Abstract:
An example fuel cell arrangement includes a fuel cell stack configured to receive a supply fluid and to provide an exhaust fluid that has more thermal energy than the supply fluid. The arrangement also includes an ejector and a heat exchanger. The ejector is configured to direct at least some of the exhaust fluid into the supply fluid. The heat exchanger is configured to increase thermal energy in the supply fluid using at least some of the exhaust fluid that was not directed into the supply fluid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase of PCT/US2011/020008 filed Jan. 3, 2011. 
     
    
       [0002]    This invention was made with government support under Contract No. DE-NT0003894 awarded by the United States Department of Energy. The Government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0003]    This disclosure relates generally to a fuel cell arrangement and, more particularly, to recycling thermal energy generated by a fuel cell stack of the fuel cell arrangement. 
         [0004]    Fuel cell stacks are well known. One type of fuel cell stack includes a plurality of individual solid oxide fuel cells. Each of the solid oxide fuel cells includes a tri-layer cell having an electrolyte layer positioned between a cathode electrode and an anode electrode. 
         [0005]    Fuel cell stacks can generate significant thermal energy. Retaining significant thermal energy in the fuel cell stack is undesirable as is known. Some fuel cell stacks rely on a fluid, such as air, to remove thermal energy from the stack. The fluid moves through the fuel cell stack and carries thermal energy away from the stack. 
         [0006]    Although the fluid is intended to cool the fuel cell stack, circulating fluid that is too cool can negatively affect the efficiency of the fuel cell stack. Accordingly, fluid that is intended to cool the fuel cell stack is typically preheated before the fluid is introduced to the fuel cell stack. A heat exchanger is often used to preheat the fluid. Heat exchangers include expensive materials and are costly to manufacture. 
       SUMMARY 
       [0007]    An example fuel cell arrangement includes a fuel cell stack configured to receive a supply fluid and to provide an exhaust fluid that has more thermal energy than the supply fluid. The arrangement also includes an ejector and a heat exchanger. The ejector is configured to direct at least some of the exhaust fluid into the supply fluid. The heat exchanger is configured to increase thermal energy in the supply fluid using at least some of the exhaust fluid that was not directed into the supply fluid. 
         [0008]    An example fuel cell arrangement includes a fuel cell stack having a multiple of solid oxide fuel cells. The fuel cell stack is configured to receive a supply fluid and to exhaust an exhaust fluid. Some of the exhaust fluid is used to heat the supply fluid within a heat exchanger. The remaining exhaust fluid is included in the supply fluid entering the fuel cell stack. 
         [0009]    An example thermal energy recycling method includes moving a supply fluid at a first temperature into a fuel cell stack and moving an exhaust fluid at a second temperature away from the fuel cell stack. The second temperature is greater than the first temperature. The method transfers thermal energy from some of the exhaust fluid to the supply fluid within a heat exchanger. The method adds some of the exhaust fluid to the supply fluid after moving the supply fluid through the heat exchanger. 
         [0010]    These and other features of the disclosed examples can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]      FIG. 1  shows a highly schematic view of an example fuel cell arrangement. 
           [0012]      FIG. 2  shows a detailed schematic view of another example fuel cell arrangement. 
           [0013]      FIG. 3  shows more detailed view of the  FIG. 2  fuel cell stack. 
           [0014]      FIG. 4  shows an example fuel cell repeater unit used in the  FIG. 3  fuel cell arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An example fuel cell arrangement  10  includes a fuel cell stack assembly  14 , a heat exchanger  18 , and an ejector  22 . A supply fluid communicates along a fluid communication path  26  from a fluid supply  30 , through the heat exchanger  18 , to the fuel cell stack assembly  14 . An exhaust fluid communicates an exhaust fluid away from the fuel cell stack assembly  14  to the ejector  22  along a fluid communication path  34 . 
         [0016]    The supply fluid is air in this example. The supply fluid absorbs thermal energy within the fuel cell stack  14 . In this example, the supply fluid exits the fuel cell stack  14  along the path  34  as the exhaust fluid. The exhaust fluid is supply fluid that has been heated by the fuel cell stack  14 . Carrying thermal energy away from the fuel cell stack  14  in the exhaust fluid cools the fuel cell stack  14 . 
         [0017]    The heat exchanger  18  helps regulate the temperature of the supply fluid entering the fuel cell stack  14 . In this example, the heat exchanger  18  heats the supply fluid. The supply fluid is then communicated to the ejector  22 . 
         [0018]    At the ejector  22 , some of the exhaust fluid is combined with the supply fluid. A recycling path  38  represents the introduction of some of the exhaust fluid to the supply fluid. 
