1. Is that even possible?
Theoretically yes, but it’s not a simple engineering challenge. Adopting unproven technologies such as hybrid propulsion is tough for any manufacturer, but at the 2019 Paris Air Show “electrification” of aircraft leaped into the spotlight after Airbus SE floated the possibility of introducing a hybrid version of its best-selling single-aisle passenger jet as soon as 2035. Smaller electric and hybrid aircraft are set to arrive sooner.
The big problem for electric motors is the size and weight of the batteries needed to power them. Current lithium-ion batteries have only about 2% the energy density of liquid jet fuel, and for an airplane, huge batteries are a non-starter. Hybrid technology mitigates that by using conventional fuels and turbines to charge smaller cells. In an airplane such as the potential Airbus model, electric engines could power the short but energy-intensive takeoffs and landings, saving vast amounts of jet fuel. Then, when the plane reaches cruising altitude, it would switch to conventional jet engines that are more efficient during that part of a flight.
3. How does that compare to a Prius?
The Prius uses what’s known as “parallel” hybrid technology, where both an electric motor powered by batteries and a conventional engine powered by gasoline are connected to the power train. They can be used interchangeably, and switched dynamically, depending on demand. There are some prototypes of parallel hybrid airplanes, but many projects use an alternative approach called “serial” hybrid, where wing-mounted engines are connected to batteries that are in turn connected to a turbine in the back of the plane run by kerosene, diesel or even hydrogen. During takeoff and landing, the battery powers the plane on its own, while at cruising altitude the turbine kicks in and recharges the batteries.
4. How soon might electric planes be in the air?
It’s a matter of size. Small robotaxis are poised to take off first, with the first commercial models of autonomous or semi-autonomous electric aircraft scheduled to hit the market in 2020. Small hybrid-electric planes ferrying 10 to 20 passengers could arrive by the middle of the next decade and bigger regional-sized aircraft carrying many as 40 passengers around 2030, according to Stephane Cueille, chief technology officer at Safran SA, a top jet engine maker.
5. What about big passenger planes?
Full-size commercial airliners will need much longer: the Airbus project mooted in Paris will take at least 15 years to reach fruition. The European aerospace giant is working with Rolls-Royce Holdings Plc and Siemens AG on developing a hybrid engine, though that power plant, due to be tested in the next two years, would produce just 2 megawatts of power, far less than what’s needed to propel even a small narrow-body model. (Siemens said on June 18 that it’s selling its share of the venture to Rolls-Royce.) Purely electrical propulsion is much further out, given that batteries are too heavy to achieve the same efficiency as fuel. Safran estimates that full-size battery-powered commercial aircraft are not feasible before 2050 at the earliest.
6. How much would they cut emissions?
That depends. As with cars, the carbon footprint of a hybrid plane depends on the source of its electricity. A serial hybrid plane that powers its battery by burning jet fuel isn’t going to save much in emissions. Using hydrogen, or making sure the batteries are originally charged using renewable energy sources, could make a difference. Even an all-electric plane would only do so much to cut emissions. In a study published inNature in 2018, researchers estimated that if all aircraft on routes shorter than 690 miles (1,110 kilometers) flew purely electrically, it would cut jet fuel use by 15%.
7. Who else is working on this?
EHang Air Mobility Group of China says it will start selling a $300,000 aircraft next year and anticipates strong demand from emergency responders, air taxi services and operators of tourist flights. Germany’s Volocopter GmbH, which is backed by Daimler AG, this year will open a “Voloport” in Singapore for public test flights and expects commercial service as early as 2021. Israel’s Eviation Aircraft Ltd. is planning a first flight later this year in the U.S. of its Alice model, followed by the assembly of more planes in Arizona and Washington state and certification around 2021. Other startups chasing the opportunity include U.S.-based Zunum Aero, MagniX Technologies Pty Ltd and the U.K.’s Faradair.
--With assistance from Benjamin Katz.
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