Abstract:
A method of fuel cell stack humidification is provided incorporating the use of an accumulation device. The method provides for feeding back humid anode exhaust gas of the fuel cell stack to the fuel cell inlet and switching the anode inlet and outlet of the fuel cell stack for achieving better homogeneity of humidity along the fuel cell channels.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to fuel cell systems, and more particularly to a fuel cell stack humidification method incorporating an accumulation device.  
         BACKGROUND OF THE INVENTION  
         [0002]    The membranes of PEM fuel cells must be kept in humid conditions in order to achieve high performance and durability. Therefore, if operated at elevated temperatures, fuel cell systems usually require a humidification device for the feed gases air and/or hydrogen. It has been shown that the fuel gas, which is fed to the anode of the fuel cell stack, requires humidification in order to prevent the fuel cell stack from drying at the fuel inlet. Along the internal channels of the fuel cell stack, there is an increase in water content that causes a humidity gradient in the electrolyte membrane, and inhomogeneous power distribution. The inhomogeneous power distribution might lead to hot spots in some areas, and to excessive water accumulation in other areas, which again has a negative affect on performance and durability. Furthermore, humidification devices have several disadvantages, especially for automotive applications of the fuel cell stack, as they are heavy, expensive, and sometimes, due to the water they contain, subject to freezing at low ambient temperatures.  
           [0003]    Previous solutions of the humidification problem involved membrane humidifiers and water injection methods, as well as humid gas recirculation. Recirculation methods take advantage of the fact that gases at the fuel cell outlets are humidified with the water produced in the fuel cell, and can be fed back at the fuel cell inlet in order to bring the humidity there without having liquid water involved. A disadvantage is the need for a recirculation pump, the power consumption of the pump and the humidity gradient in a stack along the channel. Also, the switching of oxidizing feed gas between cathode gas inlets and outlets of the fuel cell was proposed in WO/9928985A1. The advantage provided by that system is the better homogeneity of humidity in the fuel cell as the dry feed gas is alternating in one and the other direction in the channel. The feed gas, however, is suggested to be the oxidant, and is dry, which might lead to performance degradation at both gas inlets. Accordingly, it is desirable in the art to provide a method of providing homogeneous membrane humidification without having liquid water involved in the process and without requiring additional pumps or the use of additional power.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention provides a fuel cell stack humidification method incorporating an accumulation device. The method of the present invention provides for feeding back humid anode exhaust gas of the fuel cell stack to the fuel cell inlet and switching the anode inlet and outlet of the fuel cell stack for achieving better homogeneity of humidity along the fuel cell channels. The method includes operating a fuel cell stack having two openings which are each capable of serving as a fuel gas inlet and a fuel gas outlet. Fuel is supplied to one of the two openings. Exiting fuel gas from the other of the two openings is stored and the supply of fuel gas is then switched to the other of the at least two openings so that the original outlet is now serving as the fuel gas inlet and the original fuel gas inlet is now serving as the outlet. The stored exiting humid fuel gas is then introduced into the fresh supply of fuel gas going to the new inlet opening so that the stored humid exiting fuel gas can be used to humidify the PEM membrane.  
           [0005]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0007]    [0007]FIG. 1 is a schematic view of a fuel cell stack having alternating anode gas inlet and outlet passages with two expansion reservoirs according to the principles of the present invention;  
         [0008]    [0008]FIG. 2 is a schematic view of a fuel cell stack having alternating anode gas inlet and outlet passages with a single expansion reservoir;  
         [0009]    [0009]FIG. 3 is a schematic view of a fuel cell stack having alternating anode gas inlet and outlet passages with a single expansion reservoir for premixing the exhaust anode gas with fresh hydrogen according to the principles of the present invention;  
         [0010]    [0010]FIG. 4 is a schematic diagram of a fuel cell stack having alternating anode gas inlet and outlet passages with recycled anode exhaust according to the principles of the present invention; and  
         [0011]    [0011]FIG. 5 is a schematic diagram of the system of FIG. 1 with a water separator added. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0013]    With reference to FIGS. 1-5, several embodiments of the present invention will now be described. A common method between the described systems is that humid fuel gas that exits the fuel cell stack is collected in a device during one cycle of the process. Then, the flow direction of the anode gas is reversed and the stored humid gas is fed back to the former gas outlet which has become the gas inlet. The other gas inlet, which, in the second cycle, becomes the outlet, is connected to another gas storage device, which is then filled with humid exhaust gas which will be stored and then delivered to the inlet during the next cycle.  
