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

55 Protonex fuel cell for ScanEagle | Dossier “The hydrogen coming into the stack is dead-ended – the system will consume as much hydrogen as it needs for the current that is being drawn,” Osenar says. “On the cathode side, we vary the partial emission blower output based on current delivery; airflow can be increased to compensate for density altitude.” Hydrogen storage PEM cells require pure hydrogen, which some consider unsafe as a fuel. In fact though, in the event of a leak it rapidly disperses, reducing the likelihood of ignition – and, properly handled, many consider it safer than gasoline or jet fuel. Indeed, the use of PEM FCs in the US space programme since the 1960s shows that the technology is fundamentally safe and reliable, and that FCs can withstand even the g -forces associated with space flight. The downside of using pure hydrogen is getting the supply to the UAV launch site. It is straightforward to ship gaseous hydrogen in what amounts to a welding bottle, but for the ScanEagle the preferred option is to generate hydrogen at the site, extracting it from a water supply. “The electrolyser approach is mature technology,” notes Knapp. “It does take energy to split water and process the hydrogen supply, but that is negligible in the overall context – the operation of the ground station will ultimately take more power than the fuelling process.” Osenar notes that Proton Onsite is developing a 350 bar electrolyser, which within one unit produces the necessary compressed gas. An alternative is to cool it for liquid onboard storage (to liquefy hydrogen gas it must be cooled to about -423 F/-252 C). Whichever option is taken though, clearly the necessary equipment must be on site. Which option implies the more equipment, we asked? “It is about the same,” replies Knapp. “You either need a mechanical compressor or a cryo cooler, and they are about the same size and weight. “So in that sense there isn’t much difference – it is the equivalent of a dishwasher or a small refrigerator. Cryo coolers are standard equipment; every MRI machine in the medical field uses one.” To use liquid storage the craft needs a Dewar tank, which is essentially a double-walled vessel with a vacuum between the walls. “Since it is liquefied you can run it at the pressure required by the needs of the fuel cell, which is in the region of 15-20 psi,” says Knapp. “With the Dewar you essentially force it to boil off at the required heat release rate. That vapour pressure is then enough to provide the stack with the pressure it needs. Since it is operating close to atmospheric pressure you eliminate the safety concerns associated with a pressurised storage vessel.” Osenar notes that liquid hydrogen Unmanned Systems Technology | June/July 2017 Heart of the matter – a Protonex fuel cell