Start-up of high-temperature proton exchange membrane (HTPEM) fuel cell aircraft with multiple power generating units

Abstract

To provide an aircraft using propulsion systems powered using hydrogen fuel cells.SOLUTION: In an aircraft, a coolant 1917 flows through a conduit system 1902. A pump system 1904 is configured to circulate the coolant 1917 through the conduit system 1902 to fuel cell stacks 1920. A controller 1905 is configured to control a heater system 1901 to heat the coolant 1917 to form a heated coolant 1916, is configured to control the pump system 1904 to circulate the heated coolant 1916 through the conduit system 1902, and is configured to control the conduit system 1902 to circulate the heated coolant 1916 to a set of the fuel cell stacks 1920, where the heated coolant 1916 causes the set of the fuel cell stacks 1920 to reach an operating temperature 1925.SELECTED DRAWING: Figure 19

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 519,518, filed August 14, 2023, entitled "Start-Up of High Temperature Proton Exchange Membrane (HTPEM) Fuel Cell Aircraft with Multiple Power Generating Units," the entire contents of which are incorporated herein by reference.

[0002] This application is related to the following U.S. patent applications: U.S. patent application Ser. No. 18 / 610,438, filed on the same day, entitled "Pumped Two-Phase Cooling of Aircraft Electronics," Attorney Docket No. 23-0610-US-NP; U.S. patent application Ser. No. 18 / 610,526, filed on the same day, entitled "Configuration for a LH2 Fuel Cell Aircraft with Distributed Systems," Attorney Docket No. 23-1348-US-NP; U.S. patent application Ser. No. 18 / 610,580, filed on the same day, entitled "Method of Nacelle Heat Exchanger Integration for a Hydrogen Fueled Fuel Cell Powered Aircraft," Attorney Docket No. 23-1367-US-NP; and U.S. patent application Ser. No. 18 / 610,580, filed on the same day, entitled "Liquid Hydrogen Feed System for Fuel Cell Powered Aircraft." No. 18 / 610,697, entitled "Low Temperature Proton Exchange Membrane Charge Air Heat Exchanger," Attorney Docket No. 23-1368-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," Attorney Docket No. 23-1385-US-NP, filed on the same day, each of which is incorporated herein by reference in its entirety. [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, thereby reducing the carbon footprint.

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

[0006] Electric aircraft have electric propulsion systems that can be powered by hydrogen, which can be stored in tanks. The hydrogen stored in these tanks 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 a reforming process to extract hydrogen from 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 onboard the aircraft. Fuel cells are electrochemical devices that convert hydrogen and oxygen from the air into electricity to power the propulsion systems onboard the aircraft. This type of cell operates with high efficiency and minimal emissions, with water vapor being a by-product. Summary of the Invention

[0009] An embodiment of the present disclosure provides an aircraft including a heater system, a conduit system thermally connected to a fuel cell stack, a pump system, and a controller. The heater system is configured to heat a coolant. The coolant flows through the conduit system. The pump system is configured to circulate the coolant through the conduit system to the fuel cell stack. The controller is configured to control the heater system to heat the coolant to form heated coolant. The controller is configured to control the pump system to circulate the heated coolant through the conduit system. The controller is configured to control the conduit system to circulate the heated coolant to a pair of fuel cell stacks, where the heated coolant causes the pair of fuel cell stacks to reach an operating temperature.

[0010] Another embodiment of the present disclosure provides an aircraft including a fuselage, wings connected to the fuselage, engines connected to the wings, fuel cell stacks for the engines, and a fuel cell stack startup system. The fuel cell stack startup system is configured to circulate heated coolant to a pair of fuel cell stacks in the fuel cell system until the pair of fuel cell stacks reaches an operating temperature and generates power to heat the coolant to start other fuel cell stacks in the fuel cell system. Yet another embodiment of the present disclosure provides an aircraft including a fuselage, wings connected to the fuselage, engines connected to the wings, a fuel cell stack, a heater system, a conduit system connected to the fuel cell stack, a pump system, and a controller. The fuel cell stack is configured to generate electricity for the engine. The heater system is configured to heat the coolant. The coolant flows through the conduit system. The pump system is configured to circulate the coolant through the conduit system to the fuel cell stack. The controller is configured to control the heater system to heat the coolant to form heated coolant. The controller is configured to control the pump system to circulate the heated coolant through the conduit system. A controller is configured to control the conduit system to circulate heated coolant to the number of fuel cell stacks, causing the number of fuel cell stacks to reach an operating temperature with the heated coolant.

[0011] Yet another embodiment of the present disclosure provides a method for managing fuel cell stack operation for an aircraft, wherein a set of fuel cell stacks in the aircraft are heated to bring the fuel cell stacks to an operating temperature for generating electrical power.

[0012] Another embodiment of the present disclosure provides a method for managing the start-up of fuel cell stacks for an aircraft, wherein a coolant is heated by a heater to form heated coolant in a conduit system connected to the fuel cell stacks, the conduit system being controlled to circulate the heated coolant to a pair of fuel cell stacks, causing the pair of fuel cell stacks to reach an operating temperature with the heated coolant.

[0013] The novel features believed characteristic of the illustrative embodiments are set forth in the appended claims. However, the illustrative embodiments and preferred modes of use, further objects and features thereof will best be understood by reading the following detailed description of illustrative embodiments of the present disclosure when read in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is an illustration of a hydrogen fuel cell vehicle including a distributed propulsion system according to an illustrative embodiment; [Figure 2] FIG. 1 is a front view of a liquid hydrogen tank location according to an exemplary embodiment. [Figure 3] FIG. 1 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; [Figure 5] FIG. 1 is a block diagram of a heat exchange system for an aircraft in accordance with an illustrative embodiment. [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] FIG. 2 is another cross-sectional view of a portion of a nacelle including a nacelle heat exchanger according to an exemplary embodiment. [Figure 8] FIG. 2 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 exemplary embodiment. [Figure 10] FIG. 1 is an illustration of an engine including a nacelle heat exchanger according to an exemplary embodiment. [Figure 11] FIG. 1 is an illustration of an engine including a nacelle heat exchanger according to an exemplary 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. 1 is a schematic diagram of a fan configuration used in a nacelle with a nacelle heat exchanger in accordance with an exemplary 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. 1 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 exemplary 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. [Figure 23] FIG. 1 is a flow diagram of a process for circulating heated coolant in accordance with an illustrative embodiment. [Figure 24] FIG. 1 is a flow diagram of a process for delivering power to a fuel cell stack according to an exemplary embodiment. [Figure 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 start-up 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. [Figure 30] 1 is a schematic diagram of a fuel cell stack start-up system within a thermal management system according to an exemplary embodiment. [Figure 31] FIG. 1 is a block diagram of an aircraft management system in accordance with an illustrative embodiment; [Figure 32] FIG. 1 is a flow diagram of a process for air management in accordance with an illustrative embodiment. [Figure 33] FIG. 1 is a schematic diagram of an aircraft air management system for a fuel cell stack according to an illustrative embodiment. [Figure 34] FIG. 1 is a block diagram of an aircraft thermal management system in accordance with an illustrative embodiment. [Figure 35] FIG. 1 is a flow diagram of a process for thermal management in accordance with an illustrative embodiment. [Figure 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 exemplary embodiment. [Figure 38] FIG. 1 is a diagram of a hydrogen recirculation system according to an exemplary embodiment. [Figure 39] FIG. 1 is a flow diagram of a process for recycling hydrogen in accordance with an illustrative embodiment. [Figure 40] FIG. 1 is a flow diagram of a process for recycling hydrogen in accordance with an illustrative embodiment. [Figure 41] FIG. 1 is a flow diagram of a process for recycling hydrogen in accordance with an illustrative embodiment. [Figure 42] FIG. 1 is a diagram of a liquid hydrogen recirculation system for tank pressure control according to an illustrative embodiment. [Figure 43] FIG. 1 is a diagram of a gaseous hydrogen vent system according to an illustrative embodiment. [Figure 44] FIG. 1 is a diagram of a system for recirculating gaseous hydrogen for tank pressure control according to an illustrative embodiment. [Figure 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 exemplary embodiment. [Figure 49] FIG. 1 is a diagram of a hybrid two-phase (P2P) cooling loop according to an exemplary embodiment. [Figure 50] FIG. 1 is a diagram of a pumped two-phase (P2P) cooling loop with waste heat recovery according to an illustrative embodiment. DETAILED DESCRIPTION OF THE INVENTION

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

[0016] The illustrative embodiments recognize and take into account several 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 incorporating a liquid hydrogen (LH2) system into a passenger aircraft, such as a 50-passenger aircraft, can be very challenging. For example, design challenges exist in determining where to safely place large liquid hydrogen tanks and how to package the thermal management system and fuel cell stack system, both of which require a large amount of space.

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

[0019] Increasing redundancy can be achieved by increasing the number of engines. As used herein, when the term "a number of" is used with reference to an item, it 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 furthermore, the heat exchanger includes microchannel flow paths that further reduce the size. Such systems are optional in illustrative examples.

