AERIAL VEHICLES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 62/384,519 filed September 7, 2016, the contents of which are incorporated as if fully set forth herein.

TECHNICAL FIELD

[0002] The present disclosure relates to ground stations and methods to support PEM fuel ceil powered unmanned aerial vehicles (UAVs). In particular, it relates to devices and methods for monitoring and restoring the health of PEM fuel cells without adding to the mass of UAVs and ensuring quick turnaround of UAVs after they return to a home base.

BACKGROUND

[0003] Unmanned aerial vehicles (UAVs), which are also known as drones, are becoming increasingly popular for applications such as photography, surveillance, farm maintenance and pest control, atmospheric research, fire control, wildlife monitoring, package deliver}', and military purposes. These drones are either controlled from a central control station or by a person. Drones generally fall into two categories, namely, multirotor drones used generally in commercial applications and fixed wing drones used for military applications. The payloads in the drones vary depending on the end application and may comprise video cameras, reconnaissance equipment, remote sensing devices, pesticides held in a suitable container that is capable of spraying, fire retardants, packages for delivery, and the like. Drones are also equipped with navigation systems. UAVs are typically smaller than manned aircraft and may weigh, for example, between a few grams and 20 kilograms.

[0004] UAVs require power in order to provide propulsion, and power for auxiliary functions to operate payloads such as image or video capture, signal telemetry, or other onboard systems. For many applications, the computing power required on-board the vehicle in order to provide the necessary functionality may represent a significant power demand. This is particularly the case in autonomous UAVs in which an on-board control system may make decisions regarding flight path and the deployment of auxiliary functions. Although the vehicle itself is unmanned, a UAV may be piloted remotely and may still be under human control.

[0005] Some UAVs use primary batteries to provide power, although it is now more common to use secondary (rechargeable) batteries such as lithium-ion batteries. When power is supplied only by batteri es, the flight time of UAVs may be limited because of the power demands of the propulsion and other on-board systems. In recent years, photovoltaic panels have been used to extend the flight range of UAVs. However, the power generating capacity of a photovoltaic panel depends on the ambient weatlier conditions and the time of day, and, subsequently, photovoltaic panels may not be appropriate for use in all circumstances. In addition, the power generation capacity of photovoltaic panels may be inadequate for some applications in which either high power (speed) propulsion is required, or the on-board systems of the UAV that provide its functionality are particularly heavy or demand substantial electrical power. The flight time and range of UAVs are generally a function of payload (weight) and the energy (Watt-hours) available from a power supply. Other power supplies include jet engines fueled by fuels such as gasoline and jet fuel for fixed wing military applications and fuel cells fueled by hydrogen and other fuels, such as propane, gasoline, diesel, and jet fuel . In many instances, the power supplies are hybrid versions, wherein a combination of power supplies may be used. For example, when a fuel cell is used, any peak power requirement such as during take-off, may be supplemented using a battery. The U AVs typically return home, that is, to a home station or home base, after a flight to recharge or refill the power supplies.

[0006] Fuel cells are attractive power supplies for UAVs and can exceed the energy provided by batteries and extend flight time (or range) in many instances. Fuel cells are electrochemical energy conversion devices that convert an external source fuel into electrical current. Many fuel cells use hydrogen as the fuel and oxygen (typically from air) as an oxidant. The by-product for such a fuel cell is water, making the fuel cell a very low environmental impact device for generating power. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.

[0007] There is a wide variety of fuel cells available, each using a different chemistry to generate power. Fuel cells are usually classified according to their operating temperature and the type of electrolyte system that they utilize. One common fuel cell is the proton exchange membrane fuel cell (PEMFC), which uses hydrogen as the fuel and oxygen (usually air) as its oxidant. It has a high power density and a low operating temperature of usually below 80 °C. These fuel cells, which are assembled to form a PEMFC stack, are reliable and efficient, with modest packaging and system implementation requirements. A PEMFC comprises a polymeric ion transfer membrane, also known as a proton exchange membrane (PEM), within a membrane -electrode assembly (MEA), with hydrogen and air being fed to the anode side and cathode side of the membrane, respectively. A PEMFC stack comprises fluid manifolds and bipolar plates to uniformly distribute hydrogen and air to the anode and cathode of each cell respectively, conduct electrical current from cell to cell, remove heat generated in the stack, and to achieve a leak-tight seal for each cell.

[0008] In a PEMFC, hydrogen gas is fed to the anode side, ionizes and releases protons as follows:

2H2 -» 4H+ + 4e-

[0009] The protons pass through the electrolyte to the cathode side while the electrons are conducted through an external electrical circuit to the cathode side. At the cathode side, oxygen (usually from ambient air) reacts with the electrons and the protons to form water as follows:

02 + 4e- + 4H+ 2H20

[0010] Wate produced at the cathode is generally exhausted out to the ambient or condensed and used to cool the PEMFC. Although water is produced at the cathode, it is essential to maintain a water balance across the MEA. Where dry air is introduced into the cell (at the cathode) there is a tendency for the creation of an unbalanced water distribution across the membrane such that the area around the inlet port is drier than elsewhere.

Ultimately, this could mechanically stress the membrane and lead to uneven current distribution, both of which can lead to premature failure of a cell.

DISCLOSURE

[0011] According to one aspect of the disclosure, a method is provided for pre-flight checking using a ground station the health and safety of a PEM fuel cell powered UAV comprising one or more of the following steps: conducting a hydrogen leak check around the UAV and decommissioning the UAV if a leak is detected, and if a leak is not detected, temperature conditioning one or more stacks in the fuel cell power supply of the UAV to between about 20 °C and about 25 °C by exposing the fuel cell to temperature conditioned air, measuring the open circuit voltage of the one or more stacks; and if the measured open circuit voltage is above a threshold voltage (0.85V ± 2%), monitoring the cell voltages across one or more cells in the one or more stacks using optical devices; and if the optical devices suggest no significant deterioration in cell voltage, rehydrating the cells in the one or more stacks, conducting a polarization check to measure stack voltage and current at one or more points on the polarization curve, and if the measured voltage exceeds a set-point voltage (0,75V at 20 amperes) refilling or replacing the hydrogen supply.

