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

69 Fuel cells | Focus storage becomes the critical variable, making its storage the principal structural design challenge. These fuel cells minimise parasitic load losses while providing rapid electrical transient response. This means that power is available as soon as it’s needed, and at an increased level of efficiency. The cell itself operates at greater than 50% efficiency when consuming compressed oxygen rather than compressed air, accentuating the ability to scale power output with fuel cell stack size while separately scaling energy output through reactant storage sizing. Systems such as this also use a bifurcated approach that separates power output from energy output. This is the distinguishing difference between fuel cells and batteries. Batteries provide a high specific power (kW/kg) while fuel cells provide a high specific energy (kWh/kg). It is the high specific energy resulting from this bifurcated approach that provides a three-hour mission cycle, which become eight hours or more, and that simply depends on vehicle layout by minimising system inefficiencies. It is becoming clear however that the dominating factor in establishing optimum specific energies are the hydrogen and oxygen storage tank systems contained on board a UUV, but for most applications a simple compressed gas storage arrangement should suffice. One common feature in the fuel cell market is the remote, in-situ production of hydrogen from water. Modern electrolysers operate in the reverse way to a fuel cell and can generate hydrogen, at pressures of up to 250 bar, by breaking water molecules into their constituent parts. The resulting hydrogen and oxygen reactants are then stored in tanks without additional compression and supplied to the UUV for consumption in its fuel cell. This technology is already allowing mission durations of the order of months, rather than weeks. We can see this in the search for answers to the mystery surrounding the disappearance of flight MH370, with UUVs operating in the southern Pacific week in week out. It is this sort of technology that will make operations like this routine. Reformer technology One constraint on the adoption of fuel cells from the automotive industry is the use of hydrogen as a fuel. Compressed hydrogen, when used as a fuel source in unmanned ground vehicle systems, has safety issues, so the safety systems and processes that as a result need to be in place tend to limit the take-up of fuel cells. There are also high costs associated with expanding the infrastructure necessary for buying and distributing compressed hydrogen to consumers. Similarly, typical industrial-scale storage fuel cell installations require an on-site hydrogen generation plant, the infrastructure of which can add significant cost to the unmanned ground vehicle system being used. As it is, given that the likely use of unmanned ground vehicles would be either the military or other government agencies, their existing non-public infrastructure would have to be improved. One alternative to the in-situ production of hydrogen is a ‘reformer’ that allows on-site or point-of-use generation of Unmanned Systems Technology | Dec 2015/Jan 2016 Some fuel cells can use formic acid as a source of hydrogen, which is safer than traditional methods (Courtesy of Neah Power Systems) One common feature in the fuel cell market is the in-situ production of hydrogen from water, which can be stored in tanks without additional compression