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

66 December/January 2020 | Unmanned Systems Technology cell supplying gas, and the other routing exhaust out from the other end. “At any of those three stages – introduction of gases into and around the cell, transition of gases through the membrane or cell walls, and exhaust of H 2 O – a leak or contamination might occur that could destroy the cell, so solving that issue of sealing was critical to creating an SOFC with a viable time between overhauls,” Dr Kendall adds. The fuel cell In each individual cell, the anode forms the basic structure and main source of mechanical strength, with the electrolyte and cathode layered on top of that sequentially. The materials were selected and managed to achieve the power, weight and size desired. The anode material is nickel oxide, although it may alternatively be a cermet – a ceramic-metal composite – of nickel mixed with another ceramic such as yttria-stabilised zirconia (YSZ) to achieve different structural or electrical properties. As mentioned, the electrolyte must be a dense layer of ceramic that conducts oxygen ions at temperatures of 700-750 C. Suitable materials include YSZ, scandia-stabilised zirconia and gadolinium-doped ceria, although Adelan typically uses YSZ. For the cathode, it uses lanthanum strontium cobalt ferrite or lanthanum strontium manganite (LSM), depending on the performance and application required, as well as the availability and cost of either material during a given project. As with the anode, a composite cathode of LSM and YSZ can be used. At the moment, the ‘base’ tube fuel cell is roughly 15 cm long and 1 cm in diameter, which as indicated can vary depending on the UAV, with larger tubes providing more active cell reaction surface and therefore more power per cell and Wh/kg. Manufacturing them to micron- level diameters could provide a higher cell and power density within a given area, but it could also increase manufacturing costs and weight significantly. Typically, 15 to 20 of the basic cells would provide a nominal 100 W (depending on the thicknesses of the layers and the fuel feeds), with a peak of around 10 W per cell. “You can get more than that from the basic cell, but we advise engineers to overspec the system to ensure that the nominal power rating can be maintained for its lifespan – as you might for any power plant,” Dr Kendall adds. “SOFC designs often aim for lifespans in the tens of thousands of hours – operating in a highly vibratory environment like a UAV or UGV, you can still get 3000-plus hours of operation between overhauls.” Multiple cells are thus stacked together and electrically interconnected in series or parallel as the application voltage and power output requires. The electrical connections can be silver or copper wires The company first developed a fuel cell system for UASs in 2011, with the SUAV project. This was part of the EU-funded Fuel Cells and Hydrogen Joint Undertaking (FCH-JU) research programme spearheaded by Adelan and conducted with Airbus, Survey Copter and other partners. The e 1 billion FCH-JU programme project, completed in November 2015, was aimed at exploring the feasibility of using a propane- and oxygen-fuelled mSOFC module as a range extender for a commercial UAS to maximise its endurance (rather than for energy efficiency). The experiment’s platform was a DVF2000 from Survey Copter, an 11 kg fixed-wing battery electric UAV with a 3 m wingspan carrying a gyro-stabilised EO/IR payload. It also featured a cylindrical body with a 3.32 litre battery compartment and 3.88 kg of battery-carrying capacity. That gave the approximate shape, size and weight for the combined mSOFC and battery system. To supply the craft’s required nominal 170 W of power, the fuel cell module had to be designed for a peak operating power output of 310 W, with 250 W nominal power to cover a further 80 W for electrical balance-of-plant charging and other parasitic loads. The UAV also had a highly variable load profile, with the nominal operating power demand being between 10 and 20% of the maximum possible. A hybrid battery would therefore provide high specific power in short bursts, then recharge itself from the fuel cell when that power was not needed. The project selected propane as the module’s fuel because of its superior energy density over hydrogen. It enabled the system to fly for up to 5 h with a fuel-plus- tank weight of just 880 g at speeds of 54-97 kph – a significant increase over the 2 h endurance provided by the craft’s standard battery packs. Over the course of the experiment the cells were redesigned and improved to provide a nominal 8 W per cell (compared with the original 5 W each cell design provided), with 48 cells in the overall module made up of two connected stacks with 24 cells each. That power increase was achieved by changing the cell diameters from 2.3 mm to 6.8 mm, and the lengths from 56 to 149 mm. That also made the electrical and mechanical design simpler (with larger tubes and fewer of them for easier handling), as well as allowing the cells to operate at lower temperatures, at 750 C. Each cell could produce 8.84 W at 0.7 V output; eventually the cells produced more than 10 W each. The SUAV project