[0012] The disclosure provides that refilling the hydrogen supply may comprise at least one of filling the hydrogen supply with compressed gas, replacing a spent metal hydride hydrogen cartridge with a fresh cartridge, and replacing a spent chemical hydride hydrogen cartridge with a fresh cartridge. Further, hydrogen leak check and temperature conditioning steps may ^¬ be performed after temporarily enclosing the U AV in a removable enclosure. Monitoring the cell voltages using optical devices comprises at least one of monitoring the intensit ⁷ of light emitted, determining whether one or more devices are OFF, and determining whether one or more devices are flickering. Further, rehydrating the cells may comprise feeding hydrogen to the anode side of the stacks, feeding air to the cathode side of the one or more stacks by operating the fans of the one or more fuel cell stacks for about 2 seconds, shutting off the air supply to the stacks by shutting off the fans, providing a resistive load of about 2 ohms across the stack for about 4 seconds, and resuming air feed to the cathode side of the stacks.

[0013] The disclosure also provides a ground station to conduct pre-flight health and safety check of a PEM fuel cell powered U AV comprising a hydrogen supply and one or more fuel cell stacks made up of fuel cells, the ground station including one or more compressed hydrogen supply in fluid communication with the one or more fuel cell stacks of the UAV, one or more inert gas supply in fluid communication with the with the one or more fuel cell stacks of the UAV, a plurality of resistive electrical loads in electrical communication with the one or more fuel cell stacks of the UAV, a temperature conditioning module to supply temperature conditioned air to increase or decrease the temperature of the one or more stacks to a set temperature, an optical detector to monitor the intensity of light output by optical devices electrically coupled to the one or more stacks, an auxiliary power source to provide power to the UAV; and a controller to control the operation of the ground station.

[0014] The disclosure provides a ground station wherein the controller is capable of bidirectional communication with at least one or more of a controller coupled to one or more stacks and a controller that controls the operation of the UAV. ^' The controller controls the operation of at least one of hydrogen supply, inert gas supply, resistive load supply, temperature conditioned air supply, auxiliary power supply, and the operation of the optical detector. The detector to monitor the intensity of light output from optical devices comprises a charge coupled device.

[0015] According to an aspect of the present disclosure, a method for checking the status of a PEM fuel cell powered UAV having a hydrogen supply and a fuel cell stack that includes a fuel cell is disclosed. The method may include the steps of determining whether a hydrogen leak is present: exposing the fuel ceil to temperature-conditioned air, such that the fuel cell stack is between about 20 °C and about 25 °C; measuring an open circuit voltage of the fuel cell stack; monitoring the open circuit voltage using an optical device configured to detect deterioration of the vol tage in the fuel cell; rehydrating the fuel cell; measuring a stack voltage and a current at one or more points on a polarization curve; and refilling the hydrogen supply.

[0016] In some aspects, the method may also include decommissioning the U AV if a leak is detected during the determining step. The method may include the step of monitoring the open circuit voltage if the voltage is over a threshold value. In some aspects, the voltage threshold value may be 0.85 V ± 2%. The method may include the rehydrating step if the optical device detects no significant deterioration in cell voltage. The method may include the refilling step if the measured stack voltage exceeds a set-point voltage. In some aspects, the set-point voltage may be 0.75V at 20 amperes.

[0017] In some aspects, the refilling step may include at least one of: filling the hydrogen supply with compressed gas, replacing a spent metal hydride hydrogen cartridge with a fresh cartridge, and replacing a spent chemical hydride hydrogen cartridge with a fresh cartridge. The method may further include enclosing the UAV in a removable enclosure before performing the determining and exposing steps. The monitoring step may include at least one of: monitoring the intensity of an emitted light, determining whether the device is in an "OFF" configuration, and determining whether the device is flickering.

[0018] In some aspects, the rehydrating step may include: providing hydrogen to the anode side of the fuel stack; providing air to the cathode side of the fuel stack; ceasing the step of providing air; providing a resistive load of about 2 ohms across the fuel stack for about 4 seconds; and, resuming the step of providing air. The step of providing ah to the cathode may include operating a fan for about 2 seconds. Ceasing the step of providing air may include stopping operation of the fan. In some aspects, the refilling step may include refilling at least one of Type 2, Type 3, Type 4, and Type 5 cylinders with a compressed hydrogen gas. The pressure of the compressed hydrogen gas may be between about 300 bar and about 700 bar. In some aspects, the pressure of the compressed hydrogen gas may be about or exactly 700 bar.

[0019] According to another aspects of the present disclosure, a ground station for conducting a pre-fiight health and safety check of a PEM fuel cell powered UAV, the UAV having a hydrogen supply and a fuel cell stack having a fuel cell, is disclosed. The ground station includes a hydrogen supply in fluid communication with the fuel cell stack; an inert gas supply in fluid communication with the fuel cell stack; a plurality of resistive electrical loads in electrical communication with the fuel cell stack; a temperature conditioning module configured to supply temperature-conditioned air to increase or decrease the temperature of the fuel stack to a set temperature; an optical detector configured to monitor the intensity of light output by an optical device electrically coupled to the fuel cell stack; an auxiliary power source configured to provide power to the UAV; and a controller configured to control the operation of the ground station. Θ020] In some aspects, the controller may be configured to communicate with the fuel cell stack and the UAV. The controller may be configured to control at least one of the hydrogen supply, the inert gas supply, the plurality of resistive electrical loads, the temperature conditioning module, the auxiliasy power source, and the optical detector. The hydrogen supply may be configured to store compressed hydrogen at pressures between about 300 bar and about 700 bar. The inert gas supply may include at least one of helium, argon, and nitrogen. The optical detector may include a charge-coupled device. In some aspects, the ground station may include an enclosure configured to removably enclose the UAV during the pre-ftight health and safety check. [0021] Other features and advantages of the present disclosure will he set forth, in part, in the descriptions which follow and the accompanying drawings, wherein the preferred aspects of the present disclosure are described and shown, and in part, will become apparent to those skilled in the art upon examination of the following detailed description taken in conjunction with the accompanying drawings or may be learned by practice of the present disclosure. The advantages of the present disclosure may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appendant claims.

