Liquid hydrogen feed system for fuel cell powered aircraft

Abstract

To provide a thermal management system and a fuel cell system that use as little volume in each nacelle as possible in aircraft that operate using liquid hydrogen fuel.SOLUTION: An aircraft 3855 comprises: a fuselage 3862; wings; engines connected to the wings; a set of liquid hydrogen tanks 3810; a vent 3860 located on a tail 3861 of the fuselage 3862; and a conduit system 3801 connecting the set of liquid hydrogen tanks 3810 to the vent 3860. A controller 3802 is configured to control the conduit system 3801 to release gaseous hydrogen 3811 in the set of liquid hydrogen tanks 3810 to travel through the conduit system 3801 and exit at the vent 3860 in response to pressure in the set of liquid hydrogen tanks 3810 being greater than a specified tolerance.SELECTED DRAWING: Figure 38

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 519,514, entitled “Liquid Hydrogen Feed System for Fuel Cell Powered Aircraft,” filed Aug. 14, 2023, the entire contents of which are incorporated by reference herein.

[0002] This application is related to the following United States patent applications: U.S. application Ser. No. 18 / 610,438, entitled "Pumped Two-Phase Cooling of Aircraft Electronics," filed on the same day, Attorney Docket No. 23-0610-US-NP; U.S. application Ser. No. 18 / 610,526, entitled "Configuration for a LH2 Fuel Cell Aircraft with Distributed Systems," filed on the same day, Attorney Docket No. 23-1348-US-NP; U.S. application Ser. No. 18 / 610,580, entitled "Method of Nacelle Heat Exchanger Integration for a Hydrogen Fueled Fuel Cell Powered Aircraft," filed on the same day, Attorney Docket No. 23-1367-US-NP; and U.S. application Ser. No. 18 / 610,580, entitled "Start-Up of High Temperature Proton Exchange Membrane (HTPEM) Fuel Cell Aircraft with Multiple Power Generating," filed on the same day, Attorney Docket No. No. 18 / 610,742, entitled "Low Temperature Proton Exchange Membrane Charge Air Heat Exchanger," Attorney Docket No. 23-1377-US-NP, filed on the same day, U.S. patent application Ser. No. 18 / 610,816, entitled "Low Temperature Proton Exchange Membrane Charge Air Heat Exchanger," Attorney Docket No. 23-1383-US-NP, and U.S. patent application Ser. No. 18 / 610,868, entitled "Fuel Cell Aircraft Thermal Management System," filed on the same day, Attorney Docket No. 23-1385-US-NP, all of which are incorporated by reference herein in their entireties. [Technical field]

[0003] The present disclosure relates generally to aircraft, and more particularly to aircraft that use propulsion systems powered with hydrogen fuel cells. [Background technology]

[0004] Aircraft with electric propulsion systems are the latest trend that could offer many benefits for the aviation industry. Electric aircraft could bring many environmental benefits. For example, electric aircraft could result in zero emissions during flight, thus reducing the carbon footprint.

[0005] Furthermore, electric motors are generally quieter than combustion engines, and as a result, noise pollution can be reduced using this type of aircraft.

[0006] An electric aircraft has an electric propulsion system that can be powered using hydrogen, which can be stored in tanks, where the hydrogen is normally in a liquid state and is stored at cryogenic temperatures, such as -253 degrees Celsius.

[0007] The hydrogen in the fuel tank may be stored as liquid hydrogen, gaseous hydrogen, or other hydrogen carriers such as methane or other hydrocarbons in liquid or gaseous state. A reformer may be used in the reforming process to extract hydrogen from the hydrocarbon molecules.

[0008] The hydrogen in the fuel tanks can then be sent to fuel cells that are used to generate electricity to power the propulsion and other systems on board the aircraft. Fuel cells are electrochemical devices that convert hydrogen and oxygen from the air into electricity to power the propulsion systems on board the aircraft. This type of cell operates with high efficiency and minimal emissions, where the by-product is water vapor. Summary of the Invention

[0009] One embodiment of the disclosure provides an aircraft including a fuselage, a wing, an engine connected to the wing, a set of liquid hydrogen tanks, a vent located on a tail of the fuselage, a conduit system connecting the liquid hydrogen tanks to the vent, and a controller, wherein the controller is configured to control the conduit system to remove and move gaseous hydrogen in the set of liquid hydrogen tanks through the conduit system and out at the vent in response to a pressure in the set of liquid hydrogen tanks being greater than a certain tolerance.

[0010] Another embodiment of the present disclosure provides a hydrogen recirculation system including a conduit system and a controller, the conduit system including conduits and valves connected to a set of liquid hydrogen tanks of an aircraft, the controller configured to control the conduit system to: remove gaseous hydrogen from the set of liquid hydrogen tanks, route the gaseous hydrogen to a tank heat exchanger where the gaseous hydrogen is heated to form heated gaseous hydrogen, route a first portion of the heated gaseous hydrogen to a fuel cell stack, and route a second portion of the heated gaseous hydrogen back to the set of liquid hydrogen tanks.

[0011] Yet another embodiment of the present disclosure provides a hydrogen recirculation system including a conduit system and a controller, the conduit system including conduits and valves connected to a set of liquid hydrogen tanks, the controller configured to control the conduit system to remove liquid hydrogen from the set of liquid hydrogen tanks, route the liquid hydrogen to a tank heat exchanger to form heated gaseous hydrogen, route a first portion of the heated gaseous hydrogen to a fuel cell stack, and route a second portion of the heated gaseous hydrogen back to the set of liquid hydrogen tanks.

[0012] Yet another embodiment of the present disclosure provides a method for recirculating hydrogen. Gaseous hydrogen is extracted from a set of liquid hydrogen tanks connected to an aircraft. The gaseous hydrogen extracted from the set of liquid hydrogen tanks is routed through a tank heat exchanger in a nacelle for an engine of the aircraft to form heated gaseous hydrogen. A first portion of the heated gaseous hydrogen is routed to a fuel cell stack in the engine of the aircraft. A second portion of the heated gaseous hydrogen is returned into the set of liquid hydrogen tanks.

[0013] The novel features believed characteristic of the illustrative embodiments are set forth in the appended claims, however, the illustrative embodiments as well as their preferred modes of use, further objects and characteristics thereof will be best understood by reading the following detailed description of the illustrative embodiments of the present disclosure in conjunction with the accompanying drawings, in which: [Brief description of the drawings]

[0014] [Figure 1] 1 is a diagram of a hydrogen fuel cell vehicle including a distributed propulsion system according to an illustrative embodiment. [Diagram 2] FIG. 2 is a front view of a liquid hydrogen tank location according to an illustrative embodiment; [Diagram 3] FIG. 2 is an illustration of a nacelle layout in accordance with an illustrative embodiment; [Figure 4] 1 is a block diagram of a liquid hydrogen fuel cell aircraft in accordance with an illustrative embodiment; [Diagram 5] 1 is a block diagram of a heat exchange system for an aircraft in accordance with an illustrative embodiment; FIG. [Figure 6] 1 is a cross-sectional view of a portion of a nacelle including a heat exchanger according to an exemplary embodiment; [Figure 7] 4 is another cross-sectional view of a portion of a nacelle including a nacelle heat exchanger according to an exemplary embodiment; [Figure 8] 4 is another cross-sectional view of a portion of a nacelle including a nacelle heat exchanger according to an exemplary embodiment; [Figure 9]FIG. 1 is an illustration of an engine including a nacelle heat exchanger according to an illustrative embodiment. [Figure 10] FIG. 1 is an illustration of an engine including a nacelle heat exchanger according to an illustrative embodiment. [Figure 11] FIG. 1 is an illustration of an engine including a nacelle heat exchanger according to an illustrative embodiment. [Figure 12] FIG. 2 is another view of an engine including a nacelle heat exchanger in accordance with an exemplary embodiment. [Figure 13] FIG. 2 is a schematic diagram of a fan arrangement for use in a nacelle with a nacelle heat exchanger in accordance with an illustrative embodiment; [Figure 14] 1 is a cross-sectional view of a nacelle including a nacelle heat exchanger according to an exemplary embodiment; [Figure 15] 4 is an illustration of airflow through a nacelle to provide cooling for a fuel cell stack according to an illustrative embodiment. [Figure 16] FIG. 1 is a front view of a nacelle including a nacelle heat exchanger according to an exemplary embodiment. [Figure 17] FIG. 2 is an illustration of a nacelle heat exchanger according to an illustrative embodiment; [Figure 18] FIG. 1 is an illustration of airflow through a nacelle heat exchanger in accordance with an illustrative embodiment; [Figure 19] FIG. 1 is a block diagram of a starting system for an aircraft in accordance with an illustrative embodiment; [Figure 20] FIG. 1 is a flow diagram of a process for managing the start-up of a fuel cell stack in an aircraft according to an illustrative embodiment. [Figure 21] FIG. 2 is a flow diagram of a process for heating a fuel cell stack according to an illustrative embodiment. [Figure 22] FIG. 1 is a flow diagram of a process for managing the start-up of a fuel cell stack in an aircraft according to an illustrative embodiment. [Diagram 23] FIG. 1 is a flow diagram of a process for circulating heated coolant in accordance with an illustrative embodiment. [Figure 24] FIG. 2 is a flow diagram of a process for delivering power to a fuel cell stack according to an exemplary embodiment. [Diagram 25] FIG. 10 is a flow diagram of a process for stopping heating of a coolant in accordance with an illustrative embodiment. [Figure 26] FIG. 1 is an illustration of an aircraft including a propulsion and power system in accordance with an illustrative embodiment. [Figure 27] FIG. 1 is an illustration of an integrated power and propulsion unit in accordance with an illustrative embodiment. [Figure 28] 1 is a schematic diagram of a fuel cell stack starting system according to an exemplary embodiment. [Figure 29] 1 is a schematic diagram of a fuel cell stack start-up system within a thermal management system according to an exemplary embodiment. [Diagram 30] 1 is a schematic diagram of a fuel cell stack start-up system within a thermal management system according to an exemplary embodiment. [Diagram 31] FIG. 1 is a block diagram of an aircraft management system in accordance with an illustrative embodiment; [Diagram 32] FIG. 1 is a flow diagram of a process for air management in accordance with an illustrative embodiment. [Diagram 33] FIG. 1 is a schematic diagram of an aircraft air management system for a fuel cell stack according to an illustrative embodiment. [Diagram 34] FIG. 1 is a block diagram of an aircraft thermal management system in accordance with an illustrative embodiment. [Diagram 35] FIG. 1 is a flow diagram of a process for thermal management in accordance with an illustrative embodiment. [Diagram 36] FIG. 2 is a flow diagram of a process for managing the temperature of a fuel cell stack according to an exemplary embodiment. [Figure 37] 1 is a diagram of a thermal management system including a low temperature loop and a high temperature loop according to an illustrative embodiment. [Figure 38] FIG. 1 is a diagram of a hydrogen recirculation system according to an illustrative embodiment. [Figure 39] FIG. 1 is a flow diagram of a process for recycling hydrogen in accordance with an illustrative embodiment. [Diagram 40]FIG. 1 is a flow diagram of a process for recycling hydrogen in accordance with an illustrative embodiment. [Diagram 41] FIG. 1 is a flow diagram of a process for recycling hydrogen in accordance with an illustrative embodiment. [Diagram 42] FIG. 1 is a diagram of a liquid hydrogen recirculation system for tank pressure control according to an illustrative embodiment. [Diagram 43] FIG. 1 is a diagram of a gaseous hydrogen vent system according to an illustrative embodiment. [Diagram 44] FIG. 1 is a diagram of a system for recirculating gaseous hydrogen for tank pressure control according to an illustrative embodiment. [Diagram 45] FIG. 1 is a block diagram of an aircraft cooling system in accordance with an illustrative embodiment. [Figure 46] FIG. 1 is a flow diagram of a process for cooling a heat load in accordance with an illustrative embodiment. [Figure 47] FIG. 1 is a flow diagram of a process for cooling a heat load in accordance with an illustrative embodiment. [Figure 48] FIG. 1 is a diagram of a pumped two-phase (P2P) cooling loop according to an illustrative embodiment. [Figure 49] FIG. 1 is a diagram of a hybrid two-phase (P2P) cooling loop in accordance with an illustrative embodiment. [Figure 50] FIG. 1 is a diagram of a pumped two-phase (P2P) cooling loop with waste heat recovery in accordance with an illustrative embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] 1. Configuration of LH2 fuel cell aircraft including distributed systems

[0016] The illustrative embodiments recognize and take into account a number of different problems associated with aircraft operating using liquid hydrogen fuel. The problems recognized by the various illustrative embodiments are described herein.

[0017] The illustrative embodiments recognize and take into account that integrating a liquid hydrogen (LH2) system into a passenger aircraft, such as a 50-seater aircraft, can be very challenging. For example, design challenges exist in determining where to safely place the large liquid hydrogen tanks and how to package the thermal management system and fuel cell stack system, both of which require large amounts of space.

[0018] In the above exemplary embodiment, the fuel tank location is along the side of the fuselage and ends above the wings. This type of configuration allows the tank to be integrated in a safe manner while keeping added weight and wetted surface area as small as possible. In one exemplary example, six engine nacelles are used to add redundancy and allow the thermal management system and fuel cell system to be packaged together in a manner that uses the smallest possible space within each nacelle while also lowering the power requirements of the electric motors.

[0019] Improving redundancy can be achieved by increasing the number of engines. As used herein, the term "a number of" when used with reference to an item means one or more items. For example, "a number of engines" means one or more engines.

[0020] Additionally, in illustrative examples, it is possible to use high temperature proton exchange membrane fuel cells rather than low temperature proton exchange membrane fuel cells. The use of high temperature proton exchange membrane fuel cells helps reduce the size of the heat exchanger, and further helps the heat exchanger include microchannel flow paths that further reduce the size. Such systems are optional in illustrative examples.

[0021] Additional conduits in the ducting system may be implemented leading from the liquid hydrogen tank to the top of the vertical stabilizer. Such conduits may be used to provide venting to relieve excess pressure in the liquid hydrogen tank during flight. The ducting system may also be referred to as piping or a piping system in various illustrative examples.

[0022] Referring to Figure 1, a hydrogen fuel cell vehicle including a distributed propulsion system is illustrated in accordance with an exemplary embodiment. In various exemplary embodiments, the same reference number may be used in more than one figure. Such repeated use of a reference number in different figures represents the same element in the different figures.

[0023] In this illustrative example, aircraft 100 is an example of a liquid hydrogen fuel cell vehicle. Aircraft 100 is an airplane capable of carrying various numbers of passengers. For example, aircraft 100 may be an airplane designed to carry 50 passengers, 100 passengers, 130 passengers, or some other number of passengers. In other illustrative examples, aircraft 100 may be a cargo aircraft or a passenger-cargo aircraft.

[0024] As shown in this example, aircraft 100 includes a fuselage 102, which includes wings 104 and 105. In this example, engines 106 and liquid hydrogen tanks 108 are located on wings 104 and 105. As shown, engine 106 includes six engines. Engines 111, 112, and 113 are connected to wing 104, and engines 114, 115, and 116 are connected to wing 105.

[0025] In this example, when one component is "connected" to another component, the connection is a physical connection. For example, a first component, such as an engine, may be considered to be physically connected to a second component, such as a wing, by at least one of being fixed to the second component, being bonded to the second component, being welded to the second component, being fastened to the second component, or being connected to the second component in some other suitable manner. The first component may also be connected to the second component using a third component. The first component may also be considered to be physically connected to the second component by being formed as part of the second component, as an extension of the second component, or both. In some cases, the first component may be physically connected to the second component by being located within the second component.

[0026] In this example, the liquid hydrogen tank 108 is connected to the fuselage 102. The liquid hydrogen tank is located along the side of the fuselage 102 and above the wings 104. The liquid hydrogen tank is substantially cylindrical in shape. The liquid hydrogen tank may have other shapes, such as circular, rounded rectangular prism, teardrop, or other suitable shapes. In this example, the liquid hydrogen tank 108 is a cryogenic tank that carries liquid hydrogen.

[0027] In this example, liquid hydrogen tank 130 of liquid hydrogen tanks 108 is located above wing 105 and liquid hydrogen tank 131 of liquid hydrogen tanks 108 is located above wing 104. The location of liquid hydrogen tank 108 is selected to provide desired center of gravity (CG) characteristics for aircraft 100. As shown in this example, liquid hydrogen tank 108 is located below window 138 in fuselage 102. As shown in this example, window 138 is located along window line 139 on fuselage 102.

[0028] The location of these two liquid hydrogen tanks reduces the wetted surface area by placing the fuel tanks against several surfaces of the aircraft 100. In other words, the surface area of ​​the fuselage 102 surrounding the liquid hydrogen tank 108 that is in contact with the air during flight is reduced compared to other possible locations on the aircraft 100. By reducing the wetted surface area of ​​the fuselage 102 around the liquid hydrogen tank 108, improved aerodynamic performance may be achieved.

[0029] Additionally, the location of the liquid hydrogen tank 108 can protect the components from a hard landing, and the location of the liquid hydrogen tank can protect the tank from damage in a ground strike.

[0030] Further, in this example, six engines are used for the aircraft 100. Increasing the number of engines may allow for greater distribution of various components of the thermal management system and other components within the engines into smaller components that may reside within the nacelle space for an individual engine.

[0031] The distribution also provides the ability to use blown wings for higher lift at lower speeds, improved redundancy, and lower power requirements for the electric motors. Additionally, increasing the number of engines allows for smaller fuel cell stack sizes within each engine.

[0032] Additionally, utilizing six engines rather than two or four engines provides improved redundancy. As the number of engines increases, the electric motors may be smaller in size and the power requirements may be smaller. Similarly, the heat exchangers in the nacelles for the engines may be smaller in size (number). Similarly, the number of fuel cells in the fuel cell stack may be smaller in size, making it easier to package and integrate the above components into the aircraft 100.

