Protonex, a Ballard Power Systems subsidiary, has successfully completed test flights of a ScanEagle unmanned aerial vehicle (UAV) with a proton exchange membrane (PEM) fuel cell propulsion system. The ScanEagle, built by Insitu, a Boeing subsidiary, has been used by the U.S. Navy and U.S. Marine Corps since 2004 for low-altitude reconnaissance, surveillance, and target acquisition.
The aircraft has multiple payload capabilities, including high-definition imaging, and is said to operate "at a significantly lower cost" than larger UAV systems. While Insitu recently developed its own upgraded internal combustion engine (ICE) in conjunction with Orbital, its recent PEM fuel cell work with Protonex allows for swapping between the two propulsion systems.
The Protonex and Insitu development consisted mainly of packaging and integrating existing PEM fuel-cell technology with the airframe. The resulting design comprised a redundant double stack design that produces 1.2 kW of power. The fuel cell takes the place of the previous ICE’s fuel tank—placing it in the airframe’s center of gravity—with the hydrogen tank extending back to the pusher propeller. This arrangement is possible because the brushless, low-profile ring motor is integrated with the propeller. The fuel cell produces an additional 1.2 kW of heat energy; to manage the heat, the fuel-cell radiator is exposed to the airstream, surrounding a portion of the airframe.
Although 1.2 kW of power is more than enough for the ScanEagle during level flight, engineers included a regenerative hybrid battery system to augment power output to 2 kW. The extra torque is required by the direct drive, fixed-pitch propeller for launch and steep ascents. Insitu’s goal in developing a fuel-cell system for the aircraft was to avoid disrupting the stable, proven platform and to create a simple, plug-and-play solution.
While there are alternatives to PEM technology, Protonex chose to use PEM fuel cells because of technological maturity and high power density. PEM fuel cells create power through the reaction of hydrogen ions, or protons, with oxygen. The fuel cells utilize a thin Nafion electrolyte membrane to separate the hydrogen fuel from air but allow protons to pass through. The electrolyte layer is coated with a catalyst (and a small amount of platinum to maintain chemical stability) to speed the chemical reactions. Two hydrophobic gas diffusion layers are placed on top of the catalyst layers to remove water (a byproduct of the reaction) from the catalyst surface and to allow space for electron travel.
The PEM fuel cell consumes hydrogen commiserate to the current being drawn from the cell. The pure hydrogen fuel must be supplied at the launch site, and this must be performed by shipping gaseous hydrogen directly to the launch site in high pressure cylinders or by using an electrolyzer at the launch site to split a water supply into hydrogen and oxygen gasses.
Both methods require about the same amount of equipment at the launch site—a legitimate consideration for military operators in the battlefield. The ScanEagle currently uses a lightweight aluminum tank with a spun carbon fiber overlay which precludes the use of liquid hydrogen. Insitu is currently developing a heavier, high-capacity, double-walled Dewer tank in conjunction with Washington State University.
With an ICE powerplant, the ScanEagle can operate for 22 hours. Although the PEM fuel cell allows for less than half of that—approximately 10 hours—it is still well above the ScanEagle average flight time of 8.5 hours. In addition to lower operational costs, Insitu also anticipates equipping ScanEagles with PEM fuel cells to result in fewer overhauls and maintenance costs with less mechanical parts.
The initial unit cost is slightly higher than an ICE ScanEagle, but with Insitu UASs reaching a total of one million flight hours in early August 2017, the likelihood of increased PEM fuel cell ScanEagle production and usage will likely drive unit cost down.
The lower noise and vibration levels of the PEM fuel system allows ScanEagle operators to fly quieter and take clearer images. The fuel-cell system also allows for mid-air start-stop capability, offering operators the ability to for switch between gliding and powered flight.