Patent Publication Number: US-2006010866-A1

Title: Pressurized near-isothermal fuel cell - gas turbine hybrid system

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a hybrid system combining a gas turbine (GT) or a micro-turbine (MT) with a near-isothermal high-temperature fuel cell, for example a solid oxide fuel cell (SOFC), to produce electrical power.  
      Though very efficient power producers, fuel cells still generate much by-product heat that needs to be removed to avoid overheating the fuel cell. High-temperature fuel cells, such as the solid oxide fuel cell (SOFC), systems are normally designed so that the by-product heat is removed with airflow through the fuel cell. The air also serves as the reactant in the fuel cell cathode. Usually, the cooling requirement imposed on the airflow results in a much higher airflow rate than that required for the fuel cell reaction due to the poor heat transfer characteristics of air and, equally importantly, the inability of the SOFC stack to withstand a large thermal gradient or temperature rise from stack inlet to stack exhaust due to thermal stresses. The presence of large temperature gradients may be detrimental to both structural integrity and reliability of the stack. If the temperature rise is too large, differential thermal expansion of various stack components (cell, interconnect, seals, etc.) can lead to cell fracture, loss of sealing, or loss of contact between stack components, thereby leading to stack failure. In the absence of stack failure, stack service life is compromised due to the fact that cell component degradation is strongly temperature dependent. Cell degradation is much faster in the high temperature region (typically near the exhaust) than in the low temperature region (typically near the inlet), thereby over time leading to reduced stack power or system efficiency, or both. Thus, only part of the airflow through the fuel cell is used for reaction purposes with the rest of the airflow serving the stack cooling purpose. The power required for circulating this additional cooling airflow lowers the overall system efficiency.  
      Additionally, because the SOFC stack cannot withstand large temperature gradients, it is necessary to preheat the air to a temperature nearly equal to the stack temperature before it enters the stack. This heat transfer process is also inefficient, resulting in some loss of system efficiency, and is also complicated and expensive due to the need to employ high temperature materials consistent with the high operating temperatures of SOFC stacks. These problems can be solved if a more efficient fuel cell cooling method is devised.  
      In state-of-the-art systems, the task of preheating air to the fuel cell operating temperature is accomplished utilizing either the heat of compression in high-pressure systems (see, e.g., U.S. Pat. No. 5,482,791) or the gas turbine by-product heat transferred to the cathode air via a high-temperature heat exchanger (see, e.g., U.S. Pat. No. 5,413,879). The former method suffers from reduced system efficiency at low pressure, while the latter employs an unreliable component, the high-temperature heat exchanger, which is subject to high thermal stresses and high material oxidation rates due to its exposure to high temperature.  
     BRIEF DESCRIPTION OF THE INVENTION  
      In an exemplary embodiment of the invention, a system for generating power includes a turbine system including an air compressor and a turbine having an inlet and an outlet; and a fuel cell including a plurality of power-producing electrode-electrolyte assemblies and heat-conducting elements. The air compressor supplies cathode air to the fuel cell, and the cathode air is predominately heated inside the fuel cell by fuel cell by-product heat via the heat-conducting elements.  
      In another exemplary embodiment of the invention, a method of generating power utilizing the system of the invention includes the steps of supplying cathode air to the fuel cell via the air compressor; and heating the cathode air inside the fuel cell by fuel cell by-product heat via the heat-conducting elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic process diagram of a hybrid fuel cell-gas turbine system;  
       FIG. 2  is a flow diagram illustrating a flow process of the system;  
       FIG. 3  is a graphic showing the impact of air temperature rise in the stack on system efficiency;  
       FIGS. 4 and 5  show fuel cell interconnects containing heat-conducting elements; and  
       FIGS. 6 and 7  show top views of fuel cell interconnects. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The system  10  will be described with reference to  FIG. 1 . Generally, the hybrid system  10  includes a turbine component  12  and a fuel cell component. The fuel cell component includes a fuel cell  14  having a plurality of power-producing electrode-electrolyte assemblies, flow distribution assemblies, and heat-conducting elements  18 , such as heat pipes, which may or may not be connected to the flow distribution assemblies. As an alternative to heat pipes, high thermal conductance members may be used. The heat-conducting elements  18  have a high thermal conductance, which allows for an efficient transfer of fuel cell by-product heat to incoming reactants. The high thermal conductivity of the elements  18  allows for very small temperature gradients in the fuel cell, thus making the fuel cell nearly isothermal. In addition, the heat-conducting elements are typically good electrical current conductors and may serve as the fuel cell&#39;s interconnects that serve the purpose of transferring current from one cell to the next.  