[0033] In this example, venting of gaseous hydrogen (GH2) from the liquid hydrogen tanks 108 may be provided via conduits to a vent system capable of venting the gaseous hydrogen out of an area away from and above the passengers, crew, and ground crew, such as the vertical stabilizer 110.

[0034] 2 is a front view of the location of liquid hydrogen tanks, according to an exemplary embodiment, in which the front view of aircraft 100 provides an additional view of the location of liquid hydrogen tank 130 above wing 105 and the location of liquid hydrogen tank 131 above wing 104. As shown, liquid hydrogen tank 131 is located on the side of fuselage 102 of aircraft 100, above wing 104, and liquid hydrogen tank 130 is located on the side of fuselage 102 of aircraft 100, above wing 105.

[0035] These locations of the liquid hydrogen tanks 108 can reduce trimming issues and reduce hydrogen gas entrapment within the fuselage. Additionally, lower drag and lighter weight of the integrated liquid hydrogen tanks can be achieved with easier integration compared to liquid hydrogen tanks used under the wings of the aircraft. Additional tanks can be used to reduce liquid sloshing and improve redundancy.

[0036] The illustration of the aircraft 100 in Figures 1 and 2 is provided as one illustrative example and is not intended to limit the manner in which other illustrative examples may be implemented. For example, while the liquid hydrogen tank 108 is shown as including two liquid hydrogen tanks, other illustrative examples may have other numbers of hydrogen fuel tanks. For example, the liquid hydrogen tank may include four liquid hydrogen tanks, where two liquid hydrogen tanks are located on each side of the aircraft 100, above the wings and below the windows.

[0037]

[0023] Referring now to Figure 3, a nacelle layout is shown in accordance with an illustrative embodiment. Engine 300 is an example of engine 106 from Figures 1 and 2. In this example, engine 300 includes nacelle 302, electric motor 304, fuel cell stack 305, and heat exchanger 308. In this illustrative example, nacelle 302 is the structure that encloses electric motor 304, fuel cell stack 305, and heat exchanger 308.

[0038] In this example, the heat exchanger 308 is rectangular in shape and is disposed at an incline with respect to the inlet 310 aperture. The incline may have an acute angle less than 90 degrees. The shape and location of the heat exchanger 308 allows for a reduction in the forward area of ​​the nacelle 302. As the number of engines increases, a reduction in the size of the electric motor 304, fuel cell stack 305, and heat exchanger 308 may be realized.

[0039] 4, a block diagram of a liquid hydrogen fuel cell aircraft is shown in accordance with an illustrative embodiment. In this illustrative example, aircraft 100 and components of aircraft 100 depicted in FIGS. 1, 2, and 3 are an example of one embodiment of aircraft 400. In this example, aircraft 400 takes the form of a liquid hydrogen fuel cell aircraft 457 having a distributed system. Aircraft 400 may be selected from a group including an unmanned aircraft, a manned aircraft, a remotely piloted aircraft, a passenger aircraft, a cargo aircraft, a commercial aircraft, an autonomous aircraft, or other type of aircraft. In one example, aircraft 400 may be a passenger aircraft, such as a 50-seater aircraft.

[0040] In this example, aircraft 400 has several distinct components. As shown, aircraft 400 includes a fuselage 402, wings 404, engines 406, stabilizers 412, and a set of liquid hydrogen tanks 408. As used herein, when the term "a set of" is used with reference to an item, it means one or more items. For example, a set of liquid hydrogen tanks is one or more liquid hydrogen tanks.

[0041] In this illustrative example, wings 404 include first wing 403 and second wing 405, which are connected to fuselage 402. Engines 406 are connected to first wing 403 and second wing 405. In this example, engines 406 may be four engines, six engines, ten engines, or any other suitable number of engines.

[0042] In this illustrative example, a set of liquid hydrogen tanks 408 are connected to fuselage 402. A stabilizer 412 is connected to tail section 410 of fuselage 402.

[0043] In this example, each engine includes several components. For example, engine 415 of engine 406 includes a nacelle 420, an electric motor 421, a fuel cell stack 422, and a nacelle heat exchanger 423. In this example, electric motor 421, fuel cell stack 422, and nacelle heat exchanger 423 are located within nacelle 420.

[0044] The nacelle 420 is a housing that houses components for the engine 415. The housing may provide aerodynamic characteristics. The nacelle 420 may also provide structural support for the components within the engine 415.

[0045] Further in this illustrative example, a nacelle heat exchanger receives air 430 flowing through an inlet 431 in the nacelle 420. The nacelle heat exchanger 423 provides cooling for the fuel cell stack 422 in the nacelle 420. Such cooling may be accomplished in a high temperature cooling loop, such as high temperature loop 3401 in FIG. 34 below.

[0046] For example, conduit system 424 is fluidly connected to nacelle heat exchanger 423 and fuel cell stack 422. Conduit system 424 includes one or more conduits that carry a coolant that can be used to cool fuel cell stack 422. The conduits can be, for example, pipes, tubes, or other suitable structures. Conduit system 424 can also include valves that can control the flow of coolant within the conduits. Additionally, conduit system 424 can include pumps for moving fluids.

[0047] In this manner, the flow of coolant can be controlled to and through various components. Additionally, the valves described above can be used to control the flow of coolant to form various loops for cooling or heating components.

[0048] In this example, the nacelle heat exchanger 423 and the conduit system 424 form a thermal management system 425. This thermal management system 425 cools the fuel cell stack 422. The HTPEM (High Temperature Proton Exchange Membrane) fuel cells require a coolant that is too hot to act as a coolant for other engine components. Another low temperature coolant loop is required to cool engine components other than the fuel cell stack 422. This low temperature coolant loop uses its own ram air radiator or other heat exchanger or cooling method to cool the coolant. In this vehicle, the low temperature coolant is cooled as part of its own pressure control system for the tank, which requires hydrogen heating and recirculation.

[0049] In this illustrative example, a set of liquid hydrogen tanks 408 are located on the exterior of fuselage 402. The liquid hydrogen tanks store liquid hydrogen 433, which is used in fuel cell stack 422 to generate electricity to power electric motor 421.

[0050] Further, in this example, the liquid hydrogen tanks extend along the outside of the fuselage 402 and above the wings 404. Further, the set of liquid hydrogen tanks 408 may be located below windows 427 in the fuselage 402. In this example, the windows 427 are located along a line on both sides of the fuselage 402. In this example, the set of liquid hydrogen tanks 408 extend along several sides of the fuselage 402. The liquid hydrogen tanks 408 are cryogenic tanks.

[0051] In this illustrative example, aircraft 400 also includes a vent system 440. Vent system 440 is connected to the set of liquid hydrogen tanks 408. Vent system 440 operates to vent gaseous hydrogen 434 from liquid hydrogen tanks 408.

[0052] In one illustrative example, a vent system 440 extends through the fuselage 402 to a vertical stabilizer 441 within the stabilizer 412. The vertical stabilizer 441 is connected to the tail section 410 of the fuselage 402. In this example, the vent system 440 vents the gaseous hydrogen 434 through the vertical stabilizer 441.

[0053] Thus, in this example, as the number of engines 106 increases, various components may be further distributed across those engines. Such distribution may provide increased redundancy. Additionally, as the number of engines 406 increases, the size of the components may be reduced.

[0054] The placement of the liquid hydrogen tanks and the reduced size of the bundled thermal management and fuel cell systems are examples of one or more features that may result from various illustrative examples. For example, distributing the propulsion, thermal management, and fuel cell systems across a larger number of nacelles allows the electric motors in the nacelles to have smaller power requirements for the heat exchangers, and allows the fuel cells to be split into smaller sizes that are easier to manufacture and integrate into the aircraft.

[0055] The block diagrams of aircraft 400 in FIG. 4 and aircraft 100 in FIG. 1 and FIG. 2 are examples of aircraft implementations that are not meant to imply physical or architectural limitations to the manner in which an illustrative embodiment may be implemented. Other components in addition to or in place of the components shown may be used. Some components may not be required. The blocks are also presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an illustrative embodiment.

[0056] In other illustrative examples, different liquid hydrogen tank positions than over the wings and near the sides of the fuselage may be used. Additionally, in other illustrative examples, other numbers of fuel cell stacks may be used in the engine, for example, two, three, or some other number of fuel cell stacks. Additionally, other features may be implemented to further aid in reducing the space required for the thermal management system or fuel cell stacks and their packing. Features such as microchannel heat exchangers or high temperature proton exchange membrane fuel cells in the fuel cell stacks may be used.

[0057] As another example, hydrogen can be stored, and the hydrogen can be in another form different from liquid hydrogen, such as various hydrogen carriers, such as liquid or gaseous methane, or other hydrocarbon molecules. In this example, the above types of hydrocarbon fuels can be processed using reforming to separate hydrogen for use in the fuel cell stack. In other words, in addition to the liquid hydrogen tank 408, other types of storage tanks can be used that store other types of hydrocarbon fuels that can be processed to obtain gaseous hydrogen 434 for use in the fuel cell stack to generate electricity.

[0058] 2. Method of Incorporating a Nacelle Heat Exchanger for an Aircraft Powered by Hydrogen Fuel Cells

[0059] In an illustrative example, it is recognized and taken into account that a hydrogen powered aircraft may use fuel cells to drive propellers that are driven by electric motors. Fuel cells typically generate a significant amount of waste heat that is removed from the fuel cells and dissipated into the surrounding air during operation of the aircraft.

[0060] For example, fuel cell stacks utilizing both high temperature proton exchange membrane (HTPEM) and low temperature proton exchange membrane (LTPEM) fuel cells generate a significant amount of heat relative to the power generated. This heat is dumped from the engine into the ambient air. Heat dumping is more difficult during flight phases such as taxiing, takeoff, and climb. During these phases, the amount of ambient air is not always available in the desired amount to provide the desired cooling. Furthermore, during these flight phases, the temperature difference between the ambient air and the fuel cell coolant is typically less than during cruise.

[0061] This heat rejection may also be referred to as heat rejection. This heat rejection may cause significant ram drag, especially during the takeoff and climb phases. Significant ram drag may exist when adequate ram air is not available and the temperature difference between the ambient air and the fuel cell coolant is small.

[0062] One solution may involve using a large inlet area and a variable exhaust system, however, increasing the size of the forward area may lead to problems with packing the heat exchangers within the nacelle.

[0063] Ram drag is the difference in the change of momentum between the exit and the entrance of the system. It can be positive (meaning thrust production) or negative (drag on the aircraft). The required size of the current heat exchanger is considered large because the amount of heat to be removed is large and the temperature difference between the coolant and the surrounding air is smaller.

[0064] Typical fuel cells operate at efficiencies in the range of 40-60 percent. The total power required based on the size of the aircraft can be hundreds to thousands of kilowatts. The above operation implies that the amount of heat released by the fuel cells in the fuel cell stack can be of the same order of magnitude as the power generated by the aircraft.

[0065] The temperature difference between the coolant and the ambient air may be 20 degrees Celsius for low temperature proton exchange membrane (PEM) fuel cells and 70 degrees Celsius for high temperature PEM fuel cells. The present disclosure addresses issues related to siting the nacelle heat exchanger with minimal drag penalty, options related to bringing ambient air into or out of the heat exchanger for cooling, as well as other potential issues.

[0066] An illustrative example provides a fuel cell based electric airplane that uses hydrogen as fuel. In this example, a fuel cell stack is made up of fuel cells. The fuel cell stack can be located within the engine of the aircraft. In other words, the fuel cell stack is located within the same nacelle that contains the engine. The fuel cell stack is cooled by various options, and heat from the fuel cell stack needs to be rejected to the ambient air using a coolant to ambient air heat exchanger also located within the nacelle.

[0067] Furthermore, each nacelle may include a fan system. The electric fan system may include several fans whose speed can be varied according to the cooling required. In this example, the fan system increases the airflow through the heat exchanger. This increase in airflow using the fan system makes it possible to reduce the size of the heat exchanger in the nacelle.

[0068] Additionally, a fixed area nozzle may allow for useful thrust recovery from ram air, and the use of a variable area nozzle may optimize the power required by the fan system and increase the thrust contribution from the total cooling system during various flight phases.

[0069] An illustrative example provides for incorporating the above heat exchanger in the nacelle instead of inside the fuselage.

[0070] In one illustrative example, a fuel cell based electric airplane using hydrogen as fuel is illustrated. In this example, a fuel cell stack is made up of fuel cells. The fuel cell stack can be located within the engine of the aircraft. In other words, the fuel cell stack can be located within the same nacelle that contains the engine. The fuel cell stack is cooled by various options, and heat from the fuel cell stack needs to be rejected to the ambient air using a coolant to ambient air heat exchanger.

[0071] Additionally, each cell may include an electric fan system. The electric fan system may include several variable speed fans that may vary in speed according to the cooling required. Additionally, the fixed area nozzle may allow for useful thrust recovery from ambient ram air. In this example, ambient air is the air surrounding or around the aircraft. Additionally, in an illustrative example, the ram drag may be reduced by increasing the area of ​​the nacelle heat exchanger.

[0072] In this manner, a heat exchanger and fan system within the engine nacelle may be used to deliver the desired cooling airflow, and the heat exchanger may be positioned within the nacelle generally parallel to the aircraft's direction of flight.

[0073] As a result, the illustrative examples may provide at least one of drag reduction or thrust generation. In the examples, the phrase "at least one of" used with enumerated items means that various combinations of one or more of the enumerated items may be used and furthermore, only one of each enumerated item may be required. In other words, "at least one of" means that any combination of items and that some items from the enumeration may be used, but not all of the enumerated items are required. An item may be a particular object, thing, or category.

[0074] For example, without limitation, "at least one of item A, item B, and item C" may include "item A," "item A and item B," or "item B." This example may also include "item A, item B, and item C," or "item B and item C." Of course, any combination of these items may be present. In some illustrative examples, "at least one of" may be, for example, without limitation, "2 items A, 1 item B, and 10 items C," "4 items B and 7 items C," or other suitable combinations.

[0075] Furthermore, variable area nozzles can be used to reduce the power of the fan. In this example, variable area means that the cross-sectional area of ​​the nozzle can be adjusted during operation. By said area, it is possible to change the exhaust speed. Fixed area means that the cross-sectional area of ​​the nozzle remains fixed without change.

[0076] In various examples, the heat exchanger may have various shapes to reduce the forward area relative to the nacelle inlet, which may be substantially perpendicular to the inlet. The heat exchanger may also include a fan system, such as an electrically powered axial fan or radial blower, to provide desired levels of airflow during various phases of flight, including when the aircraft is on the ground.

[0077] 5, a block diagram of an aircraft heat exchange system is shown in accordance with an illustrative embodiment. In this example, heat exchange system 500 is located in nacelle 502. Nacelle 502 is an example of nacelle 420 for engine 415 of aircraft 400 in FIG. 4. In this illustrative example, nacelle heat exchanger 520 may be oriented generally parallel to a direction of flight 547 of the aircraft. For example, a length of nacelle heat exchanger 520 may be greater than a width or thickness, and the longer dimension of nacelle heat exchanger 520 may be oriented generally parallel to a direction of flight 547. Heat exchanger system 500 may also be referred to as a ram air radiator.

[0078] In this illustrative embodiment, nacelle 502 has an inlet 504 and an outlet 505. Air 530 flows into nacelle 502 through inlet 504 and exits nacelle 502 through outlet 505.

[0079] In this illustrative example, heat exchange system 500 includes a nacelle heat exchanger 520. In some illustrative embodiments, heat exchange system 500 may also include a fan system 522.

[0080] The nacelle heat exchanger operates to transfer heat away from the coolant 526 using air 530 in an airflow 532 to the nacelle heat exchanger 520 .

[0081] In this illustrative example, airflow 532 to nacelle heat exchanger 520 may take one or more of a variety of forms. For example, airflow 532 may move over a surface of nacelle heat exchanger 520, may move through nacelle heat exchanger 520, or some combination thereof. For example, nacelle heat exchanger 520 may have a solid surface over which air 530 in airflow 532 flows. In other examples, nacelle heat exchanger 520 may have conduits, channels, or apertures through which air 530 in airflow 532 moves.

[0082] The nacelle heat exchanger 520 may be constructed of a number of different materials. For example, the nacelle heat exchanger 520 may be constructed of a number of materials selected from at least one of aluminum, copper, a copper alloy, an aluminum alloy, titanium, a titanium alloy, a composite material, or any other suitable material.

[0083] The nacelle heat exchanger 520 may take a number of different forms. For example, the nacelle heat exchanger 520 may include a hollow geometric conduit 550 that includes a channel 551. In this example, the airflow 532 passes through at least one of a surface of the channel 551 and an outer surface of the hollow geometric conduit 550. The hollow geometric conduit 550 may be, for example, a cylinder, a right angle prism, an ellipse, or any other suitable shape.

[0084] In another illustrative example, the nacelle heat exchanger 520 may be comprised of concentric cylinders 552 disposed within the nacelle 502. In yet another illustrative example, the nacelle heat exchanger 520 may include a solid geometric column 553 disposed within the nacelle 502. In this example, the airflow 532 passes over the exterior of the solid geometric column 553 disposed within the nacelle.

[0085] In yet another example, the nacelle heat exchanger 520 may include a rectangular heat exchanger 555 disposed at an angle to the airflow 532. The rectangular heat exchanger may have the shape of a rectangular prism or a cube.

[0086] When used in the heat exchanger system 500, the fan system 522 operates to increase the airflow 532 to the nacelle heat exchanger 520. This increased airflow allows the size of the nacelle heat exchanger 520 to be reduced while still providing the desired level of cooling.

[0087] In this illustrative example, fan system 522 is a hardware system and includes several fans 523. Several fans 523 may be located in front of nacelle heat exchanger 520 or after nacelle heat exchanger 520. As another example, fans 523 and fan system 522 may be arranged in a ring within nacelle 502.