[0021] An additional conduit within the conduit system may be implemented leading from the liquid hydrogen tank to the top of the vertical stabilizer. Such conduit may be used to provide venting for releasing excess pressure within the liquid hydrogen tank during flight. The conduit 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, according to 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 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: engine 111, engine 112, and engine 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, a liquid hydrogen tank 108 is connected to the fuselage 102. The liquid hydrogen tank is located along the side of the fuselage 102 above the wings 104. The liquid hydrogen tank is substantially cylindrical in shape. The liquid hydrogen tank may have other shapes, such as a circle, a rounded rectangular prism, a teardrop, or other suitable shape. 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 locations of liquid hydrogen tanks 108 are 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 area by placing the fuel tanks against several surfaces of the aircraft 100. In other words, the surface area of ​​the fuselage 102 around the liquid hydrogen tank 108 that comes into contact with the air during flight is reduced compared to other possible locations on the aircraft 100. By reducing the wetted 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. Additionally, the location of the liquid hydrogen tank can protect the tank from ground strike damage.

[0030] Furthermore, 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, which may reside within the nacelle space for the individual engines.

[0031] The distribution also provides the ability to use blown wings for higher lift at lower speeds, increased 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 also be smaller in size. Similarly, the number of fuel cells in the fuel cell stack may be smaller, making it easier to integrate the components together into the aircraft 100.

[0033] In this example, venting of gaseous hydrogen (GH2) from the liquid hydrogen tank 108 may be provided via conduits to a vent system that is capable of venting the gaseous hydrogen out of an area away from and above 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 this figure, 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 for 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 aircraft wings. Additional tanks can be used to reduce liquid sloshing and improve redundancy.

[0036] 1 and 2 are provided as one illustrative example and are 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] Referring now to Figure 3, an illustration of 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 surrounds electric motor 304, fuel cell stack 305, and heat exchanger 308.

[0038] In this example, the heat exchanger 308 has a rectangular shape and is disposed at an angle relative to the opening of the inlet 310. The angle may have an acute angle of less than 90 degrees. The shape and arrangement 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] Referring now to FIG. 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 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 another 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 different 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 some examples, 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 is 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] Nacelle 420 is a housing that houses components for engine 415. This housing may provide aerodynamic characteristics. Nacelle 420 may also provide structural support for components within engine 415.

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

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

[0047] In this way, the flow of coolant to and through various components can be controlled. 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, nacelle heat exchanger 423 and conduit system 424 form thermal management system 425, which cools fuel cell stack 422. HTPEM (High Temperature Proton Exchange Membrane) fuel cells require a coolant that is too hot to function as a coolant for other engine components. A separate cryogenic coolant loop is required to cool engine components other than fuel cell stack 422. This cryogenic coolant loop uses its own ram air radiator or other heat exchanger or cooling method to cool the coolant. In this vehicle, the cryogenic coolant is cooled as part of its own pressure control system for the tank, which necessitates the heating and recirculation of hydrogen.

[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] Furthermore, in this example, the liquid hydrogen tanks extend along the outside of the fuselage 402 and above the wings 404. Furthermore, the set of liquid hydrogen tanks 408 may be located below windows 427 provided in the fuselage 402. In this example, the windows 427 are arranged 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 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, vent system 440 extends through fuselage 402 to vertical stabilizer 441 within stabilizer 412. Vertical stabilizer 441 is connected to tail section 410 of fuselage 402. In this example, vent system 440 vents gaseous hydrogen 434 through vertical stabilizer 441.

[0053] Thus, in this example, as the number of engines 106 increases, various components may be further distributed across these 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 tank and the reduced size of the thermal management and fuel cell systems when grouped together are examples of one or more features that can be obtained 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 within the nacelles to have smaller power requirements for the heat exchangers, and allows the fuel cells to be divided into smaller sizes that are easier to manufacture and integrate into the aircraft.

[0055] The block diagrams of aircraft 400 in Figure 4 and aircraft 100 in Figures 1 and 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 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.

[0056] In other illustrative examples, different liquid hydrogen tank positions than over the wings and near the sides of the fuselage may be used. Further, in other illustrative examples, other numbers of fuel cell stacks may be used within the engine, for example, two, three, or some other number of fuel cell stacks. Additionally, other features may be implemented to further assist 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 within the fuel cell stacks may be used.

[0057] As another example, hydrogen can be stored, and the hydrogen can be in another form other than 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 liquid hydrogen tank 408, other types of storage tanks can be used to 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. Nacelle Heat Exchanger Integration Method for Hydrogen Fuel Cell-Powered Aircraft

[0059] In an illustrative example, it is recognized and taken into account that a hydrogen powered aircraft may use fuel cells to drive propellers 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 ambient 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 rejected from the engine into the ambient air. Rejecting heat 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 can cause significant ram drag, especially during the takeoff and climb phases. Significant ram drag can 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 exchanger within the nacelle.

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

[0064] Typical fuel cells operate at efficiencies in the range of 40 to 60 percent. Based on the size of the aircraft, the total power required 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 on 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 locating the nacelle heat exchanger with minimal drag penalty, options related to bringing cooled ambient air into or out of the heat exchanger, as well as other potential issues.

[0066] An illustrative example provides a fuel cell based electric aircraft that uses hydrogen as fuel. In this example, the fuel cells are configured as a fuel cell stack. The fuel cell stack can be located within the aircraft's engine. In other words, the fuel cell stack is located within the same nacelle that contains the engine. The fuel cell stack can be cooled by various options, and heat from the fuel cell stack must 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, fixed area nozzles may allow for useful thrust recovery from ram air, and variable area nozzles may be used to optimize the power requirements of 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 heat exchanger described above into the nacelle instead of inside the fuselage.

[0070] In one illustrative example, a fuel cell based electric aircraft using hydrogen as fuel is illustrated. In this example, a fuel cell stack is comprised of the fuel cells. The fuel cell stack may be located within the aircraft's engine. In other words, the fuel cell stack may be located within the same nacelle that contains the engine. The fuel cell stack may be cooled by various options, and heat from the fuel cell stack must 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 can vary their 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, increasing the area of ​​the nacelle heat exchanger may reduce ram drag.

[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, illustrative examples may also provide at least one of drag reduction or thrust generation. In the examples, the phrase "at least one of" used in conjunction with enumerated items means that various combinations of one or more of the enumerated items may be used, and further, that 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 may be required. An item may be a specific 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 also be present. In some illustrative examples, "at least one of" may be, for example, without limitation, "two items A, one item B, and ten items C," "four items B and seven items C," or other suitable combinations.

[0075] Additionally, variable area nozzles can be used to reduce fan power. In this example, variable area means that the cross-sectional area of ​​the nozzle can be adjusted during operation. The area allows the exhaust speed to be changed. Fixed area means that the cross-sectional area of ​​the nozzle remains fixed and unchanged.

[0076] In various examples, the heat exchanger may have various shapes to reduce its 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 driven 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] Referring now to Figure 5, a block diagram of a heat exchange system for an aircraft is shown in accordance with an illustrative embodiment. In this example, heat exchange system 500 is located within nacelle 502. Nacelle 502 is an example of nacelle 420 for engine 415 of aircraft 400 in Figure 4. In this illustrative example, nacelle heat exchanger 520 may be oriented generally parallel to direction of flight 547 of the aircraft. For example, nacelle heat exchanger 520 may have a length greater than its width or thickness, with the longer dimension of nacelle heat exchanger 520 oriented generally parallel to direction of flight 547. Heat exchanger system 500 may also be referred to as a ram air radiator.

[0078] In this illustrative example, 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 several different materials, for example, the nacelle heat exchanger 520 may be constructed of several 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] Nacelle heat exchanger 520 can take several different forms. For example, nacelle heat exchanger 520 can include a hollow geometric conduit 550 that includes a channel 551. In this example, airflow 532 passes through at least one of the surfaces of channel 551 and the outer surface of hollow geometric conduit 550. Hollow geometric conduit 550 can be, for example, a cylinder, a rectangular prism, an ellipse, or any other suitable shape.

[0084] In another illustrative example, nacelle heat exchanger 520 may be comprised of concentric cylinders 552 disposed within nacelle 502. In yet another illustrative example, nacelle heat exchanger 520 may include a solid geometric column 553 disposed within nacelle 502. In this example, airflow 532 passes over the exterior of 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 positioned at an angle relative to the airflow 532. The rectangular heat exchanger may have the shape of a rectangular prism or 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 nacelle heat exchanger 520 is closer to inlet 504 than to outlet 505. 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. Furthermore, 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 heat transfer 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 another illustrative example, 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, conduit system 540 may use coolant 526 to cool at least one of a fuel cell stack, a motor, or an electrical system in other components 525. Further, in this illustrative example, one or more of the other components 525 may be located in various positions relative to nacelle heat exchanger 520. For example, when nacelle heat exchanger 520 takes the form of a hollow geometric conduit 550, a component such as a fuel cell stack may be located within a channel 551 in hollow geometric conduit 550.

