Uncrewed Systems Technology 046

85 Honeywell 600U, 600U-HV and 1200U fuel cells | Dossier Stack components Between each Ballard MEA is a bipolar plate (BPP) that is responsible for the flow of hydrogen and oxygen across the MEAs’ outer layers. Honeywell uses graphite plates for the ease of machining and customising they bring: metal plates for bespoke applications require enormous tooling (or re-tooling) investment and time, which the team considers to be unsuited to the fast- moving UAV market. “We supply volumes of tens to maybe 100 fuel cell products per year, and if we go up to thousands or tens of thousands then we might well convert to automated stamping of metal plates,” Robinson says. Notably, the MEAs are not manufactured with gaskets or other sealing components – they would be referred to as ‘seven-layer MEAs’ in this case – nor does Honeywell manufacture its stacks by fitting a gasket between each MEA and BPP. Instead, it uses a sealing method pioneered by Protonex that enables a stack that uses no gaskets. “For each unit, we stack up all the MEA and BPP layers; then, in a patented one- shot automated sealing method, all the ‘gaskets’ are formed in situ, around the stack, keeping its internal channels and its exterior sealed,” Robinson explains. “That is critical to allowing us to mass- produce these stacks at a fraction of the cost of the conventional approach, because we remove the labour cost of including physical gaskets in the stack- up process. “If you think about a gasket between two surfaces, the only way it works is if the surfaces are precisely flat and tensioned – too much and it flattens out and leaks, too little and you get gaps, and it leaks. Most stack producers therefore resort to very heavy tensioning hardware such as titanium through-bolts to keep sufficient pressure for sealing. Since we don’t use gaskets in our assembly, we don’t have to worry about tensioning, which is what enables us to make a liquid-cooled stack that’s so lightweight and small.” Thermal management As mentioned, the 600U and 1200U are closed-cathode, liquid-cooled systems. “As the US military needs to have reliable power from mountain to desert, they were never interested in having us develop air-cooled systems that are sensitive to environmental conditions,” Robinson says. Although the standard-issue coolant is water, the company can alter its coolant according to the expected operating environment. In its tests at sub-zero temperatures for example, it has used water-glycol as well as water- methanol mixes. “At higher temperatures, the limitation comes from the fuel cell stack’s internal environment being run at 60 º C,” Robinson adds. “The closer the ambient temperature gets to 60 º C, the larger the heat exchanger needs to be – the heat exchanger’s size correlates proportionally with the Δ T. “For most UAVs, a 45 º C maximum ambient temperature enables an appropriately small heat exchanger that’s easy to integrate, and which is not problematic to design into your aircraft. We’ve run special tests and applications with the DoD where we had to run hotter than that, and in those cases we went with a bigger heat changer to keep the stack’s heat from rising above 60 º C. “To be honest, we’ve generally found that for almost every aircraft we integrate on, we make a custom heat exchanger. We work with a number of heat exchanger companies that can Uncrewed Systems Technology | October/November 2022 Honeywell has run tests of the fuel cells in sub-zero temperatures, during which water-methanol mix has been used in the thermal management system Since we don’t use gaskets in our assembly, we don’t have to worry about tensioning, which enables us to make such a lightweight stack