Uncrewed Systems Technology 046

80 Dossier | Honeywell 600U, 600U-HV and 1200U fuel cells landing’ before then, or just as likely they’ll be obsolete well before the PEMFC nears that lifespan,” he says. That enables the Honeywell fuel cell stacks to be run far harder in terms of current than a typical automotive stack, meaning more power versus weight. The MEA is optimised to withstand the resulting environment and still achieve its 3000 hour service life. Another key operational consideration is start-up time. PEMFCs have an inherent advantage over other types such as solid- oxide fuel cells, which can take 20- 30 minutes to warm up sufficiently for take-off. The liquid circulation inside the 600U and 1200U however enables a warm-up time of about 30 seconds, during which the control systems initialise, any leftover water or hydrogen is purged, and the heat inside the stack reaches its preferred operating point. Robinson adds that when air-cooled fuel cells aren’t running, their MEAs are typically exposed to the atmosphere, as air tends to sit and flow freely in the coolant-oxidant channel. That risks drying out the MEA, increasing their start-up times because the internal environment must be re-humidified to prevent damage during power generation. “A liquid-cooled stack is completely sealed though, so the water you left in there the last time you shut it down is still there when you fire it back up,” he explains. “No humidification period is needed during start-up, which is quite a bit more convenient not only for the DoD, which often wants to fly at a moment’s notice, but for any user looking to maximise uptime.” Fluid management Hydrogen typically flows at 1 bar into the fuel cell system, via the BoP which contains a system of channels and valves for delivering hydrogen as needed for stoichiometric balance of hydrogen and oxygen. For early integration programmes, Honeywell also provides a carbon-overwrapped pressure vessel as well as the pressure regulator for stepping the stored hydrogen down to 1 bar. “In Europe, that’ll be a 5000 psi tank, or around 350 bar; in the US it’ll be a 6000 psi or 414 bar tank, simply because of differing commonly available hydrogen cylinder pressures in Europe and North America,” Robinson says. “Our pressure regulator and related components are rated for 414 bar, so they’re safe for use with European and US tanks. “We also supply a hydrogen fuel level sensor. This is a pressure gauge designed with an electrical interface to run to the aircraft so that the remaining quantity of hydrogen gas can be measured and communicated to the operator at the GCS, as well as a fill coupling so that swapping the tank or bringing the UAV to a refill station is an option for replenishing the hydrogen. “At the moment, we’re working with two pressure regulator companies, and one good thing about both systems is that they’re multi-stage regulators. Since they’re going to take gas from 5000- 6000 psi down to around 20 psi, any company trying to do that with one or two stages will struggle to keep the output pressure stable over a wide input pressure range. In addition to avoiding drift, they’re small and lightweight – they were designed expressly for UAVs.” As well as delivering hydrogen, the fuel cell stack must intermittently purge waste water and nitrogen through an exhaust valve. This process can also release a small amount of hydrogen, and in automotive fuel cells a hydrogen recirculation pump is often installed to capture expelled hydrogen gas and send it back to the system’s input point, to prevent fuel waste as well as potential safety hazards. October/November 2022 | Uncrewed Systems Technology Honeywell has put its fuel cell units through Mil-Std electromagnetic testing, shock and vibration testing, and orientation testing for different gravity conditions A liquid-cooled stack is completely sealed, so no humidification is needed, which is more convenient for users looking to maximise uptime