System and method for supplying passively filtered ram air to a hydrogen fuel cell of a UAV

An unmanned aerial vehicle (UAV) has an air-cooled fuel cell, an air channel comprising a forward facing opening for receiving ram air and connected to the air-cooled fuel cell, and a passive ram air filtration system (PRAFS) configured to filter particulate matter from ram air received into the air channel via the opening.

BACKGROUND

Unmanned aerial vehicles (“UAVs”), or drones, are usually battery powered and are therefore limited in range by battery life. Hydrogen fuel cells are being considered as an option to extend range and flight time of UAVs. Fuel cells operate by allowing an electrochemical reaction between hydrogen and oxygen, which produces electrical energy and water. In most fuel cell powered vehicles, hydrogen fuel, stored in an onboard hydrogen fuel tank, is supplied to an anode of the fuel cell and ambient air is supplied to a cathode of the fuel cell. The electrical energy produced drives a motor and the water is disposed of. The hydrogen fuel tanks are often externally coupled to the UAV or may be housed internally within a nacelle, such as described in U.S. patent application Ser. No. 16/290,704, filed Mar. 1, 2019, which is incorporated herein in by reference in its entirety. Hydrogen tanks and fuel cells, along with the electronics they power, are complex devices that may be difficult to repair at an operating location.

UAVs come in many different configurations. For example, a UAV may be configured as a conventional takeoff and landing (CTOL) aircraft or a vertical takeoff and landing (VTOL) aircraft. A CTOL aircraft generates lift in response to the forward airspeed of the aircraft. The forward airspeed is typically generated by thrust from one or more propellers. Accordingly, CTOL aircraft typically require a long runway for takeoff and landing to accommodate the acceleration and deceleration required to provide the desired lift. Unlike CTOL aircraft, VTOL aircraft do not require runways. Instead, VTOL aircraft are capable of taking off, hovering and landing vertically. One example of VTOL aircraft is a helicopter which includes one or more rotors that provide lift and thrust to the aircraft. The rotors not only enable hovering and vertical takeoff and landing, but also enable forward, backward, and lateral flight. These attributes make helicopters highly versatile for use in congested, isolated, or remote areas where CTOL aircraft may be unable to take off and land. Helicopters, however, typically lack the forward airspeed and range of CTOL aircraft. Other examples of VTOL aircraft include tiltrotor aircraft and tiltwing aircraft. Both of which attempt to combine the benefits of a VTOL aircraft with the forward airspeed and range of a CTOL aircraft. Tiltrotor aircraft typically utilize a pair of nacelles rotatably coupled to a fixed wing. Each nacelle includes a proprotor extending therefrom, wherein the proprotor acts as a helicopter rotor when the nacelle is in a vertical position and a fixed-wing propeller when the nacelle is in a horizontal position. A tiltwing aircraft utilizes a rotatable wing that is generally horizontal for forward flight and rotates to a generally vertical orientation for vertical takeoff and landing. Propellers are coupled to the rotating wing to provide the required vertical thrust for takeoff and landing and the required forward thrust to generate lift from the wing during forward flight.

Yet another example of a VTOL aircraft is a tailsitter aircraft. Tailsitter aircraft, such as those disclosed in U.S. patent application Ser. No. 16/154,326, filed Oct. 8, 2018 and U.S. patent application Ser. No. 15/606,242, filed May 26, 2017, both of which are incorporated herein by reference in their entireties, attempt to combine the benefits of a VTOL aircraft with the forward airspeed and range of a CTOL aircraft by rotating the entire aircraft from a vertical orientation for takeoff, landing, hovering, and low-speed horizontal movement, to a horizontal orientation for high speed and long-range flight.

DETAILED DESCRIPTION

While the making and using of various embodiments of this disclosure are discussed in detail below, it should be appreciated that this disclosure provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative and do not limit the scope of this disclosure. In the interest of clarity, not all features of an actual implementation may be described in this disclosure. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another.

This disclosure divulges a UAV comprising a fuel cell that is at least partially air-cooled and/or sources its cathode reactant air from a ram air duct. In the least, this disclosure enables a UAV that is powered by a fuel cell that is at least partially passively cooled by passing some ram air through a portion the fuel cell and other ram air through a bleed port so that entrained particulate matter is filtered from the ram air and directed away from the fuel cell. In other embodiments, ram air can likewise be directed through a fuel cell but only after passing through an electrostatic filter. In yet another embodiment disclosed, some ram air may be passed through an electrostatic filter before entering a fuel cell and some of the ram air can be diverted through a bleed port to take particulate matter away from the fuel cell. While the aircraft shown and discussed herein is depicted as a UAV, it should be understood that it may comprise any type of aircraft. Moreover, the systems and methods disclosed herein can be used on any vehicle or device that carries an air-cooled fuel cell and can be supplied ram air.

