exhaust gas driven turbine and also optionally by an electric motor. When the throttle is opened to accelerate the engine, both electric motors are super-energized for a short period of time to compound boost pressure to the engine. While supercharger arrangement disclosed in the patent is designed for rapidly boosting both pressure and flow, the present invention is designed for boosting pressure continuously at low flow rate, and with minimal power consumption.  
         [0027]    Although not necessary, it is possible in accordance with a preferred embodiment of the invention to recover some of the energy contained in the exhaust air from the fuel cell and, if present, the catalytic burner, by providing an expander in the exhaust line. The expander could be coupled to the electric motor driven compressor, could be in-line or in series with the electric motor driven compressors, or could be in parallel with the electric motor driven compressors.  
         [0028]    In a further preferred embodiment of the invention, which is illustrated in FIGS. 2 and 3, both compressors are provided on the same rotor shaft, and the shaft is driven by an electric motor. The motor is preferably located between the compressors. More preferably, a pully is additionally provided centrally on the shaft, with first and second electrical motors (induction motors, preferably magnetically loaded composite (MLC) motors), provided on either side of the pulley, and compressor wheels provided on the first and second ends of the rotor shaft.  
         [0029]    The two-stage compressor of the present invention represents an improvement over the closest prior art two-stage centrifugal compressor assemblies as disclosed for example in U.S. Pat. No. 6,193,473 (Mruk et al), since Mruk et al drive the compressors via a switched reluctance motor disposed between the first and second compressor casings and comprising a stator and a rotor rotatable within the stator.  
         [0030]    The present invention in contrast provides a centrally located pulley so that the compressors may be driven by the main engine, and in addition provides magnetically loaded composite (MLC) motors on either side of the pulley. MLC is a product that incorporates magnetic material into high strength and high integrity fibrous composite structures. One example of such a motor is disclosed in U.S. Pat. No. 5,477,092 (Tarrant), and one commercial source of suitable rotors is Urenco Ltd., Marlow, UK, MLC. As far as the present inventors are aware, literature describes MLC as useful for motor generator rotors, high surface speed generators, flywheels, dynamometers, self-driven rollers, transducers (linear, rotary and acoustic), actuators (linear and rotary), and magnetic bearings (passive and active), but MLC has never been used for the purposes of the present invention. Benefits of MLC include weight reduction, simplified integral design, high speed, low inertia, greater quietness both mechanically &amp; electrically, potential to reduce motor air gaps, reduced high frequency losses, i.e., no laminations, versatile magnetic patterns and numbers of poles, no PM stray fields, elimination of the back iron requirement, magnetically anisotropic/isotropic, high specific strength and stiffness, and good resistance to corrosion and chemicals. Further, in the present invention, the MLC motors provide a magnetic bearing system.  
         [0031]    The rotor is preferably connected via a one way bearing to a pulley, which allows a belt to provide compressor power and generator drive from the engine crankshaft pulley at high compressor/generator speeds and high levels of compressor power consumption.  
         [0032]    The electric motor(s) could easily be switched via power electronics to function as an electricity generator.  
         [0033]    The fuel cell system of the present invention could be a solid oxide fuel cell as described in U.S. Pat. No. 6,230,494 (wherein oxygen in the air ionizes to O −2 , producing electricity), a reformation fuel cell as described in U.S. Pat. No. 6,232,005, a proton exchange membrane fuel cell as described in U.S. Pat. No. 6,190,791, or any of the various known types, so long as the fuel cell operates under elevated air pressure.  
         [0034]    Turning now to FIG. 1, two electric motor driven compressors are shown connected in series for providing regulated high pressure low volume air flow to a fuel cell. Initially, when the system is cold and is being started up, both electromotors  1  and  2  may be energized by an external source (e.g., a battery) to drive the compressors  3  and  4  to provide the necessary system air. Alternatively, valve  5  may be opened so that the first compressor  4  is bypassed. Air (or any oxidizing gas), preferably boosted to 3 bar, leaves compressor  3 , is introduced into conduit tube  6  and preferably passes through a tubular heat exchanger  7  where it is warmed prior to being directed to the cathode side of the fuel cell BZ. Fuel gas  8  (e.g., H 2  and CO 2 ) is supplied to the anode. Once the reaction has established itself, electricity generated in the fuel cell BZ is used to drive motors  1  and  2 .  
         [0035]    Preferably, a burner catalyst KatBr is provided to remove any hydrocarbons, unburned fuel, nitric oxide, carbon monoxide and particulates from the exhaust stream prior to exiting the system, and also to generate heat which may be used to pre-heat system air in heat-exchanger  7 . The burner catalyst KatBr may be brought on-line using a fuel source such as methanol prior to startup of the fuel cell, in order to provide for pre-heating the effluent stream to the fuel cell.  
