Abstract:
A recirculating assembly is provided for a fuel cell system having an electrochemical conversion unit which receives a combustible fluid at an inlet, and supplies electric energy at an electrical output, surplus combustible fluid at a first fluid outlet, and a waste fluid at a second fluid outlet. The recirculating assembly has a compressor for increasing the pressure of the surplus combustible fluid not used by the conversion unit, so as to make it once more available to the conversion unit. The drive is powered by residual energy of the waste fluid to drive the compressor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority of European Patent Application No. 04425392.0, filed on May 31, 2004, the subject matter of which is incorporated herein by reference. The disclosure of all U.S. patents and patent applications mentioned below are also incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a recirculating assembly for fuel cell systems.  
         [0003]     The following description refers, purely by way of example, to earth vehicle applications.  
         [0004]     Fuel cell systems are known for converting chemical to electric energy capable of supplying mechanical energy to an earth vehicle by means of an electric drive motor. In view of the zero pollution factor and high energy efficiency of fuel cell systems, they are a valid alternative to internal combustion engines as a means of driving earth vehicles.  
         [0005]     A fuel cell system substantially comprises a fuel cell electrochemical conversion unit; a fuel tank supplying the conversion unit; and an electric circuit supplied by the conversion unit. To convert the chemical energy stored in the fuel to electric energy for powering the electric drive motor, a fuel-oxidizing agent is required.  
         [0006]     In earth vehicle applications, in particular, the electrochemical conversion unit is defined by a number of individual fuel cells connected electrically in series to increase the electric energy generated by the system.  
         [0007]     More specifically, each fuel cell comprises an anode and a cathode, between which is interposed an electrolyte, and on which appropriate reaction catalysts are provided. In PEM (proton exchange membrane) fuel cell systems, to which the following description refers purely by way of example, the electrolytes used for driving earth vehicles are defined by a polymer membrane permeable to protons and impermeable to electrons.  
         [0008]     In known systems, the fuel is hydrogen, and the oxidizing agent is atmospheric oxygen in the air. By means of appropriate conduits, the hydrogen and air respectively supply the anode and cathode of the individual fuel cells defining the electrochemical conversion unit. The hydrogen, with the aid of a first catalyst, ionizes at the anode to produce protons and electrons. The protons migrate to the cathode through the electrolytic membrane permeable to them; and the electrons, to which the electrolytic membrane is impermeable, migrate to the cathode via the external electric circuit, thus generating electric current capable of powering an electric motor and so driving a mechanical rotary member. In the presence of a second catalyst, the oxygen reaching the cathode reacts with the protons from the electrolytic membrane and with the electrons from the external circuit to form steam.  
         [0009]     By means of an oxidizing agent (oxygen), the chemical energy of the fuel (hydrogen) is thus convertible to electric energy, with a reaction product in the form of steam together with low-oxygen air and heat.  
         [0010]     In a fuel cell system, water is present at both the anode and cathode, and, for the system to function properly, must be distributed evenly inside the fuel cell. This is done, on the anode side, using a system for generating in the circuit a stream of hydrogen capable of distributing the water evenly throughout the fuel cell.  
         [0011]     Using compressors or pumps with rotary members and fluid tight seals for this purpose is difficult and not very practical, on account of the small size of the hydrogen molecules, which give rise to particularly dangerous leakage from the seals capable of producing violent explosions even when the fuel cell is idle.  
         [0012]     Recirculating assemblies comprising hermetically sealed, powered compressors are known, e.g. as described in U.S. patent Application Publication No. 2003/0077499, but must be supplied with energy by the fuel cell system or an external energy source.  
         [0013]     Recirculating assemblies not requiring fluidtight seals for rotary members along the hydrogen conduit are also known, e.g. as described in U.S. Pat. No. 6,013,385, and comprise one or more pressure recovery devices or so-called “eductors” located along the hydrogen conduit, at the output of the fuel cell anode. Such devices comprise a hydrogen pressure recovery nozzle, and an injection conduit for injecting a high-speed fluid into the nozzle. The nozzle comprises a converging-section portion tapering to a narrow section, from which a diverging-section portion originates; and the injection conduit provides for injecting a high-speed fluid from a tank into the narrow section of the nozzle. From the electrochemical conversion unit, the feedback hydrogen thus flows into the converging-section portion of the pressure recovery device nozzle, and, upon injection of the high-speed fluid, undergoes a pressure increase in the diverging-section portion of the nozzle.  
         [0014]     A first pressure recovery device is supplied with the feedback hydrogen at the inlet of the converging portion of the nozzle, is fed at the narrow section with high-pressure hydrogen injected from the fuel tank by the injection conduit, and provides for effectively recovering the pressure of the feedback hydrogen when the fuel cell system is in the steady operating condition.  
