UST030

26 to remove it from circulation would have consumed too much energy.” Most hydrogen fuel cell systems in the unmanned vehicle market take their oxygen from the air, but UUVs cannot do that of course. The range extender’s reactant storage in the Solus-LR therefore includes a tank of oxygen, pressurised at 335 bar, along with two hydrogen tanks at 431 bar. “All three 306 litre tanks are made from aluminium and over-wrapped with CFRP to provide extra strength without adding too much weight,” Meikle notes. “We could pressurise the hydrogen to far more than 431 bar to store more of it, but what we have now is the best balance between weight, energy density and the right ratio with stored oxygen.” The three 3.05 m-long tanks are mounted lengthways in the central section of the fuselage, the Solus-LR’s largest contiguous section. The two hydrogen tanks provide 16.3 kg of useable hydrogen gas and sit near the top, with the oxygen tank containing 129.1 kg of gas underneath. Two extra tanks sit either side of the latter, and store condensed water vapour coming out of the fuel cell. The tanks are slotted into three spar- like plates that hold them in place along the length of the hull, with two additional end-plates forming the outer faces of the section’s enclosure. At the rearmost plate, a pair of aluminium mounting brackets secure an end cap, upon which is mounted a dome-shaped titanium canister that contains the fuel cell system; it sits about 60 cm tall and has a radius of 25.5 cm. Its contents have been largely custom- designed and built in collaboration with Ballard Unmanned Systems. Inside, there are four levels of componentry mounted atop circular trays. Two 600 W FCAir600h units are mounted on the second-highest tray, along with a cooling pump, a bypass valve for the coolant and a condenser. The fuel cell modules are run in parallel to output a nominal 1200 W at 60 V, which is then run through a DC- DC converter to regulate and reduce the voltage to the 48-50 V needed for charging the battery. “Being underwater for almost the entire journey, we need that onboard oxygen supply, and we also need to manage the heat and water produced as by-products of the reactions taking place in the fuel cell, as well as any trace gases,” Meikle explains. “Fortunately though, the heat produced by the fuel cell – which is roughly equal to the electrical power generated – can be easily managed thanks to the mass of cold water surrounding the UUV during operation.” On the third tier is a hydrogen flow meter for monitoring the rate of hydrogen consumption, an oxygen flow February/March 2020 | Unmanned Systems Technology Dossier | Cellula Robotics Solus-LR The Solus-LR has a flooded hull, which helps with maintenance turnrounds and cooling the electronics and fuel cells (Courtesy of Cellula Robotics) Inside the navigation canister is the UUV’s main CPU, the GNSS receiver and other key electronics (Author’s image)