[0093] In one illustrative example, inlet 504 may have a fixed area 560. In this example, variable area nozzle 561 may be positioned between fan system 522 and outlet 505. Variable area means that the cross-sectional area of ​​variable area nozzle 561 may vary. Airflow 532 through variable area nozzle 561 may result in exhaust velocity 563 of exhaust air 554 leaving outlet 505 being greater than the free stream velocity.

[0094] The block diagram of 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, nacelle heat exchanger 520 may have other shapes, such as a prism or other suitable shapes. Furthermore, one or more nacelle heat exchangers may be used in addition to or instead of nacelle heat exchanger 520 in 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 nacelle dedicated to providing 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 elsewhere in the aircraft, such as in the fuselage, in addition to or instead of the engine nacelle. Furthermore, 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 illustrative 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 inlet 611 and outlet 612. As shown, nacelle heat exchanger 600 is a hollow geometric conduit in the shape of a cylinder 620 and has 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 the airflow through nacelle heat exchanger 600. This type of airflow may be useful when the aircraft is operating on the ground or at altitudes and speeds that do not provide the desired level of airflow or heat release 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 in accordance with an illustrative 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, fan 704 is located aft of nacelle heat exchanger 700. In other examples, fan 704 may be located forward of nacelle heat exchanger 700. In this example, other components 707 are shown as being further within channel 705 of nacelle heat exchanger 700 than other components 602 of FIG. 6 .

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

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

[0106] The shallow angle position of the nacelle heat exchanger 800 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 heat exchanger layout diagrams 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 engine 900 is shown within nacelle 902, which is shown in an exposed view of its interior and partially in phantom.

[0110] In this view, it can be seen that the nacelle heat exchanger 904 is at a shallow angle relative to the inlet 906 in the nacelle 902. In this illustrative example, the shallow angle is an angle of 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 as air flows through the inlet 906 in the nacelle 902 and past 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, fan system 910 is located aft of nacelle heat exchanger 904. Fan system 910 may be used to create or increase airflow through inlet 906 and through nacelle heat exchanger 904. Fan system 910 may be used to create or increase airflow to 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 illustrative 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 of 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 , an illustration 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] One portion of the nacelle 1102 is shown in phantom lines, 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, nacelle heat exchanger 1104 is located within nacelle 1102. In this illustrative example, nacelle heat exchanger 1104 is annular cylindrical in shape and has channels 1110 therein.

[0117] The nacelle heat exchanger 1104 is positioned within the nacelle 1102 such that components can be positioned within a channel 1110 within the nacelle heat exchanger 1104. Additionally, in this illustration, 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 within 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 illustrative 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 in accordance with 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, nacelle 1400 is a dedicated nacelle for 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 may be a hollow geometric conduit containing channels. In this example, the airflow of air 1406 passes through at least one of the surfaces within the channels or the outer surface of the hollow geometric conduit. In an illustrative example, the nacelle heat exchanger 1402 may also be a pyramidal truncated cone shape.

[0123] Additionally, 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. The nozzle may increase the exhaust velocity of the air 1406 exiting the nacelle 1400 as exhaust 1408. This higher exhaust velocity may be achieved relative to the freestream 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. Other components of the ducting system, fuel cell stack, and electric engine can also be seen in the above diagram.

[0126] 15, a diagram of airflow through a nacelle to provide cooling for a fuel cell stack is shown, 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 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 located at the outlet 1552 of the nacelle 1502.

[0127] As shown, fan 1506 can augment airflow, indicated by arrows 1500, to provide desired airflow for cooling during various phases of flight. For example, 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 heat exchanger 1504 to provide cooling for fuel cell stack 1510. ducting system 1508 can provide a flow of coolant through nacelle heat exchanger 1504, where the coolant is cooled by the airflow through 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 heat exchanger 1600 is shown. Nacelle heat exchanger 1600 is a rectangular heat exchanger. As shown, coolant enters nacelle heat exchanger 1600 through coolant inlet 1612, flows through heat exchanger 1600, and exits through coolant outlet 1610.

[0130] 18, a diagram of airflow through a nacelle heat exchanger is shown in accordance with an illustrative 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 rows of tubes 1830 stacked on top of each other to form panels 1814 for heat exchanger 1600. The coolant is cooled by airflow moving through fins 1831 within heat exchanger 1600.

[0131] The 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 how 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 are 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 other inlets and other outlets for cooling airflow to heat exchangers that cool 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 offer higher operating temperatures compared to conventional low-temperature proton exchange membrane (LTPEM) technology, providing a significant improvement in thermal drive potential and enabling a significant reduction in the size of the fuel cell aircraft thermal management system (TMS). However, when high-temperature proton exchange membrane (HTPEM) fuel cells are used in fuel cell stacks, the startup process can be time-consuming and energy-intensive, as they have a much higher minimum operating temperature of approximately 120°C. In the example above, the startup process involves bringing the fuel cell stack up to its minimum operating temperature, then supplying fuel and oxidant 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 because no power is available from the fuel cell stack before it is at operating temperature, so 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 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 area 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, which can create non-uniform temperature gradients within the stack.

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

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

[0142] For example, in an aircraft configuration with six engines located in six nacelles, there are three fuel cell power units per wing. This configuration triples 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 the startup of multiple HTPEM fuel cell stacks contained on a fuel cell-powered aircraft. In one illustrative example, a coolant heater and an existing thermal management system are used to start the HTPEM fuel cell stacks by heating and circulating fuel cell stack coolant. In this example, the coolant may be initially used to heat a subset of the total number of fuel cell stacks on the aircraft. After these fuel cell stacks reach operating temperature and begin generating electricity, this power can be used to start 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, which can be incorporated into the conduit for the coolant system. This would allow the existing cooling pump to be used to circulate coolant through the stack and start-up heater. In addition to the stack minimizing start-up time and energy required, the design would also include several bypasses to allow coolant to blow through the heater during normal operation after start-up, and bypasses to allow coolant to blow through the heat exchanger during start-up without having to heat the heat exchanger.

[0145] Another feature of the illustrative example provides a startup process that generates the amount of energy required to start 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 onboard batteries sized solely for starting 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. Additionally, when the fuel cell stacks reach the 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 even more quickly.

[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 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, thereby starting up the fuel cell stacks. This type of fuel cell stack startup is feasible for any aircraft that includes 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 can be used to simultaneously power up the entire stack 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 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 an operator is not required to rely on the energy contained within the batteries.

[0152] 19, an illustration of 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 start-up system 1900 is configured to assist or cause the fuel cell stack 1920 to reach 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 the reaction within the fuel cell stack to generate electricity from hydrogen gas and oxygen.

[0155] For example, fuel cell stack startup system 1900 circulates heated coolant 1916 to a set of fuel cell stacks 1920 in fuel cell system 1921 until the set of fuel cell stacks 1920 reaches operating temperature 1925 and generates power to heat coolant 1917 to start up other fuel cell stacks in fuel cell stacks 1920 in 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 heater system 1901, conduit system 1902, pump system 1904, and controller 1905.

[0157] In this example, heater system 1901 operates to heat coolant 1917. Heater system 1901 may include several heaters 1915. In one example, several of heaters 1915 and fuel cell stacks 1920 may 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 fired heater 1933 that uses hydrogen gas to generate heat for the heated coolant 1916. In this example, the fired heater 1933 may be a conduit or radiator thermally connected to a component that combusts hydrogen fuel. The hydrogen fuel may be delivered as a gas to the burner element.

[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, controller 1905 is a hardware system that may include software. When software is used, the processes performed by controller 1905 may be embodied in program instructions configured to run on hardware, such as a processor unit. When firmware is used, the processes performed by controller 1905 may be embodied in 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 perform the processes in controller 1905.

[0161] In this embodiment, the hardware for the controller 1905 may take the form of at least one of 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 certain processes. When a programmable logic device is used, the device may be configured to perform certain processes. The device may be later reconfigured or may be permanently set to perform certain processes. 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, processes may be implemented in organic components embedded in inorganic components, or may be entirely composed of non-human organic components. For example, processes may be implemented as circuits in organic semiconductors. 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, controller 1905 can control heater system 1901 to heat coolant 1917 to form heated coolant 1916. Controller 1905 can control pump system 1904 to circulate heated coolant 1916 through conduit system 1902. Controller 1905 can also control conduit system 1902 to circulate heated coolant 1917 to a set of fuel cell stacks 1920. In this example, heated coolant 1916 causes a set of fuel cell stacks 1920 to reach an operating temperature 1925. In this example, operating temperature 1925 is a minimum operating temperature 1923 required for a set of fuel cell stacks 1920 to begin generating electricity.

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

[0164] In one example, heater system 1901 is comprised of heaters 1915, where each fuel cell stack in fuel cell stacks 1920 and each heater in heaters 1915 is 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 sends 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] Further in this example, the controller 1905 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, and 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 generating power.