         [0036]    In order to maintain the fuel cell at idle (e.g., when the internal combustion engine of a hybrid vehicle is being used for highway driving), bypass valve  5  may be open with only electromotor  1  running. As the vehicle transitions to city driving, the internal combustion engine may be shut down and the fuel cell may be operated at high output. For this, both electromotors are energized, whereby air pre-compressed in the first compressor is further compressed or boosted in the second stage of the compressor.  
         [0037]    Since the fuel cell system can be made more responsive, the need for devices such as storage batteries and inertial flywheels is reduced.  
         [0038]    The two electromotor driven compressors can be regulated with almost instantaneous response. Regulating can be in response to vehicle electrical consumption, or in response to fuel gas input into the fuel cell, temperature, gas pedal, or any combination of these or other inputs. The power required to power the two compressors is comparatively low, and the responsiveness is greatly improved as compared to compressors which are only activated in response to, e.g., exhaust gas pressure.  
         [0039]    Turning now to the specially designed, electromotor driven, integrated two-stage compressor shown in FIGS. 2 and 3, all rotating parts are mounted on a single rotor shaft  20 . In the illustrated embodiment, the integrated two-stage compressor has a generally “barbell” shape, with the pulley  21  located centrally on the rotor shaft, centrifugal compressors on the ends of the rotary shaft, and one MLC motor provided between the pulley and each of the compressors. The rotor shaft is supported on bearings inside a housing.  
         [0040]    A belt (see FIG. 3) can be tensioned over this pulley to connect the rotor shaft of the compressor to the driveshaft of an internal combustion engine of a hybrid combustion/electric vehicle.  
         [0041]    The electric motors can be powered by batteries, by a generator associated with an internal combustion motor, or by a fuel cell. The electric motors may be any type, but for the reasons listed above are preferably constructed using magnetically loaded composite (MLC) rotor technology, with a rotor  22 ,  22 ′ coupled to the rotor shaft  20  and stators  23 ,  23 ′ connected to the housing. The MLC rotor and a stator also serve as a magnetic bearing system.  
         [0042]    Since centrifugal compressors draw air in axially and expel air radially, it is necessary to place the compressors at the first and second ends of the rotary shaft. As shown in FIGS. 2 and 3, first  30  and second  30 ′ compressor wheels are provided on the first and second ends of the rotor shaft. Both compressors draw air in axially at opposite ends of the rotor shaft. Air is drawn into first compressor  24  housing inlet  25  at P1=atmospheric pressure and is discharged at outlet  26  at P2= e.g., 2 bar. Air is conveyed along conduit  27  to second compressor  24 ′ housing inlet  28  at P2= e.g., 2 bar and is discharged at outlet  29  at P3= e.g., 3 bar.  
         [0043]    As shown in FIG. 1, air may flow through the first compressor and be precompressed prior to entering the second compressor, or may alternatively, as determined by conditions, bypass at least in part the first compressor by opening a valve located in a bypass conduit  30 .  
         [0044]    The motors could easily be switched via power electronics to function as an electricity generator, in the case that the rotor shaft is being turned either by the pulley belt which is connected to the drive shaft of the motor, or driven by an expander.  
         [0045]    The two-stage compressor of. the present invention represents an improvement over the two-stage centrifugal compressor assembly disclosed in U.S. Pat. No. 6,193,473 (Mruk et al), since Mruk et al drive the compressors via a switched reluctance motor disposed between the first and second compressor casings and comprising a stator and a rotor rotatable within the stator. The absence of a pulley in the design of Mruk et al makes it difficult to fully integrate the compressor into a fuel cell system to generate electricity to power vehicle accessories or as drive systems for propelling vehicles, and in particular, hybrid vehicles. Further, the employment of magnetically loaded composite (MLC) motors in the present invention, one on either side of the pulley, provides numerous advantages discussed above.  
         [0046]    Although a two stage compressor has been described herein with great detail with respect to an embodiment suitable for use in a fuel cell system, and particularly a fuel cell system as used to generate electricity to power vehicle accessories or as drive systems for propelling vehicles, it will be readily apparent that the two stage compressor is suitable for use in a number of other applications. Although this invention has been described in its preferred form with a certain of particularity with respect to an automotive internal combustion compressor wheel, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of structures and the composition of the combination may be resorted to without departing from the spirit and scope of the invention.  
         [0047]    Now that the invention has been described,