Researchers at the US Naval Research Laboratory (NRL) flew a fuel cell powered UAV called Ion Tiger continuously for 48 hours and 1 minute on 16-18 April 2013 using liquid hydrogen fuel in a new cryogenic fuel storage tank and delivery system.

NRL researchers had previously achieved a flight of 26 hours and 2 minutes using the same UAV in 2009 using the same vehicle, but with gaseous hydrogen stored at 5000 psi.

The cryogenic fuel system enables hydrogen to be stored in a lightweight Thermos flask style container known as a Dewar at three times the density of the gaseous hydrogen used before.

"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," said Dr. Karen Swider-Lyons, NRL principal investigator.

Longer-endurance UAVs tend to use hydrocarbon fuels, and can be noisy, inefficient and reliable, whereas traditional small electric battery-powered systems have much shorter endurances.

As transporting hydrogen would pose logistical problems, the NRL researchers propose creating liquid hydrogen (LH2) on site from water using solar or wind power to electrolyse, compress, and refrigerate the fuel.


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