[0088] In this example, the front face of the nacelle heat exchanger 520 is closer to the inlet 504 than to the outlet 505. The fan 523 may be selected from at least one of an axial fan, an annular fan, a radial blower, a rim driven fan, or any other suitable type of fan.

[0089] A coolant 526 may be used within conduit system 540 to cool various components within nacelle 502. For example, coolant 526 may be located within conduit system 540. Conduit system 540 includes conduits 541 through which a fluid may travel, where conduits 541 are pipes or tubes. Additionally, conduit system 540 may also include valves 542 for controlling the flow of fluid through conduit system 540. In this example, conduit system 540 may include a pump 543 for moving the fluid.

[0090] Conduit system 540 is a hardware system comprised of one or more conduits, which may be pipes, tubes, or other structures through which coolant 526 may flow between nacelle heat exchanger 520 and other components 525 within nacelle 502. Conduit system 540 is thermally connected to nacelle heat exchanger 520 and other components 525 within nacelle 502. In this illustrative example, "thermally connected" refers to the transfer of heat that may occur between two components.

[0091] For example, the conduits in conduit system 540 may be in contact with or in close proximity to the heat load. In other illustrative examples, the conduits are connected to the heat load such that coolant 526 flows from conduit system 540 through the heat load.

[0092] In this example, the conduit system 540 may use the coolant 526 to cool at least one of the fuel cell stack, the motor, or an electrical system in the other components 525. Further, in this illustrative example, one or more of the other components 525 may be located in various positions relative to the nacelle heat exchanger 520. For example, when the nacelle heat exchanger 520 takes the form of a hollow geometric conduit 550, a component such as a fuel cell stack may be located in a channel 551 in the hollow geometric conduit 550.

[0093] In one illustrative example, the inlet 504 may have a fixed area 560. In this example, a variable area nozzle 561 may be disposed between the fan system 522 and the outlet 505. By variable area, it is meant that the cross-sectional area of ​​the variable area nozzle 561 may vary. The airflow 532 through the variable area nozzle 561 may result in an exhaust velocity 563 of the exhaust 554 leaving the outlet 505 that is faster than the free stream velocity.

[0094] The block diagram of the heat exchange system 500 in FIG. 5 is not meant to imply physical or architectural limitations to the manner in which an example embodiment may be implemented. Other components in addition to or in place of the illustrated components may be used. Some components may be unnecessary. The blocks are also presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an example embodiment.

[0095] For example, the nacelle heat exchanger 520 may have other shapes, such as a prism, or other suitable shapes. Additionally, one or more nacelle heat exchangers may be used in addition to or instead of the nacelle heat exchanger 520 in the heat exchanger system 500. This heat exchanger system may also be referred to as a ram air radiator.

[0096] In another illustrative example, a nacelle heat exchanger may be used in a dedicated nacelle to provide cooling. In yet another illustrative example, a tip-driven fan may be used in the fan system. In yet another illustrative example, the heat exchanger including the air inlet and fan system may be located in other locations within the aircraft, such as the fuselage, in addition to or instead of the engine nacelle. Additionally, the heat exchanger may have flow directing features, such as guide vanes, to ensure that airflow enters and exits the heat exchanger in a desired direction.

[0097] 6-18, diagrams of nacelle heat exchanger configurations are shown according to exemplary embodiments, in which various layouts are shown in cross section as examples of configurations that may be used when implementing a heat exchanger within an engine nacelle.

[0098] 6, a cross section of a portion of a nacelle including a heat exchanger is illustrated in accordance with an exemplary embodiment. In this illustrative example, nacelle heat exchanger 600 is located within nacelle 610, which has an inlet 611 and an outlet 612. As shown, nacelle heat exchanger 600 is a hollow geometric conduit in the shape of a cylinder 620, having a channel 621. This hollow geometric shape may also take the form of an elliptical cylinder, a toroidal cone, a polygonal prism, or other shapes.

[0099] As shown, component 602 is positioned in nacelle heat exchanger (HEX) 600, at least one of in front of and partially within channel 621. The component may be, for example, a fuel cell power system, a fuel cell stack, an electric motor, a controller, a shaft, a propeller, or other component of an aircraft engine.

[0100] As shown, the fan 604 is located aft of the nacelle heat exchanger 600. In other examples, the fan 604 may be located forward of the nacelle heat exchanger 600.

[0101] Fan 604 operates to increase airflow through nacelle heat exchanger 600. This type of airflow may be useful when the aircraft is operating on the ground, or when the aircraft is operating at altitudes and speeds that do not provide the desired level of airflow or heat rejection required to cool the fuel cells.

[0102] 7, there is shown another cross-sectional view of a portion of a nacelle including a nacelle heat exchanger according to an exemplary embodiment. In this example, a nacelle heat exchanger (HEX) 700 is located within a nacelle 710 having an inlet 711 and an outlet 712. In this example, the nacelle heat exchanger (HEX) 700 is annular and includes a channel 705.

[0103] As shown, the fan 704 is located aft of the nacelle heat exchanger 700. In other examples, the fan 704 may be located forward of the nacelle heat exchanger 700. In this example, other components 707 are shown as being further within the channel 705 of the nacelle heat exchanger 700 as compared to other components 602 of FIG.

[0104] 8, another cross-sectional view of a nacelle including a nacelle heat exchanger is shown in accordance with an exemplary embodiment. In this example, heat exchanger 800 is located within a nacelle 801 having an inlet 811 and an outlet 812.

[0105] As shown, nacelle heat exchanger 800 is rectangular in shape and is positioned at a shallow angle within nacelle 801, where fan 802 is located aft of heat exchanger 800. The above angles are considered to be shallow angles. In this example, a shallow angle is an angle of 45° or less.

[0106] The shallow angle nacelle heat exchanger 800 position reduces the exposed forward nacelle area and reduces the degree to which the airflow turns as it passes through the nacelle heat exchanger 800, keeping the pressure drop across the heat exchanger low.

[0107] In this example, the fan 802 is shown as being located aft of the heat exchanger 800. In other examples, the fan 802 may be located forward of the nacelle heat exchanger 800.

[0108] The illustrations of the heat exchanger layouts shown in Figures 6-8 are not intended to limit the manner in which the heat exchanger design within the nacelle may be implemented, for example, one or more fans in addition to those illustrated in the examples may be used.

[0109] 9, a diagram of an engine including a nacelle heat exchanger is shown in accordance with an exemplary embodiment. As shown in this example, a side view of an engine 900 is shown within the confines of a nacelle 902, which is shown in an exposed view of its interior and partially in phantom.

[0110] In this view, the nacelle heat exchanger 904 is seen to be at a shallow angle relative to an inlet 906 in the nacelle 902. In this illustrative example, the shallow angle is up to 45 degrees. Also shown is a conduit system 908 connected to the heat exchanger 904. The conduit system 908 can circulate a fluid, such as a gas or liquid, for cooling in response to air flowing through the inlet 906 in the nacelle 902 and through the nacelle heat exchanger 904. The air flowing through the inlet 906 to the nacelle heat exchanger 904 can reduce the temperature of the coolant flowing through the conduit system 908.

[0111] In this example, the fan system 910 is located aft of the nacelle heat exchanger 904. The fan system 910 may be used to create or augment airflow through the inlet 906 and through the nacelle heat exchanger 904. The fan system 910 may be used to create or augment airflow to the nacelle heat exchanger 904 when the aircraft is stationary or moving slowly.

[0112] 10, an illustration of an engine including a nacelle heat exchanger is shown in accordance with an exemplary embodiment. In this example, engine 900 of FIG. 9 is shown in perspective view.

[0113] As shown in this figure, fan system 910 includes fan 1001, fan 1002, and fan 1003. In some illustrative examples, fan system 910 may be located forward or in front of nacelle heat exchanger 904. In other illustrative examples, other numbers of fans may be present in fan system 910. For example, fan system 910 may have one fan, five fans, or some other number of fans.

[0114] 11, a diagram of an engine including a nacelle heat exchanger is shown in accordance with an illustrative embodiment. In this illustrative example, engine 1100 is shown from a side view. Engine 1100 includes a nacelle 1102 that houses components for engine 1100. Nacelle 1102 has an inlet 1131 and an outlet 1132.

[0115] A portion of the nacelle 1102 is shown in phantom, and another portion of the nacelle 1102 is shown in cross section so that other components within the engine 1100 are visible.

[0116] As shown, a nacelle heat exchanger 1104 is located within a nacelle 1102. In this illustrative example, the nacelle heat exchanger 1104 is annular cylindrical in shape and has a channel 1110.

[0117] The nacelle heat exchanger 1104 is disposed within the nacelle 1102 such that components may be positioned within a channel 1110 in the nacelle heat exchanger 1104. Additionally, in this view, a fan 1106 is located forward of the nacelle heat exchanger 1104. In the illustrated example, the fan 1106 is an annular fan.

[0118] Additionally, in this example, an electric motor 1108 is located within a channel 1110 of the nacelle heat exchanger 1104, which can be seen in this exploded view of the nacelle heat exchanger 1104. Additionally, a portion of a fuel cell stack 1112 can also be seen as located within the channel 1110 of the nacelle heat exchanger 1104.

[0119] 12, another view of an engine including a nacelle heat exchanger is shown in accordance with an exemplary embodiment. In this view, a perspective view of engine 1100 is shown. In this view, a cross-sectional view of heat exchanger 1104 is shown. Nacelle heat exchanger 1104 is shown as two concentric cylinders 1200. In this example, cylinder 1202 is located inside cylinder 1204.

[0120] 13, a schematic diagram of a fan configuration for use with a nacelle heat exchanger in a nacelle is shown in accordance with an exemplary embodiment. As shown, fans 1300 are arranged in a 360 degree ring 1302. This arrangement of fans can be used in place of the annular fans of FIGS. 11 and 12.

[0121] 14, a cross-sectional view of a nacelle including a nacelle heat exchanger is shown according to an exemplary embodiment. In this example, a cross-sectional view of a nacelle 1400 is shown having an inlet 1407 and an outlet 1409. For example, the nacelle 1400 is a dedicated nacelle for a nacelle heat exchanger 1402.

[0122] The nacelle heat exchanger 1402 is also shown in cross section within the nacelle 1400. In this example, the nacelle heat exchanger 1402 has a conical annular shape. In other words, the nacelle heat exchanger 1402 can be a hollow geometric conduit that includes channels. In this example, the airflow by the air 1406 passes through at least one of the surfaces within the channels or the outer surface of the hollow shaped conduit. In an illustrative example, the nacelle heat exchanger 1402 can also be in the shape of a pyramidal truncated cone.

[0123] Further, in this example, the fan 1404 is shown as being aft of the heat exchanger 1402. In various examples, aft means that the fan 1404 is located after the nacelle heat exchanger 1402 with respect to the flow of air 1406 entering at the inlet 1407 and passing through the outlet 1409 as exhaust 1408.

[0124] Further, in this example, the outlet 1409 takes the form of a nozzle 1412, which may be a fixed area nozzle or a variable area nozzle depending on the particular implementation. Such a nozzle may increase the exhaust velocity by the air 1406 exiting the nacelle 1400 as exhaust 1408. This higher exhaust velocity may be achieved relative to the free stream speed of the aircraft, resulting in thrust recovery by the system.

[0125] In some illustrative examples, a fan system including one or more fans may be located ahead of or before the heat exchanger. The ducting system, fuel cell stack, and other components of the electric engine can also be seen in the above diagram.

[0126] 15, there is shown a diagram of airflow through a nacelle to provide cooling for a fuel cell stack, according to an exemplary embodiment. As shown, arrows 1500 illustrate airflow entering a substantially rectangular inlet at the bottom of a nacelle 1502. In this example, the airflow enters the nacelle 1502 through an inlet 1551, passes through the front of a nacelle heat exchanger 1504, and exits at the back of the nacelle heat exchanger 1504. The air then passes through a fan 1506 before exiting through a nozzle 1554 at an outlet 1552 of the nacelle 1502.

[0127] As shown, the fan 1506 can augment the airflow, indicated by arrows 1500, to provide the desired airflow for cooling during various flight phases. For example, the increased airflow can be achieved while the aircraft is stationary or taxiing prior to takeoff. In this example, a ducting system 1508 is present and connected to the forward end of the heat exchanger 1504 to provide cooling for the fuel cell stack 1510. The ducting system 1508 can provide a flow of coolant through the nacelle heat exchanger 1504, where the coolant is cooled by the airflow through the heat exchanger 1504.

[0128] 16, a front view of a nacelle including a nacelle heat exchanger is shown according to an exemplary embodiment. As shown, a nacelle heat exchanger 1600 is visible in an inlet 1630 of a nacelle 1631. In this example, coolant enters through a coolant inlet 1612 and exits through a coolant outlet 1610. The coolant may be cooled by the heat exchanger 1600. The coolant may be cooled by airflow entering the inlet 1630 of the nacelle 1631 and passing through the nacelle heat exchanger 1600.

[0129] 17, a diagram of a nacelle heat exchanger is shown in accordance with an exemplary embodiment. In this figure, a top view of a heat exchanger 1600 is shown. The nacelle heat exchanger 1600 is a rectangular heat exchanger. As shown, coolant enters the nacelle heat exchanger 1600 through a coolant inlet 1612, flows through the heat exchanger 1600, and exits through a coolant outlet 1610.

[0130] 18, a diagram of airflow through a nacelle heat exchanger is shown in accordance with an exemplary embodiment. In this view, arrows 1840 indicate the flow of coolant in a close-up view of section 1704 of nacelle heat exchanger 1600. In this close-up view, coolant flows through multiple rows of tubes 1830 stacked on top of each other to form a panel 1814 for heat exchanger 1600. The coolant is cooled by airflow moving through fins 1831 in heat exchanger 1600.

[0131] Various views of heat exchanger systems implemented within a nacelle in Figures 6-18 are presented as several embodiments of the heat exchange system 500 of Figure 5. These illustrations are not meant to limit the manner in which other heat exchange systems may be implemented in other illustrative examples. Other heat exchanger shapes and component locations may exist in other illustrative examples. In other examples, two heat exchangers may be implemented in one illustrative example.

[0132] In the above example, a single inlet and a single outlet for the nacelle is shown. In other examples, the nacelle may have more than one inlet and more than one outlet. For example, there may be one inlet and one outlet in the nacelle for the main airflow to the fuel cell. There may be another inlet and another outlet for cooling airflow to a heat exchanger that cools other components such as the electric motor and motor controller. In this example, a unique installation may be used where there is a dedicated nacelle for the nacelle heat exchanger, such as FIG. 14. In this example, there is only one inlet and one outlet for the cooling airflow.

[0133] 3. High Temperature Proton Exchange Membrane (HTPEM) Fuel Cell Aircraft Start-Up with Multiple Power Generation Units

[0134] High temperature proton exchange membrane (HTPEM) fuel cells, compared to conventional low temperature proton exchange membrane (LTPEM) technology, offer a much higher operating temperature, resulting in a significant improvement in thermal drive potential, allowing for a significant miniaturization of the fuel cell aircraft thermal management system (TMS). However, when high temperature proton exchange membrane (HTPEM) fuel cells are used in the fuel cell stack, the start-up process can be a time-consuming and energy-intensive process, as they have a much higher minimum operating temperature of about 120C. In the above example, the start-up process is when the fuel cell stack is brought up to its minimum operating temperature, and then fuel and oxidant are supplied so that the fuel cell stack itself begins to generate electricity.

[0135] In an illustrative example, it is recognized and taken into account that it is desirable to reduce the time and amount of electrical energy required to start up an HTPEM fuel cell stack. This time factor can be significant because start-up time is allocated as downtime in the fuel cell aircraft conceptual operations (CONOPS). Downtime is an unwanted expense for airlines to operate an aircraft.

[0136] An element of energy is required before the fuel cell stack can be started up because no power is available from the fuel cell stack before it is at operating temperature, therefore the energy consumed for start-up comes from other sources.

[0137] Batteries are one solution for this startup energy. However, batteries have undesirable weight. Furthermore, batteries have low gravimetric energy density (Wh / kg). Therefore, minimizing the startup energy from batteries is useful in reducing the aircraft weight and improving the operational performance of this type of aircraft.

[0138] The illustrative embodiments recognize and take into account that no existing solutions exist for aerospace applications. Alternative solutions may be used in terrestrial applications of HTPEM fuel cells. These solutions may include heating the stack via a resistive electrical "blanket" or "mat" surrounding the ends of the stack, or via resistive heaters embedded in the stack end plates.

[0139] The above solutions rely on heat reaching the fuel cell stack's internal active areas through the outside of the stack. This type of heating uses a more thermally resistive heat flow path, resulting in more waste heat to the surroundings and longer time for the interior of the fuel cell stack to reach operating temperature. This situation can be particularly prevalent when the fuel cell stack has a large cross-sectional area exterior, which can result in non-uniform temperature gradients within the stack.

[0140] Another alternative approach could be to use an air management system (AMS) for the fuel cell stack to push heated air through the fuel cell stack, which wastes heat by expelling the heated air through the fuel cell stack outlet, but suffers from a poor effective thermal conductivity between the heating fluid (air) and the stack media compared to a coolant.

[0141] Other solutions can utilize heating the fuel cell stack via a cooling fluid, as described in various illustrative examples. This approach works to heat the entire fuel cell stack at once, but uses significantly more energy.

[0142] For example, in an aircraft configuration with 6 engines located in 6 nacelles, there are 3 fuel cell power units per wing. This configuration results in three times the total energy utilization compared to just powering one fuel cell stack per wing (and then using the power it generates to power the other fuel cell stacks).

[0143] Thus, illustrative examples provide methods, apparatus, and systems for minimizing the total energy and time required to complete start-up of multiple HTPEM fuel cell stacks contained on a fuel cell powered aircraft. In one illustrative example, a coolant heater and existing thermal management system are used to start up the HTPEM fuel cell stacks by heating and circulating the fuel cell stack coolant. In this example, the coolant may be used initially to heat a set of the total number of fuel cell stacks on the aircraft. After these fuel cell stacks reach operating temperature and begin generating power, this power may be used to start up the remaining fuel cell power stacks.

