For some time now the United States military has been utilizing Unmanned Aerial Vehicles (UAVs) for surveillance and occasionally as offensive platforms, because they can offer a number of advantages human piloted aircraft cannot. Among these advantages is great endurance, as UAVs do not get tired, but they can run low on fuel. At the Naval Research Laboratory though, a UAV with a special fuel source was able to stay in the air for 48 hours and one minute, shattering the previous record of 26 hours and two minutes.
The UAV is called an Ion Tiger and has at its heart a fuel cell that uses hydrogen to generate the electricity needed to power its systems. To set the previous record, the Ion Tiger used a tank of gaseous hydrogen at a pressure of 5000 PSI, but for the new record the researchers used liquefied hydrogen. Like most materials, hydrogen is denser in its liquid form, so more of it could be stored on the UAV, but it is more complicated than just filling it up with LH2. Hydrogen boils at 20 K, so the storage system had to be kept very cold to minimize fuel loss, and to optimize performance the researchers tuned the rate of hydrogen boiling off to match the vehicle's fuel consumption.
The researchers are now looking into advanced systems to manufacture LH2, which could be hard to come by in combat zones. Potentially an electrolyzer powered by solar or wind energy could be used to collect hydrogen from water, before it is compressed and refrigerated for use as a fuel.
Source: Naval Research Laboratory