         [0015]     Conversely, correct hydrogen pressure recovery is not possible when hydrogen flow to the conversion unit falls below a predetermined threshold value.  
         [0016]     In this case, a second pressure recovery device must be provided, connected pneumatically parallel with the first. The second device is supplied with feedback hydrogen at the converging portion inlet by means of an appropriate regulating valve, and is fed at the narrow section with a fluid, normally water, injected at high speed by the injection conduit, so that a water tank and a power pump to pump the water are required. The limited capacity of the water tank and the need for a motor to drive the water pump therefore reduce the efficiency of the recirculating assembly. Moreover, recirculating assemblies of this sort are of only limited benefit by being complicated to produce, and by still involving a certain amount, albeit reduced, of hydrogen leakage.  
         [0017]     To eliminate the above drawbacks, recirculating assemblies, e.g. as described in U.S. patent Application Publication No. 2002/0122969, have been devised comprising a pump located between the hydrogen tank and the electrochemical conversion unit anode. The pump is driven by the pressure of the hydrogen in the tank, and provides for recovering the pressure of the feedback hydrogen by means of a pressure drop between the hydrogen tank and the pump outlet conduit supplying the electrochemical conversion unit.  
         [0018]     More specifically, the pump is supplied with hydrogen from the tank at the pressure set by a regulating valve, receives feedback hydrogen from the conversion unit at a lower pressure than required at the conversion unit inlet, and feeds the hydrogen back to the conversion unit at a lower pressure than at the pump inlet, but at a higher pressure than at the conversion unit outlet. Since pressure recovery of the feedback hydrogen between the anode output and input is brought about by the pressure drop between the hydrogen in the tank entering and leaving the pump, recirculating assemblies of the above type withdraw no energy from the fuel cell system, and require no external energy sources.  
         [0019]     Though theoretically valid, such systems are difficult to actually produce, by involving fairly considerable cost.  
       SUMMARY OF THE INVENTION  
       [0020]     It is an object of the present invention to provide a recirculating assembly for a fuel cell system, designed to eliminate the aforementioned drawbacks typically associated with known assemblies.  
         [0021]     According to the present invention, there is provided a recirculating assembly for a fuel cell system, said system comprising an electrochemical conversion unit receiving a combustible fluid at an inlet, and supplying electric energy at an electrical output, surplus combustible fluid at a first fluid outlet, and a waste fluid at a second fluid outlet; said recirculating assembly comprising: a compressor to increase the pressure of the surplus combustible fluid not used by said unit, so as to make it once more available to the unit; and a drive to drive said compressor; wherein said drive is powered by the residual energy of said waste fluid from said unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:  
         [0023]      FIG. 1  shows a diagram of a fuel cell system in accordance with the present invention;  
         [0024]      FIG. 2  shows a larger-scale axial section, with parts removed for clarity, of a portion of the recirculating assembly in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Number  1  in  FIG. 1  indicates as a whole a fuel cell system for earth vehicles, which comprises a fuel cell electrochemical conversion unit  2 ; a fuel tank  3  for supplying unit  2 ; and a recirculating assembly  4  for recirculating surplus fuel not used in the electrochemical conversion.  
         [0026]     Unit  2 , which is known and therefore only described herein as required for a clear understanding of the present invention, may operate according to any of various known construction designs and technologies, and, in the preferred embodiment described, is a proton exchange membrane (PEM) type employing hydrogen as fuel, and oxygen in atmospheric air as the oxidizing agent. The present invention, however, is not limited to the above type of electrochemical conversion unit, and may be used with electrochemical conversion units of any construction design or technology.  
         [0027]     Unit  2  is defined in known manner by a number of stacked fuel cells  5  connected electrically in series, and defines as a whole a cathode  7  and an anode  8 . An electric circuit  13  connects cathode  7  and anode  8  of unit  2 , and can thus power an electromechanical load  15 —in this case, an electric motor for driving a vehicle—by means of the energy generated by system  1 .  
         [0028]     Unit  2  is supplied at the inlet with atmospheric air by a conduit  9 , and with hydrogen from tank  3  and from recirculating assembly  4  by a conduit  10 , as explained in detail later on.  
         [0029]     More specifically, the atmospheric air feed conduit  9  enters unit  2  on the cathode  7  side, supplies the individual cells  5  with oxygen, and, on exiting cathode  7 , conducts the waste products, defined by steam and low-oxygen air, to recirculating assembly  4 . The hydrogen feed conduit  10  enters unit  2  on the anode  8  side, supplies the individual cells  5  with hydrogen, and, on exiting anode  8 , conducts the surplus hydrogen to recirculating assembly  4 .  
         [0030]     With reference to  FIGS. 1 and 2 , recirculating assembly  4  substantially comprises a compressor  12  for compressing the hydrogen from unit  2 ; and a drive member for driving compressor  12 .  