[0144] This feature involves a unique configuration of the coolant heating structure in the illustrative example. One illustrative example uses an electric coolant heater. This heater can be incorporated into the conduits for the coolant system so that the existing cooling pumps would be used to circulate the coolant to the stack and start-up heater. In addition to the stack minimizing start-up time and energy required, the structure would also include several bypasses to allow the coolant to blow the heater during normal operation after start-up, and bypasses to allow the coolant to blow the heat exchanger during start-up without having to heat the heat exchanger.

[0145] Other features in the illustrative examples provide a startup process that generates the amount of energy required to start up the fuel cell stacks: In configurations including multiple fuel cell stacks on an aircraft, a set of the total number of fuel cell stacks may be initially started using on-board batteries sized solely for starting up that set of fuel cell stacks.

[0146] Once one set of fuel cell stacks is started up, the power generated by that set of currently operational fuel cell power units can be used to start up the remaining fuel cell stacks. In addition, when the fuel cell stacks reach a minimum operating temperature required for electrochemical activity, a modest power draw can be applied to the operational fuel cell stacks which can supply fuel and air to the fuel cell stacks and generate enough heat to heat themselves up even faster.

[0147] Thus, in an illustrative example, direct heating of the coolant can be implemented using one set of fuel cell stacks activated by this type of heating to heat the other fuel cell stacks. This type of feature is made possible by the use of coolant heaters and the configuration of conduits that supply heated coolant to the fuel cell stacks.

[0148] Electrical cross ties can be used to allow the first warm and operational fuel cell power unit to power the coolant heaters and air management systems (AMS) of other fuel cell stacks that have not yet been powered up to start up the fuel cell stacks. This type of fuel cell stack start-up is feasible for any aircraft that contains multiple fuel cell stacks.

[0149] In another example, a combustion-based coolant heater using fuel, hydrogen, or otherwise, can be used to heat the coolant to bring the fuel cell stack up to operating temperature.

[0150] Additionally, in some instances, a ground power hook-up may be provided that is available to power up the entire stack in unison when the aircraft is near an available ground power hook-up on the ground.

[0151] This feature allows the on-board batteries to be sized just for starting up the first set of fuel cell power units, keeping the system lightweight, but allowing for even faster start-up when ground power hook-up is available and the operator is not required to rely on the energy contained within the batteries.

[0152] 19, a block diagram of a starting system for an aircraft is shown in accordance with an illustrative embodiment. In this illustrative example, a fuel cell stack starting system 1900 may be used in aircraft 400 of FIG. 4 to start operation of a fuel cell stack in engine 406 of FIG. 4.

[0153] In this illustrative example, fuel cell stack startup system 1900 is configured to initiate operation of fuel cell stack 1920. Each fuel cell stack in fuel cell stack 1920 may be located within a nacelle of an engine. In this example, fuel cell stack 1920 is a high temperature proton exchange membrane (HTPEM) fuel cell stack.

[0154] The fuel cell stack startup system 1900 is configured to assist or bring the fuel cell stack 1920 to an operating temperature 1925. The operating temperature 1925 is one or more temperatures at which the fuel cell stack 1920 generates electricity. This electricity can be used to power at least one motor or other component within the engine or elsewhere within the aircraft. The operating temperature is the temperature necessary to sustain a reaction within the fuel cell stack to generate electricity from hydrogen gas and oxygen.

[0155] For example, a fuel cell stack startup system 1900 circulates heated coolant 1916 to a set of fuel cell stacks 1920 in a fuel cell system 1921, which set of fuel cell stacks 1920 reach an operating temperature 1925 and generate power to heat coolant 1917 to start up other fuel cell stacks in the fuel cell stacks 1920 in the fuel cell system 1921. In this example, the power is electrical power.

[0156] In this illustrative example, fuel cell stack start-up system 1900 is comprised of several different components. As shown in this example, fuel cell stack start-up system 1900 includes a heater system 1901, a conduit system 1902, a pump system 1904, and a controller 1905.

[0157] In this example, heater system 1901 operates to heat coolant 1917. Heater system 1901 can include several heaters 1915. In one example, several of the heaters 1915 and fuel cell stacks 1920 can be present in each nacelle.

[0158] The heater system 1901 may include an electric heater 1930 connected to a battery system 1931. In another example, the heater system 1901 includes a fuel-fired heater 1933 that uses hydrogen gas to generate heat for the heated coolant 1916. In this example, the fuel-fired heater 1933 may be a conduit or radiator thermally connected to a component that combusts hydrogen fuel. The hydrogen fuel may be delivered to the burner element as a gas.

[0159] A conduit system 1902 is thermally connected to the heater system 1901 and the fuel cell stack 1920. A pump system 1904 is configured to circulate heated coolant 1916 through the conduit system 1902 to the fuel cell stack 1920.

[0160] In this illustrative example, the controller 1905 is a hardware system that may include software. When software is used, the processes performed by the controller 1905 may be implemented with program instructions configured to run on hardware, such as a processor unit. When firmware is used, the processes performed by the controller 1905 may be implemented with program instructions and data, stored in persistent memory, and executed on a processor unit. When hardware is used, the hardware may include circuitry that operates to execute the processes in the controller 1905.

[0161] In this embodiment, the hardware for the controller 1905 may take the form of at least one selected from a circuit system, an integrated circuit, an application specific integrated circuit (ASIC), a programmable logic device, or any other suitable type of hardware configured to perform some process. When a programmable logic device is used, the device may be configured to perform some process. The device may be later reconfigured or may be permanently set to perform some process. Programmable logic devices include, for example, programmable logic arrays, programmable array logic, field programmable logic arrays, field programmable gate arrays, and other suitable hardware devices. In addition, the process may be implemented in an organic component embedded in an inorganic component and entirely composed of non-human organic components. For example, the process may be implemented as a circuit in an organic semiconductor. The controller 1905 may also be selected from at least one of a single core processor, a dual core processor, a multi-processor core, a general purpose central processing unit (CPU), a graphics processor unit (GPU), a digital signal processor (DSP), or some other type of processor unit.

[0162] In this illustrative example, the controller 1905 can control the heater system 1901 to heat the coolant 1917 to form a heated coolant 1916. The controller 1905 can control the pump system 1904 to circulate the heated coolant 1916 through the conduit system 1902. Additionally, the controller 1905 can control the conduit system 1902 to circulate the heated coolant 1917 to a set of fuel cell stacks 1920. In this example, the heated coolant 1916 causes the set of fuel cell stacks 1920 to reach an operating temperature 1925. In this example, the operating temperature 1925 is a minimum operating temperature 1923 required for the set of fuel cell stacks 1920 to begin generating electricity.

[0163] In this illustrative example, the controller 1905 can control the conduit system 1902 to circulate the heated coolant 1916 to all of the fuel cell stacks 1920 in response to one of the fuel cell stacks 1920 reaching an operating temperature 1925. In this example, the heated coolant 1916 is being heated by the one of the fuel cell stacks 1920 that is generating power. The remaining fuel cell stacks reach the operating temperature 1925 from the heated coolant 1916 being heated by the one of the fuel cell stacks 1920. In this example, the heated coolant 1916 is being heated by the one of the fuel cell stacks 1920. The heater system 1901 no longer needs to heat the coolant 1917.

[0164] In one example, heater system 1901 is comprised of heaters 1915, where each fuel cell stack of fuel cell stacks 1920 and each heater of heaters 1915 are located within the nacelle. Also in this example, each fuel cell stack is thermally connected to a heater in heaters 1915 by conduit system 1902. In this example, controller 1905 routes electrical power generated by one set of fuel cell stacks 1920 to heaters 1915 for other fuel cell stacks that are not generating electricity.

[0165] In this example, the controller 1905 further controls the conduit system 1902 to bypass the heater system 1901 after activation of the fuel cell stacks 1920. Heating of the coolant 1917 by the heater system 1901 is no longer needed, where the fuel cell stacks 1920 are now operating at an operating temperature 1925. For example, the controller 1905 stops heating of the coolant 1917 by the heater 1915 in the heater system 1901 for the pair of fuel cell stacks 1920 that are generating power, in response to the pair of fuel cell stacks 1920 being generating power.

[0166] During this start-up process, controller 1905 controls conduit system 1902 to bypass nacelle heat exchanger 1960 during fuel cell stack start-up. Bypassing nacelle heat exchanger 1960 prevents nacelle heat exchanger 1960 from cooling heated coolant 1916. In this example, a nacelle heat exchanger of nacelle heat exchanger 1960 and a fuel cell stack of fuel cell stack 1920 reside within each nacelle.

[0167] In an illustrative example, the controller 1905 controls the conduit system 1902 to circulate the heated coolant to the number of fuel cell stacks, where the heated coolant causes the number of fuel cell stacks to reach an operating temperature 1925 for the number of fuel cell stacks 1920. In this example, the number of fuel cell stacks 1920 is selected from a group that includes all of the fuel cell stacks 1920 and a set of the fuel cell stacks 1920. In other words, the circulation of the heated coolant can be to all or a portion of the fuel cell stacks 1920.

[0168] Further, in this example, conduit system 1902 and pump system 1904 operate to circulate coolant 1917 to nacelle heat exchanger 1960 during operation of the aircraft's electric engine.

[0169] The block diagram of the fuel cell stack start-up system 1900 in FIG. 19 is not meant to imply physical or architectural limitations to the manner in which an example embodiment may be implemented. Other components in addition to or in place of the ones shown may be used. Some components may be unnecessary. The blocks are also presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an example embodiment.

[0170] For example, although not shown, conduit system 1902 may include conduits. Conduit system 1902 may also include valves, pumps, or any combination thereof.

[0171] Turning now to Figure 20, a flow diagram of a process for managing the startup of an aircraft fuel cell stack is shown in accordance with an exemplary embodiment. The process of Figure 20 may be implemented in hardware, software, or both. For example, the process may be performed within fuel cell stack startup system 1900 of Figure 19.

[0172] The process includes heating a set of fuel cell stacks in an aircraft (step 2000) so that the fuel cell stacks reach an operating temperature for generating electricity, after which the process ends.

[0173] Referring now to Figure 21, a flow diagram of a process for heating a fuel cell stack is shown in accordance with an exemplary embodiment. The process shown in the flow diagram is an example of an additional process for carrying out the process of Figure 20.

[0174] The process uses heat from the power generated by one set of the fuel cell stacks to heat the remaining fuel cell stacks to operating temperature (step 2100), after which the process ends.

[0175] Turning now to FIG. 22, a flow diagram of a process for managing the startup of an aircraft fuel cell stack is shown, according to an exemplary embodiment. The process of FIG. 22 may be implemented in hardware, software, or both. When implemented in software, the process may take the form of program instructions executed by one or more processor units located in one or more hardware devices in one or more computer systems. For example, the process may be implemented in the fuel cell stack startup system 1900 of FIG. 19.

[0176] The process begins by heating a coolant with a heater to form a heated coolant in a conduit system connected to the fuel cell stack, step 2200. In step 2200, the heater may be selected from at least one of an electric heater connected to a battery system or a fuel-powered heater that uses hydrogen gas to generate heat to heat the coolant.

[0177] The process controls a conduit system to circulate heated coolant to a pair of fuel cell stacks where the heated coolant brings the pair of fuel cell stacks to an operating temperature (operation 2202), after which the process ends.

[0178] 23, a flow diagram of a process for circulating heated coolant is shown, according to an example embodiment. This process is an example of additional steps that can be performed in conjunction with the steps of FIG.

[0179] In response to one set of fuel cell stacks reaching operating temperature, the process circulates heated coolant to all fuel cell stacks that have not reached operating temperature (step 2300), after which the process ends.

[0180] 24, a flow diagram of a process for delivering power to a fuel cell stack is shown, according to an exemplary embodiment. The steps of this flow diagram are an example of additional steps to carry out the steps of FIG. 23. In this example, each of the fuel cell stacks and each of the heaters are located within the nacelle. Additionally, each of the fuel cell stacks is thermally connected to the heater. This thermal connection may be made using a conduit system.

[0181] The process sends the power generated by one set of fuel cell stacks to heaters for other fuel cell stacks that are not generating power (step 2400), after which the process ends.

[0182] Turning now to Figure 25, a flow diagram of a process for stopping heating of a coolant is shown in accordance with an illustrative embodiment. The process shown in this flow diagram is one example of additional steps that can be performed along with the steps of Figure 22.

[0183] In response to the pair of fuel cell stacks generating electrical power, the process stops heating the coolant by the heating system for the pair of fuel cell stacks that are generating electrical power (step 2500), after which the process ends.

[0184] The flow diagrams and block diagrams in the various illustrated embodiments show the structure, functions, and operations of some possible implementations of the apparatus and methods in the exemplary embodiments. In this regard, each block in the flow diagrams or block diagrams may represent at least one of a module, a segment, a function, or a portion of an operation or step. For example, one or more blocks may be realized as program instructions, hardware, or a combination of program instructions and hardware. When implemented in hardware, the hardware may take the form of, for example, an integrated circuit that is manufactured or configured to perform one or more operations of the flow diagrams or block diagrams. When implemented as a combination of program instructions and hardware, the implementation may take the form of firmware. Each block in the flow diagrams or block diagrams may be realized using a special purpose hardware system that performs various operations, or a combination of special purpose hardware and program instructions executed by the special purpose hardware.

[0185] In some alternative implementations of the exemplary embodiments, the functions noted in the blocks may occur out of the order noted in the figures. For example, in some cases, two blocks shown in succession may be performed substantially simultaneously, or the blocks may sometimes be executed in reverse order, depending on the functionality involved. Furthermore, other blocks may be added in addition to the blocks noted in the flow diagrams or block diagrams.

[0186] 26, a diagram of an aircraft including a propulsion and power system is shown in accordance with an exemplary embodiment. The fuel cell stack starting system 1900 of FIG.

[0187] In this illustrative example, aircraft 2600 is an airplane with a fuselage 2630, a first wing 2632 and a second wing 2634, a propulsion and power system in the form of an engine 2601, and a liquid hydrogen tank 2602. The engine is an electric engine.

[0188] As shown, liquid hydrogen tank 2602 includes liquid hydrogen tank 2651, liquid hydrogen tank 2652, liquid hydrogen tank 2653, and liquid hydrogen tank 2654. In this illustrative example, two liquid hydrogen tanks are located above the wings on either side of aircraft 2600.

[0189] As shown in this figure, engine 2601 includes engine 2611, engine 2612, and engine 2613 connected to a first wing 2632. Engine 2601 also includes engine 2614, engine 2615, and engine 2616 connected to a second wing 2634. In this example, these engines are all electric engines.

[0190] In this example, each of the engines includes a nacelle, which includes a fuel cell stack, an air management system, a thermal management system, a motor, and a controller.

[0191] As shown, there are two starter batteries: starter battery 2603 is connected to engine 2613, and starter battery 2604 is connected to engine 2614. Starter battery 2603 may be used to start the fuel cell stack in engine 2613. Starter battery 2604 may be used in starting the fuel cell stack in engine 2614. In this example, one starter battery is located on each side of the aircraft 2600.

[0192] In this illustrative example, two starter batteries are depicted. In other illustrative embodiments, other numbers of starter batteries may be used. For example, an additional starter battery may be connected to engine 2612. In yet another illustrative example, a starter battery may be connected to engine 2616. In yet another illustrative example, a starter battery may be connected to more than one engine.

[0193] Turning now to Figure 27, an illustration of an integrated power and propulsion unit is shown in accordance with an illustrative embodiment, with engine 2700 exposed without a nacelle. In this illustrative example, engine 2700 is an integrated power and propulsion unit (IPPU) and is an example of the engine without a nacelle in engine 2601 shown in Figure 26.

[0194] Engine 2700 is an example of an engine that may implement a fuel cell stack starting system. Engine 2700 is made up of several different components.

[0195] In this illustrative example, heat exchanger 2701 is a rectangular heat exchanger that operates to remove heat from the fuel cell stack coolant and reject the heat to the ambient air. Fan system 2702 operates to increase airflow through heat exchanger 2701. This fan system may be used when the speed and temperature of the ambient air moving through the nacelle inlet is insufficient to reject heat from the coolant for cooling various components within engine 2700.

[0196] A fuel cell stack 2703 converts the hydrogen and oxygen reactants into electricity. In this example, the hydrogen is gaseous hydrogen and the oxygen is oxygen from the air. Heat and water are also obtained from the electricity generation using the hydrogen and oxygen reactants.

[0197] A motor 2704 and controller 2705 convert electricity generated by the fuel cell stack 2703 into rotational mechanical energy. This rotational mechanical energy is used to rotate the propeller. The controller 2705 controls the operation of the motor 2704.

[0198] A compressor 2706 compresses ambient air to provide sufficient oxygen reactant to the fuel cell stack cathode manifold. The compressor also operates to maintain cathode pressure for improved fuel cell stack performance. A turbine 2707 recovers energy from the air exiting the fuel cell stack 2703 through the fuel cell stack cathode outlet.

[0199] A hydrogen (H2) recirculation blower 2708 operates to recirculate excess unreacted hydrogen fuel that flows out of the fuel cell stack 2703 from the fuel cell anode outlet back into the fuel cell stack 2703 via the fuel cell anode inlet. In this example, the heater is a starter heater that heats the fuel cell stack 2703 to bring it up to an operating temperature, which is the temperature required to sustain the reaction to generate electricity from hydrogen gas and oxygen.

[0200] In this example, a hot coolant pump 2710 circulates coolant to remove heat from the fuel cell stack 2703 and transfers it to a heat exchanger 2701 for rejection to the air. A cold coolant pump 2711 circulates coolant to remove heat from cold components that require a lower level of cooling compared to the fuel cell stack 2703. Cold components are, for example, the motor 2704 and the controller 2705. The heat is transferred to a cold heat exchanger, not shown, as it is located closer to the LH2 tank in the wing.