[0166] During this startup process, controller 1905 controls conduit system 1902 to bypass nacelle heat exchanger 1960 during fuel cell stack startup. 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 one illustrative example, controller 1905 controls conduit system 1902 to circulate heated coolant to several fuel cell stacks, where the heated coolant causes the several fuel cell stacks to reach an operating temperature 1925 for several fuel cell stacks 1920. In this example, several fuel cell stacks 1920 are selected from a group including all fuel cell stacks 1920 and a set of fuel cell stacks 1920. In other words, the circulation of 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 engines.

[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 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.

[0170] For example, although not shown, conduit system 1902 may include conduits. Conduit system 1902 may also include valves, pumps, or some 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, according to an illustrative 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 so that the fuel cell stacks reach an operating temperature for generating electricity (step 2000), 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 illustrative embodiment. The process shown in this 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 fuel cell stacks to heat the remaining fuel cell stacks to operating temperature (step 2100), after which the process ends.

[0175] Turning now to Figure 22, a flow diagram of a process for managing the startup of an aircraft fuel cell stack is shown, according to an illustrative embodiment. The process of Figure 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 within one or more computer systems. For example, the process may be performed within fuel cell stack startup system 1900 of Figure 19.

[0176] The process begins by heating a coolant with a heater to form 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 for heating the coolant.

[0177] The process controls a conduit system to circulate heated coolant to a pair of fuel cell stacks, where the heated coolant causes the pair of fuel cell stacks to reach 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. 22.

[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 yet reached operating temperature (step 2300), after which the process ends.

[0180] Referring now to Figure 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 for carrying out the steps of Figure 23. In this example, each fuel cell stack of the fuel cell stacks and each heater of the heaters are located within the nacelle. Furthermore, each fuel cell stack is thermally connected to the heater. This thermal connection can be made using a conduit system.

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

[0182] 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 an example of additional steps that can be performed in conjunction with the steps of FIG. 22.

[0183] In response to one of the fuel cell stacks generating power, the process stops heating the coolant by the heating system for the one of the fuel cell stacks that is generating power (step 2500), with the process then terminating.

[0184] The flow diagrams and block diagrams in the various illustrated embodiments illustrate the structure, functionality, and operations of some possible implementations of apparatuses and methods in example embodiments. In this regard, each block in a flow diagram or block diagram 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 implemented 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 diagram or block diagram. When implemented as a combination of program instructions and hardware, the implementation may take the form of firmware. Each block in a flow diagram or block diagram may be implemented 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 concurrently, 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, an illustration of an aircraft including a propulsion and power system is shown, in accordance with an illustrative embodiment. Fuel cell stack start 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 connected to engine 2613 and starter battery 2604 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 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 depicted in accordance with an illustrative embodiment. An illustration of engine 2700 is shown without its nacelle. In this illustrative example, engine 2700 is an integrated power and propulsion unit (IPPU) and is an example of the engine without its 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 velocity 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 power generation using the hydrogen and oxygen reactants.

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

[0198] 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. Turbine 2707 recovers energy from the air exiting 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 through 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 that produces 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 the heat to a heat exchanger 2701 for rejection to the air. A cold coolant pump 2711 circulates coolant to remove heat from cold components, which require a lower level of cooling than the fuel cell stack 2703. Cold components include, for example, motor 2704 and controller 2705. The heat is transferred to a cold heat exchanger, not shown, which is located closer to the LH2 tank in the wing.

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

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

[0203] The illustration of aircraft 2600 in FIG. 26 and engine 2700 in FIG. 27 are provided as illustrations 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 aside from the six engines depicted for aircraft 2600. In other illustrative examples, one starter battery or three starter batteries may be used instead of the two starter batteries shown for aircraft 2600. In other illustrative examples, fan system 2702 may be omitted. In other illustrative examples, fan system 2702 may be located before heat exchanger 2701. In yet another illustrative example, heater 2709 may be omitted from 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 among fuel cell stacks 1920 of FIG. 19 , startup heater 2802 is an example of a heater among heaters 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 startup, 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. These valve positions cause air to flow through the startup heater 2802 instead of the ram air radiator 2815.

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

[0208] Referring now to Figure 29, a schematic diagram of a fuel cell stack startup system within a thermal management system is shown, according to an illustrative embodiment. Fuel cell stack startup system 2997 within thermal management system 2996 is an example implementation of fuel cell stack startup system 1900 of Figure 19. Fuel cell stack startup 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 Figure 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 startup system 2800 of Figure 28.

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

[0210] As shown, an HTPEM fuel cell stack 2920 is located within a nacelle 2950. Other components within the nacelle include a start-up heater 2910, a pump 2911, an air intake 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 a controller, 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, an 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 HTPEM fuel cell stack 2920 in nacelle 2950.

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

[0216] 1. A start battery is used to power a start heater 2910 and a coolant pump 2911. The starter battery can be battery 2603 in FIG. 26.

[0217] 2. Open bypass valve 2973 to allow coolant to bypass ram air radiator 2927, ensuring that 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 dissipate heat.

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

[0219] In this example, a flow of heated coolant passes through one set 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 reaches its minimum operating temperature for electrochemical activity, air (via the activation of the air management system) and fuel can be supplied to the stack, allowing the HTPEM FC stack 2920 to begin generating its own electricity. Additionally, applying power draw to the HTPEM FC stack 2920 at this point will help the HTPEM FC stack 2920 heat itself due to its inefficient operation at this initial operating temperature.

[0221] At this stage, the power generated by the HTPEM FC stack 2920 can be used to power the startup 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 a single loop. This connection can occur 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 occur in addition to or instead of using start-up heaters in the other nacelles. This connection can occur 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 way, 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. Furthermore, this type of heating also allows beneficial use of start-up heaters in other nacelles. As a result, some heaters can be eliminated, reducing aircraft weight.

[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 other nacelles 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 in which high temperature cooling loops are isolated from one another by a configuration of a conduit system 2990. There are no conduits that allow for the 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 diagrams of fuel cell stack startup system 2997 in Figures 29 and 30 are presented as diagrams of implementations of fuel cell stack startup system 1900 of Figure 19. The above diagrams are not intended to limit how other fuel cell stack systems may be implemented in other embodiments. For example, a single startup heater may be used in other fuel cell stack startup systems. In other illustrative examples, a battery may be included in the nacelle.

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

[0229] However, this compression can heat the inlet air above the operating temperature of the fuel cell stack. To avoid overheating in this situation, a cooling stage can be used.

[0230] In an illustrative example, cooling load and cooling efficiency are factors in a cooling design system, and it is desirable to reduce the overall cooling load and increase the efficiency of the cooling system.

[0231] Existing low-temperature proton exchange membrane (LTPEM) fuel cell systems require a compressor to sufficiently compress 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, the air may be cooled before entering the fuel cell stack to prevent overheating and drying out of the fuel cell stack membrane.

[0232] 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 energy recovery opportunities.

[0233] 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.

[0234] 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. The component cools the compressed air, allowing for the elimination of an air-to-liquid intercooler 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.

[0235] Cooling of the compressor exit air is currently achieved 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, little energy recovery is obtained by expansion.

[0236] The charge air heat exchanger is a heat exchanger that provides heat exchange for compressed air. This heat exchanger can cool the compressed air. The charge air intercooler is an intercooler that provides cooling for the compressed air in this example.

[0237] 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.

[0238] 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 replace an intake air intercooler, rather than simply downsizing the intake air intercooler.

[0239] 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, aircraft air management system 3100 may be implemented within aircraft 400 of Figure 4 to manage the operation of a fuel cell stack within engine 406 of Figure 4 to generate electricity to run engine 406 of Figure 4. In this example, aircraft air management system 3100 manages the temperature of air delivered to fuel cell stack 3105, which is used along with gaseous hydrogen to generate electricity to run electric motors within the aircraft. In this example, fuel cell stack 3105 includes a low-temperature proton exchange membrane fuel cell 3113.

[0240] In one illustrative example, aircraft air management system 3100 has several different components. As shown, aircraft air management system 3100 includes air heat exchanger 3102, intercooler 3103, and ducting system 3104.

[0241] In this example, conduit system 3104 is connected to air heat exchanger 3102, intercooler 3103, and fuel cell stack 3105. Heated air 3120 flows through conduit system 3104 to air heat exchanger 3102. Air heat exchanger 3102 cools the heated air 3120 to form cooler air 3122. In this example, cooler air 3122 flows from air heat exchanger 3102 through the conduit system to intercooler 3103. Intercooler 3103 further cools cooler air 3122 to form cooled air 3124. Cooled air 3124 flows from intercooler 3103 through conduit system 3104 to fuel cell stack 3105. In this example, cooled air 3124 is at an operating temperature 3125 for fuel cell stack 3105.

[0242] In one illustrative example, air heat exchanger 3102 , intercooler 3103 , ducting system 3104 , and fuel cell stack 3105 are located within a nacelle 3130 for an engine 3132 of an aircraft 3127 .

[0243] Additionally, aircraft air management system 3100 includes compressor 3107. Compressor 3107 is connected to duct system 3104 and generates heated air 3120, which flows through air heat exchanger 3102.

