Abstract:
An auxiliary power unit (APU) for an aircraft utilizing solid oxide fuel cells for providing electrical power. The solid oxide electrolytes of the fuel cells allow for reformed fuel to provide a catalyst for oxygen migration. The auxiliary power unit, utilizing solid oxide fuel cells, can also power systems of the aircraft to produce water for use on the aircraft. Waste exhaust energy may be captured from the APU by a power recovery turbine which drives a compressor to provide aircraft cabin air under increased pressure to the fuel cell, thereby increasing system efficiency. The APU may provide all of the electricity to the aircraft allowing for more efficient aircraft engine design and a decrease in aircraft engine size. Furthermore, the fuel cell APU can reduce airport ramp noise and exhaust emissions.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an auxiliary power unit for an aircraft, and particularly relates to an auxiliary power unit utilizing fuel cells for an aircraft. 
     BACKGROUND OF THE INVENTION 
     It is generally known in the art to provide electricity for different functions of an aircraft such as environmental controls or systems and avionics. In addition, other electrical equipment or outlets on the aircraft, such as power outlets for passengers&#39; use or electrical controls for controlling the aircraft, require electricity. Most often, the electricity for these systems is provided from the engines of the aircraft. The engines of the aircraft power generators which create electricity for use in these and other subsystems on the aircraft. 
     It is also known to provide an auxiliary power unit to provide electricity for use on an aircraft. The auxiliary power unit (APU) provides electricity in addition to, or supplementary to, the power produced by the engine generators. Generally, the APUs use gas turbine engines, similar to those of the engine, although smaller. An APU turbine is, therefore, limited in efficiency and creates gaseous emissions similar to the engines. The emissions of the APU at the airport are relatively high when compared to the engines. In addition, the APU reduces the overall fuel efficiency of the aircraft by converting jet fuel to electricity through internal combustion. Turbine APUs also reduce fuel efficiency of the aircraft. 
     Turbine APUs also create high levels of noise exterior to the aircraft. This requires ground crew to use extra hearing protection even when the engines of the aircraft are not running. This is generally because the turbine APUs are run at the airport to power aircraft systems prior to take off. 
     Therefore, it would be desirable to provide an APU which does not create additional noise and discomfort to the passengers or ground crew. In addition, it is desired to provide an efficient and clean source of electricity. It is also desired to use an APU that provides nearly all of the electricity required on an aircraft. Aircraft engines may be reduced in size and increased in efficiency by removing the need for the engines to provide compressed air and electricity to the aircraft. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an auxiliary power unit (APU) for an aircraft. The auxiliary power unit, utilizing solid oxide fuel cells, creates electricity through the migration of oxygen ions. In particular, the solid oxide electrolytes of the fuel cells provide for reformed fuel to cause oxygen migration across the electrolyte. The auxiliary power unit, utilizing solid oxide fuel cells, can power systems of the aircraft and produce water for use in the aircraft. Furthermore, the auxiliary power unit can provide all of the electricity to the aircraft allowing for more efficient aircraft engine design and a decrease in aircraft engine size. 
     A first preferred embodiment of the present invention provides an aircraft comprising a fuselage, wherein the fuselage comprises a cabin defining a volume of air including oxygen, and an engine for powering the aircraft in flight. A fuel supply provides fuel to the engine. Electronic components are arranged in the aircraft to control a plurality of functions of the aircraft. A power unit, comprising a fuel cell, including a solid electrolyte provides a first source of electricity for the electronic components. The fuel supply supplies the fuel to the power unit so that the power unit produces electricity. 
     A second preferred embodiment of the present invention provides an auxiliary power unit for an aircraft. The auxiliary power unit comprises a solid oxide fuel cell including a solid oxide electrolyte. A fuel supply supplies fuel to the auxiliary power unit. A fuel reformer reforms the fuel from the fuel supply to constituent elements comprising carbon monoxide and molecular hydrogen for use in the solid oxide fuel cell. An air supply system provides an oxidizer, wherein oxidizer is oxygen to the solid oxide fuel cell. A turbine powered by the solid oxide fuel cell is adapted to provide power to the oxygen supply system. The solid oxide fuel cell is adapted to allow the transport of oxygen ions from the oxygen supply system to combine with the reformed fuel to produce electricity. 
     The present invention also provides for a method of providing power to an aircraft. The method involves providing a solid oxide fuel cell comprising a solid oxide electrolyte, providing a fuel to the solid oxide fuel cell, and providing an oxidizer to the solid oxide fuel cell. Electricity is produced by passing the oxidizer through the solid oxide electrolyte. The method also involves powering a reclamation system with thermal energy from the solid oxide fuel cell to provide the oxidizer to the solid oxide fuel cell. 
