Unmanned Systems Technology 024 | Wingcopter 178 l 5G focus l UUVs insight l CES report l Stromkind KAT l Intelligent Energy fuel cell l Earthsense TerraSentia l Connectors focus l Advanced Engineering report

66 Dossier | Intelligent Energy fuel cell commercial UAVs with payloads, and other small-to-medium unmanned systems, including ground vehicles or anything with access to air for the cathode and in need of lightweight power. “You get more power density relative to volume because there’s more useful area of each cell; the different cell sizes operate at the same pressure,” says Kelly. “A 10 cm 2 cell and a 64 cm 2 cell both need a few millimetres on the edges to seal in the hydrogen and oxygen reactants, so it’s simply more efficient to maximise the active area ratio in each cell.” Dan Ninan, IE’s head of advanced concepts and prototyping, adds, “Going from 74 cells in a module to 28 or 34 cells overall also means there are fewer components, so the construction and operation are simpler, reducing the overall production cost.” Items supplied from outside the company include pressure regulators from Pressure-Tech, inlet, purge, and regulator solenoid valves from The Lee Company, cooling fan motors from T-Motor, and batteries and fuel tanks from various other manufacturers. All other parts are built in- house, including proprietary materials for the PEM, catalyst layers and gas diffusion layers for each cell. In addition to the cells, the central part of the 800 W module integrates an inlet solenoid valve for hydrogen delivery, a purge solenoid valve for regulating the exhaust, and sensors such as thermistors for monitoring the fuel cell stack. The second part of the module, the control and power electronics section, is installed in front of the central fuel cell stack. It takes readings from the internal sensors and controls the valves and air intake to maintain a consistent internal environment for producing electrical power. It also contains a DC-DC converter that regulates energy production for a steady 25 V DC power output, consistent with a 6S lithium-ion battery. The difference here, between the fuel cell and a battery, is that the fuel cell voltage will remain consistent throughout a cycle, whereas a battery’s voltage will decay. The third (and final) section of the fuel cell module is the cooling section, which sits opposite the electronics section. Here, the cooling fans draw in air through vents on the electronics section to feed oxygen into the cathode and cool the fuel cell stack. The fans also expel the purged hydrogen from the anode and water from the cathode, and disperse them into the air, preventing the creation of a flammable atmosphere. The only parts external to the fuel cell module are the fuel tank and the hybrid battery, both of which can vary in size, weight and capacity depending on the level of energy storage and redundancy desired by the end-user. Hybridising the system with a small battery is important for managing the peaks and troughs of power demand. The fuel cell’s output may not always be enough for a UAV’s changing power requirements, as with VTOL aircraft moving between hover and flight, say, or fixed-wing craft diving or flying at maximum speed. If more power is required, the battery delivers the excess. If less is needed, the fuel cell recharges the battery. The battery also provides redundant power should a problem in any other area occur, but emergency flight time is rated at 2 minutes, so typically it’s recommended that a UAV lands at its GNSS coordinate for retrieval. The typical hybrid battery on the 800 W fuel cell has an 1800 mAh capacity, measures 120 x 40 x 35 mm and weighs 300 g. Before shipping, each fuel cell unit goes through a 10-hour factory acceptance test in-house. That includes running the fuel cell at the rated power; running flight, peak and trough profiles; and charging and discharging the battery. Testing also includes certifying that the battery takes over when the hydrogen supply is exhausted, and that the fuel cell module’s fan starts up consistently and cools the fuel cell stack to the correct temperature, as well as ensuring there are no leaks in the power generation system. Standard four-corners testing is also carried out in environmental chambers and on rate tables, to validate tolerance to temperature, humidity, electromagnetic compatibility, and vibration and shock. Integration Six mounting points are designed into an aluminium bracket on top of a moulded plastic housing of the fuel cell section, with another such bracket on the underside, to give UAV designers February/March 2019 | Unmanned Systems Technology The IE fuel cell is topped with an aluminium bracket with six mounting points to promote flexible integration onto a UAV, with two fasteners typically being enough to secure the 930 g module in place