[0201] Also shown is an air management system (AMS) inlet 2731. This inlet receives air for the air management system which conditions the air for the fuel cell stack 2703. The air management system includes a compressor 2706 and an air intake heat exchanger 2714.

[0202] Further in this example, exhaust 2713 is a location for exhausting humid and dirty air from the fuel cell stack cathode outlet of the fuel cell stack 2703 to the ambient air. An air supply heat exchanger 2714 thermally conditions the compressor outlet air before it enters the fuel cell stack. In this example, the air supply heat exchanger 2714 is an air supply heat exchanger that provides heat exchange for the compressed air. This conditioning can include raising or lowering the temperature of the air so that the fuel cell stack 2703 is at operating temperature. A fuel cell air supply duct 2715 directs the compressed air from the outlet of the compressor 2706 to the fuel cell stack cathode inlet of the fuel cell stack 2703. A heater 2709 can heat a coolant in a high temperature loop as part of the start-up process to heat the fuel cell stack 2703 to operating temperature.

[0203] The illustration of the aircraft 2600 in FIG. 26 and the engine 2700 in FIG. 27 are provided as diagrams of engines and aircraft in which a starting system may be implemented. This illustration is not intended to limit the manner in which other illustrative examples may be implemented. For example, an aircraft may have four engines or some other number of engines other than the six engines depicted for the aircraft 2600. In other illustrative examples, one starter battery or three starter batteries may be used instead of the two starter batteries shown for the aircraft 2600. In other illustrative examples, the fan system 2702 may be omitted. In other illustrative examples, the fan system 2702 may be located before the heat exchanger 2701. In yet another illustrative example, the heater 2709 may be omitted from the engine 2700.

[0204] 28, a schematic diagram of a fuel cell stack startup system is shown in accordance with an illustrative embodiment. In this illustrative example, fuel cell stack startup system 2800 is an example of a simplified implementation of fuel cell stack startup system 1900 of FIG.

[0205] As shown, fuel cell stack startup system 2800 includes startup heater 2802 and conduit system 2803. These two components are used to initiate operation of high temperature proton exchange membrane (HTPEM) fuel cell stack 2801. HTPEM fuel cell stack 2801 is an example of a fuel cell stack of fuel cell stack 1920 of FIG. 19, startup heater 2802 is an example of a heater of heater 1915 in heater system 1901 of FIG. 19, and conduit system 2803 is an example of conduit system 1902 of FIG. 19. As shown, conduit system 2803 includes conduit 2810, valve 2811, and valve 2812. In other illustrative examples, these two valves can be replaced with a three-way valve.

[0206] During start-up, the HTPEM fuel cell stack 2801 is warmed up to operating temperature. As shown, valve 2811 is in a closed position and valve 2812 is in an open position. With these valve positions, air flows through the start-up heater 2802 instead of the ram air radiator 2815.

[0207] As part of the process to heat the HTPEM fuel cell stack 2801 to an operating temperature, air is heated by start-up heater 2802 and directed to the HTPEM fuel cell stack 2801. When the HTPEM fuel cell stack 2801 is at operating temperature, valve 2811 moves to an open position and valve 2812 moves to a closed position such that coolant now flows through ram air radiator 2815 instead of start-up heater 2802. Ram air radiator 2815 now operates to provide cooling to maintain the HTPEM fuel cell stack 2801 at an operating temperature.

[0208] 29, a schematic diagram of a fuel cell stack start-up system within a thermal management system is shown according to an exemplary embodiment. Fuel cell stack start-up system 2997 within thermal management system 2996 is an example of an implementation of fuel cell stack start-up system 1900 of FIG. 19. Fuel cell stack start-up system 2997 illustrates components that may be implemented in a wing that includes three nacelles. For example, the wing may be first wing 2632 of aircraft 2600 of FIG. 26. In this example, components are shown for nacelle 2950, ​​nacelle 2952, and nacelle 2954. Each of the nacelles may include the components illustrated in fuel cell stack start-up system 2800 of FIG. 28.

[0209] In this illustrative example, the flow of fluid between the components within the nacelle is facilitated using a conduit system 2990. The conduit system 2990 includes conduits such as tubes and pipes. Flow through these conduits may be controlled by valves to create various flows through the conduit system to the heat load 2933. Additionally, the conduit system may include pumps to move the fluid. The conduit system 2990 is connected to the various components. The connections may be thermal connections where the fluid is not exchanged between components. In other examples, the connections may include introducing or receiving fluid from one component.

[0210] As shown, the HTPEM fuel cell stack 2920 is located in a nacelle 2950. Other components in the nacelle include a start-up heater 2910, a pump 2911, a charge air heat exchanger (HX) 2912, a high temperature and low temperature loop heat exchanger (HT-LT HX) 2914, and a ram air radiator 2927. In this example, a liquid hydrogen heat exchanger (LH2 LX) 2913 is located closer to the LH2 tank 2971, for example in the wing. In some examples, this component may be present in each of the three nacelles or in only one of the nacelles. The nacelle also includes a heat load 2933. This heat load may include controllers, tanks, electric motors, and other components that generate heat. A pump 2925 is used to move coolant to the heat load.

[0211] In this depicted example, the HTPEM fuel cell stack 2921 is located within a nacelle 2952. Other components within the nacelle include a start-up heater 2930, a pump 2931, a charge air heat exchanger (HX) 2932, and a ram air radiator 2937. The nacelle also includes a heat load 2934. A pump 2935 is used to move coolant to the heat load.

[0212] The HTPEM fuel cell stack 2922 is located within a nacelle 2954. Other components within the nacelle include a start-up heater 2960, a pump 2961, a charge air heat exchanger (HX) 2966, and a ram air radiator 2967. The nacelle also includes a heat load 2945. A pump 2965 is used to move coolant to the heat load.

[0213] In this configuration, the high temperature (HT) cooling loop in each nacelle is isolated from the other nacelles. Isolation between the cooling loops can be achieved by closing valves 2981, 2982, and 2983.

[0214] In this example, a start-up process is performed for HTPEM fuel cell (FC) stack 2920, HTPEM fuel cell (FC) stack 2921, and HTPEM fuel cell (FC) stack 2922. The start-up process involves starting up a subset of these HTPEM fuel cell stacks. In this example, the subset of HTPEM fuel cell stacks is TPEM fuel cell stack 2920 in nacelle 2950.

[0215] Starting up the HTPEM FC stack 2920 in this configuration may be accomplished as follows.

[0216] 1. A start battery is used to power the start heater 2910 and the coolant pump 2911. The starter battery can be battery 2603 in FIG.

[0217] 2. Open bypass valve 2973 to allow coolant to bypass the ram air radiator 2927, ensuring that the ram air radiator 2927 does not heat up. At this point, this is used to heat the coolant to bring the fuel cell stack up to operating temperature. As a result, there is no need to dump heat.

[0218] As shown, flow path 2995 is a loop that allows start-up heater 2910 to heat coolant to form a heated coolant that is used to heat HTPEM fuel cell stack 2920. As shown in this flow path, coolant flows from the supply air HX 2912 to the start-up heater 2910, which heats the coolant. The heated coolant flows to the HTPEM FC stack 2920 and then back to the supply air HX 2912. In this example, a pump 2911 operates to move the coolant through this flow path.

[0219] In this example, a flow of heated coolant passes through one pair of fuel cell stacks, HTPEM FC stack 2920. Furthermore, in this example, the flow is for HTPEM fuel cell stack 2920 and not for the other fuel cell stacks, i.e., HTPEM fuel cell stack 2921 and HTPEM fuel cell stack 2922.

[0220] 3. Once the HTPEM FC stack 2920 has reached its minimum operating temperature for electrochemical activity, air (via running the air management system) and fuel can be supplied to the stack, allowing the HTPEM FC stack 2920 to begin generating electricity on its own. Additionally, applying power draw to the HTPEM FC stack 2920 at this point will help the HTPEM FC stack 2920 heat itself up due to its inefficient operation at this initial operating temperature.

[0221] At this stage, power generated by the HTPEM FC stack 2920 can be used to power start-up heaters in adjacent nacelles to begin warming up those HTPEM fuel cell stacks. Once those HTPEM fuel cell stacks reach operating temperature, air (via the single started stack powering their air management systems) and fuel can be supplied to the remaining fuel cell stacks, allowing them to begin generating power themselves.

[0222] For example, the high temperature (HT) coolant loops in each nacelle can be connected together to form one loop. This connection can be made in response to the HTPEM FC stack 2920 reaching operating temperature. In other words, coolant from that fuel cell stack can be used to heat the other fuel cell stack. This type of heating can be in addition to or instead of using start-up heaters in the other nacelles. This connection can be made by placing valves 2981, 2982, and 2983 in the open position.

[0223] As a result, heated coolant from the HTPEM FC stack 2920 can be exchanged with coolant in other HT coolant loops for other nacelles to begin heating those fuel cell stacks. In this manner, heat generated by a fuel cell stack that has reached operating temperature can be used to heat other fuel cell stacks in other nacelles to begin operation of those fuel cell stacks. Additionally, this type of heating allows beneficial use of start-up heaters in other nacelles. As a result, some heaters can be eliminated, reducing the weight of the aircraft.

[0224] In another illustrative example, this type of heating may be performed in conjunction with an electrical heating approach using a starter trigger. As a result, the use of a start-up heater and exchange of coolant with the rest of the nacelle can increase the speed at which the fuel cell stack reaches operating temperature and begins generating electricity.

[0225] Referring now to Figure 30, a schematic diagram of a fuel cell stack start-up system within a thermal management system is shown, according to an exemplary embodiment. Figure 30 illustrates a configuration of a fuel cell stack start-up system 2997 where the high temperature cooling loops are separated from each other by a configuration of a conduit system 2990. There are no conduits that allow for exchange of high temperature coolant between the fuel cell stacks in the various nacelles.

[0226] In this example, start-up of HTPEM fuel cell (FC) stack 2921 and HTPEM fuel cell (FC) stack 2922 is initiated by power sent from HTPEM fuel cell (FC) stack 2920 to start-up heaters 2930 and 2960 in response to start-up heater 2910 generating heat that causes HTPEM fuel cell (FC) stack 2920 to reach operating temperature. In this example, coolant is not exchanged between the various nacelles for use during start-up of the various HTPEM fuel cell stacks.

[0227] The diagram of the fuel cell stack start-up system 2997 in Figures 29 and 30 is presented as an implementation diagram of the fuel cell stack start-up system 1900 of Figure 19. The above diagrams are not intended to limit the manner in which other fuel cell stack systems may be implemented in other embodiments. For example, a single start-up heater may be used in other fuel cell stack start-up systems. In other illustrative examples, a battery may be included in the nacelle.

[0228] 4. Low-temperature proton exchange membrane intake air heat exchanger

[0229] Low temperature proton exchange membrane (LTPEM) fuel cells offer a zero-emission, highly efficient form of generating propulsion or system power for aircraft. When oxygen is used as one of two reactants, LTPEM fuel cell systems often utilize a compressor to maintain an adequate supply of reactant and increase performance through increased cathode pressure. In this example, the other reactant is gaseous hydrogen (H2).

[0230] However, this compression can cause the inlet air to be heated above the operating temperature of the fuel cell stack.To avoid overheating in this situation, a cooling stage can be used.

[0231] In an illustrative example, cooling load and cooling efficiency are factors in a cooling design system. It is desirable to reduce the overall cooling load of a cooling system and increase its efficiency.

[0232] Existing low temperature proton exchange membrane (LTPEM) fuel cell systems require a compressor to sufficiently pressurize ambient air to supply one of the two reactants, usually oxygen from the air, to the fuel cell stack. When the air is compressed, heat is also added. Depending on the degree of compression, cooling the air before it enters the fuel cell stack can be done to prevent overheating and drying out of the fuel cell stack membrane.

[0233] This cooling can be achieved using an air-to-liquid intercooler. However, this type of cooling places an air cooling burden on the stack cooling system. Additionally, current systems do not take advantage of the energy recovery opportunities.

[0234] The illustrative examples provide methods, apparatus, and systems for cooling incoming compressed air while enabling the capture of significant amounts of energy to improve overall system efficiency.

[0235] In one illustrative example, an intake air heat exchanger is used. This component utilizes cooled exiting stack air to cool the hot compressed air at the compressor outlet. The effect of this type of cooling is two-fold. It cools the compressed air, allowing for the reduction of air-to-liquid intercoolers compared to not having this type of cooling. Additionally, the use of an intake air heat exchanger allows for increased energy recovery by the air management system (AMS) expander by heating the stack exhaust, increasing overall system efficiency.

[0236] Cooling of the compressor exit air is currently done using charge air intercoolers. In ground vehicle fuel cell systems, the heat input to the air during compression is small due to the relatively low pressure ratio between the stack and the ambient air. As a result, for the same reason, energy recovery is not very good through expansion.

[0237] The supply air heat exchanger is a heat exchanger that provides heat exchange for the compressed air. The heat exchanger can cool the compressed air. The supply air intercooler is an intercooler that provides cooling for the compressed air in this example.

[0238] The illustrative examples recognize and take into account that systems used in ground vehicles often have no inlet air cooling or simply use a single air-to-liquid intercooler. Similarly, current systems in vehicles often do not include expanders for energy recovery.

[0239] The illustrative examples recognize and take into account that current cooling in ground vehicle systems does not account for the widely varying altitudes at which aircraft operate compared to ground targets. Aircraft operating at varying altitudes increase the need to cool the compressed air and have energy available for recovery at the stack outlet. The illustrative examples use a heat exchanger in addition to an intercooler to reduce cooling loads and increase energy recovery by the expander. In some illustrative examples, an intake air heat exchanger can take the place of an intake air intercooler, rather than simply downsizing the intake air intercooler.

[0240] Referring now to Figure 31, a block diagram of an aircraft management system is shown in accordance with an illustrative embodiment. In this illustrative example, an aircraft air management system 3100 may be implemented within the aircraft 400 of Figure 4 to manage the operation of a fuel cell stack within the engine 406 of Figure 4 to generate electricity to run the engine 406 of Figure 4. In this example, the aircraft air management system 3100 manages the temperature of air delivered to the fuel cell stack 3105, which is used along with gaseous hydrogen to generate electricity to run electric motors within the aircraft. In this example, the fuel cell stack 3105 includes a low temperature proton exchange membrane fuel cell 3113.

[0241] In one illustrative example, the aircraft air management system 3100 includes several different components. As shown, the aircraft air management system 3100 includes an air heat exchanger 3102, an intercooler 3103, and a ducting system 3104.

[0242] In this example, the conduit system 3104 is connected to the air heat exchanger 3102, the intercooler 3103, and the fuel cell stack 3105. Heated air 3120 flows through the conduit system 3104 to the air heat exchanger 3102. The air heat exchanger 3102 cools the heated air 3120 to form cooler air 3122. In this example, the cooler air 3122 flows from the air heat exchanger 3102 through the conduit system to the intercooler 3103. The intercooler 3103 further cools the cooler air 3122 to form cooled air 3124. The cooled air 3124 flows from the intercooler 3103 through the conduit system 3104 to the fuel cell stack 3105. In this example, the cooled air 3124 is at an operating temperature 3125 for the fuel cell stack 3105.

[0243] In one illustrative example, the air heat exchanger 3102 , the intercooler 3103 , the ducting system 3104 , and the fuel cell stack 3105 are located within a nacelle 3130 for an engine 3132 of the aircraft 3127 .

[0244] Additionally, the aircraft air management system 3100 includes a compressor 3107. The compressor 3107 is connected to the duct system 3104 and generates heated air 3120 that flows through the air heat exchanger 3102.

[0245] In this example, the heated air 3120 , the cooler air 3122 , and the cooled air 3124 are compressed air 3126 .

[0246] Additionally, the conduit system 3104 also guides gaseous hydrogen 3140 from the fuel system 3142 to the fuel cell stack 3105. The gaseous hydrogen 3140 can be used together with the cooled air 3124 to generate electricity to power the engines 3132 of the aircraft 3127. The above guidance is performed using separate conduits, so that the gaseous hydrogen 3140 is separate from the air flowing through the conduit system 3104. In other words, a first portion 3111 of the conduit system 3104 can be used to guide air, and a second portion 3112 of the conduit system 3104 can be used to guide the gaseous hydrogen 3140. The above two portions are separate from each other, so that the air and the hydrogen do not mix in the conduit system 3104.

[0247] In this example, a second portion 3112 of the conduit system 3104 directs the cooled air 3124 from the intercooler 3103 to a fuel system heat exchanger 3141 which heats liquid hydrogen 3143 to form gaseous hydrogen 3140. The second portion of the conduit system 3104 directs the gaseous hydrogen 3140 from the fuel system heat exchanger 3141 to a fuel cell stack 3105. The fuel cell stack 3105 uses the oxygen in the cooled air 3124 and the gaseous hydrogen 3140 to generate electricity. In this example, the air is cooled to an operating temperature 3125.

[0248] The block diagram of the aircraft air management system 3100 in Figure 31 is not meant to imply physical or architectural limitations to the manner in which an illustrative embodiment may be implemented. Other components in addition to or in place of the illustrated components may be used. Some components may be unnecessary. The blocks are also presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an illustrative embodiment.

[0249] For example, in other illustrative examples, a nacelle may have more than one fuel cell stack. As another example, conduits may be present in conduit system 3104. In addition, valves and pumps may also be present in conduit system 3104. Although not shown in this block diagram, aircraft 3127 may include other engines in addition to engine 3132.

[0250] Additionally, the aircraft air management system 3100 may heat the compressed air 3126 against the coolant flowing through the air heat exchanger 3102 according to the temperature of the compressed air to heat or cool the heated air 3120 from the compressor 3107. In this manner, the compressed air 3126 may be managed for use in the fuel cell stack 3105 by heating and cooling the compressed air 3126 against the coolant in the air heat exchanger 3102 according to the temperature of the compressed air 3126. In this manner, the temperature of the compressed air 3126 reaching the fuel cell stack 3105 may be managed to have a temperature sufficiently close to the temperature of the fuel cell stack 3105 that allows for a desired level of power generation. This type of management of the compressed air 3126 reaching the fuel cell stack 3105 is done to prevent the development of a temperature gradient that reduces efficiency in power generation. As a result, the compressed air 3126 may be heated or cooled to have a temperature relative to an operating temperature for the fuel cell stack 3105 that provides a desired level of power generation.