[0244] In this example, heated air 3120 , cooler air 3122 , and cooled air 3124 is compressed air 3126 .

[0245] Additionally, conduit system 3104 also guides gaseous hydrogen 3140 from fuel system 3142 to fuel cell stack 3105. The gaseous hydrogen 3140 can be used, along with cooled air 3124, to generate electricity to power engines 3132 of aircraft 3127. This guidance is achieved using separate conduits, which separates the gaseous hydrogen 3140 from the air flowing through conduit system 3104. In other words, a first portion 3111 of conduit system 3104 can be used to guide air, and a second portion 3112 of conduit system 3104 can be used to guide gaseous hydrogen 3140. The two portions are separate from each other, which prevents the air and hydrogen from mixing within conduit system 3104.

[0246] In this example, a second portion 3112 of conduit system 3104 directs cooled air 3124 from intercooler 3103 to fuel system heat exchanger 3141, which heats liquid hydrogen 3143 to form gaseous hydrogen 3140. The second portion of conduit system 3104 directs gaseous hydrogen 3140 from fuel system heat exchanger 3141 to fuel cell stack 3105. 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.

[0247] The block diagram of 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.

[0248] 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. Additionally, 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.

[0249] Additionally, the aircraft air management system 3100 can 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 can 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 can be managed to have a temperature sufficiently close to the temperature of the fuel cell stack 3105 to enable 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 temperature gradients that reduce efficiency in power generation. As a result, the compressed air 3126 can be heated or cooled to have a temperature relative to the operating temperature for the fuel cell stack 3105 that provides a desired level of power generation.

[0250] 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 fuel cell stack start-up system 1900 of Figure 19.

[0251] The process begins by receiving heated air (step 3200). The process cools the heated air using an air heat exchanger to form cooler compressed air (step 3202).

[0252] The process includes cooling the cooler air using an intercooler to form cooled compressed air (step 3204). The process includes sending the cooled air to the fuel cell stack (step 3206). The process then ends.

[0253] In Figure 32, the air heat exchanger, intercooler, and fuel cell stack are located within a nacelle for an aircraft engine. Ambient air is directed through a nacelle inlet into a compressor where it is increased in pressure. The now higher pressure and temperature air is then ducted to an intake air heat exchanger where it is conditioned toward the temperature of the fuel cell stack by heat exchange with the fuel cell stack cathode outlet.

[0254] The air temperature is then further adjusted toward the fuel cell stack temperature through heat exchange with fuel cell stack coolant in an intake 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. Upon exiting the stack, the contaminated air also removes some water product, preventing the fuel cell stack from flooding.

[0255] Once the air leaves the stack, it passes through an intake air heat exchanger, then passes through a turbine where it expands and recovers energy for use towards the compression process, and finally the air exits the nacelle as exhaust.

[0256] 33, there is shown a schematic diagram of an aircraft air management system according to an illustrative embodiment. Air management system 3301 operates to cool fuel cell stack 3304. In this example, air management system 3301 includes charge air exchanger (HX) 3300, charge air intercooler 3302, and ducting system 3303. Air management system 3301 also includes compressor (C) 3305, turbine (T) 3311, motor 3341, and controller 3342.

[0257] In this example, compressor (C) 3305 receives inlet air 3317 entering the nacelle and produces compressed air 3318 at a higher pressure than inlet air 3317. In this example, compressor 3305 is operated by motor 3341 under the control of controller 3342. Additionally, compressed air 3318 has a higher temperature, which may be higher than desired for use in fuel cell stack 3304. This compressed air can be cooled using charge air heat exchanger (HX) 3300 and charge air intercooler 3302 to form cooled compressed air 3319. Charge air heat exchanger (HX) 3300 initially cools compressed air 3318 to form cooler air 3321. This cooler compressed air can be further cooled by charge air intercooler 3302, which provides additional cooling, to form cooled compressed air 3319. The resulting cooled compressed air 3319 is compressed air that may have a desired temperature for use in the fuel cell stack 3304 .

[0258] The cooled compressed air 3319 can be used in the fuel cell stack 3304 along with gaseous hydrogen received from the hydrogen (H) fuel system 3320 to generate electricity used to power components such as the motor 3341 and controller 3331. The motor 3341 itself provides the mechanical energy to rotate the propeller 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 for use in the fuel cell stack 3304.

[0259] In this example, compressed air 3323 that has flowed through fuel cell stack 3304 is directed to charge air heat exchanger 3300, which heats the air as part of the process of cooling the compressed air received from compressor 3305, to form heated air 3326. This heated air is used to turn turbine 3311. In this manner, some energy recovery may be achieved using turbine 3311. Air 3328 from turbine 3311 is then directed to exit the engine as exhaust 3313.

[0260] 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.

[0261] 5. Fuel Cell Aircraft Thermal Management System

[0262] 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. Furthermore, these types of fuel cells may also provide a highly efficient way to generate propulsion and / or electrical power for aircraft systems.

[0263] Lower operating temperatures compared to conventional turbomachinery lend themselves to large and heavy thermal management system (TMS) designs, which, in an illustrative example, provide features to manage and release heat generated by fuel cell propulsion systems onboard aircraft while reducing undesired impacts on performance.

[0264] High-temperature proton exchange membrane (HTPEM) technology can be used to provide a higher thermal driving potential, which 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 exiting 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. Therefore, 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.

[0265] 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 sections 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 other than the fuel cell stack, which can be the motors, controllers, and power electronics in the nacelle.

[0266] In one example, a low-temperature (LT) cooling loop contains 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 onboard 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 dedicated nacelle heat exchangers for lower temperatures and liquid-cooled components within the nacelle.

[0267] The high temperature (HT) loop includes a coolant 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.

[0268] In this example, the high temperature loop includes a nacelle heat exchanger, which is a different heat exchanger than the one 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.

[0269] Referring now to FIG. 34 , a block diagram of an aircraft thermal management system is shown in accordance with an illustrative embodiment. In this illustrative example, an aircraft thermal management system 3400 may be implemented within aircraft 400 of FIG. 4 to provide operational thermal management of a fuel cell stack 3405 and other components that generate heat within engine 406 of FIG. 4 . In this example, aircraft thermal management system 3400 provides cooling for fuel cell stack 3405 and several heat loads 3416. In this example, fuel cell stack 3405 includes high-temperature proton exchange membrane fuel cells. Several 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.

[0270] 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 conduit system 3403.

[0271] High temperature loop 3401 is configured to manage temperature 3410 of fuel cell stack 3405 in nacelle 3406 using nacelle heat exchanger 3421. In managing temperature 3410 of fuel cell stack 3405, high temperature loop 3401 is configured to maintain fuel cell stack 3405 at operating temperature 3411. Maintaining temperature 3410 may include at least one of cooling or heating fuel cell stack 3405. Operating temperature 3411 may be a range of temperatures within operating temperature limit 3419. In this example, operating temperature limit 3419 is the maximum or minimum temperature at which fuel cell stack 3405 may operate.

[0272] In this example, high temperature loop 3401 can utilize coolant 3430 within high temperature loop 3401 to remove heat from fuel cell stack 3405. Nacelle heat exchanger 3421 can be used to cool coolant 3430 to a temperature sufficient to cool or heat fuel cell stack 3405 so that temperature 3410 of fuel cell stack 3405 is at operating temperature 3411 within operating temperature limits 3419 of fuel cell stack 3405.

[0273] 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 to the ambient air as part of the exhaust gas exiting nacelle 3406. Heat may be added to fuel cell stack 3405 in response to operating temperature 3411 being less than operating temperature limit 3419. 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.

[0274] Cryogenic loop 3402 is configured to use the heat capacity 3407 of liquid hydrogen 3408 to cool several heat loads 3416 within nacelle 3406. In this example, the heat capacity 3407 of liquid hydrogen 3408 may be used to cool coolant 3430 in cryogenic loop 3402 that is used to cool several heat loads 3416.

[0275] 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.

[0276] In this illustrative example, when charge air heat exchanger 3420 is used to add or remove heat from the exhaust of fuel cell stack 3405, heated compressed air 3440 generated by compressor 3441 can be conditioned in conduit system 3403 by charge air heat exchanger 3420 to form compressed air near operating temperature 3411. This compressed air is sent into fuel cell stack 3405 for use with gaseous hydrogen 3445 to generate electricity.

[0277] In one illustrative example, there may also be an air intercooler 3460. In this example, air intercooler 3460 receives cooled compressed air 3444 from supply air heat exchanger 3420 via conduit system 3403, further conditions the cooled compressed air 3444, and can deliver the cooled compressed air 3444 near operating temperature 3411 into fuel cell stack 3405. The cooled compressed air 3444 reacts with gaseous hydrogen 3445 in fuel cell stack 3405 to generate electricity.

[0278] In this illustrative example, heated compressed air 3440 may be based on temperature 3410 of fuel cell stack 3405. If the air in heated compressed air 3440 is too much above operating temperature 3411 of fuel cell stack 3405, heated compressed air 3440 is cooled. If the air in heated compressed air 3440 is too much below operating temperature 3411 of fuel cell stack 3405, heated compressed air 3440 is heated. Thresholds may be used to determine when a temperature is too much below or above.

