Abstract:
A method and apparatus for preventing or at least reducing condensation in a cathode exhaust conduit of a fuel cell is disclosed. The method includes introducing air into the fuel cell through an air intake conduit, removing an exhaust stream from the fuel cell through the cathode exhaust conduit and introducing excess air into the cathode exhaust conduit to prevent or reduce condensation of the exhaust stream in the cathode exhaust conduit. The apparatus includes an air intake conduit for introducing air into the fuel cell, a cathode exhaust conduit for distributing the exhaust stream from the fuel cell and an excess air diversion conduit providing fluid communication between the air intake conduit and the cathode exhaust conduit for diverting air from the air intake conduit and the cathode exhaust conduit.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to fuel cell vehicles which utilize an electrolyte polymer membrane interposed between a cathode and an anode to generate electricity as electrons are stripped from hydrogen atoms to form protons prior to passage of the protons through the membrane. More particularly, the present invention relates to a method and apparatus for preventing or at least reducing condensation of water in a cathode exhaust conduit of a fuel cell by introducing air into the cathode exhaust conduit during operation of the fuel cell.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit no harmful by-products which would otherwise contribute to atmospheric pollution.  
         [0003]     Fuel cells include three components: a cathode, ananode and an electrolyte which is sandwiched between the cathode and the anode and passes only protons. Each electrode is coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.  
         [0004]     In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer membrane serves as the electrolyte between a cathode and an anode. The polymer membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate conductivity of the membrane. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is very important for the proper functioning of the fuel cell. Irreversible damage to the fuel cell will occur if the membrane dries out.  
         [0005]     During the conversion of hydrogen and oxygen (air) to electricity, water is produced as a reaction by-product. The product water is removed from the fuel cell by a cathode exhaust conduit. Due to the moist operation conditions of the fuel cell, the operating parameters are chosen in such a manner that certain water saturation is reached at the cathode outlet.  
         [0006]     Depending on the arrangement or design of the fuel cell system, the water in the cathode exhaust can be utilized within the fuel cell system. This assists in the water management of fuel cells used in mobile applications. Depending on the conditioning of the cathode exhaust, small heat loss to the environment and condensation of the reaction product water coming from the fuel cell cannot be prevented. However, the inclusion of large quantities of liquid water must be avoided for proper operation of the fuel cell. At lower environmental temperatures, the heat loss and condensation will increase. Therefore, during winter operation of a vehicle, the formation of ice within the exhaust stream is a possibility and must be avoided.  
         [0007]     In fuel cell systems currently being developed, the cathode exhaust stream leaves the cathode exhaust conduit in a certain saturated state. With environmental temperatures falling as low as −25 degrees C. in many winter climates, such emission of the cathode exhaust stream from the cathode exhaust conduit in a saturated state renders the development of fuel cells for vehicles problematic.  FIG. 1  shows the essential operation characteristic features of a fuel cell under selected fuel cell system load operating conditions. For an operating temperature of 80 degrees C., a dew point temperature (dotted line) of 63 degrees C. is realistic. The heat transfer coefficient (solid line) for the cathode exhaust ranges from 5 to 50 kW/m 2  K.  
         [0008]     The dashed line in  FIG. 1  depicts the environmental temperatures that correspond to the minimal temperatures at which condensation of water in the cathode exhaust conduit would occur, given the selected fuel cell system load operating conditions. In the graph, a system load of 0.0 indicates no load, whereas a system load of 1.0 indicates maximum load. With increased system load, fuel cell operation results in condensation of water at progressively lower environmental temperatures. According to the graph, environmental temperatures of approximately 40 degrees C. and lower would lead to undesirable levels of condensation of the cathode exhaust stream in the cathode exhaust conduit throughout the operating range of the fuel cell. Therefore, the low environmental temperatures which are characteristic of winter temperatures in many climates would render the operation of fuel cells problematic in such climates. Thus, a method and apparatus are needed to reduce condensation of a cathode exhaust stream in a cathode exhaust conduit of a fuel cell.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention is generally directed to a novel method for reducing condensation of a cathode exhaust stream in a cathode exhaust conduit of a fuel cell. The method includes the introduction of air into the cathode exhaust conduit during fuel cell operation in order to reduce the partial pressure of the exhaust water vapor in the exhaust stream, and therefore, reduce the dew point temperature of the cathode exhaust stream. Consequently, the tendency of the cathode exhaust stream to condense from the vaporized state into the liquid state in the cathode exhaust conduit is reduced. This enables operation of the fuel cell without excessive condensation of the exhaust stream in the cathode exhaust conduit, particularly at reduced environmental temperatures. The method may further include pre-heating of the cathode exhaust stream to further reduce condensation of the cathode exhaust stream in the cathode exhaust conduit.  
