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68 V ehicle developers these days have a wider than ever range of solutions for power generation and energy storage to choose from, and hydrogen fuel cells continue to appeal as a middle ground between ICEs and battery-electric powertrains where hybrid- electric arrangements are not optimal. While ICEs generally provide the longest range, they generate significant emissions and come with greater maintenance requirements than electrical systems. By contrast, modern batteries can autonomously maintain themselves and operate smoothly for thousands of hours, while producing no emissions at the point of use. They are problematically large and heavy though – a typical lithium-ion battery stores 0.15 kWh/kg, versus 11-12 kWh/kg for gasoline and diesel. Meanwhile, an increasing number of unmanned systems in undersea and off-world applications need to endure missions as long or longer than those of solar HALE aircraft, and cannot get by on wing-borne lift and solar cells for their energy requirements. In these missions, hydrogen fuel cells make for an ideally SWaP-optimised energy and power solution. Their mean times between failures (MTBFs) can run into the tens of thousands of hours depending on the model and how it is used. Also, hydrogen’s specific energy is 33.3 kWh/kg, the highest of any commercially available fuel; it also has an energy density of 14 kWh/litre, the next highest being diesel, at roughly 10 kWh/litre. These qualities make them extremely advantageous for maximising endurance versus weight. To push that advantage further, the proton exchange membrane fuel cells (PEMFCs) from Teledyne Energy Systems are designed with ejector-driven reactant (EDR) circulation systems. These A novel reactant circulation system makes this fuel cell an ideal candidate for long-haul undersea and space missions. Rory Jackson finds out how it was developed Staying power December/January 2021 | Unmanned Systems Technology The standard EDR fuel cell is designed to use pure oxygen rather than ambient air