[0279] The heating or cooling ability of 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 by varying the amount of airflow through the nacelle heat exchanger 3421.

[0280] Thus, in this illustrative example, high temperature loop 3401 functions 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 functions 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.

[0281] Additionally, in the illustrative example, it is possible to condition the air used with gaseous hydrogen 3445 in fuel cell stack 3405 to generate electricity. This conditioning of the air from compressor 3441 can include heating or cooling according to the temperature of heated compressed air 3440 and temperature 3410 of fuel cell stack 3405.

[0282] The block diagram of 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 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.

[0283] For example, in other illustrative examples, nacelle 3406 can include one or more fuel cell stacks in addition to fuel cell stack 3405. In other examples, other heat exchangers, such as intercoolers, can be used in conjunction with supply air heat exchanger 3420 to further cool the heated compressed air 3440.

[0284] Referring 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.

[0285] 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).

[0286] The process cools several heat loads in the nacelle using a cryogenic loop that uses the heat capacity of the liquid hydrogen to cool several heat loads (operation 3502), with the process then terminating.

[0287] 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 illustrative embodiment. The process shown in this figure is an example of an implementation of step 3500 of Figure 35.

[0288] 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 to maintain the fuel cell stack at an operating temperature. This operating temperature may be a temperature range at which the fuel cell stack 3405 operates to generate electricity. Furthermore, 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.

[0289] Referring now to Figure 37, a diagram of a thermal management system including a cold loop 3781 and a hot loop 3782 is shown according to an exemplary embodiment. This diagram illustrates the hardware design contained within a single nacelle for a high temperature proton exchange membrane (HTPEM) configuration. The fuel cell stack in this example is supplied with air from the air management system and hydrogen from the fuel system, and generates electricity. This electricity is used to power an electric motor through its controller, which itself rotates a propeller.

[0290] As shown, thermal management system 3700 is located within the engine nacelle. The system includes a hot loop 3781 and a cold loop 3782. Hot loop 3781 removes heat from fuel cell stack 3710 using ram air radiator 3702.

[0291] Cryogenic loop 3782 uses the heat capacity of the liquid hydrogen in hydrogen fuel system 3711 to cool a heat load that generates less heat than fuel cell stack 3710. In other words, cryogenic 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 hydrogen fuel system 3711.

[0292] As shown, inlet air 3701 passes through 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 elevated temperature compared to inlet air 3701.

[0293] 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.

[0294] 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 .

[0295] 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 toward the fuel cell stack temperature by exchanging heat with hot HT coolant. The air can be conditioned by heating or cooling the air according to the temperature of the compressed air. This conditioning can include at least one of heating and 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.

[0296] The conditioned air is delivered to fuel cell stack 3710. Within fuel cell stack 3710, oxygen in the conditioned air is reacted with gaseous hydrogen as the air moves across the individual fuel cells within fuel cell stack 3710.

[0297] 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 turbine 3706. The air expands to recover the energy used in the compression process, and the air is exhausted as exhaust 3703.

[0298] Ram air radiator 3702 removes heat from the hot coolant in high temperature loop 3781 and heats the air before it is expelled in exhaust 3703.

[0299] Hydrogen is stored in liquid form in a tank in the hydrogen fuel system 3711. This liquid hydrogen is distributed through a fuel line into the LH2 HX 3712, which 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.

[0300] 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 routed to ram air radiator 3702 where the heat in the coolant is rejected to the ambient air via ram air radiator 3702.

[0301] 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 LH2 HX 3712, causing it to vaporize.

[0302] In this example, a motor 3750 rotates a thruster 3795. A high power bus 3786 connects the fuel cell stack 3710 to other electrical components.

[0303] In some cases, the heat load does not generate enough heat to vaporize a sufficient amount of hydrogen fuel. In this case, a 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.

[0304] A high temperature loop 3781 within the thermal management system 3700 functions to remove heat from the fuel cell, and a ram air radiator 3702 rejects the heat to the ambient air.

[0305] The LH2 HX 3712 operates to remove heat from the cryogenic coolant and cold 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 HX 3731 operates to provide additional heat 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.

[0306] 6. Liquid Hydrogen Supply System for Fuel Cell-Powered Aircraft

[0307] 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 -424°F. 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 the gas prevents undesired overpressurization of the tank.

[0308] 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).

[0309] Liquid hydrogen tanks can use venting systems to prevent overpressure due to ambient heat leaks. Due to the highly flammable nature of hydrogen, the venting system can include a burn arrestor at the end to prevent backflash up the vent line if the vented hydrogen burns. The burn arrestor design functions to trap water in the narrow passage. If a pulse of warm hydrogen is vented before the relief valve opens, this action forces any water out of the burn arrestor. This process is done to prevent water that freezes in the burn arrestor due to extremely low temperatures from venting hydrogen and blocking flow through the burn arrestor.

[0310] In addition to pushing water out, a heater can be added to the burn arrestor to prevent icing when ambient temperatures may cause freezing.

[0311] An illustrative example recognizes that a concern with safely venting hydrogen gas is venting the hydrogen gas in a manner that does not impact the aircraft and get lost underneath it 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.

[0312] 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.

[0313] Due to the temperature of liquid hydrogen, there are only two known substances that remain gaseous: hydrogen (above the vapor pressure of liquid hydrogen) and helium.

[0314] Regarding the ability to purge vent hydrogen, the first step involves performing a warm hydrogen gas pressure cycle 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.

[0315] Referring now to Figure 38, an illustration of a hydrogen recirculation system is depicted in accordance with an illustrative embodiment. In this illustrative example, 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 within set of liquid hydrogen tanks 3810.

[0316] 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 on aircraft 3855. Conduit system 3801 includes a conduit 3803 and a valve 3804.

[0317] Conduit system 3801 includes conduits and valves. The valves can be controlled by controller 3802 to direct fluid along various routes or loops. Controller 3802 is a hardware system and can include software.

[0318] In one illustrative example, controller 3802 may control conduit system 3801 to remove gaseous hydrogen 3811 from the set of liquid hydrogen tanks 3810. In one illustrative example, controller 3802 controls conduit system 3801 to perform several different processes. For example, controller 3802 routes gaseous hydrogen 3811 to tank heat exchanger 3820. Gaseous hydrogen 3811 is heated by tank heat exchanger 3820 to form heated gaseous hydrogen 3823. The tank heat exchanger may be, for example, LH2 HX 3712 in FIG. 37 . Controller 3802 routes a first portion of the heated gaseous hydrogen to fuel cell stack 3822. Controller 3802 returns a second portion of the heated gaseous hydrogen 3823 to the set of liquid hydrogen tanks 3810.

[0319] In this example, the second portion of 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 liquid hydrogen 3807 stream described above includes liquid hydrogen 3807 that flows from the set of liquid hydrogen tanks 3810, is converted to gaseous hydrogen 3811, and is heated for use in the fuel cell stack 3822. This type of stream can be used when the second portion of heated gaseous hydrogen 3823 is not present in a sufficient quantity required for use in the fuel cell stack 3822.

[0320] Additionally, controller 3802 may control the pressure within liquid hydrogen tanks 3810. For example, controller 3802 may route gaseous hydrogen 3811 within set of liquid hydrogen tanks 3810 via conduit system 3801 to vent 3860 in response to the pressure within set of liquid hydrogen tanks 3810 being greater than a particular tolerance. In this example, vent 3860 is considered to be part of hydrogen recirculation system 3800. Thus, in this example, excess gaseous hydrogen is routed from set of liquid hydrogen tanks 3810 to vent 3860 within aircraft 3855 in response to the pressure within set of liquid hydrogen tanks 3810 being greater than a particular tolerance.

[0321] Vent 3860 may be located in various locations on aircraft 3855. For example, vent 3860 may be located in tail section 3861 of fuselage 3862 of aircraft 3855. For example, vent 3860 may be located in vertical stabilizer 3865 on tail section 3861 of fuselage 3862. Vertical stabilizer 3865 is a vertical stabilizer connected to tail section 3861 of fuselage 3862.

[0322] In another illustrative example, controller 3802 may perform similar operations on liquid hydrogen 3807 stored in set of liquid hydrogen tanks 3810. Liquid hydrogen 3807 may be used to maintain a desired pressure level within 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.

[0323] In this example, controller 3802 can control conduit system 3801 to perform processes. For example, controller 3802 can remove liquid hydrogen 3807 from a set of liquid hydrogen tanks 3810 and route liquid hydrogen 3807 to tank heat exchanger 3820 to form heated gaseous hydrogen 3823. Controller 3802 can route a first portion of heated gaseous hydrogen 3823 to fuel cell stack 3822 and return a second portion of heated gaseous hydrogen 3823 to the set of liquid hydrogen tanks 3810.

[0324] For example, vent 3860 may be in other locations. In other illustrative examples, vent 3860 may be in other stabilizers within vertical stabilizer 3865. In yet another illustrative example, vents may be located on the wings of aircraft 3855.