     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 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a schematic view of an auxiliary power unit according to the present invention placed in the tail section of an aircraft; 
     FIG. 2 is a diagrammatic view of an auxiliary power unit placed in an aircraft including electrical transmission lines to various aircraft portions; 
     FIG. 3 is a schematic view of a simplified auxiliary power unit including fuel stacks according to a first embodiment of the present invention; and 
     FIG. 4 is detailed schematic view of an auxiliary power unit as illustrated in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. Although the following description describes solid oxide fuel cells, it is understood that this is merely exemplary of fuel cell types which may be used as auxiliary power units in aircraft. 
     With reference to FIG. 1, an auxiliary power unit  10  for an aircraft  12  is shown. The auxiliary power unit (APU)  10  utilizes fuel cells and is generally included in a tail section  14  of the aircraft  12 . More particularly, the APU  10  is generally placed within an envelope defined by a first cross-sectional area  16   a  and a second cross-sectional area  16   b  within the tail section  14 . 
     The APU  10 , utilizing fuel cells, generally includes a water sump  18 , a recirculation plenum  20  and a heat exchanger complex  22 . These surround a fuel cell stack  24  which is abutted by air manifolds  26  and intermingled with fuel manifolds  28 . These portions generally define the APU  10  and will be discussed in greater detail herein. 
     With continuing reference to FIG.  1  and further reference to FIG. 2, the APU  10  is placed within the aircraft  12  to provide auxiliary power to the aircraft  12  and its electronic components. As discussed above, the APU  10  is generally placed within a tail section  14  of the aircraft  12 . The APU  10  provides electricity to either charge a battery  30  or to power various electronic components through a direct current (DC) power unit  32 . Alternatively, the APU  10  output may be converted to an alternating current (AC) with an AC converter  34 . The electricity is transmitted to electrical components through transmission lines  35 . The electronic components include an electric starter  36  for an engine  38  to assist in initial start-up. Other electronic components include avionics such as those to control the wing flaps  40 . Power can also be routed to gear lift motors  48  to power a gear lift  50  to raise and lower a landing gear  51 . Also electricity can be routed to environmental control systems  52  to power the environmental systems, such as air conditioning and air recirculation subsystems. Therefore, the APU  10  can provide power to all subsystems of the aircraft  12  which require electricity through the electrical transmission lines  35 , while unneeded power is stored in the battery  30  until it is required. 
     If the APU  10  is not intended to provide all of the power needed for the aircraft  12 , then a generator  42  is implemented and powered by the engines  38 . Power can then be diverted from the generator  42  and controlled by AC regulator  44 , or DC power can be provided with a DC unit  46 . It will be understood that the starter  36  and the generator  42  may be the same component depending upon whether the single component is driving or being driven by the engine  38 . It will also be understood that the APU  10  can be designed in such a way as to provide all electrical power necessary for powering all the electrical subsystems on the aircraft  12 . With the appropriate APU  10 , generators  42 , which use part of the power provided by the engines  38  to create additional electricity, may not be necessary for normal flight. 
     With reference to FIG. 3, the APU  10  preferably includes planar solid oxide fuel cell stacks (SOFC)  24 . Although described in greater detail below, the SOFCs  24  utilize a hydrocarbon fuel and oxygen to produce electricity. The SOFCs  24  are completely solid state with no moving mechanical parts and do not require pure hydrogen to produce electricity, as do some other fuel cell technologies. The electricity can be communicated directly as DC power or converted to AC with the AC converter  34 , alternatively electricity can be stored in the battery  30 . A voltage stepper may also be provided to regulate the electricity produced by the APU  10 . An integrated fuel processor and air preheat system (processor system)  56  is provided to first process and reform a fuel from a fuel supply  58  which is pumped to the processor system  56  with a fuel pump  59 , and preheat air before they enter the SOFCs  24 . 
     Because the APU  10  is not 100% efficient, excess heat is produced from energy which is not converted into electricity. The SOFCs  24  also operate optimally only under pressure higher than ambient. This excess energy is provided through waste energy line  60  to a turbine  62 . The turbine  62  powers a compressor  64  to compress air from the cabin  66  to provide compressed air to the processor system  56 . It is understood that air contains oxygen; therefore the compressor also supplies oxygen to the processor system and the SOFC  24 . After fuel from the fuel supply  58  and air from the cabin  66  is compressed, the reformed fuel is provided to the SOFCs  24  through a reformed fuel line  68  while the air is provided through a compressed air line  70 . An SOFC system controller  72  controls the operation of the APU  10 . In particular, the SOFC system controller  72  can control the amount of fuel or air being provided to the SOFC  24  so that an efficient production of energy is created. 
