Abstract:
An apparatus and method for improving the efficiency and energy output of a power unit are disclosed. The power unit employs a fuel cell configured to chemically convert combustible fuel into electrical energy. A waste fuel burner is configured to receive uncombusted fuel emitted by the fuel cell, and the waste burner combusts the uncombusted fuel to generate heated gas. A turbine is configured to receive and be driven by the heated gas. The turbine is further configured to drive a drive shaft which is coupled to a compressor. The compressor is configured to compress a pressurized source of oxidizing gas for supplying compressed oxidizing gas to the fuel cell such that efficiency of the fuel cell is improved. A generator also may be coupled to the drive shaft, and the generator is configured to generate electrical power from the drive shaft being turned by the turbine.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to aircraft electrical power supplies and, more specifically, to providing a more efficient source of electrical power for aircraft.  
         BACKGROUND OF THE INVENTION  
         [0002]    As is understood in the art, aircraft may use two types of auxiliary power units (APUs)—turbine-driven APUs and fuel cell APUs—to provide electricity to onboard systems. Turbine-driven APUs burn jet fuel to drive a turbine, which in turn drives a generator to produce electrical energy. Fuel cells chemically convert fuel into electrical energy. Both types of APUs may be used to power onboard systems while the aircraft is on the ground, as well as when the aircraft is in flight.  
           [0003]    As useful as both types of APUs may be, however, each has disadvantages. Turbine-driven APUs may contribute appreciable pollution to an airport environment, because the burning of fuel to drive the turbines leaves some jet fuel uncombusted and produces oxides of nitrogen (NO X ). Fuel cell APUs, on the other hand, offer lower emissions than turbine-driven APUs, but tend not to be efficient at high altitudes. In turbine-powered systems and fuel cels, attempts are made to exploit waste heat generated from the burning of combusted fuel by turbine-driven APUs or exothermically produced by fuel cell APUs to heat buildings or water for ground facilities.  
           [0004]    In both cases, aircraft or other systems could benefit from improved APUs. More efficient fuel cell APUs, particularly at higher altitudes, would reduce the demand for other sources of electrical power.  
           [0005]    Thus, there is an unmet need in the art for an improved APU and power generation method to cost-effectively provide sufficient electrical power while generating less pollution and consuming less fuel than current systems.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides an apparatus and a method for improving the efficiency and energy output of a power unit. The present invention improves efficiency of a chemical process for converting combustible fuel into electrical energy by providing a supply of compressed air to the process. Combustible fuel not consumed by the chemical process is collected and burned to drive a turbine which, in turn, drives a compressor to supply the compressed air. In addition, an electrical generator can be coupled to the turbine to produce additional electrical energy. A starter motor also may be coupled with the compressor to compress air during a starting process.  
           [0007]    An exemplary embodiment of the present invention is a power unit in which a fuel cell is configured to chemically convert combustible fuel into electrical energy. A waste fuel burner is configured to receive uncombusted fuel emitted by the fuel cell, and the waste burner combusts the uncombusted fuel to generate heated gas. A turbine is configured to receive and be driven by the heated gas. The turbine is further configured to drive a drive shaft which is coupled to a compressor. The compressor is configured to compress a pressurized source of oxidizing gas for supplying compressed oxidizing gas to the fuel cell such that efficiency of the fuel cell is improved.  
           [0008]    The present invention may also incorporate a fuel reformer which can extract reactants from waste heat produced by the fuel cell. The present invention also may supply water to the reformer to facilitate the extraction of reactants. If desired, the water may be supplied by a water separator which extracts water from the heated gas. Water not used by the reformer may provide a water source for other purposes.  