         [0019]    Introducing some exhaust fluid to the supply fluid increases the amount of thermal energy in the supply fluid. Accordingly, the heat exchanger  18  is not exclusively relied on to preheat the supply fluid. For example, if 20% of the supply fluid is provided by redirecting the exhaust fluid through the ejector, then the heat exchanger must only heat up 80% of the supply fluid. Further, if a desired temperature of the supply fluid entering the fuel cell stack  14  is 700 degrees Celsius, and desired exhaust fluid exiting the fuel cell stack  14  is 825 degrees Celsius, then the heat exchanger need only heat its supply fluid to 669 degrees Celsius, which when mixed with the redirected exhaust will be the desired inlet temperature of 700 degrees (this example makes the simplifying assumption that the two streams are relatively similar in heat capacity). 
         [0020]    Referring to  FIG. 2 , a fuel cell arrangement  10   a  includes a fuel cell stack  14   a  having a plurality of cathodes  42  and a plurality of anodes  46 . A cathode blower  50 , such as a fan, moves fluid along a path  26   a  to the cathodes  42 . The path  26   a  communicates the fluid through a heat exchanger  18   a.    
         [0021]    Additional fluid is introduced to the path  26   a  at an ejector  22 . The fluid moving along the path  26   a  entrains the additional fluid from fluid moving along an exhaust path  34 . The entrained fluid moves along a recycling path  38   a  to the supply path  26   a.    
         [0022]    In one example, 20% of the flow in the exhaust path  34   a  at the fuel cell stack  14   a  is introduced to the supply path  26   a  at the ejector  22   a.  In such the example, 20% of the fluid flowing from the ejector  22   a  to the cathodes  42  is recycled fluid that has already passed through the cathodes  42 . The remaining fluid in the exhaust path  34   a  communicates from near the ejector  22   a  through the heat exchanger  18   a.    
         [0023]    Thermal energy is transferred from the fluid in the exhaust path  34   a  to the fluid in the supply path  26   a  within the heat exchanger  18   a.  In this example, the heat exchanger  18   a  includes a substantial amount of nickel. Notably, the size of the heat exchanger  18   a  is smaller that in the prior art because the heating requirement is not as great as in the prior art. 
         [0024]    This arrangement makes the heat exchanger smaller in three ways. First, less fluid needing thermal energy transfer flows through the heat exchanger because some is provided through the ejector. Second, the fluid in the heat exchanger need only be raised to a lesser temperature, in that it will be mixed with a hotter fluid to obtain the desired temperature. Third, the difference in temperature between the two fluids within the heat exchanger is greater (the colder fluid having been heated to a lesser value), which enables the heat exchanger to transfer heat more proficiently. 
         [0025]    After moving through the heat exchanger  18   a,  additional thermal energy in the fluid moving along the exhaust path  34   a  may be used as customer heat at  54 , or expelled into the surrounding environment at  58 . 
         [0026]    In this example, the ejector  22   a  is downstream from the heat exchanger  18   a  relative to flow along the supply path  26   a.  Other examples may include other arrangements of the ejector  22   a  relative to the heat exchanger  18   a.    
         [0027]    The example ejector  22   a  utilizes the flow of fluid along the supply path  26   a  to withdraw fluid from the exhaust path  34   a.  The blower  50  provides the head for the ejector  22   a  to withdraw the fluid from exhaust path  34   a.  The ejector  22   a  is subsonic in this example, which facilitates lowering the head requirement on the blower  50 . 
         [0028]    Although described as an ejector, a person having skill in this art and the benefit of this disclosure would understand that other devices may be suitable for removing fluid from the exhaust path  34   a  for introduction to the supply path  26   a.    
         [0029]    Referring to  FIGS. 3 and 4  with continued reference to  FIG. 2 , the example fuel cell stack assembly  14   a  includes a solid oxide fuel cell (SOFC)  62  positioned between a SOFC  62   a  and a SOFC  62   b.  A first metal plate  66  and a second metal plate  70  are secured at opposing ends of the fuel cell stack assembly  14   a.  Electrons travel from the SOFC  62   a,  to the SOFC  62 , to the SOFC  62   b,  and to the second metal plate  66 , which provides electric power from the SOFC  62  along path  74  in a known manner. The SOFC  62  is also referred to as the fuel cell stack repeater unit in some examples. 
         [0030]    The example SOFC  62  includes a tri-layer cell  78 . This example includes an electrolyte layer  82  positioned between a cathode electrode layer  86  and an anode electrode layer  90 . The fluid cools the fuel cell stack  14   a  by carrying thermal energy from the cathodes  42  of the SOFC  62 , the SOFC  62   a,  and the SOFC  62   b.    
         [0031]    Features of the disclosed examples include increasing thermal energy in a supply fluid by introducing a heated fluid exhausted from a fuel cell stack. Introducing the heated fluid lowers the heating requirements at the heat exchanger and reduces the airflow required to provide the supply fluid. 
         [0032]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.