DRAWINGS

[0022] The foregoing aspects and many of the attendant advantages of this disclosure will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0023] FIGS, 1A-1C show representations of PEM fuel cell powered UAVs.

[0024] FIG. 2 shows a schematic diagram, of an exemplary ground station for supporting the operation of a fuel cell powered UAV.

[0025] FIG. 3 shows an exemplar ' PEM fuel cell power supply stack module for use in a UAV.

[0026] FIG. 4 illustrates an exemplary- pre-flight procedure for a PEM fuel cell powered UAV.

[0027] The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the disclosure. All reference numerals, designators and callouts in the figures and Appendices are hereby incorporated by this reference as if fully set forth herein. The failure to number an element in a figure is not intended to waive any rights. Unnumbered references may also be identified by alpha characters in the figures.

FURTHER DISCLOSURE

[0028] The following detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the ground stations and methods may be practiced. These embodiments, which are also referred to herein as "examples" or "options," are described in enough detail to enable those skilled in the art to practice methods and devices disclosed. The embodiments may be combined, other embodiments may be utilized or structural or logical changes may be made without departing from the scope of the disclosure. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the disclosure is defined by the appended claims and their legal equivalents.

[0029] In this document, the terms "a" or "an" are used to include one or more than one, and the term "or" is used to refer to a nonexclusive "or" unless otherwise indicated. In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation.

[0030] Particular aspects of the disclosure are described below in considerable detail for the purpose for illustrating the principles and operation of ground stations for PEM fuel cell powered UAVs. However, various modifications may be made, and the scope of the disclosure is not limited to the exemplary aspects described. In general, the expression "fuel cell" is used herein to encompass either a single fuel cell or a plurality of individual fuel cells assembled to form a fuel cell stack.

[0031] Schematic representations of an exemplary UAV 100 that comprises a fuel cell power supply 160, are shown in FIG. 1A-1C. UAV 100 in FIG. 1A-1C may be a quadcopter and compri ses four propulsion modules 120 coupled to a body 101 by a plurality of struts 130, which may also be referred to the arms, or limbs of the UAV. The number of propulsion modules in UAV 100 may vary depending on the aerodynamic design, payload, and flight time required. A controller 180 (FIG. 2), navigation components such as GPS (not shown), and an auxiliary power supply 207, such as a battery power supply may be provided within body 101.

[0032] The fuel cell power supply 160 may be a PEM fuel cell power supply. The fuel cell power supply 160 may be removably coupled to body 101 and electrically coupled to a fuel ceil power supply controller 164 via a suitable electrical adapter or plug 162. Each propulsion module 120 may comprise a motor that is capable of driving a respective rotor 1 10. The stmts provide mechanical support, and also provide for conduits to carry the cables (not shown) that provide electrical and control communication between the controller 180 and the propulsion modules 120. The rotors 110 provide thrust and lift for the UAV. The exemplary UAV 100 also comprises a plurality of leg members 140 to support the UAV during landing and to protect the payload during landing.

[0033] Hydrogen feed to the PEM fuel cell power supply 160 is supplied by hydrogen supply 150, which may be removably mounted on saddles 151 that are mechanically coupled to body 101. Hydrogen supply 50 may also be removably mounted to the body 10 using brackets, ties, and the like. The hydrogen supply 150 comprises a hydrogen connection assembly 152 that is capable of mating with a first end of a hydrogen supply conduit 153 usmg quick connect/disconnect fittings, magnetic couplings and the like. The hydrogen connection assembly 152 may comprise at least one of a pressure regulator, solenoid valve, shut off valve, and pressure relief valve (not shown) to ensure that hydrogen at the desired flow rate and pressure is routed to the power supply 160. Θ034] Tire components that comprise the hydrogen connection assembly 152 may be electrically actuated by a signal from the controller 180. The second end of the hydrogen supply conduit 153 that is opposite the first end is capable of mating with fuel cell connection assembly 161 . The fuel cell connection assembly 161 may comprise at least one of a pressure regulator, solenoid valve, shut off valve, and pressure relief valve (not shown) to ensure that hydrogen at the desired flow rate and pressure is routed to fuel ceil power supply 160. The components included in the fuel cell connection assembly 161 may be electrically actuated by a signal from the controller 180.

[0035] A payload (not shown), may include one or more cameras and may be removably coupled to the fuel cell power supply 160 or to a frame 170 (FIG. 1C) that may be connected to body 101. The payload is capable of communicating with at least one of the controller 180 and the fuel cell power supply 160.

[0036] The controller 180 is capable of controlling at least one of propulsion modules 120, the operation of the fuel cell power supply 160, the operation of the hydrogen supply 150, the operation of the payload, and the auxiliary power supply 207, such as a rechargeable battery, which is configured to store excess power generated by the fuel cell power supply 160.

[0037] The auxiliary power supply 207 may also be used to power at least one of the UAV 100 during a transient power period, such as take-off, or when the fuel ceil power supply 160 is producing less power than expected, and the payload. Auxiliary power supplies may also comprise super capacitors and primary batteries. Exemplary systems and methods for operating a device using a fuel cell power supply and an auxiliary power supply to power a load (device such as UAV 100) are disclosed in commonly owned U.S. Pat. No. 9,356,470 and U.S. Pat. Pub. No. 20160164285, which are both incorporated by reference herein in their entirety.

