Back in 2009, the US Naval Research Laboratory (NRL) set an endurance record for electric unmanned aerial vehicles (UAVs) when its fuel cell-powered Ion Tiger aircraft managed to stay aloft for 26 hours and 1 minute. Now, NRL has announced that the same aircraft has trounced that record, by remaining in flight for 48 hours and 1 minute.
In its previous record-breaking flight, the UAV’s Polymer Electrolyte Membrane (PEM) fuel cells were running on compressed gaseous hydrogen stored at 5,000 psi. On its latest flight, which took place from April 16th to 18th, cryogenic liquid hydrogen was used instead. Liquid hydrogen has three times the density of gaseous hydrogen, thus its ability to power a much longer flight.
One challenge lay in developing a lightweight dewar (a double-walled insulating vessel) for carrying as much of the fuel as possible, while keeping it sufficiently cold. If the liquid hydrogen were to get too warm, it would boil off at a rate faster than the UAV could use it.
NRL is particularly interested in using fuel cells for such drone aircraft, as internal combustion engines are inefficient and loud (and thus not particularly stealthy), while battery electric UAVs have a very limited range. The researchers are hoping to develop a system in which an onboard electrolyzer and energy source (such as solar or wind) could be used to electrolyze, compress, and refrigerate liquid or gaseous hydrogen from water.
Source: Naval Research Laboratory
"The Ion Tiger is a UAV research program that merges two separate efforts - UAV technology and fuel cell systems. The Ion Tiger weighs approximately 37 pounds and carries a 4- to 5-pound payload. The 550 Watt (0.75 horsepower) fuel cell onboard the UAV has about four times the efficiency of a comparable internal combustion engine."
So multiply the 3 time energy density by 4 times the engine efficiency and you have to carry 12 times the weight of petrol to equal the useful energy of the hydrogen/fuel cell combo. I'm guessing that at the slow speeds this flies at, the added drag is more than made up for by the reduction of lift needed, hence a reduction in the lift induced drag.
As a hydrogen aside, in the mid '70s I lived in Palo Alto, CA and would often see huge liquid hydrogen tankers passing through on El Camino Real. This disturbed me so I wrote the fire and safety official in Palo Alto to express my concern. I was assured by letter that liquid hydrogen presented no danger to the large metropolitan area on either side of El Camino Real from San Francisco to San Jose (my house was one block off of it.) I actually considered hijacking one, taking it to some wide open space and setting it off to make an unequivocal point.
Sure wish I still had that letter. :-)
Even if your numbers are right it is still more cost effective to use ICE.
re; DonGateley
The hydrogen transport trucks are less dangerous than diesel transport trucks. Hydrogen being lighter than air dissipates much faster and does not contaminate anything. Cost is the issue not safety.
With hydrogen having to be stored in the fuselage means the wing structure has to carry the entire load to the center whereas with diesel (or Avgas) stored in the wing the load is distributed reducing the structural requirements. Also the smaller space requirement reduces airframe mass and aerodynamic drag.