Referring toFIG.1, a tailsitter UAV100, operable to transition between thrust-borne lift in a VTOL orientation and wing-borne lift in a biplane orientation, are depicted. In the VTOL orientation, thrust modules126provide thrust-borne lift. In the biplane orientation, the thrust modules126provide forward thrust and the forward airspeed of UAV100provides wing-borne lift, enabling UAV100to have a high speed and/or high endurance forward-flight mode.

UAV100is a mission-configurable aircraft operable to provide high-efficiency transportation for diverse payloads. Based upon mission parameters, including flight parameters such as environmental conditions, speed, range, and thrust requirements, as well as payload parameters such as size, shape, weight, type, durability, and the like, UAV100may selectively incorporate a variety of thrust modules having different characteristics and/or capacities. For example, the thrust modules operable for use with UAV100may have different thrust types including different maximum thrust outputs and/or different thrust vectoring capabilities including non-thrust vectoring thrust modules, single-axis thrust vectoring thrust modules such as longitudinal thrust vectoring thrust modules and/or lateral thrust vectoring thrust modules, and two-axis thrust vectoring thrust modules which may also be referred to as omnidirectional thrust vectoring thrust modules. In addition, various components of each thrust module may be selectable including the power plant configuration and the rotor design. For example, the type or capacity of the fuel cell system in a thrust module may be selected based upon the power, weight, endurance, altitude, and/or temperature requirements of a mission. Likewise, the characteristics of the rotor assemblies may be selected, such as the number of rotor blades, the blade pitch, the blade twist, the rotor diameter, the chord distribution, the blade material, and the like.

In the illustrated embodiment, UAV100includes an airframe112including wings140and160each having an airfoil cross-section that generates lift responsive to the forward airspeed of UAV100when in the biplane orientation. Wings140and160may be formed as single members or may be formed from multiple wing sections. The outer skins of wings140and160are preferably formed from high strength and lightweight materials such as fiberglass, carbon fiber, plastic, aluminum, and/or another suitable material or combination of materials. As illustrated, wings140and160are straight wings. In other embodiments, wings140and160could have other designs such as polyhedral wing designs, swept wing designs, or another suitable wing design.

Extending generally perpendicularly between wings140and160are two truss structures depicted as pylons118and120that can comprise and/or carry tanks125for carrying fuel, such as, but not limited to, gaseous hydrogen for powering a fuel cell26d.

Wings140and160and pylons118and120preferably include passageways operable to contain flight control systems, energy sources, communication lines and/or other desired systems. In the illustrated embodiment, thrust modules126are fixed pitch, variable speed, omnidirectional thrust vectoring thrust modules.

As illustrated, thrust modules126are coupled to the outboard ends of wings140and160. While not shown, additional thrust modules126may be coupled to central portions of wings140and160. Thrust modules126are independently attachable to and detachable from airframe112such that UAV100may be part of a man-portable aircraft system having component parts with connection features designed to enable rapid assembly/disassembly of UAV100. Alternatively, or additionally, the various components of UAV100.