         [0010]     The present invention is further directed to a novel apparatus for reducing condensation of a cathode exhaust stream in a cathode exhaust conduit of a fuel cell. The apparatus includes an air inlet conduit for distributing oxygen to a cathode in a polymer-electrolyte-membrane (PEM) fuel cell. A cathode exhaust conduit extends from the fuel cell to carry an exhaust stream from the cathode. An excess air diversion conduit extends from the air inlet conduit to the cathode exhaust conduit. During operation of the fuel cell, excess air is diverted from the air inlet conduit to the cathode exhaust conduit in order to reduce condensation of the exhaust stream in the cathode exhaust conduit by reducing the partial pressure of the exhaust stream. A heating element may be provided in thermal contact with the cathode exhaust conduit to pre-heat the exhaust stream prior to mixing of the exhaust stream with the excess air.  
         [0011]     In the graph of  FIG. 6 , the impact of adding excess air to the cathode exhaust on the dew point temperature is shown. The upper dashed line represents the dew point before mixing excess air to the cathode exhaust. The middle or sloped line shows the excess air mixed with the cathode exhaust. This mixing of the excess air with the cathode exhaust leads to an allowable ambient temperature of −20° C., which is represented by the bottom dashed line. Accordingly, feeding excess air to the cathode exhaust reduces the allowable ambient temperature from 40 . . . 3° C. (function of system load) to −20° C. over the complete operation range.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0013]      FIG. 1  is a graph which illustrates typical operating data for a standard fuel cell system;  
         [0014]      FIG. 2  is a schematic view of an air intake and exhaust conduit system for a fuel cell provided with an excess air supply system according to the present invention;  
         [0015]      FIG. 3  is a graph which illustrates typical fuel cell operating data of a fuel cell system with additional electrical heating of the exhaust stream without the effect of excess air in the exhaust conduit of the system, according to the present invention;  
         [0016]      FIG. 4  is a schematic view of an air intake and exhaust conduit system for a fuel cell provided with an alternative excess air supply system according to the present invention;  
         [0017]      FIG. 5  is a schematic view of an air intake and exhaust conduit system for a fuel cell provided with another alternative excess air supply system according to the present invention; and  
         [0018]      FIG. 6  is a graph which illustrates typical fuel cell operating data of a fuel cell system with the effect of feeding excess air into the exhaust conduit of the system according to the present invention, wherein the upper dashed line is the dew point before mixing excess air to the cathode exhaust and the middle (sloped) line is the excess air mixed with cathode exhaust leading to an allowable ambient temperature of −20 degrees C. (represented by the lower dashed line). 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     Referring initially to  FIG. 2 , an air intake and exhaust conduit system for a fuel cell according to the present invention is generally indicated by reference numeral  10 . The system  10  includes a polymer-electrolyte-membrane (PEM) fuel cell  14 , which may be conventional and includes a cathode (not shown), an anode (not shown) and an electrolyte membrane (not shown) sandwiched between the cathode and the anode. An air intake conduit  12  is provided for receiving a continuous air stream  22  from an air compressor (is shown without a number). An inlet segment  12   a  branches from the air intake conduit  12  and is provided in fluid communication with the cathode of the PEM fuel cell  14 . A valve  17  may be provided in the cathode exhaust conduit  16  to control the rate of flow of the exhaust gas stream through the cathode exhaust conduit  16 . A downstream segment  16   a  of the cathode exhaust conduit  16  may extend from the outlet of the valve  17 .  
         [0020]     According to the present invention, an excess air diversion conduit  18  branches from the air intake conduit  12 , typically at the inlet segment  12   a , and is provided in fluid communication with the downstream segment  16   a  of the cathode exhaust conduit  16 . A valve  19  may be provided in the excess air diversion conduit  18  for purposes which will be hereinafter described.  
         [0021]     During operation of the PEM fuel cell  14 , a continuous air stream  22  is distributed from the air compressor (shown without number) through the air intake conduit  12 . Operating air from the air stream  22  is distributed by the inlet segment  12   a  into the fuel cell  14 , whereas excess air  26  from the air stream  22  is distributed by the excess air diversion conduit  18  into the downstream segment  16   a  of the cathode exhaust conduit  16 . In the PEM fuel cell  14 , a catalyst coated on the anode (not shown) splits hydrogen into electrons and protons. The electrons from the hydrogen are distributed as electric current from the anode, through a drive motor (not shown) and then to the cathode (not shown), whereas the protons migrate from the anode, through the electrolyte membrane (not shown) to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the operating air to form the exhaust gas stream  28 . The cathode exhaust conduit  16  and downstream segment  16   a  distribute the exhaust stream  28 , in vapor form, from the PEM fuel cell  14 .  
