Abstract:
A power production unit onboard a motor vehicle, including a fuel cell including at least one orifice for evacuating off-gases, in particular of air and water vapor, which are discharged into a discharge pipe. A condenser liquefies the water vapor, and a compressor is interposed upstream of the condenser, liquid water being diverted from the discharge pipe to a liquid water circuit. The compressor compresses the off-gases so that the dew point of the water vapor is higher than the temperature of the condenser.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an electricity-generating installation on board a motor vehicle, of the type equipped with a fuel-cell stack. 
     The invention relates more particularly to an electricity-generating installation on board a motor vehicle, of the type equipped with a fuel-cell stack provided with at least one orifice for evacuation of residual gases, which are composed mainly of air and water vapor and which are discharged into an evacuation conduit in which there is disposed a condenser that liquefies the water vapor, and in which a compressor is interposed upstream from the condenser, the liquid water being diverted from the evacuation conduit to a liquid water circuit. 
     2. Description of Related Art 
     Fuel-cell stacks are used in particular to supply the electrical energy necessary for propulsion of motor vehicles. The fuel-cell stack is then mounted on board the vehicle. 
     A fuel-cell stack is composed mainly of two electrodes, an anode and a cathode, which are separated by an electrolyte. This type of stack permits direct conversion, to electrical energy, of the energy produced by the following oxidation-reduction reactions: 
     a reaction of oxidation of a fuel, which continuously feeds the anode; and 
     a reaction of reduction of an oxygen carrier, which continuously feeds the cathode. 
     The fuel-cell stacks used to supply electrical energy on board motor vehicles are generally of the solid-electrolyte type, especially with an electrolyte formed by a polymer membrane. Such a stack uses especially hydrogen (H 2 ) and oxygen (O 2 ) as the fuel and oxygen carrier respectively. 
     In contrast to combustion engines, which discharge a non-negligible quantity of polluting substances with the exhaust gases, the fuel-cell stack offers in particular the advantage of discharging mainly water, which is produced by the reduction reaction at the cathode. 
     The stack also discharges part of the oxygen carrier that has not reacted in the form of cathode evacuation gas, and it may also discharge part of the fuel that has not reacted in the form of anode evacuation gas. In the latter case, the fuel is generally burned before being discharged to the atmosphere in the form of water vapor. 
     In addition, the oxygen carrier of a stack of the type described in the foregoing can be ambient air, the oxygen (O 2 ) of which undergoes reduction. 
     The oxygen carrier is generally humidified before being injected at the cathode, so that the membrane of polymer material is not damaged, for example by drying. This humidification operation is also applied to the fuel when the latter leaves the anode via an anode evacuation orifice. 
     The water necessary for humidification of the membrane is generally recovered at the stack outlet, and more particularly in the cathode evacuation gases, which contain water in liquid or vapor form, produced by the reaction of reduction of the oxygen carrier at the cathode. 
     Water recovery at the cathode outlet effectively has the advantage that there is no need for frequent replenishment of the water reserves of the vehicle. In addition, if sufficient water can be recovered to humidify the membrane, it is not necessary that the vehicle be equipped with a large-volume water reservoir. 
     To recover the water produced at the cathode, it is known that a condenser can be disposed in the stream of cathode evacuation gas. For optimal operation of the fuel-cell stack being fed with oxygen carrier and fuel under atmospheric pressure, this type of condenser generally requires a cooling source, whose temperature must be maintained between 20 and 30° C. 
     This solution is not usable, because motor vehicles are generally designed to operate in an environment whose temperature may vary between −20° C. and approximately 45° C. The use of a condenser therefore necessitates the use of a costly air-conditioning device, which is not available in all vehicle models. 
     It is known, therefore, to raise the oxygen-carrier pressure in the cathode circuit while retaining the condenser. By raising the pressure of the evacuation gases containing the water vapor, the dew point temperature of the water vapor is also effectively raised. The dew point temperature is the temperature at which the water vapor condenses. A condensation fog is then deposited on the surfaces whose temperature is lower than the dew point. 
     Thus, when the evacuation gases are injected into the condenser, at a pressure of 4 bar, for example, the cooling source of the condenser must then be maintained at a temperature of approximately 60° C. in order to function in optimum manner. It is much easier to maintain the cooling source of the condenser at a higher temperature than the ambient temperature. 
