Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

56 Dossier | Protonex fuel cell for ScanEagle offers a higher volumetric density, at around 71 g/litre, compared with a maximum of 25 g/litre for a vessel containing gas compressed at 350 bar. The hydrogen gas could be pressurised to 700 bar but higher pressure implies a heavier container. “As such, the weight metrics don’t get appreciably better,” notes Osenar. “You might get 33-36 g/litre but you pay a weight penalty for that, which counts more in the UAV world than in road vehicles for example, where 700 bar is often used for its lower volume.” Knapp says, “The liquid hydrogen tank for the ScanEagle is not yet complete for evaluation against the current 350 bar compressed gas tank. We get 350 g stored as liquid in a 5 litre tank, and anticipate that in terms of flight hours it will get us into the eight- to ten-hour realm.” A spun carbon fibre over aluminium vessel is currently used, which is made by HyPerComp Engineering, a specialist in filament-wound composite vessels. The alternative Dewar tank is a development by Insitu in conjunction with Washington State University. FC control Knapp says the FC’s 1.2 kW output “is good for level flight with power to spare, and then there is a hybrid battery system to boost power to 2 kW for launch and high climb rates.” Using a battery allows the size and weight of the system to be optimised. Knapp says, “We have developed what is essentially a power management system, with a power conditioning block [PCB]. “That takes care of the voltage regulation of the blend of the two dc power sources. It feeds power from the battery to the fuel cell to get it started, and supplies power to the rest of the aircraft all the time, which it can take from the fuel cell or the battery. “The PCB can also supply power to the battery and take power from the motor operated as a generator; we call it ‘the node’. We vary the current to the electric motor to provide the torque needed by the direct drive, fixed-pitch propeller to obtain the speed required of it at any given instant. We don’t have a variable-pitch propeller or any sort of gearbox – we keep it as simple and therefore as light as possible. “Once it is running, the stack is producing power that goes directly to the PCB, which feeds power to the motor’s electronic speed controller in accordance with autopilot throttle demand. At the same time, the block is monitoring the battery and supplying enough current to keep it in a full state of charge. “If the electric motor is producing a surplus of power – say when the aircraft is in a descent – the power management system includes a regeneration circuit within the motor controls, which pushes power back to the PCB. If the battery is fully charged, the PCB will force the motor controls to push the excess electricity to a resistor bank – essentially burning off surplus power. But we don’t anticipate that happening very often.” The low-profile ring motor (supplied by ThinGap) is three-phase. It is a permanent magnet brushless machine that has 26 poles and can be run as a motor or a generator. “It works by the speed controller regulating phases in the magnet system,” Knapp says. “We have worked closely with experts at Boeing to develop the motor and power block controls.” Packaging in the ScanEagle For the ScanEagle the FC is plug and play. “Essentially we have taken the space the fuel tank occupies and installed the fuel cell into it,” notes Knapp. “The fuel cell is the largest mass item in our system, and taking the space of the gas tank puts it at the aircraft’s centre of gravity.” June/July 2017 | Unmanned Systems Technology A PEM fuel cell creates electricity through a chemical reaction of positively charged hydrogen ions (protons) with oxygen. Unlike a battery, it needs a continuous source of hydrogen and oxygen to sustain the chemical reaction, but so long as those inputs are supplied it will generate electricity. The PEM cell has a negative terminal (the anode) and a positive terminal (the cathode) separated internally by an electrolyte membrane. The membrane conducts protons while blocking the passage of electrons. It also acts as a barrier film separating the gases on either side. A hydrogen supply is fed to the anode, and an air supply to the cathode. At the anode the hydrogen atoms are stripped of their negatively charged electrons, leaving protons, which pass through the membrane. In the meantime the electrons, which are unable to pass through the membrane but are attracted to re-join the protons at the cathode, travel to the cathode via an external circuit, in so doing generating a flow of dc electricity. At the cathode the oxygen in the air supply is split into two negatively charged oxygen ions, and this attracts the protons through the membrane. Each oxygen atom then combines with two protons and two electrons to form water, a by-product of the fuel cell. Given a supply of pure hydrogen, the only other by-product is heat. So there are two low-temperature electrochemical reactions in the fuel cell – a hydrogen oxidation reaction at the anode and an oxygen reduction reaction at the cathode. Normally these reactions would occur very slowly at a temperature somewhere in the region of 60-100 C, so each terminal is coated on one side with a catalytic layer that speeds up the reaction. PEM fuel cell basics