Referring now to Prior ArtFIG.2, a prior art thrust module26for use in a UAV substantially similar to UAV100is shown to include a nacelle26athat houses components including a fuel cell system26b, an electronic speed controller26c, gimbal actuators (not shown), an electronics node26f, sensors, and other desired electronic equipment. Nacelle26aalso supports a two-axis gimbal26gand a propulsion system26hdepicted as an electric motor26iand a rotor assembly26j(not shown). As the power for each thrust module26is provided by fuel cell system26b, housed within respective nacelles26a, UAVs such as UAV100can have a distributed power system for a distributed thrust array. In this embodiment, electrical power may be supplied to any electric motor26i, electronic speed controller26c, electronics node26f, gimbal actuators, flight control system, sensor, and/or other desired equipment from any fuel cell system26b. Fuel cell system26bis configured to produce electrical energy from an electrochemical reaction between hydrogen and oxygen. Fuel cell system26bincludes a fuel cell26dwhich includes a cathode configured to receive oxygen from the ambient air, an anode configured to receive hydrogen fuel, and an electrolyte between the anode and the cathode that allows positively charged ions to move between the anode and the cathode. While fuel cell26dis described in the singular, it should be understood that fuel cell26dmay include a fuel cell stack comprising a plurality of fuel cells in series or parallel to increase the output thereof. Fuel cell system26breceives hydrogen fuel from fuel tank25. Hydrogen fuel is delivered from fuel tank25to the anode of fuel cell26dthrough a supply line26tcoupled to a pressure regulator26u, which is coupled to stem27of tank25. Pressure regulator26uis configured to reduce the pressure of the hydrogen fuel from fuel tank25to a desired pressure in supply line26tthat is suitable for use at the anode of fuel cell26d. Pressure regulator26umay also have a filling port26vcoupled thereto. Filling port26vis configured to enable refilling of fuel tank25without uncoupling tank25from nacelle26a. Filling port26vmay allow for autonomous refilling of tank25when a UAV such as UAV100lands on a landing pad configured for the same. Alternatively, or additionally, thrust module26may include a pressure regulator28ucoupled to a stem29of tank25, and a filling port28vcoupled to pressure regulator28u. Filling port28vextends from pressure regulator28uto the exterior surface of tail section28, thereby enabling refilling of tank25without uncoupling tank25from a tail section such as tail section28.

Oxygen from the ambient air is delivered to the cathode of fuel cell26dvia an air channel26w. Air channel26wmay serve two functions, supplying oxygen to the cathode and cooling fuel cell26d. As such, air channel26wis configured to direct air from outside of nacelle26ato the cathode of fuel cell26dand/or to a heat transfer surface of fuel cell26d. The heat transfer surface of fuel cell26dmay comprise a heat exchanger or any surface configured to enhance heat removal therefrom. Moreover, when fuel cell26dis an open-cathode air-cooled unit, the airflow delivered to the cathode by air channel26wmay serve as both the cathode reactant supply and cooling air. That is, air ducted to a single location may deliver oxygen to the cathode and cool fuel cell26d. Air channel26wincludes a forward-facing opening26xpositioned behind rotor assembly26jsuch that ram air and propeller wash is driven through air channel26wby rotating rotor blades26r. This is particularly helpful when a UAV such as UAV100is operating in the VTOL orientation, as it insures sufficient airflow for oxygen supply and/or cooling purposes. Fuel cell system26bfurther includes an electrical energy storage device26yconfigured to store and release the electrical energy produced by fuel cell26d. Electrical energy storage device may comprise a battery, a supercapacitor, or any other device capable of storing and releasing electrical energy. Alternatively, the electrical energy produced by fuel cell26dmay be directly supplied to the electrical components.

Operation of fuel cell system26bis controlled by electronics node26f. Electronics node26fpreferably includes non-transitory computer readable storage media including a set of computer instructions executable by one or more processors for controlling the operation of thrust module26. These operations may include valve and solenoid operations to adjust the flow of hydrogen fuel from supply line26tto the anode, battery management, directing electrical energy distribution, voltage monitoring of fuel cell26d, current monitoring for fuel cell26dand electrical energy storage device26y, etc.

Referring back toFIG.1, because forward flight of UAV100in the biplane orientation utilizing wing-borne lift requires significantly less power than VTOL flight utilizing thrust-borne lift, the operating speed of some or all of thrust modules126may be reduced. In certain embodiments, some of the thrust modules126could be shut down during forward flight. While UAV100may be reconfigured with different numbers or types of thrust modules126to satisfy different flight requirements, UAV100may also be configured to allow fuel cell system26bto switch between operating on oxygen from ambient air and operating on oxygen provided by an on board oxygen tank such as the system disclosed in U.S. patent application Ser. No. 16/214,735, filed on Dec. 10, 2018, which is incorporated herein by reference in its entirety. Operating a fuel cell on oxygen, rather than air, can increase the power produced by the fuel cell, at sea level, by 15 to 20 percent. As such, the increased power of the oxygen mode may be used in the VTOL orientation and air mode may be used in the biplane orientation. It may be desirable for UAVs such as UAV100to have an oxygen tank that is remote from the thrust modules. Accordingly, a remote oxygen tank may be located anywhere on UAV100, for example, one or more of tanks125may be configured to store and distribute pressurized oxygen to thrust modules126when needed. In this configuration, UAV100includes a supply line coupled between the remote oxygen tank and the cathode of fuel cell26d. The supply line may be uninterrupted between the remote oxygen tank and the cathode, which would require a user to manually attached the supply line to the cathode when coupling thrust module126to UAV100. Alternatively, the thrust module126and UAV100may include complimentary rapid connection interfaces that include not only electrical and mechanical connections, but also include gaseous connections for automated, or quick-connection, of separate portions of the supply line. The connections between wings140and160, pylons118and120, thrust modules126, and payload130of UAV100are each operable for rapid on-site assembly through the use of high-speed fastening elements.