[0251] Referring now to Figure 32, a flow diagram of a process for air management is shown, according to an exemplary embodiment. The process of Figure 32 may be implemented in hardware, software, or both. For example, the process may be performed within the fuel cell start-up system 1900 of Figure 19.

[0252] The process begins by receiving heated air (step 3200). The process includes cooling the heated air using an air heat exchanger to form cooler compressed air (step 3202).

[0253] The process then cools the cooler air using an intercooler to form cooled compressed air (step 3204). The process then delivers the cooled air to the fuel cell stack (step 3206). The process then ends.

[0254] In Figure 32, the air heat exchanger, intercooler, and fuel cell stack are located in a nacelle for an aircraft engine. Ambient air is directed through the nacelle inlet into the compressor where it is pressurized. The now higher pressure and temperature air is ducted to the intake air heat exchanger where it is conditioned towards the temperature of the fuel cell stack by heat exchange with the fuel cell stack cathode outlet.

[0255] The temperature of the air is then further adjusted towards the fuel cell stack temperature via heat exchange with the fuel cell stack coolant in the charge air intercooler before the air is delivered to the stack. Within the fuel cell stack, oxygen in the air reacts with hydrogen as the air travels across the individual fuel cells. While exiting the stack, the contaminated air also removes a certain amount of water product to prevent the fuel cell stack from flooding with water.

[0256] Once the air leaves the stack, it passes through an intake air heat exchanger, then through a turbine where it expands and energy is recovered for use towards the compression process, and finally, the air exits the nacelle as exhaust.

[0257] 33, a schematic diagram of an aircraft air management system is shown, according to an exemplary embodiment. Air management system 3301 operates to cool a fuel cell stack 3304. In this example, air management system 3301 includes a charge air exchanger (HX) 3300, and a charge air intercooler 3302, and a ducting system 3303. Air management system 3301 also includes a compressor (C) 3305, a turbine (T) 3311, a motor 3341, and a controller 3342.

[0258] In this example, a compressor (C) 3305 receives inlet air 3317 entering the nacelle and produces compressed air 3318 at a higher pressure than the inlet air 3317. In this example, the compressor 3305 is run by a motor 3341 under the control of a controller 3342. Additionally, the compressed air 3318 has a higher temperature, which may be higher than desired for use in the fuel cell stack 3304. This compressed air can be cooled using a supply air heat exchanger (HX) 3300 and a supply air intercooler 3302 to form cooled compressed air 3319. The supply air heat exchanger (HX) 3300 initially cools the compressed air 3318 to form cooler air 3321. This cooler compressed air can be further cooled by the supply air intercooler 3302, which provides additional cooling, to form cooled compressed air 3319. The result is cooled compressed air 3319 which may have a desired temperature for use in the fuel cell stack 3304 .

[0259] The cooled compressed air 3319 can be used in the fuel cell stack 3304 along with gaseous hydrogen received from a hydrogen (H2) fuel system 3320 to generate electricity used to power components such as a motor 3341 and a controller 3331. The motor 3341 itself provides the mechanical energy to rotate the thruster 3362. The hydrogen fuel system 3320 can include a liquid hydrogen tank, a recirculation system, a cooling system, and other components. In this example, a liquid hydrogen heat exchanger converts the liquid hydrogen to gaseous hydrogen used in the fuel cell stack 3304.

[0260] In this example, compressed air 3323 that has flowed through the fuel cell stack 3304 is directed to the air intake heat exchanger 3300 which heats it as part of the process of cooling the compressed air received from the compressor 3305 to form heated air 3326. This heated air is used to turn the turbine 3311. In this manner, some energy recovery may be achieved using the turbine 3311. Air 3328 from the turbine 3311 is then directed to exit the engine as exhaust 3313.

[0261] A thermal management system operates to remove heat from the fuel cell stack 3304, motor 3341, and controller 3331, and a ram air radiator 3350 rejects this heat to the ambient air. The thermal management system uses a liquid hydrogen heat exchanger (LH2 HX) 3335 to warm and vaporize liquid hydrogen fuel for use in the fuel cell stack 3304.

[0262] 5. Fuel Cell Aircraft Thermal Management System

[0263] Both low temperature proton exchange membrane (LTPEM) and high temperature proton exchange membrane (HTPEM) fuel cells offer a zero carbon emission method for powering aircraft. Additionally, this type of fuel cell may also provide a highly efficient way to generate propulsion and / or electrical power for aircraft systems.

[0264] Lower operating temperatures compared to conventional turbomachinery lend themselves to large and heavy thermal management system (TMS) designs, and in an illustrative example, provide features to manage and dump heat generated by fuel cell propulsion systems onboard aircraft while reducing undesirable effects on performance.

[0265] High temperature proton exchange membrane (HTPEM) technology can be used to provide a higher thermal driving potential that allows for significant downsizing of the thermal management system. In this example, the thermal driving potential is the temperature difference between the ambient air and the coolant leaving the fuel cell. However, other electric propulsion elements such as the motor, its controller, and other power electronics typically operate at temperatures much closer to the LTPEM fuel cell. Thus, an aircraft powered by HTPEM fuel cells can use a variety of approaches to thermal management beyond having a single shared system for all electric propulsion elements.

[0266] In this illustrative example, a dual loop configuration is utilized to maintain separate temperature control of the HTPEM fuel cell stack and the cooler motors and controllers, cooling fans, and propulsion in the nacelle used for the fuel cell air management system. In other words, one loop can be used for the HTPEM fuel cell stack and the other loop can be used for heat generating elements separate from the fuel cell stack, which can be the motors, controllers, and power electronics in the nacelle.

[0267] In one example, a low temperature (LT) cooling loop includes a coolant that cools heat generating components, also referred to as heat loads. The coolant in this loop is cooled by the heat capacity of the on-board liquid hydrogen fuel. This type of cooling ensures that sufficient heat is available to vaporize the liquid hydrogen for the fuel cell stack. This type of cooling can also eliminate the need for a dedicated nacelle heat exchanger for lower temperatures and liquid cooled components in the nacelle.

[0268] The high temperature (HT) loop includes a coolant that is used to manage the temperature of the fuel cell stack. Managing the temperature can include at least one of heating and cooling the fuel cell stack to maintain the fuel cell stack at an operating temperature. In this example, there can be a range of operating temperatures.

[0269] In this example, the high temperature loop includes a nacelle heat exchanger, which is a different heat exchanger than that used in the low temperature loop in the engine nacelle. Separate low temperature (LT) and high temperature (HT) cooling loops are used to cool the fuel cell stack and the lower temperature thermal loads, which are lower in temperature than the fuel cell stack. These separate cooling loops eliminate the need for a separate low temperature heat exchanger and improve the effectiveness and efficiency of the cooling elements by reducing the mass and drag created by the thermal management system.

[0270] 34, a block diagram of an aircraft thermal management system is shown in accordance with an exemplary embodiment. In this illustrative example, an aircraft thermal management system 3400 may be implemented in the aircraft 400 of FIG. 4 to provide on-the-fly thermal management of the fuel cell stack and other components that generate heat in the engine 406 of FIG. 4. In this example, the aircraft thermal management system 3400 provides cooling for the fuel cell stack 3405 and a number of heat loads 3416. In this example, the fuel cell stack 3405 includes a high temperature proton exchange membrane fuel cell. The number of heat loads 3416 include at least one of an electric motor, a power circuit, a controller, power electronics, or any other component that generates heat.

[0271] In this illustrative example, aircraft thermal management system 3400 includes a hot loop 3401 and a cold loop 3402. Hot loop 3401 and cold loop 3402 are located within a ducting system 3403.

[0272] The high temperature loop 3401 is configured to manage the temperature 3410 of the fuel cell stack 3405 in the nacelle 3406 using the nacelle heat exchanger 3421. In managing the temperature 3410 of the fuel cell stack 3405, the high temperature loop 3401 is configured to maintain the fuel cell stack 3405 at an operating temperature 3411. Maintaining the temperature 3410 may include at least one of cooling or heating the fuel cell stack 3405. The operating temperature 3411 may be a range of temperatures within an operating temperature limit 3419. In this example, the operating temperature limit 3419 is the maximum or minimum temperature at which the fuel cell stack 3405 may operate.

[0273] In this example, the high temperature loop 3401 can utilize a coolant 3430 in the high temperature loop 3401 to remove heat from the fuel cell stack 3405. The coolant 3430 can be cooled to a temperature sufficient to cool or heat the fuel cell stack 3405 using the nacelle heat exchanger 3421, This causes the temperature 3410 of the fuel cell stack 3405 to have an operating temperature 3411 that is within the operating temperature limits 3419 of the fuel cell stack 3405 .

[0274] In this example, heat is removed in response to operating temperature 3411 being greater than operating temperature limit 3419. In this example, the temperature of coolant 3430 is less than operating temperature 3411. In this case, heat is removed from fuel cell stack 3405 and sent into the ambient air as part of the exhaust air leaving nacelle 3406. In response to operating temperature 3411 being less than operating temperature limit 3419, heat may be added to fuel cell stack 3405. In other words, coolant 3430 may be warmer than operating temperature limit 3419. For example, coolant 3430 may be warmer than operating temperature limit 3419 during start-up of the fuel cell stack.

[0275] The cryogenic loop 3402 is configured to use the heat capacity 3407 of the liquid hydrogen 3408 to cool several heat loads 3416 in the nacelle 3406. In this example, the heat capacity 3407 of the liquid hydrogen 3408 may be used to cool a coolant 3430 in the cryogenic loop 3402 that is used to cool several heat loads 3416.

[0276] The heat capacity 3407 of the liquid hydrogen 3408 is the energy required to convert the liquid hydrogen 3408 to gaseous hydrogen 3445. Converting the liquid hydrogen 3408 to gaseous hydrogen 3445 cools the coolant. As a result, this cooled coolant can be used to cool the heat load 3416.

[0277] In this illustrative example, when the supply air heat exchanger 3420 is used to add or remove heat from the exhaust of the fuel cell stack 3405, heated compressed air 3440 generated by the compressor 3441 can be conditioned in the conduit system 3403 by the supply air heat exchanger 3420 to form compressed air close to the operating temperature 3411. This compressed air is sent into the fuel cell stack 3405 for use with gaseous hydrogen 3445 to generate electricity.

[0278] In an illustrative example, there may also be an air intercooler 3460. In this example, the air intercooler 3460 may receive cooled compressed air 3444 from the supply air heat exchanger 3420 via the conduit system 3403, further condition the cooled compressed air 3444, and deliver the cooled compressed air 3444 near the operating temperature 3411 into the fuel cell stack 3405. The cooled compressed air 3444 reacts with gaseous hydrogen 3445 in the fuel cell stack 3405 to generate electricity.

[0279] In this illustrative example, the heated compressed air 3440 may be based on the temperature 3410 of the fuel cell stack 3405. If the air in the heated compressed air 3440 is too much above the operating temperature 3411 of the fuel cell stack 3405, the heated compressed air 3440 is cooled. If the air in the heated compressed air 3440 is too much below the operating temperature 3411 of the fuel cell stack 3405, the heated compressed air 3440 is heated. A threshold value may be used to determine when the temperature is too much below or above.

[0280] The ability to heat or cool the heated compressed air 3440 can be controlled by the temperature of the coolant, which can be controlled by varying the rate at which the coolant is pumped and / or the amount of airflow through the nacelle heat exchanger 3421.

[0281] Thus, in this illustrative example, high temperature loop 3401 serves to cool fuel cell stack 3405. In some cases, this loop may heat fuel cell stack 3405, such as during start-up of fuel cell stack 3405. In another loop, low temperature loop 3402 serves to cool a set of heat loads 3416. As a result, these two loops can provide flexibility to cool various components that may require different levels of cooling.

[0282] Additionally, in an illustrative example, it may be possible to condition the air used along with the gaseous hydrogen 3445 in the fuel cell stack 3405 to generate electricity. This conditioning of the air from the compressor 3441 may include at least one of heating and cooling according to the temperature of the heated compressed air 3440 and the temperature 3410 of the fuel cell stack 3405.

[0283] The block diagram of the aircraft thermal management system 3400 in FIG. 34 is not meant to imply physical or architectural limitations to the manner in which an illustrative embodiment may be implemented. Other components in addition to or in place of the components shown may be used. Some components may be unnecessary. The blocks are also presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an illustrative embodiment.

[0284] For example, in other illustrative examples, the nacelle 3406 can include one or more fuel cell stacks in addition to the fuel cell stack 3405. In other examples, other heat exchangers, such as intercoolers, can be used in conjunction with the intake air heat exchanger 3420 to further cool the heated compressed air 3440.

[0285] With reference now to Figure 35, an illustration of a flow diagram of a process for thermal management is shown in accordance with an illustrative embodiment. The process of Figure 35 may be implemented in hardware, software, or both. For example, the process may be performed within aircraft thermal management system 3400 of Figure 34.

[0286] The process begins by managing the temperature of the fuel cell stack in the nacelle using a nacelle heat exchanger in a high temperature loop (operation 3500).

[0287] The process includes cooling several heat loads in the nacelle using a cryogenic loop that uses the heat capacity of the liquid hydrogen to cool the several heat loads (operation 3502). The process then ends.

[0288]

[00103] Referring now to Figure 36, a flow diagram of a process for managing the temperature of a fuel cell stack is shown, according to an exemplary embodiment. The process shown is an example of an implementation of the process 3500 of Figure 35.

[0289] The process includes maintaining the fuel cell stack at an operating temperature (step 3600). The process then ends. In step 3600, air is heated by a coolant such that the fuel cell stack is maintained at an operating temperature. The operating temperature may be a range of temperatures at which the fuel cell stack 3405 operates to generate electricity. Additionally, the operating temperature may be a temperature that provides a desired level of power generation. In other words, there may be other temperatures at which the fuel cell stack 3405 is unable to generate power at a desired level.

[0290] Referring now to Figure 37, there is shown a diagram of a thermal management system including a cold loop 3781 and a hot loop 3782 according to an exemplary embodiment. This diagram illustrates a hardware design contained within a single nacelle for a high temperature proton exchange membrane (HTPEM) configuration. The fuel cell stack in this example is fed with air from the air management system and hydrogen from the fuel system to generate electricity. The electricity is used to power an electric motor through its controller, which itself rotates a propeller.

[0291] As shown, a thermal management system 3700 is located within the engine nacelle. The system includes a hot loop 3781 and a cold loop 3782. The hot loop 3781 removes heat from the fuel cell stack 3710 using a ram air radiator 3702.

[0292] The cold loop 3782 uses the heat capacity of the liquid hydrogen in the hydrogen fuel system 3711 to cool a heat load that generates less heat than the fuel cell stack 3710. In other words, the cold loop 3780 cools a coolant that is heated by the heat load as part of the process of producing gaseous hydrogen from the liquid hydrogen in the hydrogen fuel system 3711.

[0293] As shown, inlet air 3701 passes through a ram air radiator 3702 to cool a coolant that is part of a high temperature cooling loop. In this example, the coolant is used to cool a fuel cell stack 3710. The result of this cooling is exhaust air 3703, which is at an increased temperature compared to the inlet air 3701.

[0294] As shown, air management system 3704 includes compressor 3705, turbine 3706, motor 3707, and controller 3708. The air management system can be used to generate compressed air for use in fuel cell stack 3710. The air can be used along with gaseous hydrogen from hydrogen (H2) fuel system 3711 to generate electricity. Liquid hydrogen from hydrogen fuel system 3711 is converted to gaseous hydrogen using liquid hydrogen heat exchanger (LH2 HX) 3712.

[0295] In this example, inlet air 3701 is received as ambient air at the nacelle inlet and directed to compressor 3705 and ram air radiator 3702 .

[0296] In the compressor 3705, the air pressure is increased to form compressed air. The compressed air also has a higher temperature. This compressed air is sent to the charge air intercooler 3720 where the compressed air is conditioned towards the fuel cell stack temperature by exchanging heat with a hot HT coolant. The air may be conditioned by heating or cooling the air according to the temperature of the compressed air. This conditioning may include at least one of heating or cooling by the charge air intercooler 3720. The heating or cooling is based on the temperature of the coolant relative to the temperature of the compressed air.

[0297] The conditioned air is delivered to the fuel cell stack 3710. Within the fuel cell stack 3710, oxygen in the conditioned air is reacted with gaseous hydrogen as the air moves across the individual fuel cells in the fuel cell stack 3710.

[0298] Air exits the fuel cell stack 3710. This air is contaminated and may contain some amount of water products to prevent the fuel cell stack 3710 from flooding with water. As the air leaves the fuel cell stack 3710, it passes through a turbine 3706. The air expands to recover energy used in the compression process and the air is exhausted as exhaust 3703.

[0299] Ram air radiator 3702 removes heat from the hot coolant in hot loop 3781 and heats the air before it is exhausted in exhaust 3703.

[0300] Hydrogen is stored in liquid form in the tanks of the hydrogen fuel system 3711. This liquid hydrogen is distributed through fuel lines into the LH2 HX 3712. This component heats the liquid hydrogen to form gaseous hydrogen. The gaseous hydrogen is heated using heat exchange with a cryogenic coolant in the cryogenic loop 3782. The gaseous hydrogen can then be sent to the fuel cell stack 3710.

[0301] In this example, high temperature (HT) loop 3781 utilizes liquid coolant flowing through fuel cell stack 3710 to remove heat generated during the fuel cell reaction. The heated coolant is sent to ram air radiator 3702 where the heat in the coolant is rejected to the ambient air via ram air radiator 3702.