         [0031]     An important aspect of the present invention lies in the drive member being defined by a turbine  11  powered by the energy of the waste produces from unit  2 .  
         [0032]     More specifically, turbine  11  and compressor  12  are angularly connected by a permanent-magnet or electromagnetic joint  16  described in detail later on, and define as a whole a turbocharger  6  of axis A.  
         [0033]     Turbine  11  comprises a spiral casing  18 , and an impeller  19  rotating about axis A inside casing  18 . More specifically, casing  18  defines a circumferential inlet section  18   a  for the waste products from unit  2 ; and an outlet section  18   b  for expelling the products expanded by turbine  11  from system  1 .  
         [0034]     More specifically, impeller  19  of turbine  11  is fitted to a shaft  14  projecting from turbine  11 . In a preferred embodiment of the present invention, impeller  19  may be coated with, or made entirely of, corrosion-resistant material, e.g. teflon, to safeguard against corrosion by the deionized steam in turbine  11 . Alternatively, impeller  19  may be surface-treated to resist such corrosion.  
         [0035]     Compressor  12  comprises a spiral casing  20 , and an impeller  21  rotating about axis A inside casing  20 . More specifically, casing  20  defines an axial inlet section  20   a  for the feedback hydrogen from unit  2 ; and a circumferential outlet section  20   b  for feeding the compressed hydrogen to unit  2  along a recirculating conduit  25  connected to conduit  10 .  
         [0036]     More specifically, impeller  21  is fitted to a shaft  17  projecting from compressor  12  and angularly connectable to shaft  14  by means of joint  16 . In a preferred embodiment of the present invention, impeller  21  may be coated with, or made entirely of, corrosion-resistant material, e.g. teflon, to safeguard against corrosion by the deionized steam in compressor  12 . Alternatively, impeller  21  may be surface-treated to resist such corrosion.  
         [0037]     Joint  16  will be described, purely by way of example, with reference to a known permanent-magnet, axial-flow, electric machine not described in detail. Joint  16  transmits motion from shaft  14  to shaft  17 , while keeping them a predetermined axial distance apart, and with no contacting mechanical parts, and, more specifically, comprises a disk  22   a  fitted to the opposite end of shaft  14  to turbine  11 , and a disk  22   b  fitted to the opposite end of shaft  17  to compressor  12 . Disks  22   a  and  22   b  have a number of permanent magnets, and are maintained a predetermined axial distance apart.  
         [0038]     Using joint  16  as described above, which provides for transmitting motion between shafts  14  and  17  with no mechanical contact, the waste product expansion environment can be separated in fluidtight manner from the recirculated hydrogen compression environment, so that shaft  14  and relative disk  22   a  are housed in a first casing  23  integral with casing  18  of turbine  11 , and shaft  17  and relative disk  22   b  are housed in a second casing  24  separate from casing  23  and integral with casing  20  of compressor  12 .  
         [0039]     As such, the hydrogen compressed in compressor  12  and the waste products can in no way react chemically with each other to impair the efficiency of recirculating assembly  4  and, therefore, of system  1 .  
         [0040]     In actual use, impeller  19  of turbine  11 , powered by the waste products from conduit  9 , rotates shaft  14  and disk  22   a  integral with shaft  14 ; disk  22   a  rotates disk  22   b  by means of joint  16 , which thus makes rotation about axis A of shaft  14  of turbine  11  angularly integral with similar rotation about axis A of shaft  17  of compressor  12 ; and disk  22   b  rotates impeller  21  of compressor  12  by means of shaft  17 , so as to increase the pressure of the feedback hydrogen fed to compressor  12  by conduit  10 .  
         [0041]     The waste products from turbine  11  constitute the exhaust of system  1 , whereas the hydrogen from compressor  12  is fed by recirculating conduit  25  into conduit  10  where it mixes with the hydrogen from tank  3  to supply unit  2 .  
         [0042]     To better regulate system  1 , provision may be made, immediately downstream from tank  3 , for a regulating valve  26  and a flow gauge  27  for regulating the amount of fuel required to supplement the hydrogen made available to unit  2  by recirculating assembly  4 .  
         [0043]     The advantages of recirculating assembly  4  according to the present invention will be clear from the foregoing description.  
         [0044]     In particular, recirculating assembly  4  employs the residual energy in the waste products to drive shaft  14  by means of turbine  11 , and, by means of joint  16 , to drive compressor  12  to bring the feedback hydrogen to the pressure necessary to supply unit  2 , and therefore requires no additional energy sources.  
         [0045]     Recirculating assembly  4  therefore provides for enhancing the efficiency of fuel cell system  1 , with no increase in cost or complexity.  
         [0046]     Clearly, changes may be made to recirculating assembly  4  as described and illustrated herein without, however, departing from the scope of the accompanying claims.  
         [0047]     The invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art, that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications that fall within the true spirit of the invention.