In November of 2005, the NRL conducted its first UAV flight with the Spider-Lion, using commercial parts (such as a paintball gun compressed air tank to hold the compressed hydrogen, and aluminum foil and tubing for a radiator). It flew for 3 hours and 19 minutes, without a payload.
In 2009 the NRL flew the Ion Tiger, a much larger UAV capable of carrying a 5 pound payload. An October flight stayed aloft for a record 23 hours and 17 minutes, while a November flight set a new record of 26 hours and one minute. The Ion Tiger included a number of improvements, such as a more efficient fuel cell, and custom-fabricated compressed hydrogen tanks.
I thought a comparison of the two UAV fuel cell power plants would show how much a technology can change in four years. The following table summarized what I have found:
In 2005 a UAV could fly without a payload for 3 hours; by 2009 a UAV could carry a 5 pound payload for a full day. Furthermore, the 2009 UAV could carry the 2005 UAV as its payload!
Relative to the 2005 UAV, a 2009 UAV weighed six times as much, but also had six times the power (resulting in the same power density), and carried 12.7 times the energy (for a significantly longer flight duration). By using specialized components, rather using generic components (i.e. a paintball gun compressed air tank to hold the compressed hydrogen, and aluminum foil and tubing for a radiator), the 2009 UAV's fuel cell system level power density was twice that of the 2005 craft.
All in all, this shows the rapid progress possible with a motivated sponsor (NRL) and a promising technology (hydrogen-air fuel cells). I wonder what the NRL will prove in 2013, if they keep on this trajectory!