The Naval Research Llaboratory has designed and flown a Hydrogen Fuel Cell Powered UAV on an endurance flight that lasted 23 hours and 17 minutes...
The "ION TIGER" UAV uses a 3/4 horsepower engine that has so far shown to be 4 times more efficient than comparable combustion engines and 7 times more efficient than battery operated systems.
The Navy is researching various power systems for future use in Drones that can be used for surveillance and attack missions...
Obviously the technology can be used for civilian uses as well as it is becomes more practical to use...