Referring now toFIG.3, thrust module126is shown. Thrust module126is substantially similar to thrust module26, but comprises a passive ram air filtration system (PRAFS)200. PRAFS200comprises an air channel202through which oxygen from the ambient ram air is delivered to the cathode of fuel cell26d. Air channel202can serve two functions, supplying oxygen to the cathode and cooling fuel cell26d. As such, air channel26wis configured to direct air from outside of nacelle26ato the cathode of fuel cell26dand/or to a heat transfer surface of fuel cell26d. The heat transfer surface of fuel cell26dmay comprise a heat exchanger or any surface configured to enhance heat removal therefrom. Moreover, when fuel cell26dis an open-cathode air-cooled unit, the ram airflow delivered to the cathode by air channel202may serve as both the cathode reactant supply and cooling air. That is, ram air ducted via air channel202can deliver fuel oxygen to the cathode as well as cool fuel cell26d. Air channel202includes a forward-facing opening204positioned behind rotor assembly26jsuch that ram air and propeller wash is driven through air channel202by movement of the UAV100in a gaseous environment such as ambient air and from propeller wash of rotating rotor blades26r. As compared to air channel26w, air channel202can be relatively larger and/or can comprise a relatively larger forward-facing opening204(as compared to forward-facing opening26x). In this embodiment, ram air entering air channel202via air channel202can be delivered to at least two different paths, namely, a fuel cell path206(substantially similar to operation of thrust module26) and an entrainment path208. Entrainment path208includes passage through an entrainment air channel210that connects air channel202to an entrainment air exit212. In this embodiment, entrainment air channel210is configured to direct some of the air entering air channel202via opening204out of thrust module126via a path other than through fuel cell26d. In this embodiment, the entrainment path208connects air channel202to the environment external to the thrust module126. In this embodiment, both the air channel202and the entrainment air channel210are sized, provided shape profiles, and otherwise configured to generate a particulate matter separation force on particulate matter that enters air channel202. The particulate matter separation force generally urges particulate matter (such as matter that is undesirable for passage through fuel cell26d) along entrainment path208that leads from air channel202to entrainment air exit212via entrainment air channel210. In this embodiment, the air exiting thrust module126via entrainment air exit212occurs at a significantly lower mass flow rate as compared to the mass flow rate of air exiting thrust module via fuel cell26d. Entrainment air channel210and/or portions of air channel202can be configured to utilize funnel-like profiles, nozzle-like profiles, vortex-inducing profiles, and/or any other suitable shape to force particulate matter away from the fuel cell path206and toward and/or into the entrainment path208. In this embodiment, passively received ram air is utilized to direct particulate matter away from fuel cell26d, thereby providing a passively achieved filtration of the ram air entering thrust module126.

Referring now toFIG.4, an alternative embodiment of a thrust module300is shown. Thrust module300is substantially similar to thrust module126, but further comprises a passive ram air filtration system (PRAFS)302. PRAFS302comprises an air channel304through which oxygen from the ambient ram air is delivered to the cathode of fuel cell26d. Air channel304can serve two functions, supplying oxygen to the cathode and cooling fuel cell26d. As such, air channel26wis configured to direct air from outside of nacelle26ato the cathode of fuel cell26dand/or to a heat transfer surface of fuel cell26d. The heat transfer surface of fuel cell26dmay comprise a heat exchanger or any surface configured to enhance heat removal therefrom. Moreover, when fuel cell26dis an open-cathode air-cooled unit, the ram airflow delivered to the cathode by air channel304may serve as both the cathode reactant supply and cooling air. That is, ram air ducted via air channel304can deliver fuel oxygen to the cathode as well as cool fuel cell26d. Air channel304includes a forward-facing opening306positioned behind rotor assembly26jsuch that ram air and propeller wash is driven through air channel304by movement of a UAV such as UAV100in a gaseous environment such as ambient air and from propeller wash of rotating rotor blades26r. As compared to air channel26w, air channel304can be relatively larger and/or can comprise a relatively larger forward-facing opening306(as compared to forward-facing opening26x). In this embodiment, ram air entering air channel304via opening306can be delivered to fuel cell26dalong a fuel cell path308(substantially similar to operation of thrust module26) that passes through an electrostatic filter310. Electrostatic filter310is configured to capture particulate matter in the ram air, thereby preventing the particulate matter from entering fuel cell26d.