[0302] The cryogenic loop 3782 uses a liquid coolant to remove heat from a cryogenic thermal load, which may be, for example, a motor 3750, a controller 3751, and other power electronics such as a direct current to direct current (DC / DC) converter 3752. The heat in the coolant is transferred to the liquid hydrogen in the LH2 HX 3712, causing it to vaporize.

[0303] In this example, a motor 3750 rotates a pusher 3795. A high power bus 3786 connects the fuel cell stack 3710 to other electrical components.

[0304] In some cases, the heat load does not generate enough heat to vaporize a sufficient amount of hydrogen fuel. In this case, the high temperature to low temperature heat exchanger (HT-LT HX) 3731 can provide supplemental heating of the hydrogen. In this example, the HT-LT HX 3731 transfers heat from the hot coolant to the cold coolant to prevent insufficient liquid hydrogen vaporization, thus avoiding thermal shock damage to the fuel cell stack.

[0305] A high temperature loop 3781 within the thermal management system 3700 serves to remove heat from the fuel cell, and a ram air radiator 3702 rejects the heat to the surrounding air.

[0306] The LH2 HX3712 operates to remove heat from the cryogenic coolant and cryogenic loop 3782, which itself cools thermal loads such as the motor 3750 and controller 3751, and also warms and vaporizes the hydrogen fuel before it enters the fuel cell stack 3710. The HT-LT HX3731 operates to provide additional heat that is used to warm the hydrogen fuel when the lower thermal loads do not provide enough heat. In one illustrative example, this feature may be implemented within the nacelle.

[0307] 6. Liquid Hydrogen Supply System for Fuel Cell Powered Aircraft

[0308] In this illustrative example, hydrogen for a fuel cell powered aircraft is stored as liquid hydrogen. Hydrogen may be stored as a liquid at approximately -218°C, or approximately -424F. The storage tank is a liquid hydrogen tank, such as a cryogenic tank that stores hydrogen under pressure. Various techniques may be used to maintain pressure within the liquid hydrogen tank, ensure that venting can be done safely without freezing the vent outlet, and direct the vent gas away from the aircraft. This venting of gas prevents undesired overpressurization of the tank.

[0309] Maintaining pressure in the fuel tank can be accomplished by taking liquid hydrogen from a common fuel line, running it through a heat exchanger, then compressing it to 0.34 bar, or about 5 psi, and pumping it back to the top of the tank. The system can be activated or deactivated based on a set of pressure transducers operated by voting logic (a majority of the pressure sensors must be below the pressurization system activation point).

[0310] Liquid hydrogen tanks may use a venting system to prevent overpressurization due to ambient heat leakage. Due to the highly flammable nature of hydrogen, the venting system may include a burn arrestor at the end to prevent back flashing up the vent line if the vented hydrogen burns. The burn arrestor design works to trap water in a narrow passageway. If a pulse of warm hydrogen is vented before the relief valve opens, this action will push any water out of the burn arrestor. This process is done to prevent water that freezes in the burn arrestor due to the extremely low temperatures from venting hydrogen and blocking flow through the burn arrestor.

[0311] In addition to pushing water out, a heater can be added to the burn arrestor to prevent ice buildup when ambient temperatures may cause freezing.

[0312] An illustrative example recognizes that a concern with safely venting hydrogen gas is to vent the hydrogen gas in a manner that does not impact the aircraft and does not get lost under the aircraft when the aircraft is on the ground. The reason you do not want to impact the aircraft is that if the vent gas were to combust, damage could occur to the exterior surfaces of the aircraft that it comes into contact with.

[0313] In an illustrative example, by venting hydrogen gas out the top of the vertical stabilizer, when the aircraft is on the ground, the vent gas will warm to a point where it does not affect any surfaces of the aircraft and is less dense than air, thus causing it to rise.

[0314] Due to the temperature of liquid hydrogen, there are only two known substances that remain gaseous: hydrogen (at or above the vapor pressure of liquid hydrogen) and helium.

[0315] Regarding the ability to purge vent hydrogen, the first step involves running a pressure cycle of warm hydrogen gas to bring the system to approximately -160° C., or approximately -320 F. At these temperatures, nitrogen gas can be safely used to finish warming up and inerting the system.

[0316] Turning now to Figure 38, an illustration of a hydrogen recirculation system is depicted in accordance with an illustrative embodiment. In this illustrative example, a hydrogen recirculation system 3800 may be implemented within aircraft 400 of Figure 4 to recirculate hydrogen from set of liquid hydrogen tanks 408 of Figure 4. In this example, liquid hydrogen 3807 is stored in set of liquid hydrogen tanks 3810.

[0317] In this illustrative example, hydrogen recirculation system 3800 includes a conduit system 3801 and a controller 3802. In this example, conduit system 3801 is connected to a set of liquid hydrogen tanks 3810 in aircraft 3855. Conduit system 3801 includes a conduit 3803 and a valve 3804.

[0318] A conduit system 3801 includes conduits and valves. The valves can be controlled by a controller 3802 to direct fluids along various routes or loops. The controller 3802 is a hardware system and can include software.

[0319] In one exemplary example, the controller 3802 may control the conduit system 3801 to take gaseous hydrogen 3811 from the set of liquid hydrogen tanks 3810. In one exemplary example, the controller 3802 controls the conduit system 3801 to perform several different processes. For example, the controller 3802 sends the gaseous hydrogen 3811 to a tank heat exchanger 3820. The gaseous hydrogen 3811 is heated by the tank heat exchanger 3820 to form heated gaseous hydrogen 3823. The tank heat exchanger is, for example, the LH2 HX3712 of FIG. 37. The controller 3802 sends a first portion of the heated gaseous hydrogen to the fuel cell stack 3822. The controller 3802 returns a second portion of the heated gaseous hydrogen 3823 to the set of liquid hydrogen tanks 3810.

[0320] In this example, the second portion of the heated gaseous hydrogen 3823 is a quantity of heated gaseous hydrogen 3823 that maintains a pressure level within the set of liquid hydrogen tanks 3810 that allows the flow of liquid hydrogen 3807 to the fuel cell stack 3822. The above-mentioned liquid hydrogen 3807 stream includes liquid hydrogen 3807 flowing from the set of liquid hydrogen tanks 3810 that is converted to gaseous hydrogen 3811 and heated for use in the fuel cell stack 3822. This type of stream may be used when the second portion of the heated gaseous hydrogen 3823 is not present in a sufficient quantity required for use in the fuel cell stack 3822.

[0321] Additionally, controller 3802 may control the pressure within the liquid hydrogen tanks 3810. For example, controller 3802 may route gaseous hydrogen 3811 within the set of liquid hydrogen tanks 3810 to vent 3860 via conduit system 3801 in response to the pressure within the set of liquid hydrogen tanks 3810 being greater than a particular tolerance. Vent 3860 is considered, in this example, to be part of hydrogen recirculation system 3800. Thus, in this example, excess gaseous hydrogen is routed from the set of liquid hydrogen tanks 3810 to vent 3860 within aircraft 3855 in response to the pressure within the set of liquid hydrogen tanks 3810 being greater than a particular tolerance.

[0322] Vent 3860 may be in various locations on aircraft 3855. For example, vent 3860 may be located in tail 3861 of fuselage 3862 of aircraft 3855. For example, vent 3860 may be located in vertical stabilizer 3865 on tail 3861 of fuselage 3862. Vertical stabilizer 3865 is a vertical stabilizer connected to tail 3861 of fuselage 3862.

[0323] In another illustrative example, the controller 3802 may perform similar operations on liquid hydrogen 3807 stored in the set of liquid hydrogen tanks 3810. The liquid hydrogen 3807 may be used to maintain a desired pressure level in the set of liquid hydrogen tanks 3810. This use of liquid hydrogen 3807 may be used instead of or in addition to operating with gaseous hydrogen 3811 to maintain a desired pressure level.

[0324] In this example, the controller 3802 can control the conduit system 3801 to perform the process. For example, the controller 3802 can remove liquid hydrogen 3807 from the set of liquid hydrogen tanks 3810 and route the liquid hydrogen 3807 to a tank heat exchanger 3820 to form heated gaseous hydrogen 3823. The controller 3802 can route a first portion of the heated gaseous hydrogen 3823 to the fuel cell stack 3822 and route a second portion of the heated gaseous hydrogen 3823 back to the set of liquid hydrogen tanks 3810.

[0325] For example, the vent 3860 may be in other locations. In other illustrative examples, the vent 3860 may be in other stabilizers in the vertical stabilizer 3865. In yet another illustrative example, the vent may be located on a wing of the aircraft 3855.

[0326] The block diagram of aircraft cooling system 3800 in FIG. 38 is not meant to imply physical or architectural limitations to the manner in which an example embodiment may be implemented. Other components in addition to or in place of the illustrated components may be used. Some components may be unnecessary. The blocks are also presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an example embodiment.

[0327] Referring to Figure 39, a flow diagram of a process for recirculating hydrogen is shown in accordance with an exemplary embodiment. The process of Figure 39 may be implemented in hardware, software, or both. For example, the process may be performed in hydrogen recirculation system 3800 of Figure 38. The process may be used to manage liquid hydrogen in liquid hydrogen tanks of an aircraft.

[0328] The process begins by extracting gaseous hydrogen from a set of liquid hydrogen tanks connected to the aircraft (operation 3900). The process routes the gaseous hydrogen extracted from the set of liquid hydrogen tanks through a tank heat exchanger in a nacelle for an engine of the aircraft to form heated gaseous hydrogen (operation 3902).

[0329] The process sends a first portion of the heated gaseous hydrogen to a fuel cell stack in an engine of the aircraft (step 3904). The process sends a second portion of the heated gaseous hydrogen back into a set of liquid hydrogen tanks (step 3906). The process then ends.

[0330] Referring to Figure 40, a flow diagram of a process for recirculating hydrogen is shown in accordance with an exemplary embodiment. The process of Figure 40 may be implemented in hardware, software, or both. For example, the process may be performed in hydrogen recirculation system 3800 of Figure 38. The process may be used to manage liquid hydrogen in liquid hydrogen tanks of an aircraft.

[0331] The process includes removing liquid hydrogen from a set of liquid hydrogen tanks (step 4000). The process includes sending the liquid hydrogen to a tank heat exchanger to form heated gaseous hydrogen (step 4002).

[0332] The process sends a first portion of the heated gaseous hydrogen to the fuel cell stack (operation 4004). The process returns a second portion of the heated gaseous hydrogen to the set of liquid hydrogen tanks (operation 4006). The process then ends.

[0333] Referring to Figure 41, a flow diagram of a process for recirculating hydrogen is shown in accordance with an exemplary embodiment. The process of Figure 41 may be implemented in hardware, software, or both. For example, the process may be performed in hydrogen recirculation system 3800 of Figure 38. The process may be used to manage liquid hydrogen in liquid hydrogen tanks of an aircraft.

[0334] In response to the pressure in the set of liquid hydrogen tanks being greater than a particular tolerance, the process routes gaseous hydrogen in the set of liquid hydrogen tanks to a vent via a conduit system (operation 4100), with the process then terminating.

[0335] 42, a diagram of a liquid hydrogen recirculation system for tank pressure control is shown, according to an exemplary embodiment. As shown in this example, a recirculation system 4200 for recirculating gaseous hydrogen (GH2) can be implemented to control the temperature in liquid hydrogen tank 4251 and liquid hydrogen tank 4252.

[0336] This process may involve drawing gaseous hydrogen from the top of a liquid hydrogen tank and then sending the gaseous hydrogen through a heat exchanger 4201 as it travels down towards the wing. The heat exchanger 4201 has heat from a cold coolant line 4203 from the engine.

[0337] The gaseous hydrogen is split into two parts at flow control valve 4202 after heat exchanger 4201. In this example, most of the gaseous hydrogen goes towards the fuel cell stack via line 4204, and some of the gaseous hydrogen is pumped back into the liquid hydrogen tank via return line 4205 by gaseous hydrogen pump 4206 and gaseous hydrogen pump 4207. Returning heated gaseous hydrogen to the liquid hydrogen tank affects how much liquid hydrogen (LH2) is boiled off in the liquid hydrogen tank and affects the tank pressure. In this example, the more boil-off, the greater the resulting pressure increase.

[0338] In one example, the process may be performed as follows. 1. Extract gaseous hydrogen from the liquid hydrogen tank. 2. Route the gaseous hydrogen through a heat exchanger 4205 that contains hot coolant from the fuel cell stack as the heating fluid. The hot coolant can be heated by a heat load in the cryogenic loop. 3. Heated gaseous hydrogen is output from the heat exchanger 4205. 4. Split the gaseous hydrogen at flow control valve 4202. One portion of the gaseous hydrogen is directed for recirculation in the liquid hydrogen tank and another portion of the gaseous hydrogen is sent to the fuel cell stack. 5. Direct a portion of the gaseous hydrogen for recirculation to gaseous hydrogen pump 4206 and gaseous hydrogen pump 4207 for return to the liquid hydrogen tank. 6. The gaseous hydrogen is returned to the liquid hydrogen tank, creating bubbles in the liquid hydrogen to maintain pressure in the cryogenic tank.

[0339] In Figure 43, a diagram of a vent system for gaseous hydrogen is shown, according to an exemplary embodiment. The vent system 4300 provides the ability to vent gaseous hydrogen (GH2) in response to overpressure. The vent system allows the gaseous hydrogen to be released from the liquid hydrogen tank 4310 and removed from the aircraft. In flight, this venting may occur via a vent 4304 in the vertical stabilizer 4306. In this example, the vent 4304 is connected to the liquid hydrogen tank 4310 by a conduit system 4312.

[0340] Referring now to Figure 44, a diagram of a system for recirculating gaseous hydrogen for tank pressure control is shown, according to an exemplary embodiment. In an illustrative example, the same reference number may be used in more than one figure. Such repeated use of a reference number in different figures represents the same element in the different figures. This figure is a more detailed view of the components within section 4350 of Figure 43.

[0341] In addition to the components for recirculating gaseous hydrogen, the diagram also shows components that can be used to recirculate liquid hydrogen (LH2). In this example, the liquid hydrogen is drawn from the bottom of the liquid hydrogen tank 4310. A portion of the liquid hydrogen is sent to the engine, where a portion of the liquid hydrogen passes through a heat exchanger 4401 to warm it up and cause a phase change to gaseous hydrogen (GH2). The heat exchanger 4401 has heat from a cryogenic coolant line 4402 from the engine. The liquid hydrogen is then pumped back into the liquid hydrogen tank 4310. Adding heated GH2 back to the liquid hydrogen tank 4310 affects how much liquid hydrogen boils off in the liquid hydrogen tank 4310, which in turn affects the pressure of the liquid hydrogen tank.

[0342] With respect to implementing the recirculation using liquid hydrogen, the process may be carried out as follows. 1. Extract liquid hydrogen from the liquid hydrogen tank 4310. 2. Liquid hydrogen is input through a heat exchanger 4401 that contains hot coolant from a heat load such as an engine as the heating fluid. 3. Output gaseous hydrogen from heat exchanger 4401. 4. Split the gaseous hydrogen at flow control valve 4407. One portion of the gaseous hydrogen is directed for recirculation within the liquid hydrogen tank via refill line 4408 leading to the refueling port of the liquid hydrogen tank 4310, and another portion of the gaseous hydrogen is sent to the fuel cell stack via fuel cell stack line 4415. 5. Gaseous hydrogen for recirculation is sent to gaseous hydrogen (GH2) pump 4405 and gaseous hydrogen pump 4406. 6. Return a portion of the gaseous hydrogen for recirculation back into the liquid hydrogen tank 4310, generating bubbles in the liquid hydrogen to maintain pressure in the liquid hydrogen tank 4310.

[0343] Alternatively, instead of liquid hydrogen, GH2 can be drawn from the liquid hydrogen tank 4310 and a similar process can be carried out using gaseous hydrogen.

[0344] Additionally, a pressure relief valve 4420 can open to relieve pressure by venting the hydrogen gas. This valve can open when the pressure is greater than some selected threshold. The hydrogen gas can then flow to a vent 4304 in the vertical stabilizer 4306 of FIG. 43.

[0345] 7. Two-phase cooling of aircraft electronics pumps

[0346] Conventional single-phase cooling systems are unable to thermally manage an electrically charged propulsion system within the size, weight, and power (SWaP) envelope that is acceptable for use on aircraft, such as liquid hydrogen-powered aircraft.

[0347] Current solutions pump liquid through a cooling loop using a ram air radiator. Ram air radiators for electrified propulsion systems and high power electronics are large in size and weight and require large amounts of cooling airflow. This situation results in packaging difficulties and potential aerodynamic drag penalties.

[0348] The solutions currently used in aircraft are single-phase liquid or gas cooling solutions used for aircraft components such as power electronics, fuel cells, batteries, and electric motors and drives, which are ineffective in handling the large amounts of heat from electrical components of the above types that typically operate below 100 degrees Celsius.

[0349] In one or more illustrative examples, a series of components including a liquid pump, an evaporator, a condenser, and an accumulator may be used to provide the desired cooling with a pumped two-phase cooling loop.

[0350] In an illustrative example, a fluid that undergoes a phase change at an appropriate temperature and pressure is used for thermal management of aircraft electronics and power systems. A heat source and a heat sink may be configured to function as an evaporator and a condenser, respectively.

[0351] In various illustrative examples, a waste heat recovery (WHR) system may be added to the thermal management system downstream of the evaporator. In other illustrative examples, a system for generating thrust from ram air heated by a condenser may also be used.

[0352] With reference to Figure 45, an illustration of a block diagram of an aircraft cooling system is shown in accordance with an illustrative embodiment. In this illustrative example, aircraft cooling system 4500 may be implemented within aircraft 400 of Figure 4 to cool heat load 4501. Aircraft cooling system 4500 may be located within a nacelle of an engine of the aircraft.

[0353] The thermal load 4501 can take a number of different forms. For example, the thermal load 4501 can be selected from at least one of a fuel cell, a fuel cell stack, a power circuit, an electric motor, a computer system, an electronics enclosure, a laser, or any other component that generates heat.