[0325] The block diagram of aircraft cooling system 3800 in Figure 38 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.

[0326] Referring to Figure 39, a flow diagram of a process for recirculating hydrogen is shown in accordance with an illustrative 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. This process may be used to manage liquid hydrogen in liquid hydrogen tanks on an aircraft.

[0327] The process begins by extracting gaseous hydrogen from a set of liquid hydrogen tanks connected to the aircraft (operation 3900). The process then routes the extracted gaseous hydrogen 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).

[0328] The process sends a first portion of the heated gaseous hydrogen to a fuel cell stack in an engine of the aircraft (operation 3904). The process returns a second portion of the heated gaseous hydrogen to a set of liquid hydrogen tanks (operation 3906). The process then ends.

[0329] Referring to Figure 40, a flow diagram of a process for recirculating hydrogen is shown in accordance with an illustrative 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. This process may be used to manage liquid hydrogen in liquid hydrogen tanks of an aircraft.

[0330] The process involves removing liquid hydrogen from a set of liquid hydrogen tanks (step 4000). The process involves sending the liquid hydrogen through a tank heat exchanger to form heated gaseous hydrogen (step 4002).

[0331] 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.

[0332] Referring to Figure 41, a flow diagram of a process for recirculating hydrogen is shown in accordance with an illustrative 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. This process may be used to manage liquid hydrogen in liquid hydrogen tanks on an aircraft.

[0333] In response to the pressure in the set of liquid hydrogen tanks being greater than a specified 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.

[0334] 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.

[0335] 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.

[0336] The gaseous hydrogen is split into two portions 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) boils off (vaporizes) in the liquid hydrogen tank and affects the tank pressure. In this example, the more boil-off, the greater the resulting pressure increase.

[0337] 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 heat exchanger 4205, which contains hot coolant from the fuel cell stack as the heating fluid. The hot coolant can be heated by the heat load in the cryogenic loop. 3. Heated gaseous hydrogen is output from 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 the other 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.

[0338] 43 shows a diagram of a vent system for gaseous hydrogen, according to an exemplary embodiment. Vent system 4300 provides the ability to vent gaseous hydrogen (GH2) in response to overpressure. This vent system allows the gaseous hydrogen to be released from the liquid hydrogen tank 4310 and removed from the aircraft. During flight, this venting can occur via a vent 4304 in the vertical stabilizer 4306. In this example, vent 4304 is connected to the liquid hydrogen tank 4310 by a conduit system 4312.

[0339] Referring now to Figure 44, a diagram of a system for recirculating gaseous hydrogen for tank pressure control is shown, according to an illustrative 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 found in section 4350 of Figure 43.

[0340] This diagram also shows components that can be used to recirculate liquid hydrogen (LH2) in addition to components for recirculating gaseous hydrogen. In this example, 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 it passes through a heat exchanger 4401 to warm it 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.

[0341] Regarding carrying out the recycle using liquid hydrogen, the process can 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 which contains hot coolant from a heat load such as an engine as the heating fluid. 3. Gaseous hydrogen is output from the heat exchanger 4401. 4. The gaseous hydrogen is split at the 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. Send gaseous hydrogen for recirculation to gaseous hydrogen (GH2) pump 4405 and gaseous hydrogen pump 4406. 6. A portion of the gaseous hydrogen for recycling is returned to the liquid hydrogen tank 4310, creating bubbles in the liquid hydrogen to maintain pressure within the liquid hydrogen tank 4310.

[0342] 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.

[0343] Additionally, a pressure relief valve 4420 can open to vent the hydrogen gas, thereby relieving pressure. 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.

[0344] 7. Two-phase cooling of aircraft electronics pumps

[0345] Conventional single-phase cooling systems are unable to thermally manage electrified propulsion systems within the size, weight, and power (SWaP) envelope acceptable for use in aircraft, such as liquid hydrogen-powered aircraft.

[0346] Current solutions pump liquid through a cooling loop using a ram air radiator. Ram air radiators for electrically charged 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.

[0347] 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. These solutions are ineffective at handling the large amounts of heat from these types of electrical components, which typically operate at temperatures below 100 degrees Celsius.

[0348] 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 using a pumped two-phase cooling loop.

[0349] 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.

[0350] 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.

[0351] Referring 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 an engine nacelle of the aircraft.

[0352] The thermal load 4501 can take several 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.

[0353] The aircraft cooling system 4500 can take several 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.

[0354] Conduit system 4515 includes conduit 4527 and may also include valve 4517. Conduit system 4515 is connected to evaporator 4511, condenser 4512, accumulator 4513, and pump system 4514.

[0355] In this example, evaporator 4511 is thermally connected to a set of heat loads 4501 and is capable of using liquid 4516 to cool the set of heat loads 4501 within aircraft 4502. In response to cooling the set of heat loads 4501, liquid 4516 forms vapor 4519.

[0356] 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, whereby cooling vapor 4519 forms liquid 4516. In this example, condenser 4512 also rejects 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.

[0357] In this example, accumulator 4513 receives liquid 4516 from condenser 4512 and stores liquid 4516. A pump system is a physical system that includes one or more pumps. Pump system 4514 pumps the liquid 4516 stored in accumulator 4513 to evaporator 4511.

[0358] Additionally, in this example, aircraft cooling system 4500 also includes a generator 4530 and a turbine 4531. In this example, turbine 4531 is connected to the generator and to conduit system 4515. Turbine 4531 is physically connected to generator 4530 and may rotate generator 4530 to generate electricity. Turbine 4531 is connected to conduit system 4515 such that steam 4519 flows through turbine 4531, causing turbine 4531 to rotate.

[0359] The turbine 4531 rotates in response to receiving steam 4519 from the evaporator 4511 to generate electricity for the generator 4530 , where a condenser 4512 receives the steam 4519 from the turbine 4531 .

[0360] In another illustrative example, aircraft cooling system 4500 includes condenser 4512, evaporator 4511, and pump system 4514. In this example, two liquids, namely, first liquid 4541 and second liquid 4542, flow through conduit system 4515. The two liquids flow separately from each other through conduit system 4515. In other words, the liquids do not mix. In this example, a first pump 4544 within pump system 4514 pumps first liquid 4541, and a second pump 4545 within pump system 4514 pumps second liquid 4542.

[0361] In this example, a first pump 4544 pumps a first liquid 4541 to a set of heat loads 4501 in a liquid loop 4550. The set of heat loads 4501 heat the first liquid 4541, whereby heated liquid 4546 is formed.

[0362] Evaporator 4511 is a separate or separated evaporator, where first liquid 4541 (heated liquid 4546) and second liquid 4542 can flow through the components without mixing with each other. In this example, evaporator 4511 receives heated liquid 4546 and cools the heated liquid 4546. Cooling of heated liquid 4546 causes second liquid 4542 in liquid-vapor loop 4551 to form vapor 4552.

[0363] 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.

[0364] 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 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.

[0365] Referring now to Figure 46, a flow diagram of a process for cooling a heat load is shown in accordance with an illustrative 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. This process may be used to cool a heat load within an aircraft.

[0366] 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 (operation 4600). The process uses a condenser to cool the vapor to form a liquid (operation 4602).

[0367] The process receives the liquid from the condenser (operation 4604). The process stores the liquid in an accumulator (operation 4606).

[0368] The process pumps the liquid stored in the accumulator to the evaporator (operation 4608), with the process then terminating.

[0369] 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.

[0370] 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).

[0371] 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.

[0372] Referring now to FIG. 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, accumulator 4808 is a reservoir that stores liquid. Pump 4802 can pump liquid from accumulator 4808 to evaporator 4804. 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 fuel cells, fuel cell stacks, electric motors, computer systems, electronics enclosures, or lasers. The set of thermal loads may include fuel cells, fuel cell stacks, electric motors, electronics enclosures, computer systems, lasers, or other suitable types of components that generate heat.

[0373] When cooling the heat load, 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 that is stored in the accumulator 4808, and the liquid can be recirculated by the pump 4802 to provide further cooling within the pumped two-phase cooling loop 4800.

[0374] 49, a diagram of a hybrid two-phase (P2P) cooling loop is shown, in accordance with an illustrative embodiment. In this example, the hybrid two-phase (P2P) cooling loop 4900 includes a pump 1 4902, a pump 2 4904, an evaporator 4906, and a condenser 4908.

[0375] In this example, pump 1 4902 pumps a first liquid in liquid loop 4901 that has been condensed by evaporator 4906 to 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.

[0376] 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.

[0377] In this example, in a two-phase (P2P) loop 4915, a heated liquid is heated in an evaporator 4906 to form vapor 4903. This 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 can minimize the use of potentially volatile working fluids. Additionally, this configuration can be used with heat loads that cannot directly contact the liquid used as a coolant or refrigerant.

[0378] 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 within evaporator 4906 cools the first liquid in liquid loop 4901.

[0379] 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.