     The turbine  62 , powered by the waste energy through the waste energy line  60 , can also power other systems. In particular, the turbine  10  can power a water separator  74 . Exhaust from the APU  10  is provided to the water separator  74  through an exhaust line  76 . The exhaust from the APU  10  includes combustion materials from the SOFC  24 , which is essentially combustion products from the hydrocarbon fuel and the cabin air, which are essentially carbon dioxide and water. Therefore, the water separator  74  can separate the water to provide it to a water container  78 . Therefore, even though the APU  10  may not be 100% efficient in producing electricity, a greater operational or overall efficiency can be achieved by using the excess heat energy and exhaust from the APU  10  to power the turbine  62  to separate water from the exhaust, which may be used on the aircraft  12 . 
     With reference to FIG. 4, the APU  10  is shown in greater detail. The APU  10  may be enclosed in a pressure vessel  79 . The pressure vessel  79  encloses at least the SOFCs  24  and the processor system  56 . Among other things, the pressure vessel  79  decreases the chance of gas leakage due to pressure differentials during operation. In addition, the pressure vessel  79  reduces the mechanical strength requirements of both the SOFC  24  and the processor system  56  for installation into an aircraft  12 . 
     The SOFC  24  includes a plurality of a planar solid oxide fuel cell  80 , nevertheless only one is illustrated for clarity. Although only planar solid oxide fuel cells  80  are described herein, it will be understood the SOFC  24  may be in any appropriate arrangement. Each fuel cell  80  may be surrounded by a barrier  82  which is impermeable to the oxidizer and fuel which are provided to the fuel cell  80 . This creates discrete fuel cells  80  which may be placed in series. 
     Each fuel cell  80  also includes a solid oxide layer  90  with a reformed fuel layer  92  adjacent the solid oxide layer  90  and an oxidizer layer  94  adjacent the solid oxide layer  90 , but opposite the reformed fuel layer  92 . The solid oxide layer  90  provides a physical barrier between the reformed fuel layer  92  and the oxidizer layer  94 . The solid oxide layer  90  also performs the function of an electrolyte in the fuel cell  80 . In addition, a first electrode  96 , which is generally porous, is disposed adjacent one side of the solid oxide layer  90 , for example in the fuel layer  92 . A second electrode  98 , also generally porous, is disposed adjacent a second side of the solid oxide layer  90 , for example in the oxidizer layer  94 . Generally, the first and second electrodes  96  and  98  are porous and span the space between the barrier  82  and the solid oxide layer  90 . In this way the oxidizer layer  94  and the fuel layer  92  flow through the respective electrodes  96  and  98 . 
     The solid oxide layer  90  may be any appropriate solid electrolyte such as a metal oxide. One exemplary material is zirconium oxide that has been doped with a rare earth metal. Exemplary rare earth metals include yttrium, scandium, or cerium. Other exemplary metal oxides also include calcium oxide. It is preferred to have the dopant comprise between about 5% and about 20% by weight of the solid oxide layer  90 . It will be understood, however, that any appropriate solid oxide may be used in the SOFC  24 . 
     It will be understood that the SOFC  24  may contain any number of solid oxide layers  90  and a limited number is illustrated simply for clarity. A plurality of the fuel cells  80  are placed adjacent one another to form the SOFC  24 . It will also be understood that the configuration of the SOFC  24  may compromise any appropriate configuration to provide the APU  10 . 
     The reformed fuel is reformed in the processor system  56  to break the fuel into substantially carbon monoxide, H 2 , or hydrogen atoms which are components of hydrocarbon fuels. Although the processor system  56  is described herein using steam, it will be understood that other reformation systems may be used. For example, an auto thermal, thermal decomposition, or partial oxidation techniques may also be used to reform the fuel for the APU  10 . The constituents of the reformed fuel are the fuel for the SOFC  24  and are present in the fuel layer  92 . The oxidizer layer  94  includes air from the cabin  66  that has been compressed to concentrate the oxygen present in air in the cabin  66 . The oxygen in the air is the oxidizer in oxidizer layer  94 . It will be understood, however, that another oxidizer or oxygen from a different source may be used in oxidizer layer  94 . The solid oxide layer  90  allows the oxygen ions present in the oxidizer layer  94  to transport through the solid oxide layer  90  to combine with the fuel in the fuel layer  92 . The migration of oxygen ions across the solid oxide layer  90  produces, by creating an electrical potential, electricity. The potential is created between the two electrodes  96 ,  98  in the fuel cell  80 . In the fuel layer  92  the oxygen combines with carbon monoxide, H + , or H 2  to form CO 2  and H 2 O. 