         [0014]    With reference to FIG. 1, a first embodiment of the present invention will now be described. As shown in FIG. 1, a fuel cell stack  10  is provided including a first opening  12  and second opening  14  which each serve as inlet and outlet openings for the anode gas. A hydrogen tank  16  is provided in connection with passages  18 ,  20  which are connected to respective anode gas openings  12 ,  14 . Each of the passages  18 ,  20  is provided with a venturi tube  22 ,  24  which are each provided in fluid communication with a respective expansion reservoir  26 ,  28 . A valve system is provided including a first flow control valve  30  provided in the first passage  18  and a second flow control valve  32  provided in the second passage  20 . The first passage  18  is also connected to a first exhaust passage  36 . The second passage  20  is connected to a second exhaust passage  38 . The exhaust gas in the first and second exhaust passages  36 ,  38  is handled by an exhaust system which can, for example, provide a constant gas stream to the environment or to the cathode, or may provide a pulsed gas stream to the environment or to the cathode which is controlled by the system.  
         [0015]    During a first cycle of operation, the first flow control valve  30  is in an open condition and the second flow control valve  32  is in a closed position so that hydrogen from hydrogen tank  16  flows through the first passage  18  to supply hydrogen to the fuel cell stack  10  via the first opening  12 . The hydrogen gas passes through the fuel cell stack  10  and is exhausted through the second opening  14  where it is directed to the second passage  20 . As the exhaust anode gas passes by the venturi tube  24 , the expansion reservoir  28  is provided with humid exhaust anode gas via the venturi tube  24 .  
         [0016]    During a second cycle of operation, the flow control system is switched so that the flow control valve  32  is opened and the flow control valve  30  is closed to cause hydrogen flow from hydrogen tank  16  through second passage  20  and into the fuel cell stack  10  via the second opening  14  which now serves as the fuel cell inlet  14 . As the hydrogen flows past the venturi tube  24 , the stored humid exhaust anode gas within the expansion reservoir  28  is sucked into and mixed with the fresh hydrogen gas which is delivered to the fuel cell stack  10 . As the anode exhaust now exits through the first opening  12 , serving as the outlet, humid exhaust anode gas is extracted from the passage  18  by the venturi tube  22  and stored in the first expansion reservoir  26 . The flow control system is then switched again so that the first flow control valve  30  is opened and the second flow control valve  32  is closed to cause hydrogen from the hydrogen tank  16  to flow through the passage  18  into the first opening  12  which now is serving as the fuel cell stack  10  inlet. The stored humid anode exhaust gas is then reintroduced into the fresh hydrogen provided in the first flow passage  18 .  
         [0017]    This two-cycle system is continued so that humid exhaust gas is continually reintroduced into the inlet of the fuel cell stack  10  while the inlet is alternated between the first and second openings  12 ,  14 . By alternating the inlets and reintroducing humid exhaust anode gas, the present invention provides higher humidity homogeneity in the fuel cell stack which has a positive impact on the cell performance and durability.  
         [0018]    Liquid water may occur within the anode gas stream. Too much liquid water could have a negative impact on the operation of the fuel cell system. To drain the liquid water, a water separator  35  could be integrated into the anode system, as illustrated in FIG. 5. It should be understood that each of the embodiments of the present invention may employ a water separator as shown.  
         [0019]    With reference to FIG. 2, an alternative arrangement of the fuel cell stack humidification system will now be described wherein common reference numerals are utilized to identify common elements as described with reference to FIG. 1. The fuel cell stack  10  is again provided with first and second openings  12 ,  14 , which each serve as inlet and outlet openings for the anode gas of the fuel cell stack  10 . The hydrogen tank  16  is provided in fluid communication with first and second passages  18 ,  20  which are in communication with the first and second openings  12 ,  14 , respectively. A flow control system is provided including a first flow control valve  30  disposed in the first passage  18  and a second flow control valve  32  disposed in the second passage  20 .  
         [0020]    The embodiment of FIG. 2 differs from the embodiment of FIG. 1 in that a single expansion reservoir  40  is provided in communication with first and second venturi tubes  42 ,  44  provided in the first passage  18  and second passage  20 , respectively. The expansion reservoir  40  has two chambers  40 A,  40 B which each communicate with a respective venturi tube  42 ,  44 .  
         [0021]    During operation, the fuel cell system is operated in a first cycle by opening the first flow control valve  30  and closing a second flow control valve  32  so that hydrogen flows from hydrogen tank  16  to passage  18  into the first opening  12  which serves as an inlet during the first cycle. Humid exhaust anode gas exits the fuel cell stack  10  through the opening  14  which is functioning as the outlet and through passage  38 . The humid anode exhaust gas passes through venturi  44  which siphons off humid exhaust gas to fill chamber  40 B of the single expansion reservoir  40 . During a second cycle, the flow control valve  30  is closed and the flow control valve  32  is opened so that hydrogen is supplied from hydrogen tank  16  through passage  20  to opening  14  which now serves as the fuel cell stack  10  inlet port. The humid exhaust gas stored in storage chamber  40 B of expansion reservoir  40  is reintroduced into the fresh hydrogen supply passing through passage  20  and into the fuel cell stack  10 . Humid anode exhaust gas exits the fuel cell stack  10  through opening  12  which now serves as the outlet in connection with passage  36  and some of the exhaust anode gas is diverted by the venturi tube  42  into the first chamber  40 A of the expansion reservoir  40 . The stored humid anode exhaust gas in the chamber  40 A is later used when the opening  12  is serving as the fuel cell stack anode gas inlet.  