[0038] The fuel cell power supply 160 may be provided with a fuel cell power supply controller 164, in which case, the controller 180 is capable of communicating with the fuel cell power supply controller 164 in a bidirectional manner. Alternatively, the fuel cell power supply controller 164 may be used to control the components in the fuel ceil connection assembly 161 and the hydrogen connection assembly 152 instead of the controller 180. The UAV 100 typically returns home after a flight, that is, to a home station or home base (not shown), after a flight to recharge or refill the power supplies.

[0039] As shown in FIG. 1A, the fuel cell power supply 160 may be located below the hydrogen supply 150 with reference to the UAV 100 being in a stationary position on the ground. Alternatively, the fuel cell power supply 160 may be located above the hydrogen supply 150 (FIG. IB). The fuel cell power supply 160 may comprise one or more fuel cell stack modules 163 depending on the power requirement of UAV 100, which in turn, may depend on the mass of the payload. The fuel cell stack module 163 may be an open cathode PEMFC fuel cell stack module. A plurality of hydrogen supplies 150 may be employed depending on the flight time required and the mass budget that is available to the fuel supply for a given payload mass.

[0040] Alternatively, the fuel cell power supply 160 and the hydrogen supply 150 may be mounted adjacent to each other (FIG. IC). A suitable payload may be coupled to the frame 170 (FIG. IC), which in turn is supported by body 101. In FIG. IC, the UAV 100 comprises a single fuel cell power supply 160, which may include a single fuel cell stack. In this case, the terms ''fuel cell power supply" and "fuel cell stack" may be interchangeably used.

[0041] The fuel cell power supply 160 may comprise one or more open cathode PEMFC fuel cell stack modules 163. The power output as a function of cumulative time of service from the fuel cell stack module 163 is dependent on various factors, such as the ambient temperature, humidity, and number of start/stops, one or more of which may impact the water management within the ME A. To ensure reliable operation of the fuel cell power supply 160, it is desirable to check the condition (health) of the fuel cell stack modules 163 when the UAV 100 returns to the home base using a ground station 200 to either condition stacks 166, or replace one or more of the fuel cell stack module 163. In particular, for long duration flights, it is desireable to condition the stacks 166 prior to take-off. In this disclosure, conditioning of the stack 166 may include the conditioning of one or m ore fuel cells that comprise the stack.

[ΘΘ42] Disclosed are exemplary ground stations 200 that comprise devices and methods to monitor and maintain at least one of the health of fuel cell supply 160, the hydrogen supply 150, the pay load, and the propulsion module 120 when the UAV 100 returns to the home base or as it approaches the home base. A primary function of the ground station 200 (FIG. 2) is to enable health and safety checks of the components that comprise UAV 100, in particular, that of the fuel cell power supply 160, provide for automated or semi -automated corrective actions, and ensure quick turnaround of the UAV 100 back into flight service.

[0043] FIG. 2 shows that a link 208 may be employed to couple a ground station controller 206 in the ground station 200 to at least one of the fuel cell power supply control ler 164 and the controller 180 in the body 101. The link 208 may include a power link to provide power from an auxiliary power source 207 or from a wall supply (after rectification and voltage reduction) (not shown) to the UAV 1 0, for example, during troubleshooting of the UAV 100, and a communication link (not shown) that provides bidirectional communication between the ground station controller 206 and at least one of the controller 180 and the fuel ceil power supply controller 164.

[0044] Bidirectional communication may be achieved using wired or wireless

communication. I e link 208 enables the ground station controller 206 to control various functions as described below. The described functions need not be performed in the sequence described, and further, not all functions may be be performed using the ground station 200. To perform one of more of these functions, the ground station 200 may have an enclosure 220 that may temporarily enclose the UAV 100. When the UAV 100 returns to the home base, it may be directed to land on a surface of the ground station 200, for example, on top of the ground station 200, or on a platform (not shown) that extends from a surface of the ground station 200. The platform may be able to retract into the station 200 or be folded when not in use. Further, the ground station 200 may be capable of communicating with and sharing information with one or more remote servers using wireless means or via wires, for example via Ethernet, to enable management of logistics and inventory including the UAV

I I 100, various components, such as the fuel cell power supply 160, the hydrogen supply 150, and the like. The remote servers may be capable of directing UAV traffic between home bases, managing flight paths, and communicating with ground stations.

Temperature conditioning (cooling or heating) PEMFC power supply

[0045] Temperature conditioning of the PEMFC power supply 160, generally refers to cooling or heating the one or more stack modules to a desired temperature. Once the UAV 100 is powered off at the home base, the ground station 200 may provide for a

heating/cooling module 204. The heating/cooling module 204 may be configured to cool the PEM fuel cell power supply 160, particularly when the UAV 100 returns to the home base after an extend flight or if ambient temperature is high.

[0046] Cooling the fuel cell power supply 160 to between 20 °C and 25 °C may be achieved by connective cooling using an air blower that directs an air stream to the fuel cell power supply 160. The temperature of the fuel cell power supply 160 may be monitored using infrared temperature readers, by reading thermocouples that may be temporarily attached the skin of the fuel cell power supply 160, and by reading thermocouples that may be provided in the fuel cell power supply 160, in the vicinity of the one or more stacks 166 for troubleshooting and maintenance purposes, if needed, the air may be chilled using the heating/cooling module 204 before being directed to the fuel cell power supply 160.

Alternately, the fuel cell power supply 160 may be temporarily enclosed in a cooling jacket (not shown) that has a coolant that is chilled using the heating/cooling module 204. The cooling jacket may serve as a portable refrigeration system.

[0047] The operation of the heating/cooling module 204 is controlled using the ground station controller 206. Portable compressors and refrigeration systems may be supplied by Aspen Compressors (Marlborough, MA). Alternately, a jacket that employs one or more of phase change materials may also be used. Phase-change material containing cooling jackets and blankets may be supplied by Phase Change Energy Solutions (Asheboro, NC). The fuel ceil power supply 160 may also be cooled using evaporatively cooled enclosures as may be supplied by NanoCool, LLC ( Albuquerque, NM).