         [0022]     As the exhaust stream  28  flows through the downstream segment  16   a  of the cathode exhaust conduit  16 , the excess air  26  flowing into the downstream segment  16   a  from the excess air diversion conduit  18  combines and mixes with the vaporized exhaust gas  28 . The excess air  26  reduces the partial vapor pressure of the exhaust gas  28  in the downstream segment  16   a . This lowers the dew point of the exhaust stream  28  and prevents condensation of the exhaust stream  28  from the vapor to the liquid state in the downstream segment  16   a . Accordingly, the downstream segment  16   a  discharges the excess air  26  and exhaust stream  28  into the atmosphere as a vaporized air/water mixture  30 . It is understood that the excess air  26  can be introduced into any portion of the cathode exhaust conduit  16  or downstream segment  16   a  thereof from either the excess air diversion conduit  18 , as heretofore described, or from an air source (not shown) which is separate from the air stream  22 .  
         [0023]     The quantity of excess air  26  which is required to maintain the exhaust stream  28  in a vaporized state throughout the downstream segment  16   a  depends in large part on the loss of heat from the exhaust stream  28 . The quantity of excess air  26  which is required can be reduced if the cathode exhaust conduit  16  is properly insulated to minimize the quantity of heat loss. Furthermore, the quantity of excess air  26  necessary to maintain the exhaust stream  28  in a vaporized state is typically inversely related to the ambient temperature, with larger quantities of excess air  26  necessary in colder ambient temperatures.  
         [0024]     Referring again to  FIG. 2 , a heating element  20  (shown in phantom) may optionally be provided in thermal contact with the cathode exhaust conduit  16 , according to the knowledge of those skilled in the art, to heat the exhaust stream  28  flowing therethrough and maintain the exhaust stream  28  in the vaporized state. Accordingly, the heat applied to the exhaust stream  28  using the heating element  20 , in combination with the excess air  26  applied to the exhaust stream  28 , maintains the exhaust stream  28  in the vaporized state as the excess air  26  mixes with the exhaust stream  28  and the air/water mixture  30  is discharged from the downstream segment  16   a.    
         [0025]     The graph of  FIG. 3  illustrates typical fuel cell operating data of a fuel cell system with additional electrical heating of the exhaust stream  28  in the cathode exhaust conduit  16 , according to the present invention. The line-connected circles indicate the allowable ambient temperatures for operation of the fuel cell  14  using the flow of excess air  26  to the downstream segment  16   a , without heating of the exhaust stream  28  using the heating element  20 , throughout the system load range of the fuel cell  14 . Accordingly, such operation of the fuel cell  14  without operation of the heating element  20  is effective to prevent condensation of the exhaust stream  28  to a temperature range as low as from about 40 degrees C. to about 5 degrees C., as indicated by the line-connected circles. On the other hand, operation of the fuel cell  14  using the heating element  20  to pre-heat the exhaust stream  28 , in addition to distribution of the excess air  26  into the downstream segment  16   a , is effective to prevent condensation of the exhaust stream  28  to a temperature range as low as from about 30 degrees C. to about −1 degrees C. over the system load range of the fuel cell  14 .  
         [0026]     Referring next to  FIG. 4 , an alternative air intake and exhaust conduit system for a fuel cell according to the present invention is generally indicated by reference numeral  10   a . The system  10   a  is similar in design to the system  10  heretofore described with respect to  FIG. 2 , except that the excess air diversion conduit  18  is provided in fluid communication with the segment of the cathode exhaust conduit  16  which is upstream of the valve  17 , instead of with the downstream segment  16   a  of the cathode exhaust conduit  16 . Operation of the system  10   a , with or without the heating element  20 , is similar to that described with respect to the system  10 . The heating element  20  is optional and may be provided in thermal contact with the cathode exhaust conduit  16 , according to the knowledge of those skilled in the art.  
         [0027]     Referring next to  FIG. 5 , a preferred embodiment of the air intake and exhaust conduit system for a fuel cell according to the present invention is generally indicated by reference numeral  10   b . The system  10   b  is similar in design to the system  10  heretofore described with respect to  FIG. 2 , except that the excess air diversion conduit  18  branches from the air intake conduit  12  upstream of the inlet segment  12   a . A heating element  20  may optionally be provided in thermal contact with the cathode exhaust conduit  16 , according to the knowledge of those skilled in the art. Operation of the system  10   a , with or without the heating element  20 , is similar to that described with respect to the system  10 .  
         [0028]     In the embodiments shown in  FIGS. 4 and 5 , excess air is supplied to the cathode exhaust conduit  16  from a port between the air compressor and charge air cooler in the air intake conduit  12 . This results in additional heat being supplied to the exhaust stream in the cathode exhaust conduit  16 .  
         [0029]     Referring next to the graph of  FIG. 6 , the effect of adding excess air to the cathode exhaust on the dew point temperature of the exhaust is shown. The upper dashed line represents the dew point (° C.) before mixing excess air to the cathode exhaust. The middle or sloped line represents the excess air (g/s) mixed with the cathode exhaust. This mixing of the excess air with the cathode exhaust leads to an allowable ambient temperature of −20° C., which is represented by the bottom dashed line. Accordingly, feeding excess air to the cathode exhaust reduces the allowable ambient temperature from 40 . . . 3° C. (function of system load) to −20° C. over the complete operation range.  
         [0030]     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.