         [0022]    With reference to FIG. 3, a fuel cell stack  10  is provided with a first opening  12  and a second opening  14  which each serve as an inlet and an outlet for the anode gas provided to the fuel cell stack  10 . A hydrogen tank  16  provides hydrogen to the fuel cell stack  10 . A first passage  50  and a second passage  52  are connected to the hydrogen tank  16  and in connection with an expansion reservoir  40 . The expansion reservoir  40  includes a first chamber  40 A and a second chamber  40 B which are in fluid communication with the first passage  50  and second passage  52 , respectively. In this embodiment, the exhaust gas and the fresh hydrogen is first mixed in the expansion reservoir  40  before the mixture enters the fuel cell stack  10 . A venturi tube  54  is in communication with the first passage  50  for providing mixed exhaust gas and hydrogen to the first opening  12  and a second venturi tube  56  is provided in communication with the second passage  52  for providing mixed exhaust gas and hydrogen to the second opening  14  of the fuel cell stack  10 .  
         [0023]    During operation, the first chamber  40 A of the expansion reservoir  40  is filled with mixed exhaust gas and hydrogen while fresh hydrogen is supplied through passage  52  through the open flow control valve  32  while flow control valve  30  is in a closed position. The introduction of fresh hydrogen into chamber  40 B of expansion reservoir  40  forces the piston  46  of the expansion reservoir  40  to move in the direction of arrow A forcing the stored mixture of hydrogen and exhaust gas through opening  12  which now serves as the inlet of the fuel cell stack  10 . The passage of hydrogen through passage  52  causes the exhaust gas exiting the opening  14  which is serving as the outlet port in connection with passage  38  to be mixed with the fresh hydrogen through venturi tube  56  which is then supplied to chamber  40 B of expansion reservoir  40 . The inlet and outlet are then reversed by closing flow control valve  32  and opening flow control valve  30  which directs the hydrogen from hydrogen tank  16  through passage  50  and into chamber  40 A of expansion reservoir  40 . The introduction of hydrogen into the expansion chamber  40 A causes piston  46  to move downward opposite the direction of arrow A forcing stored hydrogen and humid exhaust gas out of chamber  40 B and into the opening  14  which is now serving as the fuel cell stack inlet. As the hydrogen gas goes through passage  50 , it is mixed with exhaust gas via the venturi tube  54  and then enters chamber  40 A of expansion reservoir  40 . This cycle is repeated continuously in order to provide higher humidity and homogeneity in the fuel cell stack  10 .  
         [0024]    With reference to FIG. 4, the fuel cell stack  10  is provided with an opening  12  and an opening  14  which are each capable of serving as an inlet and an outlet for the anode gas of the fuel cell stack  10 . A hydrogen tank  16  is provided for supplying hydrogen to the fuel cell stack anode passage. A first passage  18  is connected between the hydrogen tank  16  and first opening  12 , and a second passage  20  is provided between the hydrogen tank  16  and second opening  14  of the fuel cell stack  10 . A first flow control valve  30  is provided in the first passage  18  and a second flow control valve  32  is provided in the second passage  20 . A venturi tube  60 ,  62  is provided in each of the openings  12 ,  14 . A recycle passage  64  is in communication with the venturi tubes  60 ,  62 .  
         [0025]    During operation, hydrogen is provided from hydrogen tank  16  through the first opening  12  of the fuel cell stack  10  which is serving as the anode gas inlet. The flow control valve  30  is in an open position and the flow control valve  32  is in a closed position so that hydrogen flows through passage  18 . The opening  14  is serving as the outlet along with passage  38  for the humid exhaust anode gas of the fuel cell stack  10 . As the humid exhaust gas passes through venturi tube  62  provided in the second opening  14 , the humid exhaust gas is drawn through the recycle passage  64  to the first venturi tube  60  provided in the inlet  12  of the fuel cell stack  10 . The humid exhaust anode gas is mixed with fresh hydrogen from passage  18  and is delivered to the fuel cell stack  10 . In a second step of the cycle, the flow control valve  30  is closed and the flow control valve  32  is opened to supply hydrogen through passage  20  to opening  14  which now serves as the fuel cell stack inlet. The opening  12  then serves as the fuel cell stack anode gas outlet and exhaust gas passing through venturi tube  60  is drawn through recycle passage  64  to the inlet port  14  for mixing the humid exhaust anode gas with fresh hydrogen.  
         [0026]    The systems disclosed provide higher humidity homogeneity in the fuel cell, thus having a positive impact on fuel cell performance and durability. The pressure in the hydrogen tank  16  is used as an energy source so no additional electrical power for hydrogen recirculation and no recirculation pump is needed. The result is a higher system efficiency and lower cost at the same time.  
         [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.