[0048] When the UAV 100 operates in cold weather, the fuel cell power supply 160 may be heated to between about 20 °C and about 25 °C. Air may be heated by the heating/cooling module 204 that may employ a heat exchanger (not shown) to heat the air stream. Heat may be provided by electrical heaters or by combustion of a fuel, for example, combustion of natural gas.

[0049] To enable quick heating or cooling of the fuel cell power supply 160 within a short time, preferably a time period of less than 5 minutes, the UAV 100 may be temporarily enclosed in the enclosure 220 as previously described. The enclosure 220 provides for a conduit or conduits (not shown) in fluid communication with wann air for heating or chilled air for cooling from the ground station 200 and also a conduit for venting the air.

Hydrogen leak check of fuel cell power supply

[0050] Once the fuel cell power supply 160 is at the desired temperature, and prior to shutting off the air flow to the enclosure 220, hydrogen as a continuous flow or as a pulse from a hydrogen storage 202 in the ground station 200 may be fed to the fuel ceil power supply 60 to ensure that the one or more stacks 166 are leak-tight against the hydrogen. To enable the flow of hydrogen to the fuel cell power supply 160 when the UAV 100 is powered off, the hydrogen supply conduit 153 may be temporarily disconnected from the fuel cell connection assembly 161, and an alternate conduit from the hydrogen storage 202 may be connected to the fuel cell connection assembly 161 .

[0051] Other means may be employed to provide hydrogen from, the hydrogen storage 202 for testing purposes that may not require disconnection of the hydrogen supply conduit 153. For example, the hydrogen supply conduit 153 may provide a tee connection (not shown) for temporarily accepting hydrogen from the hydrogen supply 202. A tee connection may also be provided in the fuel cell connection assembly 161. A hydrogen detector (not shown) located in the air stream that is exiting the hydrogen detector may be used to detect the presence of hy drogen in the air stream, which may be indicative of a leak in the fuel cell power supply 160 and/or a leaking stack 166. Generally, hydrogen concentrations in air of about 4000 ppm (10% of the lower explosive limit of hydrogen) may suggest a hydrogen leak.

[0052] If a hydrogen leak is detected, hydrogen flow from the hydrogen storage 202 is stopped by the ground station controller 206, and the UAV 100 may be decommissioned (taken off flight service). Air flow to the enclosure 220 may also be stopped. An inert gas from an inert gas storage 203 may be used to purge the enclosure 220 prior to

decommissioning and to replace the fuel cell power supply 160. For this purpose, the ground station 200 may be configured to provide a supply of the inert gas, such as helium, argon, and nitrogen, and suitable gas conduits, pressure gauges, regulators and fkudic components, such as shut off valves.

[0053] The ground station controller 206 may be configured to receive information from tlie hydrogen detector and to control the operation of the hydrogen storage 202 and the inert gas storage 203.

Measuring open circuit voltage (OCV) of stack

[0054] OCV may be measured using a voltage meter using methods that are well known to persons of ordinary skill in the art. The ground station controller 206 independently, or via the fuel cell power supply controller 164, may control operation of the stack 166 for OCV measurement. Hydrogen is supplied by the hydrogen source 202. OCV below about 0.85 V/cell ± 2% (a threshold voltage) may indicate end-of-life status the stack 166 and may- require replacement of the fuel cell power supply 160. Voltage in this disclosure typically indicates direct current (DC) voltage.

Cell voltage monitoring using optical devices

[0055] Control parameters, such as cell voltages across fuel cells in the stack, impedance of fuel cells, and resistance of fuel cells, may be monitored to determine whether conditioning of the stack 166 is required, or whether tlie stack 166 should be replaced; that is, to determine tlie state of health of tlie stack 166. More typically, cell voltage monitoring (CVM) is used for stack health monitoring. CVM may use a ribbon cable (not shown) and the like to make an electrical connection between each fuel cell of stack 166 or a pre -determined number of cells, and controller 164. A CVM cable increases the mass of the PEM fuel cell power supply 160, and subsequently the mass of the UAV 100, and reduces the mass available for the payioad. CVM therefore may be undesirable for monitoring the health of the stack 166 in the PEM fuel cell power supply 160. Θ056] In some aspects, bipolar plates (not shown) in the stack 166 are made of lightweight metals such as titanium. In such aspects, the heavier CVM cable may also damage the plates. Instead of a CVM cable, ceil voltages may be measured by employing optical devices, such as light emitting diodes (LEDs), for providing an optical signal corresponding to cell voltages. The intensity of the optical signal may be proportional to the measured voltages and may be used to monitor change in ceil voltages. The optical signal may be detected by a remote detector 209 housed in the ground station 200. The remote detector 209 may convert the optical signals back to electrical signals for processing by the ground station controller 206. The remote detector 209 may include one or more mirrors (not shown) to scan for optical signals and a charged coupled device (CCD). U.S. Pat. No. 7,687,174 discloses a voltage monitoring system for measuring the voltage of the fuel cells that employs optical devices. Typically, LEDs "wink out," thai is flicker or turn off, when a cell voltage is below a threshold value, generally below 0.75 V, in which case, the fuel cell power supply 160 may have to be replaced. The disclosure provides for monitoring the cell voltages using optical devices by one or more of the following actions: monitoring the intensity of light emitted, determining whether one or more devices are in an ' ^" OFF" configuration, and determining whether one or more devices are flickering.

