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

“Most hydrogen-electric vehicles carry a battery or supercapacitor, as that allows a much faster power response than fuel cells, which are relatively sluggish because they depend on a chemical process to output current. We need to be able to replicate the power curves and ramp rates of turboprops if we’re going to achieve stable hydrogen- electric flight, in particular during take- off.” For thermal management of the cell, a liquid-cooling loop is integrated to enable piping of heat to the end-user’s preferred heat sink (with the cell’s coolant typically exiting the outlet at around 80 C). In ZeroAvia’s case, this was a radiator installed behind the propeller, with the prop’s downwash of air being used to create additional airflow for cooling the radiator. As Renz explains, “Thermal management was something we worked on extensively to ensure we reject as much waste heat as possible. A number of liquid-cooling loops were used to cool the motors, inverters and the cell stack. A water-glycol mix was used for all the electronics but not the cell stack, for which a special low-conductivity cooling liquid recommended by the fuel cell manufacturer was used.” As indicated, electricity from the fuel cell and a battery pack (rated for up to 250 kW power output) runs to the inverters and electrical motors for achieving the necessary ramp rate for flight operations. It also helps to stabilise the supply voltage coming from the fuel cell stack, and some open-cathode PEM fuel cells will periodically be disconnected from their load so that a process called ‘current pulsing’ can be carried out. This consists largely of carrying out short-circuits inside the stack to rejuvenate its humidity and maintain its conductivity and power output during lengthy operations. The inverters were selected to match the power performance of the electric motor system and for compatibility with the input voltage from the battery and the output voltage needed for the motor (the latter of which runs from 400 to 800 V AC). The drive system consists of two motors driving a single output shaft for the propeller; each is rated to 130 kW maximum power, and combined these produce up to 250 kW. Each e-motor is a COTS product supplied by an undisclosed company, although ZeroAvia comments that the supplier traditionally produces systems for the automotive sector, and it chose this motor for its reliability, input voltage, weight and by extension its power-to- weight ratio. It is a permanent magnet synchronous motor, configured as a radial-flux machine with an internal rotor and external stator for power density. “The power of the electric motors directly determines how large of an aircraft you can lift, as well as how many passengers you can transport, while the energy density of the fuel tank is key for how long and how far you can transport them,” Renz says. “Again, that is why a hydrogen-electric powertrain makes so much sense to us for aviation – you’re matching the high energy density of hydrogen gas with the high power of electric machines.” The powertrain (encompassing the fuel cell, battery, inverters and electric motors) is installed inside the nose of the Piper aircraft. The propeller is mounted on the motor shaft, but rather than use the stock propeller, ZeroAvia has installed a COTS electrically actuated variable-pitch system, to maximise the aerodynamic efficiency of the prop blades’ angles of attack across the test aircraft’s flight envelope, through taxiing, take-off, cruise and descent. Hydrogen infrastructure While one of the biggest criticisms levelled at proponents of hydrogen- electric power is the lack of availability of hydrogen gas compared with, say, ZeroAvia’s origins ZeroAvia was founded in November 2017 by Val Miftakhov, who became its CEO in May 2019. He had previously founded eMotorwerks, a company focused on developing EV charging infrastructure, and had served as CEO there for 7 years before taking the reins at ZeroAvia. Outside his work life, Miftakhov has piloted ixed-wing and rotary aircraft, and is familiar with the engineering of such vehicles, including their power and energy density needs. This experience, combined with his professional background and track record, convinced him that commercial aviation could not be decarbonised through batteries alone, and that developing battery- electric planes while waiting for cell technology to advance to the necessary energy density levels was impractical. Through early discussions with his team at ZeroAvia, hydrogen was put forward as the most practical means of realising zero-emissions long-haul lights, especially in terms of the technological readiness and commercial availability of powertrain components, and how easily a hydrogen-electric powertrain could be scaled up from a six-seater aircraft to it something akin to an Airbus A320. A demonstrator for green hydrogen production is used by a mobile fuelling truck with Type 4 hydrogen tanks to refuel the Piper Malibu 54 Autumn 2021 | E-Mobility Engineering