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6 Solid hydrogen storage pellets and fuel cell technology are set to significantly increase the flight duration potential of electric UAVs. Consider, for example, AeroVironment’s Raven RQ-11, a UAV widely used for reconnaissance by the US military and other operators worldwide. Hand launched, it has a 1.37 m wingspan and weighs just under 2 kg, carrying a surveillance camera and motor powered by a lithium-ion battery giving it an endurance of 60-90 minutes. Replacing the battery with a pellet-fed hydrogen fuel cell promises to triple its endurance. The pellets have been developed by Cella Energy, which has been working with systems integrator L2 Aerospace to replace the battery in a Raven with a far greater source of electrical power for comparable weight. There is wide interest in proving the technology for the growing number of civilian and commercial applications, as it can be applied to all electric-powered unmanned craft. To this end, Cella is part of a further collaboration, co-funded by the UK’s innovation agency, Innovate UK, to develop a 200 W hydrogen system for a larger unmanned marine survey aircraft used by the Scottish Association for Marine Science, with fuel cell integration by Arcola Energy. A test flight is planned by summer 2015. Central to the concept is the use of the pellets instead of a tank of hydrogen to supply the fuel cell. Cella Energy’s pellets are solid capsules that combine ammonia borane and a polymer, and which release hydrogen gas when heated to 100 C. Whereas hydrogen gas normally has to be compressed and consequently stored in a strong and hence heavy tank, the pellets can be stored on the craft in an easily replaceable cartridge at ambient pressure and temperature. The pellets are heated sequentially throughout a mission as dictated by the UAV’s electronic control system, so as to provide a constant supply of hydrogen from this wing structure location to the fuel cell. Only a few per cent of the total energy output of the fuel cell is needed to heat the pellets. The net result is a system that can match the weight of the normal lithium-ion battery while promising sufficient energy to keep the motor running for up to three times longer. Mission-critical info for UST professionals The pros of pellets Cella Energy scientists working on the hydrogen-powered Raven UAV Power supply November 2014 | Unmanned Systems Technology Platform one