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

68 membrane electrode assembly and two flow-field plates. A single fuel cell delivers typically a voltage between 0.5 and 1 V, which is too low for most applications so, as with batteries, individual cells are stacked to achieve a higher voltage and power. This is called a fuel cell stack. PEM fuel cells do not require corrosive fluids, like some fuel cells. They only need hydrogen, oxygen from the air, and water to operate. The power output of a given fuel cell stack will depend on its size. Increasing the number of cells in a stack increases the voltage, while increasing the surface area of the cells increases the current. One 10 kW full stack measures 190 × 270 x 520 mm, and weighs 35 kg. A stack is finished with end plates and connections for ease of use. Advantages include a fuel cell stack power density as high as 1000 W/kg with a potential for automated cell fabrication and assembly, which brings about a true potential for low-cost stack manufacture. Drawing elsewhere from industry to provide commercial off-the-shelf fuel cells can also have an effect on UAV efficiencies and performance. One recent test flight using this technology achieved the world’s longest multi-rotor UAV flight, flying for 3 hours, 43 minutes and 48 seconds, beating its own previous world record of 2:12:46. This extended duration translates into a more attractive flight system that will go a long way to meeting the demands of customers who are always looking for additional capabilities. Based on fuel cell system technology similar to that used in the automotive industry, specific attention has been paid to operation in ambient temperature extremes (high and low), high altitudes and environments where significant airborne contaminants such as dust are present. Underwater systems The design compromises in autonomous unmanned underwater vehicles are not as critical as those with UAVs, since being in water mitigates the issue of weight when sub-surface. This allows the list of potential applications to grow as fuel cell technology develops, the extra capability driving bigger and more effective unmanned sub-surface systems. By having a wholly independent UUV, the operator is able to release it from its support vessel without the risk of having a tethered vehicle in rough weather. Most operators and UUV manufacturers are therefore looking for ways to increase autonomy and time at station depth to achieve a more cost-effective performance. With battery power, a typical mission allows a cycle time of only three hours: an hour down to station, an hour at station then another hour back up to the support vessel for recharge. However, a fuel cell system increases the time at station depth, how much longer depending on the amount of fuel carried – often enough for months of autonomous operation. This additional storage can be accomplished by adding separate containers or increasing storage pressure, or both. Typical modern UUV fuel cells are designed to operate with a minimal amount of system components, the complexity from extra parts outweighing the advantage they can provide. This will supply a more reliable and cost-effective power and energy system over a more complex system. A consequence of this is that with a fixed-volume fuel cell sized for a given power level, the reactant Dec 2015/Jan 2016 | Unmanned Systems Technology The US Navy is testing different fuel cell technologies in its Large Displacement UUV Innovative Naval Prototype, which will start trials in California in 2016 (Courtesy of USN Office of Naval Research) Battery power allows a typical UUV cycle time of only three hours, however a fuel cell will increase the time at station depth