A team from the US Naval Research Laboratory (NRL) is claiming an electric-powered unmanned air vehicle endurance breakthrough by keeping its "Ion Tiger" test aircraft aloft for two full days, using only 500g (18 ounces) of liquified hydrogen to feed its fuel cell.
The trick that nearly doubled flight time - from 26h 2min in 2009 to 48h 1m - was to replace gaseous hydrogen compressed to 5,000psi with liquified hydrogen. The aircraft, about the size of a catapult-launched Insitu ScanEagle, was otherwise unchanged.
The cryogenic liquid - stored in a insulated tank the size of a thermos flask and lighter than the compressed air tank - has to be stored at 20 kelvin (-253˚C). But it packs far more energy into the tank.
The aircraft's electric motor is powered directly by power from the fuel cell, with no need for the extra mass of batteries. Fuel cells, fed with hydrogen gas, produce electricity and, as waste products, water vapour and heat.
"Polymer fuel cells load very quickly, so you don't need to hybridise it. Some of the other fuel cell UAVs have used much more complicated schemes," says principal investigator Karen Swider-Lyons.
Apart from keeping the fuel cold, success therefore depends on matching the rate of boil-off of the cryogenic hydrogen to the vehicle's fuel consumption.
"Liquid hydrogen coupled with fuel-cell technology has the potential to expand the utility of small unmanned systems by greatly increasing endurance while still affording all the benefits of electric propulsion," says Swider-Lyons.
Like some automotive systems, larger test aircraft such as AeroVironment's Global Observer and Boeing's Phantom Eye have used gaseous hydrogen as fuel for internal combustion engines. Others have used hydrogen fuel cells to replenish a traditional battery. But the NRL's direct electric drive system maximises the energy density potential of hydrogen fuel cells, which can achieve 34,000 watt hours/kg of fuel.
As Swider-Lyons notes, long endurance is possible with conventional, hydrocarbon-fueled systems, but these are usually loud, inefficient and unreliable in this aircraft class. Similarly, small, electric, battery-powered systems are limited to endurances of only several hours.
To address the logistics of in-theatre supply of liquid or gaseous hydrogen, NRL proposes in-situ separation of hydrogen from water - leaving oxygen and hydrogen to feed the fuel cell. Such an electrolysis system would require only water for feedstock and electricity, possibly from solar or wind power. The concept could be ideal for small reconnaissance aircraft to be operated from ships, where electricity is plentiful.