     However, such a solution requires that all feed circuits of the fuel-cell stack be kept under pressure, with the risk of degradation of the said stack. It is therefore necessary to use a non-negligible portion of the energy supplied by the stack to compress the oxygen-carrier and fuel circuits, to the detriment of the efficiency of the fuel-cell stack. 
     SUMMARY OF THE INVENTION 
     To overcome these problems, the invention proposes an electricity-generating installation on board a motor vehicle, which installation is of the type described in the foregoing, characterized in that the compressor compresses the residual gases in such a way that the dew point temperature of the water vapor is higher than the temperature of the condenser. 
     According to other characteristics of the invention: 
     the installation is provided with a turbine, which is interposed in the evacuation conduit downstream from the condenser and which drives the compressor; 
     the turbine and the compressor comprise a turbine compressor; 
     the installation includes a reformer, which feeds the fuel-cell stack with fuel and which discharges the exhaust gases under pressure and injects them into the turbine. 
     The invention also relates to a method for electricity generation on board a motor vehicle, of the type equipped with a fuel-cell stack, the method operating by liquefying the water vapor by a condenser disposed into an evacuation conduit in which the residual gases are discharged via at least one orifice for evacuation of the residual gases, which are composed mainly of air and water vapor; by diverting the liquid water from the evacuation conduit to a liquid water circuit; and by compressing the residual gases by the compressor in such a way that the dew point temperature of the water vapor is higher than the temperature of the condenser. 
     The present invention is applicable to a vehicle equipped with such an electricity-generating installation or using such a method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary electricity-generating installation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Other characteristics and advantages of the invention will become evident upon reading the detailed description hereinafter, for the understanding of which reference will be made to the single attached figure, which is a schematic representation of the electricity-generating installation constructed according to the teachings of the invention. 
       FIG. 1  illustrates an electricity-generating installation  10 , which in the present case is mounted on board a motor vehicle. Installation  10  is provided mainly with a fuel-cell stack  12 , the electrolyte of which in the present case is a polymer membrane  14 . 
     Fuel-cell stack  12  is provided with an anode  16  and a cathode  18 . Cathode  18  is fed continuously by an oxygen carrier, which in the present case is air. Anode  16  is fed continuously by a fuel, which in the present case is mainly hydrogen (H 2 ). The flowrates of oxygen carrier and fuel are in the present case regulated as a function of the electrical power required for the motor vehicle. 
     Fuel-cell stack  12  is traversed by a first cathode circuit  20  of oxygen carrier as illustrated by a thin continuous line in  FIG. 1 , and it is traversed by a second anode circuit  22  of fuel as illustrated by a thick continuous line. 
     Cathode circuit  20  is provided in particular with a cathode feed conduit  24 , which is connected to a cathode feed orifice  26  in order to feed cathode  18  with air. Cathode  18  is provided with a cathode evacuation orifice  28 , via which the cathode evacuation gases or residual gases, or in other words the gases that have not been consumed by the cathode, are evacuated into a cathode evacuation conduit  30 . 
     Cathode evacuation conduit  30  is connected to a compressor  32 , which in the present case is driven mechanically by a turbine  34 . Compressor  32  is designed to compress the cathode evacuation gases, which are then guided via a water-recovery conduit  36  to a condenser  38 . Turbine  34  and compressor  32  in the present case comprise a turbine compressor  40 . 
     Condenser  38  is intended to collect the liquid water contained in the compressed cathode evacuation gases. 
     After passage into condenser  38 , the cathode evacuation gases are then expelled into the atmosphere via a gas-expulsion conduit  42  in which turbine  34  is interposed. 
     Anode circuit  22  is provided in particular with a reservoir  44  containing a customary fuel, which in the present case is gasoline and which is situated upstream from anode  16 . The gasoline is guided via a gas-transport tube  46  from reservoir  44  to a reformer  48 , which is designed to extract the hydrogen (H 2 ) from the gasoline. 
     Reformer  48  discharges a reformate containing hydrogen (H 2 ) into an anode feed tube  50 , which is connected to an anode feed orifice  52 , which opens into anode  16  of fuel-cell stack  12 . 
