Unmanned Systems Technology Dec/Jan 2020 | Phoenix UAS | Sonar focus | Construction insight | InterGeo 2019 | Supacat ATMP | Adelan fuel cell | Oregon tour | DSEI 2019 | Copperstone Helix | Power management focus

63 is poor in energy density [kWh/kg] and price [$/kWh] relative to our mSOFC technology because of the weight and cost of pressurised hydrogen gas in cylinders these days. That is sure to change as supplies increase, but there is no surety about when hydrogen capacity will increase to the scale required. “For clients hoping to minimise their UAV operating costs, using an SOFC with propane for example as fuel poses the better option. Heavy fuels can also be used by technicians preferring to operate with those, for example defence UAS operators. Multi-fuel systems in general can fit in better with established fuel infrastructures than those based strictly on hydrogen or gasoline,” Dr Kendall says. In site trials, the mSOFC has generated far less greenhouse gases than ICEs and hybrid electric engines (and even less than some PEMFCs using hydrogen generated mostly from natural gas). That makes the technology ideal for operating UAVs across low-emissions airspace over towns or airports without emitting vehicle tailpipe pollutants such as carbon monoxide, nitrogen oxides and sulphur oxides, which are likely to be increasingly heavily regulated or banned in these areas. The standard mSOFC module is a 5 kg cuboid enclosed system (or 1.5 kg if manufactured using carbon fibre in place of metal) outputting a nominal 100 W up to a peak of about 200 W. It incorporates up to 20 fuel cells arranged into sub- stacks, a fuel storage and feed system, fans for air intake and cooling, a catalytic reformer, a heat recuperator, an exhaust port and a battery cable connector. Dr Kendall and her team emphasise that the base system is a starting point: the cells can be modified in their geometries and physical volumes to achieve different rates of power generation and specific fuel consumption. Altogether, the mSOFC and balance-of-plant (BoP) can be tailored – and have been, as in the SUAV project outlined in the sidebar on page 66 – to fit into different power plant bays and give different power-to-weight ratios. Operation SOFCs and PEMFCs work on similar principles. They both consist of a positive terminal (cathode) and a negative terminal (anode) with an electrolyte layer between the two. Both also use oxygen as an input on the cathode side and hydrogen on the anode side. However, PEMFCs operate through a hydrogen oxidisation reaction at the anode and an oxygen reduction reaction at the cathode, generating a DC current from the electrons stripped from the hydrogen atoms at the anode. SOFCs on the other hand take electrons from the oxygen at the cathode, and the oxygen ions react with hydrogen or carbon atoms to form water and small amounts of CO 2 on the anode side. These reactions occur in SOFCs through metallised ceramic layers, rather than the carbon and platinum membranes common to PEMFCs. As with PEMFCs, their internal environment must reach the optimum temperature range for the hydrogen and oxygen reactions to occur. But while PEMFCs typically begin and sustain power generation between 50 and 100 C, SOFCs must be heated to between 500 and 1000 C for the solid ceramic layers to allow oxygen ions to pass through. In the mSOFC, a 700-750 C environment is needed to enable oxygen ions to flow from the outside of the tubes to the inside. Some SOFC designs heat up as slowly as 1 C/minute, to prevent build- ups of heat and thermal shock, but that makes them unfeasible for vehicular applications. The tubular geometry of the mSOFCs, on the other hand, enables them to withstand thermal shocks, so they can be heated in a matter of minutes rather than hours. Furthermore, the high operating Adelan solid oxide fuel cell | Dossier Unmanned Systems Technology | December/January 2020 Much of Adelan’s testing and experimentation to develop the mSOFC has been conducted in partnerships with local academia, such as the University of Birmingham, Aston University and Keele University The mSOFCs can vary significantly in size and dimensions depending on end-user requirements. Notably, the cells have been stacked into cylindrical modules to fit into fixed-wing UAV fuselages