           [0009]    A form of the present invention may be used as an aircraft auxiliary power unit. The power unit suitably compresses air to provide a pressurized input gas to the fuel cell to enhance production of electrical power. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0011]    [0011]FIG. 1 is a block diagram of a hybrid solid oxide fuel cell auxiliary power unit using an embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a flowchart of a method using an embodiment of the present invention to produce auxiliary power;  
         [0013]    [0013]FIG. 3 is graph plotting increasing power performance versus atmospheric pressure attending a fuel cell; and  
         [0014]    [0014]FIG. 4 is a graph plotting total mass and effective total mass versus cruise time for auxiliary power units. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 is a block diagram of one exemplary embodiment of a hybrid fuel cell/turbine driven auxiliary power unit (APU)  100  of the present invention. A fuel cell  104  has an anode portion  108  and a cathode portion  112  which together provide direct current (DC) power. The DC power provided can, in turn, be supplied to an inverter  116  to provide alternating current (AC) power to electrical systems  118 . Thus, both DC and AC power are provided by an embodiment of the present invention. In one presently preferred embodiment, the fuel cell is a solid oxide fuel cell (SOFC). Although the exemplary embodiment herein described refers to an aircraft application, embodiments of the present invention also could be used in other transportation systems, such as ships, tanks, military vehicles, trains, and others. Embodiments of the present invention also could be used for ground-based power generating applications.  
         [0016]    The fuel cell  104  receives a supply of jet fuel  120 . The jet fuel supply  120  is routed through a heat exchanger  124  and a reformer  128 . The heat exchanger  124  receives hot exhaust  132  generated by the anode portion  108  of the fuel cell  104  or the fuel cell stack (not shown) itself, and that heat is applied to the jet fuel supply  120  to increase the temperature of the jet fuel supply  120  to expedite the chemical processes of the reformer  128 . The reformer  128  reforms the now-heated, albeit heated below a coking point, jet fuel supply  120  into hydrogen and carbon monoxide to be used by the fuel cell  104 . The reformer may be a steam reformer, a catalytic partial oxidation reformer, an autothermal reformer, or another similar type of reforming apparatus which can convert fuel into the hydrogen and carbon monoxide reactants used by the fuel cell  104 .  
         [0017]    The fuel cell  104  also receives a supply of input gas  136  which, in embodiments of the present invention, is a pressurized supply. In one presently preferred embodiment, a compressor  140 , driven by a drive shaft  144  through a process which will be further described below, compresses the input gas supply to provide a denser and more component-rich input gas supply to the fuel cell  104 . As previously mentioned, currently conceived fuel cells are inefficient at high altitudes. This inefficiency owes in part to low atmospheric pressure. Pressurizing the supply of input gas  136  improves efficiency of the chemical reactions in the fuel cell  104 .  
         [0018]    In the embodiment shown in FIG. 1, a raw gas input supply suitably can be aircraft cabin air  148 . Using aircraft cabin air  148  that is already pressurized increases, for example, the pressure of input gas to the fuel cell from a pressure of about 3.5 pounds per square inch (PSI) at a typical commercial airline cruising altitude of about 35,000 feet to about 10.9 PSI or more, that is a typical cabin pressure. Moreover, further compressing the gas using the compressor  140  can compress the aircraft cabin air  148  around another 2.5 to 5.0 times, further expediting the chemical processes of the fuel cell  104 .  
         [0019]    Using aircraft cabin air  148  has another advantage in that it is relatively rich in water vapor, and water is used in the chemical conversion made by the reformer  128 . By advantageously using the aircraft cabin air  148  which is richer in water vapor than high-altitude atmospheric air and compressing it further, the dense, rich input gas supply  136  makes the fuel cell  104  even more efficient.  
         [0020]    As shown in FIG. 1, the compressed input gas supply  136  is also passed through a heat exchanger  152  which is warmed by hot exhaust  156  produced by the cathode portion  112  of the fuel cell  104  or can be warmed by the heat lost from the fuel cell stack (not shown) itself. Increasing the temperature of the input gas supply  136 , as will be appreciated by one ordinarily skilled in the art, further enhances the chemical processes performed by the fuel cell  104  and reduces thermal gradients to the fuel cell stack (not shown).  
         [0021]    As shown in FIG. 1, the exhaust  132  generated by the anode portion  108  of the fuel cell  104  and the exhaust  156  produced by the cathode portion  112  of the fuel cell are collected and combusted in a burner  160 . The exhaust  132  of the anode portion  108  contains uncombusted jet fuel which was not converted by the chemical reactions within the fuel cell  104 . In one presently preferred embodiment of the present invention, capturing and combusting this uncombusted jet fuel in the burner  160  results in a pressurized heated gas stream  164  which can forcibly drive a turbine  168 . The turbine  168  is coupled to the drive shaft  144 , driving that drive shaft  144  so that it can impart the mechanically converted energy for other uses. One such use, as previously described, is turning the compressor  140  to pressurize the input gas supply  136  to the fuel cell  104 .  