Rehydrating fuel cells in stack

[0057] Conditioning of the stack 166 may be necessitated by drying out of one or more cells in the stack 166. This may require the stack 166 to be rehydrated. During PEMFC operation, product water must be exhausted from the MEA at the same time that oxygen is transported to the cathode side of the MEA. However, it is also important that the MEA remains suitably hydrated to ensure that the internal electrical resistance of the ceil remains within tolerable limits. Failure to control MEA humidifi cation may lead to hot spots and to potential cell failure and/or poor electrical cell performance. A key function during the fuel ceil electrochemical reaction in a PEMFC is the proton migration process via the PEM. The proton exchange process will only occur when the solid state PEM is sufficiently hydrated. With insufficient water present the water drag characteristics of the membrane will restrict the proton migration process leading to an increase in the internal resistance of the cell. With over-saturation of the PEM there is the possibility that excess water will "flood" the electrode part of the MEA and restrict gas access to the so called three phase reaction interface. These events may have a negative effect on the overall performance of the fuel cell.

[0058] As shown in FIG. 3, fuel cell stack module 163 may comprise a plurality of open cathode fuel ceil stacks 166. Suitable air and hydrogen distribution components (flow field plates) may be used to ensure uniform distribution of air to the cathode side of the ceils and hydrogen to the anode side of the cells. Air may be fed by a low-pressure air source such as a fan 165 or a plurality of fans 165 (FIG. 1 C). Air provides the dual function of stack cooling and supplying oxygen to the cathode of each cell in the stack 166. The dual purpose of the air flow may lead to a conflict in air flow requirements. A very high stoichiometric air flow across the cathode electrodes is required for cooling, and, depending on ambient conditions and stack temperature, this may result in a low membrane water content (resulting in low performance), or in extreme cases, in a continual net water loss from, the fuel ceil stack .166 over time. This may eventually result in the failure of one or more cells in the stack 166. This is because for a set stack power output a balance will be achieved between the water content of the fuel cell polymer membranes and the rate of water removal by the flow of air. A lower current, high air flow, and a warmer stack will tend to reduce the membrane water content, and conversely, a higher current, lower air flow and a cooler stack will increase the membrane water content. Methods to rehydrate open catiiode PEMFC stacks during operation are disclosed in commonly owned U.S. Pat. No. 8,263,277, which is incorporated by reference herein in its entirety. The ground station 200 may recondition the stack 166 by one or more of the following methods: Θ059] (a) Using one or more electrical (resistive) loads 201, temporarily increasing the current drawn from the stack 166 during pre-determined rehydration intervals to increase the hydration level of the fuel cells, and maintaining the current demand to a load external to the fuel cell assembly during the rehydration intervals. Typically, this step involves running fans 165 for about 2 seconds, waiting until the fans stop, and shorting the stack 166 for about 4 seconds using a load of about 2 ohms.

[0060] (b) modulating air flow through the stack 166 on a periodic basis independent of current demand to provide rehydration intervals that increase the hydration level of the fuel cells, and maintaining the current demand to the one or more loads external to the fuel cell assembly during the rehydration intervals.

[0061] Conditioning of the stacks 166 prior to first use in the UAV 100 or reconditioning of the stacks 166 may also be accomplished by introducing small quantities of oxidant, such as air, into the hydrogen fuel feed to the anode side of the cells in the stack 166. This generates water vapor and heat to pre-condition the fuel delivered to the anode. The hydrogen gas for preconditioning may be supplied from the hydrogen supply 202 in the ground station 200. Pre-conditioning may assist in hydration control of the membrane and temperature control of the MEA for optimum stack performance. Methods for pre-conditioning are disclosed in commonly owned U.S. Patent No. 7,785,746, which is incorporated by reference herein in its entirety. [0062] Further, conditioning the stack 166 may also be done by purging the anode side using hydrogen prior to start-up as disclosed in commonly owned U.S. Pat. No. 9,276,277, and U.S. Pat. Pub. No. 20160240875, which are both incorporated by reference herein in their entirety.

[0063] In some aspects of the disclosure, certain process steps may be conducted in response to a preceding process step. In some aspects, the UAV may be decommissioned if a leak is detected. In some aspects, monitoring the open circuit voltage occurs only if the voltage is over a threshold value. In some aspects, the step of rehydrating is conducted only if the optical device detects no significant deterioration in cell voltage. In some aspects, the refilling step is conducted only if the measured stack voltage exceeds a set-point voltage.

Polarization Check

[0064] Instead of running tests (known to those of skill in the art) to obtain a complete polarization curve (current v. voltage) for the stack 166, the ground station 200 may be used to check current and voltage at discrete points. For the exemplary 32-cell stack shown in FIG. 1C, 0.75V at 25 amps (set point voltage at 25 amps) would indicate that the stack 166 is healthy.

Refill hydrogen supply

[0065] Once the fuel cell power supply 160 has passed one of more of the checks described above, the hydrogen supply 150 in the UAV 100 may be refilled or replaced. The hydrogen supply 150 may comprise at least one of a compressed hydrogen storage supply, chemical hydride hydrogen storage, metal hydride hydrogen storage, hydrogen stored in metal organic frameworks, hydrogen stored in carbon nanotubes, and cryogenic hydrogen storage.

[0066] When the hydrogen supply 150 stores compressed hydrogen, hydrogen may be stored at pressures of up to about 900 bar. The density of compressed hydrogen at about 300 bar is about 20 kg/m3 and at about 700 bar is about 40 kg/ni3. Therefore, it may

advantageous to store compressed hydrogen at higher pressures. As known to those skilled in the art, suitable supply cylinders may be of Type 2 cylinders (up to 300 bar), Type 3 cylinders (up to 700 bar), Type 4 (up to 700 bar) and Type 5 cylinders. Type 2 cylinders may include a metal shell (e.g. aluminum) having a fiber winding around the shell. Type 3 cylinders may be made of composite material (fibers) and have a metal liner. Type 4 cylinders may be made of composite fibers and have a polymer (plastic) liner. Type 5 cylinders may be composite cylinders and have no liner, and therefore may be lighter compared to cylinder Types 2-4.