      The fuel cell  14  has fuel (anode) and air (cathode) chambers that provide the reactants required for the fuel cell reaction. While the fuel cell is nearly isothermal due to the heat conduction elements  18 , the waste heat must still be removed from the stack to prevent the stack from overheating and attaining a temperature higher than desired. The byproduct heat of the fuel cell  14  necessitates the use of excess cathode air for temperature control and cooling purposes, but not for the purpose of minimizing temperature gradients, as the heat conducting elements accomplish this purpose. In order to maintain the fuel cell operating temperature, the air used in the fuel cell  14  cathode absorbs byproduct heat and is heated to a temperature just below the fuel cell operating temperature. Because the cathode air is used for reaction purpose and heat removal purpose, but not thermal gradient control purposes as in conventional systems, lower air flows and temperatures are possible, thereby increasing system efficiency, as shown in  FIG. 3 . Because the cell is held nearly isothermal by the heat conducting elements  18 , cooler air can be introduced into the fuel cell without damaging the cells for heat removal purposes than can be used in conventional systems. The fuel cell by-product heat is then conducted via the heat-conducting elements and other stack components to directly heat the fuel cell cathode air. The solution herein heats the air directly utilizing the fuel cell by-product heat and thus eliminates the need for a high-temperature heat exchanger while operating the system at a reasonably low pressure to achieve high system efficiency.  
      In a preferred embodiment, a GT compressor  24  of the turbine component  12  supplies the fuel cell with air. An external fuel processor or reformer  26  partially or fully converts fuel to a hydrogen-containing gas (fuel conversion in the external fuel processor can range from 0% to 100%) before feeding it to the fuel cell  14 . The preferred embodiment of the fuel processor  26  is a steam reformer. The remaining fuel may be processed in the fuel cell  14  to produce more hydrogen-containing gas. The fuel cell  14  produces electrical power from the GT air and the converted fuel. All or part of the fuel cell by-product heat is conducted to the inlet airflow thus heating it to nearly the fuel cell operating temperature and removing byproduct heat from the system.  
      A schematic of a fuel cell interconnects containing heat-conducting elements is shown in  FIGS. 4-7 . In  FIG. 4 , a cross sectional view of a fuel cell interconnect  50 , often called a bipolar plate, is shown. The anode flow field is shown at the top surface of the interconnect  50  and serves the purpose of directing anode gas to the adjacent cell. The cathode flow field is shown at the bottom surface of the interconnect  50  and serves the purpose of directing cathode gas to the adjacent cell. In the core of the plate  50  are the heat-conducting elements  18 . Alternatively, the heat conducting elements  18  can be located in the cathode flow field as shown in  FIG. 5  (or less preferentially in the anode flow field). The top surface of the interconnect interfaces to the anode side of a cell. The cell and interconnect  50  comprise a repeat unit within the stack. The bottom face of the interconnect  50  interfaces to the cathode side of an adjacent cell.  
      Shown in  FIGS. 6 and 7  are top views of a fuel cell interconnect  50  containing heat-conducting elements  18 . The interconnect  50  is shown in two configurations, whereby the heat-conducting elements either begin and end within the active area of the fuel cell ( FIG. 6 ), or alternatively, begin in the active area of the fuel cell and end in the air inlet manifold ( FIG. 7 ). Heat generated within the anode and cathode of the cell during electrochemical operation is conducted through the interconnect to the heat conducting elements, and is transferred in the plane of the interconnect, thereby minimizing temperature gradients within the cell and interconnect while simultaneously transferring heat to the cathode gas (the air).  
