Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

52 Dossier | Protonex fuel cell for ScanEagle 4.0 litre hydrogen tank and could run for six to eight hours, whereas the equivalent battery-powered version would last about 90 minutes. This fuel cell was also used in a custom- designed US Navy fixed-wing UAV, the Ion Tiger. This 17 ft wingspan craft flew for 26 hours in November 2009 while carrying a 5 lb payload using 350 g of hydrogen compressed to 350 bar in a 22 litre carbon-aluminium pressure vessel. In May 2013 the Ion Tiger flew for 48 hours and 1 minute using liquid hydrogen fuel contained in a cryogenic fuel storage tank developed by the US Navy Research Laboratory. By contrast, the Ion Tiger would be able to fly for only about four hours had it been designed to use lithium-ion batteries. The ScanEagle fuel cell The ScanEagle has essentially the same PEM FC as the Ion Tiger, albeit with the stack doubled up. “Protonex added cells to boost the voltage a bit,” says Jeffrey Knapp, chief engineer, advanced development, at Insitu. “Plus each stack runs independently for redundancy; each stack can provide sufficient power to sustain cruise.” In addition to the twin stacks, the FC unit incorporates the balance of plant, consisting of the stack control circuit boards, pumps, valves and other support items. The unit produces 1.2 kW of power and in view of this it is also generating 1.2 kW of heat (its total energy output is 2.4 kW), and its coolant system incorporates a radiator mounted around the fuselage, in the airstream. Each stack includes a number of cells connected in series to provide a useable working voltage (much like a battery). At the heart of each cell is a very thin membrane made of Nafion, which has to keep the hydrogen and air separate while letting protons pass rapidly through. It is that proton conduction that is Nafion’s key attribute, and it was the development of Nafion by DuPont that kick-started modern PEM technology. Each side of the membrane is coated with a catalyst that is carbon black with a small amount of platinum. Osenar notes that the key is getting the quantity of platinum “low enough to keep the cost realistic. We use carbon in almost powder form with nanometre-scale platinum nodules deposited on it. But you can only go so far in reducing the platinum before you lose [chemical] stability.” On top of the catalyst layers are two gas diffusion layers, made out of carbon and very porous and hydrophobic. Osenar explains, “The gas diffusion layer allows the hydrogen or oxygen to diffuse to the catalyst sites, and also allows the electrons to move around – it is electrically conductive and gas permeable. As it is also hydrophobic, when we make water at the catalyst surface, the water is expelled.” June/July 2017 | Unmanned Systems Technology The underside of the ScanEagle’s fuel cell, showing the four circular heat exchangers and the tubular humidifiers to the right of them