     After part of the hydrogen (H 2 ) has been consumed, the residual fuel is in the present case injected via an anode evacuation tube  54  into a burner  56 , which in the present case is integrated into reformer  48 , in order to be burned therein. The exhaust gases resulting from this operation are then evacuated via an exhaust orifice  58  of burner  56  into an exhaust tube  60  of burner  56 , which tube is connected to expulsion conduit  42  upstream from turbine  34 . After their passage into turbine  34 , the exhaust gases are thus discharged into the atmosphere together with the cathode evacuation gases. 
     Installation  10  is also provided with a water-distribution circuit  62 , which is illustrated by a broken line in the figure. Water-distribution circuit  62  is provided with a water reservoir  64 . Water reservoir  64  is fed with water by condenser  38  via a water flow conduit  66 . 
     The water recovered by condenser  38  is then distributed via a water-distribution system  68  to reformer  48  as well as to a device  70  for humidifying the fuel and the oxygen carrier, which device is disposed in cathode feed conduit  24  and in anode feed tube  50 . The distribution of water is achieved in the present case by means of a water pump  72 . 
     We will now describe the operation of such an installation  10 , and in particular the operation of the water-recovery device. 
     In anode circuit  22 , the gasoline is passed into reformer  48  via gasoline-transport tube  46 . The product of the reforming operation is known as “reformate”. 
     The reformate is composed mainly of hydrogen (H 2 ), carbon monoxide (CO), carbon dioxide (CO 2 ), nitrogen (N 2 ) and water (H 2 O). The reformate is the fuel that feeds anode orifice  52  via anode feed tube  50 . 
     In the present case it is injected at anode  16  under a pressure of approximately 1 bar after passage into the humidification device. 
     In cathode circuit  20 , the atmosphere air is admitted into cathode feed conduit  24 . The air, which in the present case is at atmospheric pressure, or in other words approximately  1  bar, is then introduced to cathode  18  via cathode feed orifice  26  after passage into the humidification device. 
     Fuel-cell stack  12  is then fed with fuel and with oxygen carrier. Oxidation reactions at anode  16  and reduction reactions at cathode  18  then permit the generation of electrical energy. 
     When the fuel is in contact with anode  16 , 70% to 95% of the hydrogen (H 2 ) is oxidized in the present case. The remainder of the fuel is discharged in the form of anode evacuation gas to burner  56  via anode evacuation tube  54 . 
     During contact of the air with cathode  18 , a portion of the oxygen (O 2 ) contained in the air is reduced to water. The surpluses of air and water are then evacuated in the form of cathode evacuation gas via cathode evacuation conduit  30  to compressor  32 . The water is present in the cathode evacuation gases in the form of liquid and in the form of vapor. The cathode evacuation gases in the present case have a temperature of approximately 70° C. 
     The cathode evacuation gases then contain the water in the form of liquid and vapor. 
     In compressor  32 , the cathode evacuation gases are then compressed to a pressure of 4 bar, for example, such that the dew point temperature of the water vapor, as explained hereinabove, is higher than the temperature of condenser  38 , or in other words approximately 60° C. 
     After compression, the cathode evacuation gases are injected into condenser  38 . Condenser  38  is maintained at a temperature below the dew point temperature of water, so that the water contained in the cathode evacuation gases is completely liquefied. 
     The water is then diverted to water reservoir  64  via water-flow conduit  66 . 
     After passage into condenser  38 , the cathode evacuation gases are injected into turbine  34 . 
     Advantageously, turbine  34  and compressor  32  are situated close to condenser  38 , so that the cathode evacuation gases arriving at turbine  34  suffer only small pressure losses. Thus the cathode evacuation gases are capable of supplying a non-negligible portion of energy for driving compressor  32 . 
     Exhaust tube  60  of burner  56  is in the present case connected to turbine  34 , in order to furnish additional energy sufficient for compressor  32  to compress the cathode evacuation gases to the desired pressure. 
     With such an installation  10 , it is not necessary to pressurize the entire fuel-cell stack  12 , which leads to non-negligible pressure losses between compressor  32  and turbine  34 .