[0067] Hydrogen may be delivered to the home base as commercial gas cylinders (K-size), which may be coupled to form a pack comprising a number of cylinders. Hydrogen may also be delivered in tube trailers, which comprise a bundle of pressurized tubes. Hydrogen may also be delivered as a cryogenic liquid, in which case boil-off from the liquid storage is compressed to a desired pressure at the home base. Hydrogen from cylinders, liquid storage, or tube trailers is compressed to a desired pressure, if required, at the home base, and routed to the hydrogen storage 202 in the ground station 200 for filling the hydrogen supply 150. The hydrogen storage 202 may include a plurality of storage vessels (not shown). The ground station controller 206 may be capable of estimating filling parameters, such as amount of hydrogen remaining in the hydrogen supply 150 when the UAV 100 returns to the home base after a flight, and the amount of hydrogen that needs to be provided to the hydrogen supply 150 for the next flight. The ground station controller 206 may be capable of recording (in a suitable memory device) the number of times the hydrogen supply 150 in a particular UAV 1 0 has been filled to enable a decision to be made regarding when to replace a used hydrogen supply 150 with a new hydrogen supply 150.

[0068] Suitable identification devices, such as RFID tags, machine readable barcodes, and similar unique identifiers, may be used on the hydrogen supply 150 to enable the ground station controller 206 to scan and identify the hydrogen supply 150 using methods and devices that are known to those skilled in the art (e.g. scanners that may be provided by the ground station 200). Similar identification devices may also be employed in or on the UAV 100. The ground station controller 206 may also be capable of controlling the operation of shut-off valves and the operation of safety valves that may be coupled to a fill conduit (not shown) that extends between the hydrogen storage 202 and the hydrogen connection assembly 152 of the hydrogen supply 150. The ground station 200 may also comprise hydrogen leak sensors (not shown) to abort filling of the hydrogen supply 150 in the event of a hydrogen leak.

[0069] Hydrogen may also be generated on-site using reformation of hydrocarbon fuels or using an electrolyzer. Hydrogen is purified and compressed to a desired pressure at the home base. Reformation of hydrocarbon fuels and purification of hydrogen is disclosed in commonly owned U.S. Pat. No, 8,372,170 and U.S. Pat. No. 7,670,587, which are both incoiporated by reference herein in their entirety.

[0070] The disclosure provides that the hydrogen supply 150 may comprise a metal hydride storage cylinder instead of storing compressed hydrogen. Metal hydrides, such as MgH2, NaAlH4, and LaNi5H6, may be used to store hydrogen and supply hydrogen on-demand reversibly. However, metal hydride systems often suffer from poor specific energy (i.e., a low hydrogen storage to metal hydride mass ratio) and poor input/output flow characteristics. Hydrogen flo characteristics are driven by the endothermic properties of metal hydrides (the internal temperature drops when removing hydrogen and rises when recharging with hydrogen). Because of these properties, metal hydrides tend to be heavy and require complicated systems to rapidly charge and/or discharge them. For example, commonly owned U.S. Patent No. 7,271,567 describes a system designed to store, and then controllable release hydrogen gas from a cartridge containing a metal hydride. U.S. Pat. No. 7,271,567 is incoiporated by reference herein in its entirety.

[0071] The disclosed system also monitors the level of remaining hydrogen capable of being delivered to the fuel cell by measuring the temperature and/or the pressure of the metal hydride fuel itself and/or by measuring the current output of the fuel cell to estimate the amount of hydrogen consumed. The hydrogen supply 150 comprising a metal hydride cartridge may need to be refilled or replaced each time the UAV 100 returns to the home base. Refilling of metal hydride tanks is time consuming and may not generally be a suitable function for the ground station 200. Rather, the ground station 200 may be used to replace the metal hydride cartridges in a safe and reliable manner. The ground station controller 206 maybe configured to determine the level of hydrogen remaining in the hydrogen supply 150, decide whether a replacement is required by taking into account the flight requirements of the UAV 100, and, if a replacement is necessary, disconnecting the spent hydrogen supply 150, replacing it with a fresh cartridge, performing a leak check, and storing at least one operating parameter and at least one cartridge identifier in a suitable memory device (not shown) located in or on the ground station 200, the fuel cell power supply controller 164, or in a remote location (e.g., a remote server). The ground station controller 206 may also be configured to authenticate new cartridges.

[0072] The disclosure provides that the hydrogen supply 150 may comprise a chemical hydride cartridge that is capable of releasing hydrogen when initiated by heat, using a hydrolysis reaction, or a combination of both. In the case of chemical hydride-based hydrogen supply 150, the ground station 200 provides for the exchange of a depleted chemical hydride hydrogen supply 150 with a fresh supply. Chemical hydrides, such as lithium hydride (LiH), lithium aluminum, hydride (LiAlH4), lithium borohydride (LiBH4), sodium hydride (NaH), sodium borohydride (NaBH4), and the like, may be used to store hydrogen gas non-reversibly. Chemical hydrides produce hydrogen upon reaction with water (hydrolysis) as shown below for two exemplar chemical hydrides:

[0073] (Sodium borohydride) NaBH4 + 2H20 -» NaB02 + 4H2

[0074] (Lithium aluminum hydride) LiAlH4 + 4H20 4H2 + LiOH + Al(OH)3

[Θ075] To reliably control the reaction of chemical hydrides with water to release hydrogen gas from a fuel storage device, a catalyst and additives that control the pH of the water feed may be employed. An inert stabilizing agent may also be used to inhibit the early release of hydrogen gas from the hydride. A disadvantage of the hydrolysis route is the need for providing a water supply, which adds to the weight and volume of the fuel cell power system. Hydrogen may also be stored in chemical hydrides that release hydrogen by thermolysis. Examples of such hydrides include aiane and ammonia borane, which release hydrogen when heated to 50 °C to 300 "C, and more preferably between 90 °C to 170 "C. Commonly owned U.S. Pat. No. 9,276,278 and U.S. Pat. No. 9,269,975 disclose methods and cartridges for generating hydrogen by thermolysis and are both incorporated by reference herein in their entirety. Controller 206 may also be configured to authenticate new cartridges. Methods and devices to authenticate hydrogen cartridges (metal hydride and chemical hydride cartridges) are disclosed in U.S. Pat. No. 9,111,124, U.S. Pat. Pub. No. 20150041354, and U.S. Pat. Pub. No. 20150093667, which are all incorporated by reference herein in their entirety.