     The heat exchanger complex  22 , in the processor system  56 , may further include a first heat exchanger  22   a  and a second heat exchanger  22   b . A recirculation plenum  20  is also diagrammatically shown to illustrate the capability of the APU  10  to reuse resources such as water or air. The water sump  18  provides water to the system. Water provided in the water sump  18  may either come from the water supply  78 , from the water separator  74 , or from an onboard water supply. A jet pump  100  pumps water from the water sump  18  into the processor system  56 . The first heat exchanger  22   a  heats the water and jet fuel to bring them up to the temperature necessary to vaporize the liquid mixture into gaseous form. The second heat exchanger  22   b  further heats the steam and jet fuel mixture before entry into the mixing plenum  104 . The mixing plenum  104  allows for a remix of the steam and jet fuel mixture from the first heat exchanger  22   a  to the second heat exchanger  22   b  ensuring that a proper mix has been accomplished before entry into the steam reformer  102 . This helps to prevent fuel coking at too high of a temperature and to ensure that enough steam is produced to prevent fuel coking. 
     The steam reformer  102  breaks the fuel into simpler molecular components to provide the proper components to the SOFCs  24 . A preheater  106  provides additional energy to the steam reformer  102  and to the second heat exchanger  22   b  to ensure that the water is converted to steam for use in the steam reformer  102 . Additionally, fuel and air may be transferred between the recirculation plenum  20  and the preheater  106  to allow for an initial heat up of the SOFCs  24  for initial start-up. 
     After the fuel has been reformed in the steam reformer  102  it travels through a reformed fuel line  108  to be fed into the SOFCs  24 . A valve  110  controls the flow of reformed fuel through the reformed fuel line  108  and air through an air line  112 . The valve  110  is controlled by the SOFC system controller  72  to create the proper mix of fuel and air in the SOFC  24  to ensure the most efficient power production. 
     The SOFCs  24  work most efficiently when they are between about 600° C. and 1000 (° C.) (about 1112° F. and about 1832° F.). Therefore, if the SOFCs  24  have cooled due to not being operated or because of a cool environment, they are most efficient after being first heated. The preheater  106  assists in heating the SOFCs  24  to ensure that they reach the optimal operating temperature quickly. Fuel is supplied to the preheater  106  from the fuel supply  58  to be used to heat the water for use in the steam reformer  102 . Therefore, the fuel and oxygen may also be heated before entering the SOFC  24 . 
     Though any appropriate configuration may be used to provide the fuel cell  80  of the APU  10 , the solid oxide fuel cells  80  are preferred for aircraft because fuel cells  80  do not require pure hydrogen for proper operation. In particular, the reformed jet fuel from the jet fuel supply  58 , which is already provided on an aircraft  12 , can be used to provide the hydrogen and carbon monoxide to cause the migration of oxygen ions across the solid oxide layer  90 . Generally, preferred are carbon monoxide and atomic hydrogen, but molecular hydrogen is also usable. Therefore, rather than requiring the inclusion of another fuel source, particularly a highly explosive hydrogen fuel source, reformed jet fuel can be used to power the SOFCs  24 . Furthermore, the APU  10  allows for the recirculation and re-use of cabin air from the cabin  66  to provide the oxygen for the SOFCs  24 . Again, rather than providing a pure oxygen source, compressed air from the cabin  66  or reformed water may be used to provide the oxygen source for the SOFCs  24 . 
     Furthermore, the turbine  62  of the APU  10  is part of a reclamation system. The reclamation system allows for the reuse of waste thermal energy produced by the APU  10  to power a turbine to reform exhaust produced by the APU  10 . The turbine  62  powers the compressor  64  to convert the exhaust into usable water. In particular, the exhaust of the APU  10  using SOFCs  24  generally consists substantially only of H 2 O and CO 2 . Therefore, the H 2 O may be separated, with the water separator  74  and stored in the water container  78 , to provide water for use in the aircraft  12 . Furthermore, the water can be used for other uses in the aircraft  12  such as general humidity control in the environment control systems  52  and other gray water uses. 
