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While fuel cells offer higher energy densities than batteries, their poor power densities have so far made them unsuitable for applications that require high power-to-weight ratios, such as multi-copter UAVs (writes Peter Donaldson). However, they also produce water and heat as by-products of the recombination of hydrogen and oxygen, and that generates electric current. Now a team of inventors from the Guangdong Hop Energy Technology Company has patented a way to harness the high-temperature steam from a methanol fuel cell for propulsion, and to help drive the reforming process that extracts the gaseous hydrogen from the methanol. The result is a hybrid fuel cell electric and rotary jet propulsion system for a multi-rotor UAV. While this novel invention recovers and uses otherwise wasted energy, and benefits from the use of methanol as an energy-dense fuel that is much easier to handle than pure hydrogen, it seems to do so at the cost of considerable complexity. That is because the system includes a methanol tank, a reformer – which needs an initial energy input from a battery-powered heater – to separate the hydrogen and associated liquid and gas, as well as feed pipes, valves and pumps. It also needs an air intake to provide the atmospheric oxygen to the fuel cell. It also needs heat exchangers, plus an electric motor or motors driven by the fuel cell’s electrical output. On top of that it needs a means of ducting steam exhaust from the fuel cell to sustain the methanol-hydrogen reforming process and for rotary jet propulsion to supplement electrical power. In some variants, mechanical drives form a rotating jet system for the individual rotors, as drawings indicate that these are not actually tip-jet driven – as might be expected – probably to simplify the plumbing. Such drives might also take power from a central electric motor (on a common shaft with the jet drive) to the rotors, or to individual motors under each rotor. The patent was granted in China in November 2017, and given the number CN206645005U. Fuel cell tech steams ahead Airborne vehicles