Uncrewed Systems Technology 046

79 a period of time upside down or in different gravity conditions, despite the fluidic challenges of maintaining stable hydrogen, oxygen and waste water flows amid changes in gravity. “On that note, we have also made a unit for an uncrewed space launch vehicle, which underwent immense g -forces during lift-off, so we know how to make fuel cells for punishing applications. “We’ve put a lot of expertise and investment into hydrogen test capabilities, and Honeywell has considerable internal facilities and knowhow for testing to FAA certification. We’re only now beginning to understand what BVLOS UAS certification requirements will be, but we’re well- prepared for whatever the case.” Architecture and operation Around the fuel cell stack is a BoP consisting of control electronics, a blower, humidifier and other electronic and electromechanical parts. The 600U measures 24.1 x 17 x 14 cm, while the 600U-HV is 23.1 x 15.5 x 13.7 cm, and the 1200U is 43.2 x 26.67 x 16.5 cm. Each is built largely as a single, monolithic unit that is integrated for simplicity as well as to minimise cabling requirements, but it can also be designed as a cube-shaped system for multi-rotors or as a longer, thinner package more suited to fixed-wing aircraft. There are several systems for mounting the fuel cells in different ways, the most widely used being an aluminium tray with mounting holes on its top, bottom and sides for bolting into any fuselage. While the overwhelming majority of parts in the 600U and 1200U are made in-house by Honeywell or its partners, one item that Honeywell does not make are the membrane electrode assemblies (MEAs), given the delicate balancing of materials and understanding required to fabricate such items. “Even when we were still Protonex, we used a COTS automotive-grade MEA,” Robinson recounts. “When we were part of Ballard, they made us a special custom MEA that was optimised for UAVs, and although we’re now part of Honeywell, we continue to use that MEA primarily; it’s a five-layer component supplied by Ballard.” As discussed in UST 14, those five layers consist of a central Nafion membrane that’s responsible for conducting protons and hence outputting current. They are coated on each side with a catalyst layer made from carbon black and platinum, and with a carbon gas diffusion layer over each coating. Also, being a liquid-cooled fuel cell, it is a closed-cathode system, meaning the oxidant and coolant flow channels are separate rather than being the same. That gives advantages such as more optimised and responsive cooling and oxidant supply rates, easier humidity control, less need for current pulsing, and a wider environmental operating range than open-cathode PEMFCs – the 600U and 1200U are recommended for use between 5 º C and 45 º C, compared with an upper limit of 35 º C in most open- cathode systems. “That’s a very conservative environmental operating range,” Robinson adds. “We’ve done a lot of testing down to -20 º C. Also, the MEA has some inherent features that are optimised for UAV operations. “Consider a typical automotive PEM fuel cell stack, you tend to run it very conservatively. Because you don’t pull a lot of power from it, you operate it fairly high on the voltage-current curve. You do that because you want it to last tens of thousands of hours, maybe even 100,000. “The thing about this market is that nobody flies a UAV for 100,000 hours. Even the most robust commercial uncrewed aircraft will undergo either an overhaul or potentially an ‘unplanned Honeywell 600U, 600U-HV and 1200U fuel cells | Dossier Uncrewed Systems Technology | October/November 2022 Protonex spent the early 2000s developing numerous small fuel cell technologies to enable reliable electric power for the US Department of Defense (DoD) in extreme environments. UAVs emerged as the most viable application of these technologies, and in 2015 Protonex was acquired by Ballard, but Protonex saw applications in larger-scale urban air taxis and even crewed aircraft as a potentially critical direction to move towards. Their development was driven strongly by the DoD, as well as some civil and commercial organisations. “Honeywell was one of our partners and customers that we were working with on hydrogen-powered UAV solutions, and Ballard’s long-term strategy did not include the US DoD or the aviation market,” recalls Phil Robinson, senior director of engineering for zero-emissions aviation at Honeywell Aerospace. “It was therefore decided that the business originally started at Protonex would be better placed at Honeywell, not Ballard,” he says. “Honeywell has long been closely aligned with the aviation world and has had a lot of success with DoD programmes, so it acquired the UAV business in 2020. “The big advantage for us since then has been Honeywell’s expertise in what it takes to build, and most of all certify, aircraft-grade equipment. Commercial users increasingly want to fly BVLOS, and there’s going to be a wave in the near future of uncrewed aircraft well over 25 kg carrying Lidar, multi-sensor gimbals and cargo deliveries. So, knowledge of how to engineer and manufacture subsystems in a way to ensure they can be certified by the FAA and similar organisations is really critical.” Protonex company history, from Ballard to Honeywell