Referring now toFIG.5, an alternative embodiment of a thrust module400is shown. Thrust module400is substantially similar to thrust module26, but comprises a passive ram air filtration system (PRAFS)402. PRAFS402comprises an air channel404through which oxygen from the ambient ram air is delivered to the cathode of fuel cell26d. Air channel404can serve two functions, supplying oxygen to the cathode and cooling fuel cell26d. As such, air channel26wis configured to direct air from outside of nacelle26ato the cathode of fuel cell26dand/or to a heat transfer surface of fuel cell26d. The heat transfer surface of fuel cell26dmay comprise a heat exchanger or any surface configured to enhance heat removal therefrom. Moreover, when fuel cell26dis an open-cathode air-cooled unit, the ram airflow delivered to the cathode by air channel404may serve as both the cathode reactant supply and cooling air. That is, ram air ducted via air channel404can deliver fuel oxygen to the cathode as well as cool fuel cell26d. Air channel404includes a forward-facing opening406positioned behind rotor assembly26jsuch that ram air and propeller wash is driven through air channel404by movement of a UAV such as UAV100in a gaseous environment such as ambient air and from propeller wash of rotating rotor blades26r. As compared to air channel26w, air channel404can be relatively larger and/or can comprise a relatively larger forward-facing opening406(as compared to forward-facing opening26x). In this embodiment, ram air entering air channel404via opening406can be delivered to at least two different paths, namely, a fuel cell path408(substantially similar to operation of thrust module26) and an entrainment path410. Entrainment path410includes passage through an entrainment air channel412that connects air channel404to an entrainment air exit414. In this embodiment, entrainment air channel412is configured to direct some of the air entering air channel404via opening406out of thrust module400via a path other than through fuel cell26d. In this embodiment, the entrainment path410connects air channel404to the environment external to the thrust module400. In this embodiment, both the air channel404and the entrainment air channel412are sized, provided shape profiles, and otherwise configured to generate a particulate matter separation force on particulate matter that enters air channel404. The particulate matter separation force generally urges particulate matter (such as matter that is undesirable for passage through fuel cell26d) along entrainment path410that leads from air channel404to entrainment air exit414via entrainment air channel412. In this embodiment, the air exiting thrust module400via entrainment air exit414occurs at a significantly lower mass flow rate as compared to the mass flow rate of air exiting thrust module via fuel cell26d. Entrainment air channel412and/or portions of air channel404can be configured to utilize funnel-like profiles, nozzle-like profiles, vortex-inducing profiles, and/or any other suitable shape to force particulate matter away from the fuel cell path408and toward and/or into the entrainment path410. In this embodiment, passively received ram air is utilized to direct particulate matter away from fuel cell26d, thereby providing a passively achieved filtration of the ram air entering thrust module400. While this embodiment is substantially similar to thrust module126, thrust module400further comprises an electrostatic filter416disposed within air channel404so that ram air entering air channel404cannot reach either of fuel cell26dor entrainment path410without first passing through electrostatic filter416. Accordingly, this embodiment provides two passive features for filtering particulate matter from ram air and thereby preventing the filtered particulate matter from entering fuel cell26d.

While the embodiments described above discuss PRAFSs used in conjunction with air-cooled fuel cells, the same PRAFSs can alternatively be used in conjunction with open cathode hydrogen fuel cells. Utilizing the passive filtration of the PRAFSs disclosed herein can reduce cathode catalyst contamination and enhance the durability of the fuel cell stack. In some embodiments, geometry can be added to the air ducts to accomplish basic levels of filtration without the added mass or pressure drop of typical filtration elements. In some embodiments, a bleed mass flow rate of air carrying particulate matter through the entrainment air channels can be about 1% to about 2% of the mass flow rate of air passed through the fuel cells, thereby enhancing fuel cell stack life with no additional pressure drop and very little added mass. In some embodiments, the electrostatic filters can be provided as relatively course charged screens configured to a attract larger particles with minimal pressure drop and only some added mass. This course electrostatic filter can require less maintenance than a typical paper filter element with less pressure drop and wider service intervals. In some embodiments, the electrostatic filters can be charged and/or powered by battery and/or fuel cell power with only a low power draw.