The aviation industry is looking to make flying greener. One idea that’s picking up speed: hybrid airplanes.
Electric planes have attracted attention and investment. But planes are harder to fully electrify than cars. The weight of a battery is a bigger challenge if you’re looking to get off the ground—the batteries needed for planes are over 40 times as heavy as the equivalent amount of fossil fuel—and range limitations pose severe problems. While pulling off the road to recharge a car is relatively easy, landing a plane to periodically juice up would make air travel unbearable.
Over the long term, the aviation industry is exploring other solutions, like biofuels or hydrogen. But some aviation companies are betting on a nearer-term approach: hybrid systems that combine electric batteries and traditional fuel, allowing planes to fly more miles more cleanly. The need for a solution is especially pointed since the aviation industry—responsible for about 2% of greenhouse-gas emissions—has set a goal of zero emissions by 2050.
For the most part, these hybrid planes either have electric and combustion engines alongside each other, or combine them both into one. Some companies envision using the technology for 100-passenger planes that can fly up to 1,000 miles, though not for decades. More are working on getting planes with from nine to 50 passengers to market before the end of the decade. This method could cut from about 5% to 50% of emissions, compared with traditional planes of similar size.
“If the focus is decarbonizing aviation, electricity [from batteries only] is definitely not going to be a big player. Hybrid-electric could be a bigger player,” says Jayant Mukhopadhaya, senior researcher for aviation at the nonprofit International Council on Clean Transportation.
To give an idea of the limitations of battery power, the companies trying to make all-electric work generally limit their designs to smaller airplanes that travel fairly short distances. For one, there is electric-plane company Joby Aviation JOBY 4.05%increase; green up pointing triangle. Joby’s plane, which can take off and land vertically like a drone to get out of tight spaces, can handle four passengers up to about 100 miles. The company, along with rivals such as Archer Aviation, aims to launch electric vertical takeoff and landing (eVTOL) taxi services in several U.S. and international cities within about two years, with typical routes up to 50 miles.
Here’s a look at some of the hybrid technologies that might be taking to the air in years to come.
Two types of engines, side by side
In the simplest hybrid approach, planes use kerosene-powered gas turbines running side by side with electric motors.
Sweden’s Heart Aerospace is developing a 30-passenger plane, the ES-30, with two traditional turboprop engines and two all-electric engines driving propellers. The plane runs solely on electric power for as long as possible, then the combustion engines take over. The aircraft could fly up to 124 miles on electric power, says co-founder and CEO Anders Forslund. Running the combustion engines along with electric power could extend the range to about 500 miles.
Fuel savings would be as high as 95% on shorter flights; on the longest, they would dip to 34% because of the increased use of the gas-turbine engines, according to the company.
NASA is testing a similar type of combination, fitting two electric motors from a company called MagniX with two gas-turbine engines on a 50-seat De Havilland Dash 7 plane, the agency says.
All-in-one engines
Another version of the hybrid approach combines combustion and electric power into the same engine—a bit like basic hybrid cars. In essence, the electric power only kicks in when the engines need it most, such as taking off or increasing speed in flight, instead of helping to power the plane for the entire flight. The rest of the time, the plane relies on a smaller and more-efficient conventional engine that is optimized for cruising.
That is the strategy that aerospace company Ampaire is using. Co-founder and CEO Kevin Noertker expects fuel savings of roughly 50% on average. Ampaire will start by retrofitting popular models of planes that have long been in service. It aims to get a modified nine-seat Cessna Grand Caravan to customers within two years and a 19-seat De Havilland Canada DHC-6 Twin Otter around a year afterward. NASA is testing the all-in-one strategy, too, retrofitting a roughly 30-passenger Saab 340 plane by replacing one of its two turboprop engines with a GE GE 0.99%increase; green up pointing triangle Aerospace hybrid engine.
The biggest jet-engine makers, such as GE Aerospace and RTX RTX 1.20%increase; green up pointing triangle, are also working on hybrid models. But, in those cases, the electric motors would provide minor assistance, such as for taxiing. Bio-based synthetic fuel is the main strategy for further greening large jet engines in the future.
Batteries and a generator
Another take on hybrids—known as the series hybrid—uses batteries or a generator powered by a combustion engine inside the plane to run electric motors. The aircraft has the option to run on either.
Whisper Aero is developing a 19-passenger plane with this technology, with a range of 230 miles on battery alone, and 500 miles if the generator is used. That’s for 2034. The company plans to retrofit a traditional nine-seat model to get to market in five years. It recently released a concept for a series hybrid electric jet that could handle 100 passengers and over 700 miles. But that is at least 20 years away.
Series hybrids open up an intriguing possibility for hybrid airplanes: distributed electric propulsion. Because the propellers or jet engines are all run on relatively small and light electric motors, planes can have many of them placed all over the wings instead of just one or two in total. Planes can use this array of motors to improve airflow, for instance. Whisper plans to have between 22 and 30 small electric jet engines on its first plane.
Distributed electric propulsion is essential to series hybrid planes from Electra. The company’s test plane uses eight propellers to blow air over the wings, simulating the lift it would get from barreling down the runway. This “blown lift” method lets it take off after just 150 feet (and at a slower speed), versus thousands of runway feet with traditional engines, says founder and CEO John Langford. He bills it as a way to get in and out of tight spots—like a vertical takeoff and landing system—but without the heavy power drain of vertical flight.
The Electra plane uses batteries only when extra power is required, such as takeoff, allowing it to run a smaller combustion engine-generator combo than would otherwise be needed, for promised emission savings of 40%.
In the coming months, Electra will unveil the design of its nine-seat production-level plane—with a range of 500 miles—which it aims to deliver to customers in 2028.
https://www.wsj.com/science/environment/hybrid-electric-airplanes-9ba2ad22