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

54 The oxygen feed comes from what Osenar describes as a “partial emission blower”. “That is a high-speed air blower, sort of a cross between a fan and a supercharger. It is driven by an electric motor [powered by the stack] and increases the pressure of the air fed to the stack,” he says. “It runs at between 20,000 and 60,000 rpm, which provides between 1.0 and 1.5 psi of pressurisation. That is actually far higher than you can achieve with a fan system. “The stack has channels for each layer, whereby air is moved on one side of the membrane [the hydrogen passing on the other]. There is a pressure drop associated with those serpentine channels that you have to overcome; you can get higher performance from higher pressure. “We focused on designing the stack and the air mover in tandem, to get the right balance between performance and weight. The blower we designed weighs only 85 g including its motor. The lightest we could find on the market at the time was 450 g.” On the thermal side the aim is to maintain an operating temperature of around 60 C. “As with higher pressure, with higher temperature you can get more performance, but the balance of plant gets heavier,” Osenar notes. “We have an electric coolant pump that sends coolant through each stack and out to the radiator. The coolant doesn’t come into contact with anything electrical so it can be distilled water, if necessary mixed with antifreeze. There is a diverter valve, so if it is cold the coolant is recirculated through the stack rather than travelling through the radiator.” The membranes have to be kept humidified. “There is a delicate balance,” reports Osenar. “You mustn’t have too much water or the gas won’t get to where it needs to be, so you have to get the water out. At the same time though it mustn’t be too dry. “We use what is called an exchange humidifier, which looks like a heat exchanger. The air coming out of the stack is hot and moist [since water is a by-product of the reaction]: we pass that over a membrane surface in the humidifier and bring the cold, dry air in on the other side of the membrane. “That way we exchange both heat and humidity, keeping up the temperature and humidity of the incoming air. “With its passive operation the exchange humidifier is nice and simple. Its operation causes a pressure drop, so it did have to be optimised for the entire system.” On the hydrogen side, the storage at 350 bar is metered down to 1-2 bar by a lightweight regulator before it is fed to the FC, the pressurisation of the tank providing the supply of gas. June/July 2017 | Unmanned Systems Technology The fuel cell module designed to slot into the ScanEagle The hydrogen is liquefied, so you can run it at the pressure required by the needs of the fuel cell, which is about 15-20 psi