ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

management system to confirm that the regulator is working correctly. If hydrogen is heading towards the cell at excess pressure, a check valve can shut off the flow from the lines, or a safety valve on the cell stack can purge any pressure build-up inside the system. Meanwhile, a pressure sensor in the tank gauges how much fuel is left inside the hydrogen tank, or detects if the pressure drops too quickly, as that can indicate a leak. “We also have a vent line for the tank and hydrogen manifold section,” Renz adds. “That’s there to ensure that if there ever is a leak, it gets vented in a dedicated spot, to safely lead the hydrogen outside the aircraft and prevent an unsafe build-up of gas from occurring inside the fuselage or nacelle.” Oxygen is supplied to the cathode using the ambient air outside the aircraft. A compressor is used to draw in the air and pressurise it to provide a mass flow about twice that of the hydrogen going to the anode side before it enters the fuel cell. Given that at the cruising altitude of 10,000 ft the ambient air pressure drops to 0.7 atmospheres, it could be assumed that the air compressor needs to work harder to provide the same mass of airflow. However, given that the power requirement (and hence the required amount of oxygen) at altitude during cruise is also about 70% of that needed during take-off, the compressor tends to work fairly similarly across the aircraft’s full range of operations. Hydrogen-electric powertrain ZeroAvia has partnered with Swedish company PowerCell for the supply of hydrogen fuel cells for this project. In the case of the Piper Malibu test aircraft, the powerplant used was a PowerCell MS-100 cell (a product since rebranded as the Power Generation System 100), which has an expected lifetime of 20,000 hours and can be operated in temperatures from -30 to +45 C. This standard fuel cell system outputs up to 100 kW net power, weighs 170 kg and measures 696 x 674 x 606 mm. Power is typically supplied over a 300 V bus (up to a maximum of 570 V DC, and down to a minimum of 250 V) at a rated current of 375 A, with 420 A as the maximum current and 60 A as the minimum. The cell’s rated efficiency at 100 kW output is 45%. At this power output, roughly 122 kW of heat is produced. System efficiency at between 40 and 60 kW power output reaches about 58%. “There were many reasons for choosing this fuel cell, but most important was the power density, particularly the ramp rate – how quickly it can react to a change in load,” Renz says. PEM fuel cell operation Electricity in the ZeroAvia powertrain is generated by a proton exchange membrane or polymer electrolyte membrane (PEM) fuel cell. Current is produced following an electrochemical reaction of hydrogen and oxygen, which takes place inside the membrane electrode assembly (MEA) at the core of each cell structure. The MEA consists of a semi-permeable PEM layer, sandwiched between an anode and a cathode layer, which serve to di use the reactant gases. Each of the latter two layers also has a catalyst layer embedded in it, typically platinum or a similar substance, which plays a key role in the electrochemical reaction. Hydrogen gas lows into the PEM’s anode sides, while oxygen lows into the cathode sides, each typically being distributed via a low ield installed into two plates that enclose each cell to maintain pressure. The catalyst layer splits the hydrogen gas molecules at the anode into protons and electrons. The protons pass through the central semi-permeable membrane before reacting with the oxygen molecules on the cathode side to form water that leaves the cell via the exhaust. The electrons low along a circuit, which produces an electric current from the cell stack. Also installed around the cell are a number of sensors and other systems to ensure the balance of heat and humidity (for ionic conductivity of the membrane) as well as reactant low rate and pressure (corresponding to the required current) inside the stack of cells are correct. There are additional control systems for managing the power output and by extension the rate at which reactants are consumed; collectively these systems are referred to as the balance of plant. A 100 kW PEM fuel cell provided the power generation in ZeroAvia’s Piper Malibu, although the powertrain concept can be scaled up modularly as needed 52 Autumn 2021 | E-Mobility Engineering Digest | ZeroAvia hydrogen-electric aircraft