[0076] FIG. 4 is an exemplary flow chart that illustrates the functions that may be performed by the ground station 200. When it is evident that the fuel cell power supply 160 is malfunctioning (e.g., the controller 180 has directed the UAV 100 back to the home base), a hydrogen leak check is first performed using a hydrogen detector or sniffer to determine any gross leak in the fuel cell power supply 160. If the fuel cell power supply 160 fails the test, the UAV 100 may be decommissioned to replace the fuel cell power supply 160. If the malfunction is not caused by hydrogen leak, the UAV 100 may be enclosed in the enclosure 220, and another leak test may be performed after the stack temperature has been conditioned to between about 20 °C and about 25 °C. Hie UAV 100 may be removed from the enclosure 220 upon completion of the temperature conditioning and hydrogen leak checking steps.

[0077] When the UAV 100 is needed for an emergency flight (e.g., to deliver fire retardants to control a fire), a shortened version of a health check may be conducted. Hie UAV 100 may be returned to flight if it passes the hydrogen leak check and after refilling the hydrogen supply 150. Alternately, the UAV 100 may be returned to flight if, in addition to passing the hydrogen leak check, the optical devices suggest that the cells in the fuel cell power supply 160 are operating normally, and after refilling the hydrogen supply 150. Alternative variations of the steps described in FIG. 4 may be implemented depending on the intended use of the UAV 100.

[0078] Other pre-flight checks may include cleaning of stack air filters in the fuel cell power supply 160, checking the payload and refilling payload when the payload includes fire retardants, pesticides, and the like, and uploading new flight information to the controller 180 using wired or wireless means, such as Bluetooth or near field communication (NFC) methods that are known to those of ordinary skill in the art. The ground station 200 may also comprise a graphical user interface 205 to enable the user to start the pre-flight check protocol (e.g., as described in FIG. 4) or to choose one or more functions.

[0079] The ground station 200 may be operated in a semi-automated or fully automated mode using robotic aims and the like to assist in performing or to perform one or more of the above described functions.

[0080] The disclosed methods and devices comprising the ground station 200 is not restricted to multirotor UAVs. The disclosed methods and devices may also be applicable to fixed wing UAVs that may employ closed cathode PEM fuel cells.

[0081] A plurality of ground stations 200 may service multiple UAVs at a service station instead of a home base.

[0082] Aspects of some exemplars include a PEMFC stack in a fuel-cell-powered UAV may preferably be checked each time the UAV returns to the home base and prior to each take-off. Further, these health and safety checks may preferably be done quickly to prevent unnecessary downtime of the UAV. Additional functions that may be performed using the ground station at the home base, in an automated or semi-automated manner, include filling of hydrogen supplies, replacement of hydrogen cartridges, checking payload, leak -checking the PEMFC stack, and cooling the PEMFC stack. Ground stations that provide these functions while communicating with the UAV via power, communication, and data links may be advantageous to enable widespread use of PEMFC powered UAVs.

[0083] While the methods and fuel cell power systems have been described in terms of what are presently considered to be the most practical and preferred implementations, it is to be understood that the disclosure need not be limited to the disclosed implementations. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. The present disclosure includes any and all implementations of the following claims.

[0084] It should also be understood that a variety of changes may be made without departing from the essence of the disclosure. Such changes are also implicitly included in the description. They still fall within the scope of this disclosure. It should be understood that this disclosure is intended to yield a patent covering numerous aspects of the disclosure both independently and as an overall system and in both method and apparatus modes.

[0085] Further, each of the various elements of the disclosure and claims may also be achie ved in a variety of manners. This disclosure should be understood to encompass each such variation, be it a variation of an implementation of any apparatus implementation, a method or process implementation, or even merely a variation of any element of these.

[0086] Particularly, it should be understood that as the disclosure relates to elements of the disclosure, the words for each element may be expressed by equivalent apparatus terms or method terms - even if only the function or result is the same. Such equi valent, broader, or even more generic terms should be considered to be encompassed in the description of each element or action. Such terms can be substituted where desired to make explicit the implicitly broad coverage to which this disclosure is entitled.

[0087] It should be understood that all actions may be expressed as a means for taking that action or as an element which causes that action.

[0088] Similarly, each physical element disclosed should be understood to encompass a disclosure of the action which that physical element facilitates. [0089] In addition, as to each term used it should be understood that unless its utilization in this application is inconsistent with such interpretation, common dictionary definitions should be understood as incorporated for each term and all definitions, alternati ve terms, and synonym s such as contained in at least one of a standard technical dictionary recognized by artisans and the Random House Webster's Unabridged Dictionary, latest edition are hereby incorporated by reference.

[0090] To the extent that insubstantial substitutes are made, to the extent that the applicant did not in fact draft any claim so as to literally encompass any particular implementation, and to the extent otherwise applicable, the applicant should not be understood to have in any way intended to or actually relinquished such coverage as the applicant simply may not have been able to anticipate all eventualities; one skilled in the art, should not be reasonably expected to have drafted a claim that would have literally encompassed such alternative implementations.

[0091] Further, the use of the transitional phrase "comprising" is used to maintain the "open-end" claims herein, according to traditional claim interpretation. Thus, unless the context requires otherwise, it should be understood that the term "compromise" or variations such as "comprises" or "comprising," are intended to imply the inclusion of a stated element or step or group of elements or steps, but not the exclusion of any other element or step or group of elements or steps. Such terms should be interpreted in their most expansive forms so as to afford the applicant the broadest coverage legally permissible.