[0380] In this example, a turbine 5010 is located between an evaporator 5004 and a condenser 5006. Liquid delivered to the evaporator by a pump 5002 cools a heat load 5011. In this example, steam 5031 produced 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, which generates electricity in response to use of the steam 5031 from the turbine 5010. The steam 5031 then flows to a condenser 5006 where it condenses into a liquid 5032, which is stored in an accumulator 5008. As shown, a waste heat recovery system 5050 includes the turbine 5010 and the generator 5012.

[0381] 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.

[0382] 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 above its saturation temperature, respectively.

[0383] Some features of the embodiments are described in the following clauses. These clauses are examples of features and are not intended to limit other embodiments.

[0384] 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 forms disclosed. 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," "including," "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.

[0385] Numerous modifications and variations will be apparent to those skilled in the art. Furthermore, 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 others skilled in the art in understanding 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 heater system (1901) configured to heat a coolant (1917); a conduit system (1902, 2990) thermally connected to a fuel cell stack (1920), the coolant (1917) flowing through the conduit system (1902, 2990); a pump system (1904) configured to circulate the coolant (1917) through the conduit system (1902, 2990) to the fuel cell stack (1920); A controller (1905), controlling the heater system (1901) to heat the coolant (1917) to form a heated coolant (1916); controlling the pump system (1904) to circulate the heated coolant (1916) through the conduit system (1902, 2990); and controlling the conduit system (1902, 2990) to circulate the heated coolant (1916) to a set of the fuel cell stacks (1920), where the heated coolant (1916) causes the set of the fuel cell stacks (1920) to reach an operating temperature (1925); A controller (1905) configured to: Aircraft (199, 400, 3127, 3855, 4502) equipped with

2. The controller (1905), 2. The aircraft of claim 1, further configured to control the conduit system to circulate the heated coolant to all of the fuel cell stacks in response to the set of the fuel cell stacks reaching the operating temperature, wherein the heated coolant is heated by the set of the fuel cell stacks generating electrical power and the remaining fuel cell stacks reach the operating temperature.

3. the heater system (1901) is comprised of a heater (1915), each of the fuel cell stacks (1920) and each of the heaters (1915) are located within a nacelle (2950, ​​2952, 2954), each of the fuel cell stacks is thermally connected to a heater of the heaters (1915), and the controller (1905) 13. The aircraft of claim 1, further configured to transmit electrical power generated by the set of fuel cell stacks to the heaters for other fuel cell stacks that are not generating electrical power.

4. The controller (1905), 3. The aircraft (199, 400, 3127, 3855, 4502) of claim 2, further configured to: stop heating the coolant (1917) by the heating system for the set of the fuel cell stacks (1920) that are generating electrical power in response to the set of the fuel cell stacks (1920) generating electrical power.

5. 2. The aircraft (199, 400, 3127, 3855, 4502) of claim 1, wherein the conduit system (1902, 2990) and the pump system (1904) circulate the coolant (1917) to a nacelle (2950, ​​2952, 2954) heat exchanger during operation of an electric engine (106, 406, 2601) of the aircraft (199, 400, 3127, 3855, 4502).

6. The controller (1905), 2. The aircraft of claim 1, configured to control the conduit system to bypass a nacelle heat exchanger during start-up of the fuel cell stack.

7. The controller (1905), 10. The aircraft of claim 1, configured to control the conduit system to bypass the heater system after activation of the fuel cell stack.

8. 10. The aircraft of claim 1, wherein the heater system includes an electric heater connected to a battery system.

9. The heater system (1901), 10. The aircraft (199, 400, 3127, 3855, 4502) of claim 1, including a combustion heater that uses hydrogen gas to generate heat for heating the coolant (1917).

10. 10. The aircraft (199, 400, 3127, 3855, 4502) of claim 1, wherein each fuel cell stack is located within a nacelle (2950, ​​2952, 2954) for an engine.

11. 2. The aircraft of claim 1, wherein the fuel cell stack is a high-temperature proton exchange membrane (HTPEM) fuel cell stack.

12. An aircraft (199, 400, 3127, 3855, 4502), A fuselage (102, 402, 2630, 3140, 3862), a wing (404) connected to said fuselage (102, 402, 2630, 3140, 3862); an engine (106, 406, 2601) connected to said wing (404); a fuel cell stack (1920) configured to generate electricity for the engine (106, 406, 2601); a fuel cell stack startup system (1900, 2800, 2997) configured to circulate heated coolant (1916) to a set of fuel cell stacks (1920) in a fuel cell system (1921) such that the set of fuel cell stacks (1920) reach an operating temperature (1925) and generate power to heat the coolant (1917) to start up other fuel cell stacks (1920) in the fuel cell system (1921); Aircraft (199, 400, 3127, 3855, 4502) equipped with

13. The fuel cell stack starting system (1900, 2800, 2997), a heater system (1901) configured to heat a coolant (1917); a conduit system (1902, 2990) connected to the fuel cell stack (1920), the coolant (1917) flowing through the conduit system (1902, 2990); a pump system (1904) configured to circulate the coolant (1917) through the conduit system (1902, 2990) to the fuel cell stack (1920); A controller (1905), controlling the heater system (1901) to heat the coolant (1917) to form a heated coolant (1916); controlling the pump system (1904) to circulate the heated coolant (1916) through the conduit system (1902, 2990); and controlling the conduit system (1902, 2990) to circulate the heated coolant (1916) to the set of fuel cell stacks (1920), where the heated coolant (1916) causes the set of fuel cell stacks (1920) to reach the operating temperature (1925); A controller (1905) configured to:

13. The aircraft (199, 400, 3127, 3855, 4502) of claim 12, comprising:

14. An aircraft (199, 400, 3127, 3855, 4502), A fuselage (102, 402, 2630, 3140, 3862), a wing (404) connected to said fuselage (102, 402, 2630, 3140, 3862); an engine (106, 406, 2601) connected to said wing (404); a fuel cell stack (1920) configured to generate electricity for the engine (106, 406, 2601); A heater system (1901) for heating a coolant (1917); a conduit system (1902, 2990) connected to the fuel cell stack (1920), the coolant (1917) flowing through the conduit system (1902, 2990); a pump system (1904) configured to circulate the coolant (1917) through the conduit system (1902, 2990) to the fuel cell stack (1920); A controller (1905), controlling the heater system (1901) to heat the coolant (1917) to form a heated coolant (1916); controlling the pump system (1904) to circulate the heated coolant (1916) through the conduit system (1902, 2990); and controlling the conduit system (1902, 2990) to circulate the heated coolant (1916) to the number of fuel cell stacks (1920), where the number of fuel cell stacks (1920) reach an operating temperature (1925) through the heated coolant (1916); A controller (1905) configured to: Aircraft (199, 400, 3127, 3855, 4502) equipped with

15. 15. The aircraft of claim 14, wherein the number of fuel cell stacks is selected from a group including all of the fuel cell stacks and a set of the fuel cell stacks.

16. 1. A method of managing operation of a fuel cell stack (1920) for an aircraft (199, 400, 3127, 3855, 4502), comprising: A method comprising: heating (2000) a set of the fuel cell stacks (1920) in the aircraft (199, 400, 3127, 3855, 4502) such that the fuel cell stacks (1920) reach an operating temperature (1925) for generating electrical power.

17. 17. The method of claim 16, further comprising heating (2100) remaining fuel cell stacks to the operating temperature (1925) using heat resulting from the power generated by the set of fuel cell stacks (1920).

18. 1. A method of managing startup of a fuel cell stack (1920) for an aircraft (199, 400, 3127, 3855, 4502), comprising: heating (2200) a coolant (2200) with a heater (1915) to form a heated coolant (1916) in a conduit system (1902, 2990) connected to a fuel cell stack (1920); controlling (2202) the conduit system (1902, 2990) to circulate the heated coolant (1916) to a set of the fuel cell stacks (1920), the heated coolant (1916) causing the set of the fuel cell stacks (1920) to reach an operating temperature (1925); A method comprising:

19. 20. The method of claim 18, further comprising circulating (2300) the heated coolant (1916) to all of the fuel cell stacks (1920) that have not reached the operating temperature (1925) in response to the set of fuel cell stacks (1920) reaching the operating temperature (1925).

20. Each fuel cell stack of the fuel cell stacks (1920) and each heater of the heaters (1915) are located within a nacelle (2950, ​​2952, 2954), each fuel cell stack being thermally connected to a heater, and the method further comprising:

20. The method of claim 18, further comprising sending (2400) the power generated by the set of fuel cell stacks (1920) to heaters (1915) for other fuel cell stacks (1920) that are not generating power.

21. 20. The method of claim 18, further comprising: stopping (2500) the heating of the coolant (1917) by the heating system for the set of the fuel cell stacks (1920) that are generating the electrical power in response to the set of the fuel cell stacks (1920) generating electrical power.

22. 20. The method of claim 18, wherein the heater (1915) is selected from at least one of an electric heater (1930) connected to a battery system (1931) or a fuel-powered heater that uses hydrogen gas to generate heat for heating the coolant (1917).

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