      In the case where the heat conducting elements  18  are heat pipes, their condenser sections are located adjacent the air inlet manifold to enable heat transfer from the heat pipes to the relatively cold inlet air, while the evaporator sections absorb the fuel cell byproduct heat and conduct it to the condenser sections. While the condenser section is located in proximity to the air inlet manifold, it may or may not extend all the way into the manifold as shown in  FIGS. 6-7 . In the case where the heat conducting elements are high conductance members, the cross sectional area and thermal conductivity of the members are chosen and arranged within the stack so as to transfer heat from the hot regions of the fuel cell to the cold regions of the fuel cell by thermal conduction.  
      As would be apparent to those of ordinary skill in the art, the heat conducting elements are not necessary in each interconnect. Rather, for example, the heat conducting elements may be placed in alternate ones of the interconnects (every 3rd or 5th), or another combination.  
      The turbine component  12  also includes a GT turbine  28  which together with the compressor  24  generates AC power via a known generator  25  and inverter  27 . Any remaining waste fuel cell heat may be transported to other parts of the system to improve system efficiency.  
      The system supplies air and fuel to the fuel cell  14  at pre-determined flow rates and appropriate pressure and temperature. With continued reference to  FIG. 1  and with reference to  FIG. 2 , the GT compressor  24  supplies cathode air to the fuel cell  14  (step S 1 ). Fuel (such as natural gas) is supplied by a fuel compressor  40  via a fuel clean up system  41 , which removes constituents from the fuel that may harm the fuel reformer or fuel cell (for example, sulfur containing compounds), to the fuel processor  26  (a.k.a. the reformer) that uses steam reforming, auto-thermal reforming, partial-oxidation, or other known processes to convert the fuel into a gas containing hydrogen (step S 2 ). The cell is held nearly isothermal by the heat conducting elements. The air is heated up to the fuel cell operating temperature inside the fuel cell  14  using the fuel cell by-product heat transferred to the inlet air by the heat-conducting elements  18  and other components of the fuel cell (step S 3 ). The air temperature rise from fuel cell  14  inlet to exhaust is preferably greater than 25° C., more preferably between about 25-500° C., and most preferably about 100-400° C.  
      The reformed fuel stream is supplied to the fuel cell  14 , where it is electrochemically reacted with oxygen in the supplied air to produce electrical power (step S 4 ) via an inverter  27 ′. Any unused fuel is oxidized in a tail gas combustor  32  downstream of the fuel cell  14 , and the exhaust stream exchanges heat with the fuel processor  26  (step S 5 ). The tail gas combustor  32  exhaust, after being directed to the fuel processor and exchanging heat with the fuel processor, is exhausted from the fuel processor  26  and expands in the GT turbine  28  to produce more power (step S 6 ).  
      Any residual by-product heat produced during the fuel cell electrochemical reaction is transferred to the incoming reactants, such as air, inside a low temperature recuperator  38  or is used to produce steam in the steam generator  44  for the fuel processor  26  (step S 7 ). Water is extracted in the condenser  48  and stored in a water tank  49  for the system exhaust and is delivered to the steam generator  44  via a water pump  51  (step S 8 ).  
      An advantage of transferring the by-product heat directly to the incoming air within the stack is the elimination of the need to pre-heat the air with other means, such as high-temperature heat exchangers, that historically have been shown to be unreliable. Analyses have shown that the steady-state system efficiency of this concept may be between about 60 and 68%.  
      The system utilizes exhaust heat from separate power generating components, resulting in a high-temperature fuel cell-GT hybrid system design with a near-isothermal fuel cell design allowing increased overall system efficiency.  
      While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.  
      Other such embodiments might include introducing fuel into the fuel cell that is colder than that introduced into conventional systems as an alternative to, or in combination with, the introduction of air colder than that allowed by conventional systems. The inventions described are applicable to SOFC, MCFC, and phosphoric acid fuel cells.