         [0022]    In a presently preferred embodiment of the present invention, another use of the mechanical energy produced by the turbine  168  and imparted to the drive shaft  144  is the turning of a starter/generator  172  which is also coupled to the drive shaft  144 . As part of its generator function, the starter generator  172  converts the mechanical energy manifested in rotation of the drive shaft into electric power, providing another source of electricity for aircraft systems. As shown in FIG. 1, all of the DC output of the fuel cell  104  is shown as being supplied to the inverter  116  to provide AC current to the aircraft systems  118 , and the inherently AC output of the starter/generator  172  is also shown as being provided directly to aircraft systems  118 . However, as will be appreciated by one ordinarily skilled in the art, electric power output by the fuel cell  104  and/or the starter generator  172  can be divided, directed, and/or converted as needed to provide quantities of AC and DC electric power as desired for various aircraft systems.  
         [0023]    Another function of the starter/generator  172  in a presently preferred embodiment of the invention is as a starter. When the burner  168  is not burning collected uncombusted jet fuel, there is no heated gas stream  164  to drive the turbine  168  to drive the drive shaft  144  to turn the compressor  140 . Because a pressurized input gas supply  136  will enhance functioning of the fuel cell  104 , the starter/generator  172  can be supplied with a source of electricity (not shown) to drive the drive shaft  144  to which it is coupled and, thus, turn the compressor  140  to create a pressurized input gas supply  136 .  
         [0024]    Another aspect of a presently preferred embodiment of the present invention is a water separator  180 . The water separator  180  receives a turbine exhaust flow  184  which will be moisture rich from the burning of the jet fuel in the burner  160 . The water separator  180  extracts a supply of collected water  188  from the turbine exhaust flow  184 . The collected water supply  188  extracted by the water separator  180  can be used in the aircraft routed to the reformer  128  to enhance the production of reactants for the fuel cell, as will be understood by one ordinarily skilled in the art. An exhaust  196  composed of effluent from the fuel cell  104  and/or water and water vapor not extracted by the water separator  180  can be released as exhaust.  
         [0025]    [0025]FIG. 2 is a flowchart illustrating a method  200  for producing electrical power using a combination of fuel cell and turbine-driven power generation. At a block  210 , input gas to be used for the fuel cell reaction is mechanically compressed to make the input gas more dense and rich in input gas components used by the fuel cell. As previously described, gas input to this step may be an already pressurized source of gas, such as pressurized cabin air. Also as previously described, a starter/generator  172  (FIG. 1) might be used to turn a compressor  140  to compress input gas when no other source of mechanical energy is available for that process. At a block  220  (FIG. 2), combustible fuel is chemically converted into electricity, as might be performed by a fuel cell. At a block  230 , fuel not combusted in the chemical conversion step at the block  220  is then combusted to increase energy content of input gas. At a block  240 , in a presently preferred embodiment, mechanical energy produced by the combustion of uncombusted fuel also is used to mechanically produce electricity, such as by driving a starter/generator  172  (FIG. 1) as previously described. This process may repeat as desired.  
         [0026]    [0026]FIG. 3 is a graph  300  showing effects of atmospheric pressure on fuel cell performance by plotting percentage power increase on a Y-axis versus fuel cell pressure on an X-axis. As can be seen from the graph  300 , increasing pressure supplied to the fuel cell increases power output, particularly in a range from approximately one to three atmospheres. Accordingly, because an embodiment of the present invention increases fuel cell pressure by 2.5 to 5.0 times ambient pressure, embodiments of the present invention improve power production of the fuel cell.  
         [0027]    [0027]FIG. 4 is a graph  400  plotting total mass and effective total mass versus airplane cruise time. A standing weight of a standard turbine-powered APU  420  and an embodiment of the present invention  410  are plotted, along with an effective total mass of an embodiment of the present invention  440 . Advantageously, the effective total mass for the fuel cell APU decreases with cruise time because of the fuel saved and the water created as a result of the fuel cell processes. According to one embodiment of the present invention, the effective total mass  440  drops more sharply because of its greater efficiency and, at a point after about four hours of cruise time  450 , the effective total mass  440  of the embodiment of the present invention becomes less than that of the turbine-powered APU  420 . This improvement in effective total mass does not take into account the reduced pollution previously described.  
         [0028]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.