     An SOFC  24  is theoretically approximately 45-55% efficient. Due to certain inefficiencies and other physical constraints, however, the APU  10  is generally approximately 30-40% efficient. It is this waste thermal energy which provides the energy required to power the turbine  62  to power the compressor  64 . The turbine  62  is an expansion turbine which uses hot gases to expand through turbine blades to spin the blades. This provides kinetic energy for things such as powering the compressor  64 . 
     It will be understood that varying configurations of the APU  10  can be used to provide different amounts of electricity to aircraft  12 . Therefore, the aircraft  12 , where the APU  10  only provides a portion of the electricity to the aircraft  12 , especially when the aircraft  12  is on the ground, can be provided. When the APU  10  is provided in the aircraft  12 , especially when the aircraft  12  is in flight, the engines  38  may power a generator  42  to provide electricity necessary for the aircraft  12 . During this time, the APU  10  can charge the battery  30  to provide additional power for later use. 
     The APU  10  can also be designed to provide all the electricity at all times so that the engines  38  never need to provide electricity to the aircraft  12 . In this way, the engines  38  can be decreased in size and increased in efficiency since they will not need to power generators  42  to provide electricity. Therefore, the overall efficiency of the components of the aircraft  12  can be increased by using the APU  10  to provide all the electricity necessary to power the electrical subsystems of the aircraft  12 . 
     It will also be understood that the foregoing description of the preferred embodiments is merely exemplary. For example, the processor system  56  could be integrated further into the SOFCs  24 . Therefore, rather than having separate components separated physically from the SOFCs  24 , the processor system  56  can be designed so that it is interspersed with the SOFCs  24 . Furthermore, the size or number of SOFCs  24  can be augmented depending upon the amount of electricity required for a particular application. 
     The mixture of the fuel in the SOFC  24  may include between about 2-3% excess fuel to resist corrosion of the electrodes  96  and  98 . In addition, the SOFC  24  is preferably pressurized to a pressure between about 20 and about 30 psi to increase efficiency of the SOFCs  24 . The air from the cabin  66 , which is compressed by the compressor  64 , may both pressurize and cool the SOFCs  24 . Generally, the greatest electrical density is formed on the SOFC  24  when the pressure is between about 20 psi and about 30 psi. Therefore, not only does the air provided by the compressor provide oxygen to the oxidizer layer  94 , it also cools the SOFCs  24  by pulling air through the heat exchangers  97 ,  98 . This air can also be used to pressurize the SOFCs  24  to increase the electrical density of the SOFCs  24 . 
     It will also be understood that the solid oxide layers  90  may be formed from any appropriate material such as those that are doped with calcium oxide and scandium oxide. The solid oxide layer  90  provides the electrolyte that ions of oxygen may transfer through. The solid oxide layer  90  also separates the reformed fuel layer  92  from the oxidizer layer  94 . Therefore, the oxygen ions must cross through the solid oxide layer  90 , thereby creating a charge differential and the flow of electricity. Although the SOFC  24  has been described herein, it will be understood that various other fuel cells may be used. Generally, a fuel cell causes the migration of an oxidizer, such as oxygen ions, across an electrolyte to produce electricity. Other possible fuel cells include PEM fuel cells, which require hydrogen ions as the fuel source alone and carbon must be scrubbed from the fuel before entering the fuel layer  92 . 
     The APU  10  of the present invention thus provides an efficient and quiet source of electricity to the aircraft  12 . In particular, the APU  10  is entirely solid state. Therefore, the APU  10  will not produce any vibrations or noise to disturb the passengers in the cabin  66 . Additionally, the APU  10 , using SOFCs  24 , is theoretically approximately between about 30 and about 60 percent efficient. Even though physical and mechanical constraints presently provide APUs  10  which are generally between about 30 and about 40 percent efficient. Turbines are generally between about 20 and about 30 percent efficient. Therefore, the APU  10 , according to the present invention, provides electricity to the aircraft  12  at an increased efficiency of between about 10 and about 20 percent. This increases overall fuel efficiency of the aircraft  12  and provides a cleaner source of electricity to the aircraft  1   
     The APU  10  also assists in decreasing undesirable emissions such as nitrous oxide emissions. Because nitrous oxides are generally produced at elevated temperatures, generally above 1500° C., substantial amounts of nitrous oxides are not produced in the APU  10 . This is because the SOFC  24  operates at a temperature generally below about 1400° C. Therefore, nitrous oxide emissions are substantially eliminated and much below the emissions of gas turbine power units. 
     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.