34 this too reflects recent changes across our industry. Rotron for instance (first covered in our fourth issue, Autumn 2015, and revisited in the 49th) recently expressed interest in a hydrogen-burning engine, following Mazda’s success with creating a hydrogen Wankel. Such changes are not limited to UAVs or automotive. For instance, the Mayflower autonomous ship (issue 42, February/March 2022) – the largest and best-known USV we have examined in detail – is powered by a biodiesel generator, minimising the harm it poses to the waters and sea life that so many organisations hope to study using it. And as mentioned in this issue’s USV Insight (see page 62), Demcon plans to use hydrogenated vegetable oils to power its next-generation 7 m USV. Perhaps most striking of the evolutions in UAV power is regarding hydrogen. Throughout the automotive world, one still need not look far to find critics of hydrogen-electric powertrains, but among uncrewed vehicles, hydrogen is widespread. We have featured nine UAV fuel cell companies from around the world over the past 6 years, and we know of many others. A lot of uncrewed vehicles successfully run using proton exchange membrane (PEM) fuel cells to turn their hydrogen fuel into electric current. Cellula Robotics’ Solus-LR (issue 30, February/March 2020), Zephyr Flight Labs’ Z1 UAV (issue 36), and Auve Tech’s Iseauto autonomous taxi (issue 43, April/May 2022) for instance all operate using hydrogen fuel, with other users on the sea, air and land making headlines. In future, asmore fuel cell companies scale up production – as Toyota and Intelligent Energy (issue 24, February/March 2019) have – the reduced costs, easier availability and high efficiency of hydrogenelectric power will bed it in as amainstream technology in autonomous systems. Volumetric energy density remains a bottleneck for using hydrogen though, as overly large fuel tanks are problematic for any engineer to integrate. This will continue driving experiments with the cryogenic storage of liquid hydrogen, with Intelligent Energy achieving the world record for multi-copter flight in 2019 using such a solution, and storing compressed hydrogen at 700 bar or higher is expected to become common as regulations permit. Energy-efficient futures Necessity is the mother of invention, as the saying goes, and the world sorely needs more efficient vehicles. That is one reason why, despite the pace of innovation in fuel engines, many of the engine and hybrid power companies we periodically interview acknowledge that electric powertrains will one day completely replace IC ones. It might take a few decades or so, but eventually all uncrewed systems will be electric. That is not to say however that key innovations in IC powertrains won’t carry over into the electrified world. Motor/ generators and power management systems for instance have taken much from IC-powered systems’ r&d, and this will no doubt filter into higher competitiveness in the production and performance of such components. Further ahead, there are potentially huge gains to bemade in recuperating or recycling heat. Whether IC, hybrid, hydrogen-electric or battery electric, every power system in an uncrewed vehicle generates heat. Even transistors, computers and other electronics June/July 2023 | Uncrewed Systems Technology Special review 50 issues of Uncrewed Systems Technology The Mayflower autonomous ship runs on a biodiesel generator and solar cells (Courtesy of Mayflower) In 2019, an Intelligent Energy fuel cell and a MetaVista liquid hydrogen canister were combined to break a major endurance record (Courtesy of Intelligent Energy)
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