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

are similar to batteries in that respect. If you want more energy in a battery, or more power from a fuel cell, you just add more cells until you reach your target capacity.” Fuel and reactant management Energy on board the aircraft is stored as gaseous hydrogen, pressurised to 350 bar and kept in cylindrical tanks. For test operations, the tanks are stored inside the cabin, but in future they are expected to be stored on hardpoints under or over the wings, to minimise the number of modifications to the actual airframe. Customisations of the tanks have been minimal, with most focus here having been on achieving the dimensions needed for the tanks ZeroAvia wanted to meet the needs of its airframe. A Type 4 tank is used in the Piper aircraft. Such tanks combine a carbon composite outer layer (principally providing strength) with a plastic liner that largely serves to prevent leakage. These materials enable a satisfactory gravimetric energy density in terms of how much hydrogen can be stored per kilo of tank weight. “In our first test flight, we had about 1.5 kg of hydrogen on board, of which we used roughly half,” Renz notes. “Ultimately though, we’d like to have up to 15-20 kg of hydrogen, which will enable us to fly 200-300 miles.” He adds however that this topic is something the company is discussing with regulators, as the standardised types of hydrogen tanks available were designed with automotive safety requirements in mind, which could mean needing different designs and certifications. For the hydrogen gas to be used by the PEM fuel cell, its pressure needs to be lowered from 350 bar. During cruise, the required pressure is typically just under 10 bar, although over the cell’s range of operation, 8-12 bar might be needed. To enable this, the gas leaving the tank passes through a pressure regulator to achieve accurate depressurisation, before running through flexible steel tubing towards the cell’s anode. “There are quite a few other systems throughout the hydrogen manifold to ensure safe and accurate pressure regulation,” Renz says. “We have solenoid valves on the fuel tank to control the flow of hydrogen into the pressure regulator, as well as pressure sensors on either side of the regulator.” Placing a sensor after the regulator in the manifold allows the cell’s Julian Renz, head of partnerships and special projects at ZeroAvia. “But there’s a lot of vapourware in electric aviation, a lot of lofty goals and ambitions, and our CEO Val Miftakhov wants to approach our ultimate goal with a practical strategy, which is why we started with fitting our prototype powertrain on a Piper Malibu Mirage. It’s a well-known and certified aircraft, and if we need to certify some of our powertrain components, doing so is easiest when flying an aircraft with less than 20 seats.” Flying a six-seater aircraft of that size typically satisfies regulators that a company is capable of transporting 10- 20 passengers at a time. ZeroAvia therefore plans a stepping- stone approach to its technology demonstrators, both to prove their safety case to certification authorities and to gather real-world testing data on different powertrain components and how best to configure, arrange and run them. The maiden flight (including taxiing, take-off and landing), around ZeroAvia’s UK headquarters in Cranfield, England, was part of the UK government- supported HyFlyer r&d project to decarbonise medium-range small passenger aircraft. The flight lasted about 8 minutes, reaching a peak altitude of 1000 ft and a top speed of 100 knots. Before successfully completing the HyFlyer I project in June 2021, ZeroAvia carried out a series of progressively longer and more advanced flight patterns that established the capability for long-distance flights using hydrogen- only propulsion. “While we’re now working with a 250 kW powertrain, our next demonstrators will use a 600 kW electric engine – with one electric powertrain on a 10-seater aircraft, and two powertrains in wing-mounted nacelles on a 20-seater,” Renz says. “Our powertrain is designed to be modularly scalable; hydrogen fuel cells ;Oe 7iWer 4alibu»s Taiden fliNOt tooR Wlace around Aero(via»s <2 OeadXuarters in CranÄeld ,nNland Autumn 2021 | E-Mobility Engineering 51 Digest | ZeroAvia hydrogen-electric aircraft