Unmanned Systems Technology 005 | Selex ES Falco UAV | Sense and avoid systems | RCV Engines DF70 | DSEI show report | Fuel cells | CUAV Expo, InterDrone and CUAV Show reports | SLAM

66 Focus | Fuel cells proven to work, smaller UAVs can achieve longer durations, and in conjunction with higher efficiency fuel cells, flights of up to four hours are possible, against the matter of minutes previously achievable. This concept has been demonstrated at test facilities looking into this technology, although with a payload of about 1 kg the flight time has been reduced to about two- and-a-half hours, which for the size of UAV under test is still a considerable advantage. The ability to scale up this technology into bigger and bigger systems will be challenged by being able to maximise the volume of gas across the structural weight needed to carry it. There is effort now to try to succeed in this. The potential over batteries of a gaseous storage system is already being demonstrated, even though current prototypes are having to use commercial off-the-shelf gas pressure vessels and tubular frames. With bespoke materials and design though the potential for further design gains are possible. Supporting this design of UAV is the world’s highest power density fuel cell, at around 1 W/g for sub-kilowatt systems. It is more efficient than a traditional fuel cell, and has far fewer of the peripheral parts that are typically used to manage water and gases in the cell; instead it uses algorithms to manage the chemical reactions inside the cell precisely. Similar to the level of maturity of the integrated gaseous structure, this is also currently in development. Despite the potential advantages with a gaseous-fuelled UAV, one problem this UAV system could face are the restrictions on transporting battery technology around the world. One way of circumventing these regulations is by using a process of electrolyser-based hydrogen manufacturing in situ, using pure water. This replenishment of hydrogen, in up to 30 minutes, is faster than the several hours taken for battery charging using solar power, notwithstanding the logistics of manufacturing the hydrogen in situ and that a solar panel, while easier to use, will take longer to charge. With charging rates like that, the focus is then on fuel production rather than battery capability. PEM fuel cells Proton exchange membrane fuel cells, also known as polymer electrolyte membrane (PEM) fuel cells, were developed in the automotive industry. The possibilities of using such a system in unmanned ground vehicles is a direct carry-over from the commercial world. Essential parts of PEM fuel cells are the membrane electrolyte assembly (MEA), and bipolar plates to separate the MEAs. The MEA consists of two electrodes: the anode and the cathode. These are porous carbon electrodes that are each coated on one side with a small amount of platinum catalyst and separated by a proton exchange membrane. Bipolar plates, also known as flow-field plates, are positioned on either side of the MEA. They help distribute gases and serve as current collectors. The plates contain a fine mesh of gas channels, through which hydrogen gas is directed to the anode, and air flows through the channels to the cathode. At the cathode, the oxygen in the air forms water with the protons that come through the membrane and the electrons coming from the external circuit. The airflow removes this water, which can then be redirected to provide system cooling, although this cooling is sometimes achieved by humidifiers. The design of the bipolar plate is critical for the correct operation of the fuel cells. A single fuel cell consists of the Dec 2015/Jan 2016 | Unmanned Systems Technology A typical fuel cell for a UAV (Courtesy of Energyor) Combined with higher efficiency fuel cells, flights of up to four hours are possible, against the matter of minutes that previously could be achieved