Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

66 Dossier | Northwest UAV NWFC-1500 fuel cell Stack design While the catalyst layers on either side of the proprietary PEM layer are not pure platinum (as that would be prohibitively expensive), the company’s focus on certified aerospace missions has motivated it to use a mixture of platinum black and carbon black, with a proprietary ratio it judges to be higher in platinum than the average in commercially available PEMFCs. “We don’t go light on the platinum,” Ratcliffe says. “For our customers, the ability to rapidly start, stop, and ramp up power is more important than a small increase in price derived from having more platinum, so we use more than is typical to step the rate up and down at which the fuel cell consumes fuel and reactant. “And while we use COTS gas diffusion layers [GDLs], we’ve put a lot of r&d into optimising the surface geometries of the custom bipolar plates [BPPs] enclosing our membrane electrode assemblies [MEAs] to ensure efficient distribution of hydrogen and air across each cell to obtain the power densities and ramp rates we wanted, while still keeping the metallic BPPs as lightweight, compact and manufacturable as possible.” Also, to maintain a consistent and uniform vertical pressure on the stack, to seal the cell layers and their fluids tightly together, a titanium band is wrapped around the stack. Titanium was chosen for its high strength-to-weight ratio and tensile strength, and the band features numerous holes cut along its length to save further weight, with FEA and other analyses being performed to identify where material could be removed without sacrificing strength or rigidity. The band is compressed mechanically around the stack during the final stages of manufacturing before its ends are welded together. Thermal management Down the years, this magazine has covered fuel cells designed to use a single airstream for cooling and oxidant supply (open cathode), as well as those that use separate airstreams (closed cathode). Ratcliffe says the NWFC-1500 falls into the latter category. The former is doubtlessly easier to engineer, but the latter comes with numerous key benefits that are vital to his company’s end-users. In principle, a closed-cathode fuel cell can operate across a much wider range of ambient temperatures, since the system is not compelled to continue cooling an already cold fuel cell when used in colder climates, while in warmer ones the cooling system can work harder without causing the cathode side of the PEM stack to exceed its recommended airflow requirement relative to power output. As a result, the NWFC-1500 can run in environments from -5 C to +45 C, whereas open-cathode cells can rarely be used in temperatures below 0 C or above 35 C. Given that many of NWUAV’s customers operate in arctic, desert, mountainous and other extreme environments, the latter option would not have been acceptable. To achieve its flexible cooling and operating capabilities, the NWFC-1500 has a liquid-cooling circuit, which the company says is superior to air cooling for numerous reasons. For one, the higher rate of thermal transfer of liquids helps to shrink the size of the PEM stack, as there is less need for a significant gap between the cell layers to ensure sufficient cooling. “It also means we can control the December/January 2022 | Unmanned Systems Technology A titanium band serves to package and tighten the stack We don’t go light on the platinum. The ability to rapidly start, stop and ramp up power is more important than a small price increase