[0354] The aircraft cooling system 4500 may take a number of different forms. In one illustrative example, the aircraft cooling system 4500 includes an evaporator 4511, a condenser 4512, an accumulator 4513, a pump system 4514, and a conduit system 4515. The flow of fluids, such as liquid 4516 and vapor 4519, between the various components is facilitated by the conduit system 4515.

[0355] The conduit system 4515 includes a conduit 4527, and may also include a valve 4517. The conduit system 4515 is connected to an evaporator 4511, a condenser 4512, an accumulator 4513, and a pump system 4514.

[0356] In this example, the evaporator 4511 is thermally connected to the set of heat loads 4501 and is capable of using liquid 4516 to cool the set of heat loads 4501 within the aircraft 4502. In response to cooling the set of heat loads 4501, the liquid 4516 forms steam 4519.

[0357] Condenser 4512 is a hardware component that cools vapor 4519 to form liquid 4516. In one example, condenser 4512 can be a ram air condenser that uses air entering the nacelle inlet or a condenser that does not use ram air. Condenser 4512 receives vapor 4519 and cools vapor 4519 where cooling vapor 4519 forms liquid 4516. In this example, condenser 4512 also releases heat from vapor 4519 into the nacelle airflow. Nacelle airflow is air that flows through the nacelle and exits the nacelle at the nacelle outlet.

[0358] In this example, an accumulator 4513 receives liquid 4516 from the condenser 4512 and stores the liquid 4516. A pump system is a physical system that includes one or more pumps. A pump system 4514 pumps the liquid 4516 stored in the accumulator 4513 to the evaporator 4511.

[0359] Additionally, in this example, the aircraft cooling system 4500 also includes a generator 4530 and a turbine 4531. In this example, the turbine 4531 is connected to the generator and to the conduit system 4515. The turbine 4531 is physically connected to the generator 4530 and may rotate the generator 4530 to generate electricity. The turbine 4531 is connected to the conduit system 4515 such that steam 4519 flows through the turbine 4531 to rotate the turbine 4531.

[0360] The turbine 4531 rotates in response to receiving the steam 4519 from the evaporator 4511 to generate electricity for the generator 4530. Here, a condenser 4512 receives steam 4519 from a turbine 4531 .

[0361] In another illustrative example, an aircraft cooling system 4500 includes a condenser 4512, an evaporator 4511, and a pump system 4514. In this example, two liquids, a first liquid 4541 and a second liquid 4542, flow through a conduit system 4515. The two liquids flow separately from each other through the conduit system 4515. In other words, the liquids do not mix. In this example, a first pump 4544 in the pump system 4514 pumps the first liquid 4541, and a second pump 4545 in the pump system 4514 pumps the second liquid 4542.

[0362] In this example, a first pump 4544 pumps a first liquid 4541 to a set of heat loads 4501 in liquid loop 4550. The set of heat loads 4501 heat the first liquid 4541 whereby heating the first liquid 4541 forms a heated liquid 4546.

[0363] The evaporator 4511 is a separate or separate evaporator where the first liquid 4541 (heated liquid 4546) and the second liquid 4542 can flow through the components without mixing with each other. In this example, the evaporator 4511 receives the heated liquid 4546 and cools the heated liquid 454. The cooling of the heated liquid 4546 causes the second liquid 4542 in the liquid-vapor loop 4551 to form vapor 4552.

[0364] Further in this example, condenser 4512 receives vapor 4552 and cools vapor 4552. Cooling vapor 4552 forms second liquid 4542. Second pump 4545 pumps second liquid 4542 from condenser 4512 to evaporator 4511.

[0365] The block diagram of the aircraft cooling system 4500 in FIG. 45 is not meant to imply physical or architectural limitations to the manner in which an example embodiment may be implemented. Other components in addition to or in place of the illustrated components may be used. Some components may be unnecessary. The blocks are also presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an example embodiment.

[0366] Referring now to Figure 46, a flow diagram of a process for cooling a heat load is shown in accordance with an exemplary embodiment. The process of Figure 46 may be implemented in hardware, software, or both. For example, the process may be performed in aircraft cooling system 4500 of Figure 45. The process may be used to cool a heat load within an aircraft.

[0367] In this process, a liquid is used in an evaporator to cool a set of heat loads, Here, the liquid forms a vapor when cooling a set of heat loads (step 4600). The process uses a condenser to cool the vapor to form a liquid (step 4602).

[0368] The process receives liquid from the condenser (operation 4604). The process stores the liquid in an accumulator (operation 4606).

[0369] The process then pumps the liquid stored in the accumulator to the evaporator (operation 4608), after which the process ends.

[0370] Referring now to Figure 47, a flow diagram of a process for cooling a heat load is shown in accordance with an illustrative embodiment. The process of Figure 47 may be implemented in hardware, software, or both. For example, the process may be performed in aircraft cooling system 4500 of Figure 45. The process may be used to cool a heat load within an aircraft.

[0371] The process includes pumping a first liquid to a set of heat loads where the set of heat loads heat the first liquid where heating the first liquid forms a heated liquid (step 4700). The process includes cooling the heated liquid in an evaporator where cooling the heated liquid causes the heated liquid in the evaporator to form a vapor (step 4702).

[0372] The process includes sending the vapor to a condenser (step 4704). The process includes cooling the vapor in the condenser to form a second liquid (step 4706). The process includes pumping the second liquid from the condenser to an evaporator (step 4708). The process then ends.

[0373] 48, a diagram of a pumped two-phase (P2P) cooling loop is shown, according to an exemplary embodiment. As shown, pumped two-phase cooling loop 4800 includes a pump 4802, an evaporator 4804, a condenser 4806, and an accumulator 4808. In this example, the accumulator 4808 is a reservoir that stores liquid. The pump 4802 can pump liquid from the accumulator 4808 to the evaporator 4804. The evaporator 4804 cools a set of thermal loads 4805 within the aircraft. The set of thermal loads 4805 within the aircraft is selected from at least one of a fuel cell, a fuel cell stack, an electric motor, a computer system, an electronics housing, or a laser. The set of thermal loads may include a fuel cell, a fuel cell stack, an electric motor, an electronics housing, a computer system, a laser, or any other suitable type of component that generates heat.

[0374] When cooling the heat loads, the evaporator 4804 generates vapor from a liquid coolant delivered to the condenser 4806. The condenser 4806 operates to cool the vapor from the evaporator 4804 via an airflow moving through the condenser 4806. The cooled vapor becomes a liquid and is stored in the accumulator 4808, which can be recirculated by the pump 4802 to provide further cooling within the pumped two-phase cooling loop 4800.

[0375] 49, a diagram of a hybrid two-phase (P2P) cooling loop is shown in accordance with an exemplary embodiment. In this example, the hybrid two-phase (P2P) cooling loop 4900 includes pump 1 4902, pump 2 4904, an evaporator 4906, and a condenser 4908.

[0376] In this example, pump 1 4902 pumps a first liquid in liquid loop 4901 that has been condensed by evaporator 4906 to a heat load 4910. In this example, heat load 4910 heats the first liquid. The heated liquid flows to evaporator 4906 where it is cooled for use by pump 1 4902.

[0377] This flow of liquid forms a liquid loop 4901 in a hybrid two-phase (P2P) cooling loop 4900. In this example, the evaporator 4906 may be located away from the heat load 4910. For example, the evaporator 4906 may be located closer to the condenser 4908.

[0378] In this example, in a two-phase (P2P) loop 4915, a heated liquid is heated in an evaporator 4906 to form a vapor 4903. The vapor is sent to a condenser 4908. This type of configuration may be desirable when the second liquid is more volatile or more difficult to handle. This configuration utilizing two liquids allows for the use of a potentially volatile working fluid to be minimized. Additionally, this configuration can be used with heat loads that do not allow for direct contact with the liquid used as the coolant or refrigerant.

[0379] Vapor 4903 is sent to condenser 4908, which cools the vapor to form a second liquid. Pump 2 4904 pumps the second liquid back to evaporator 4906. Heat exchange in evaporator 4906 cools the first liquid in liquid loop 4901.

[0380] 50, a diagram of a pumped two-phase (P2P) cooling loop with waste heat recovery is shown in accordance with an illustrative embodiment. As shown, pumped two-phase cooling loop 5000 includes a pump 5002, an evaporator 5004, a condenser 5006, and an accumulator 5008.

[0381] In this example, the turbine 5010 is located between the evaporator 5004 and the condenser 5006. Liquid pumped to the evaporator by the pump 5002 cools the heat load 5011. In this example, steam 5031 generated by the evaporator 5004 that cools the heat load 5011 may be used to rotate the turbine 5010. The turbine 5010 is connected to a generator 5012 that generates electricity in response to the use of the steam 5031 from the turbine 5010. The steam 5031 then flows to the condenser 5006 and condenses into liquid 5032, which is stored in the accumulator 5008. As shown, the waste heat recovery system 5050 includes the turbine 5010 and the generator 5012.

[0382] Thus, in one or more illustrative examples, this type of cooling system may be used in place of other cooling systems that use single-phase liquids. A liquid may be selected that evaporates when cooling a heat load.

[0383] Additionally, the vapor 5031 generated by the evaporator 5004 may be a superheated gas. Similarly, the liquid 5032 generated by the condenser 5006 may be subcooled. The terms superheated and subcooled refer to raising or lowering the liquid temperature, respectively, above its saturation temperature.

[0384] Certain features of the embodiments are described in the following clauses. These clauses are examples of features and are not intended to limit other embodiments.

[0385] The description of various exemplary embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the embodiments to the disclosed forms. Components that perform actions or steps are described by various examples. In an exemplary embodiment, a component may be configured to perform the described actions or steps. For example, the component may have a structural configuration or design that provides the component with the ability to perform the actions or steps described as being performed by the component in the exemplary examples. Furthermore, to the extent that the terms "includes," "has," "contains," and variations thereof are used herein, such terms are intended to be inclusive in the same manner as the term "comprises" as an open transition phrase, without excluding any additional or other elements.

[0386] Numerous modifications and variations will be apparent to those skilled in the art. Additionally, various exemplary embodiments may provide different features as compared to other preferred embodiments. The selected embodiments have been chosen and described in order to best explain the principles and practical applications of the embodiments and to facilitate an understanding by others skilled in the art of the disclosure of the various embodiments and various modifications suitable for the particular use contemplated.

Claims

1. An aircraft (199, 400, 3127, 3855, 4502), A fuselage (102, 402, 2630, 3140, 3862), Wings (404), an engine (106, 406, 2601) connected to said wing (404); A set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310), a vent (2860, 4304) located on the tail section (410, 3861) of said fuselage (102, 402, 2630, 3140, 3862); a conduit system (3801) connecting said set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) to said vent (2860, 4304); a controller configured to control the conduit system (3801) to release gaseous hydrogen (3811) in the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) to move through the conduit system (3801) and out at the vent (2860, 4304) in response to a pressure in the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) being greater than a certain tolerance; Aircraft (199, 400, 3127, 3855, 4502) equipped with

2. 2. The aircraft (199, 400, 3127, 3855, 4502) of claim 1, wherein a set of the liquid hydrogen tanks (108, 408, 2602, 3810, 4310) are present along several sides of the fuselage (102, 402, 2630, 3140, 3862) above the wings (404) of the aircraft (199, 400, 3127, 3855, 4502).

3. 2. The aircraft (199, 400, 3127, 3855, 4502) of claim 1, wherein the vent (2860, 4304) is located in a vertical stabilizer (410, 3861) (110, 441, 3865, 4306) of the tail section (410, 3861) of the fuselage (102, 402, 2630, 3140, 3862).

4. The controller: Extracting gaseous hydrogen (3811) from the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310); sending the gaseous hydrogen (3811) extracted from the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) through a tank heat exchanger (3820) to form heated gaseous hydrogen (3811); Passing a first portion of the heated gaseous hydrogen (3811) to a fuel cell stack in the engine (106, 406, 2601); returning a second portion of the heated gaseous hydrogen (3811) into the set of liquid hydrogen tanks (108, 408, 2602, 3810); 2. The aircraft (199, 400, 3127, 3855, 4502) of claim 1, configured to control the conduit system (3801) to perform

5. The controller: Extracting liquid hydrogen (3807) from the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310); directing the liquid hydrogen (3807) extracted from the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) through a tank heat exchanger (3820) to form heated gaseous hydrogen (3811); Passing a first portion of the heated gaseous hydrogen (3811) to a fuel cell stack in the engine (106, 406, 2601) of the aircraft (199, 400, 3127, 3855, 4502); returning a second portion of the heated gaseous hydrogen (3811) into the set of liquid hydrogen tanks (108, 408, 2602, 3810); 2. The aircraft (199, 400, 3127, 3855, 4502) of claim 1, configured to control the conduit system (3801) to perform

6. 5. The aircraft (199, 400, 3127, 3855, 4502) of claim 4, wherein the second portion of the heated gaseous hydrogen (3811) is a quantity of the heated gaseous hydrogen (3811) that maintains a pressure level within the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) that allows the flow of liquid hydrogen (3807) to the fuel cell stack.

7. 1. A hydrogen recirculation system (3800), comprising: a conduit system (3801) including conduits and valves connected to a set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) of an aircraft (199, 400, 3127, 3855, 4502); A controller, Extracting gaseous hydrogen (3811) from said set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310); Sending the gaseous hydrogen (3811) to a tank heat exchanger (3820), where the gaseous hydrogen (3811) is heated to form heated gaseous hydrogen (3811); sending a first portion of the heated gaseous hydrogen (3811) to a fuel cell stack; and returning a second portion of said heated gaseous hydrogen (3811) to said set of liquid hydrogen tanks (108, 408, 2602, 3810). a controller configured to control the conduit system (3801) to: A hydrogen recirculation system (3800) comprising:

8. 8. The hydrogen recirculation system (3800) of claim 7, wherein the second portion of the heated gaseous hydrogen (3811) is a quantity of the heated gaseous hydrogen (3811) that maintains a pressure level within the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) that enables the flow of liquid hydrogen (3807) to the fuel cell stack.

9. The controller:

8. The hydrogen recirculation system (3800) of claim 7, configured to control the conduit system (3801) to route gaseous hydrogen (3811) in the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) through the conduit system (3801) to a vent (2860, 4304) in response to a pressure in the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) being greater than a certain tolerance.

10. 10. The hydrogen recirculation system (3800) of claim 9, wherein the vent (2860, 4304) is located in a tail section (410, 3861) of a fuselage (102, 402, 2630, 3140, 3862).

11. 11. The hydrogen recirculation system (3800) of claim 10, wherein the vent (2860, 4304) is located in a vertical stabilizer (410, 3861) (110, 441, 3865, 4306) of the tail section (410, 3861) of the fuselage (102, 402, 2630, 3140, 3862).

12. 1. A hydrogen recirculation system (3800), comprising: a conduit system (3801) including conduits and valves connected to a set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310); A controller, removing liquid hydrogen (3807) from said set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310); passing said liquid hydrogen (3807) to a tank heat exchanger (3820) to form heated gaseous hydrogen (3811); sending a first portion of the heated gaseous hydrogen (3811) to a fuel cell stack; and returning a second portion of said heated gaseous hydrogen (3811) to said set of liquid hydrogen tanks (108, 408, 2602, 3810). a controller configured to control the conduit system (3801) to: A hydrogen recirculation system (3800) comprising:

13. 13. The hydrogen recirculation system (3800) of claim 12, wherein the second portion of the heated gaseous hydrogen (3811) is a quantity of the heated gaseous hydrogen (3811) that maintains a pressure level within the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) that enables the flow of liquid hydrogen (3807) to the fuel cell stack.

14. The controller:

13. The hydrogen recirculation system (3800) of claim 12, configured to control the conduit system (3801) to route gaseous hydrogen (3811) in the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) through the conduit system (3801) to a vent (2860, 4304) in response to a pressure in the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) being greater than a certain tolerance.

15. 15. The hydrogen recirculation system (3800) of claim 14, wherein the vent (2860, 4304) is located in a tail section (410, 3861) of a fuselage (102, 402, 2630, 3140, 3862).

16. 16. The hydrogen recirculation system (3800) of claim 15, wherein the vent (2860, 4304) is located in a vertical stabilizer (410, 3861) (110, 441, 3865, 4306) of the tail section (410, 3861) of the fuselage (102, 402, 2630, 3140, 3862).

17. 1. A method for recycling hydrogen, comprising the steps of: Extracting gaseous hydrogen (3811) from a set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) connected to an aircraft (199, 400, 3127, 3855, 4502); - routing the gaseous hydrogen (3811) extracted from the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) through a tank heat exchanger (3820) in a nacelle of an engine (106, 406, 2601) of the aircraft (199, 400, 3127, 3855, 4502) to form heated gaseous hydrogen (3811); Passing a first portion of the heated gaseous hydrogen (3811) to a fuel cell stack in the engine (106, 406, 2601) of the aircraft (199, 400, 3127, 3855, 4502); returning a second portion of the heated gaseous hydrogen (3811) into the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310); A method for recycling hydrogen, comprising:

18. 18. The method of claim 17, wherein the second portion of the heated gaseous hydrogen (3811) is a quantity of the heated gaseous hydrogen (3811) selected to maintain a pressure level within the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) that enables the flow of liquid hydrogen (3807) to the fuel cell stack.

19. 20. The method of claim 17, wherein extracting gaseous hydrogen (3811) comprises extracting the gaseous hydrogen (3811) located within the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310).

20. 18. The method of claim 17, further comprising, in response to a pressure in the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) being greater than a tolerance, routing excess gaseous hydrogen (3811) from the set of liquid hydrogen tanks (108, 408, 2602, 3810, 4310) to a vent (2860, 4304) in the aircraft (199, 400, 3127, 3855, 4502).

21. 21. The method of claim 20, wherein the vent (2860, 4304) is located in a stabilizer.

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