One of Europe's largest airlines, EasyJet, announced on Wednesday that it is aiming to begin service with electric-powered airplanes within the next decade. EasyJet will be collaborating with an aviation startup called Wright Electric to make this vision a reality.

The companies have ambitious goals: they want to build airplanes with room for 120 and 220 passengers and a range of 335 miles. That's so ambitious, in fact, that I was a little skeptical that anyone should take it seriously.

The fundamental problem is a matter of physics: the energy density of jet fuel is way, way higher than the energy density of batteries. As a result, while a conventional airplane can travel thousands of miles before refueling, electric airplanes can only travel a fraction of that distance before they run out of juice.

Yet there's significant room for improvement in electric airplane technology, argued NASA scientist Sean Clarke in a Thursday email to Ars.

"Electric propulsion systems may be relevant in the marketplace sooner than you might expect, because they can be much more efficient," Clarke told Ars.

Not only is battery performance steadily improving, Clarke argues, but there are ways to improve the performance of electric motors and thereby squeeze more range out of existing battery technology.

And there's another way to capture most of the benefits of electric airplanes while still achieving reasonable range. A Seattle startup called Zunum Aero is developing an electric airplane that combines battery power with a conventional generator. Zunum expects its first airplane, which it's aiming to release in the early 2020s, will have a range of 700 miles—far enough to serve many popular short-haul routes in the United States and around the world.

Electric airplanes push the limits of battery technology

Anna Zvereva

NASA

Zunum

Wright Electric

Jet fuel has a specific energy of 12,000 watt-hours per kilogram, Clarke told Ars. For comparison, battery systems work out to around 200 watt-hours per kilogram. In other words, jet fuel is about 60 times as efficient for storing energy as batteries are. This is somewhat offset by the fact that electric motors are about three times more efficient than jet engines. But that still means that you can go a lot further with a kilogram of jet fuel than you can with a kilogram of batteries.

According to Clarke, one of the best electric airplanes on the market, the Alpha Electro, has a range of around 80 miles. That's a tiny fraction of the range of conventional airplanes powered by jet fuel—and much less than the 335 mile goal EasyJet touted in its press release.

Clarke, a leader of NASA's own experimental X-57 electric airplane project, told Ars that, despite the limitations of battery power, electric airplanes have real promise.

For starters, battery technologies have been improving at around seven percent per year. If that pace of progress continues, batteries will hold about twice as much energy a decade from now as they do today.

Also, there are efficiency gains to be had by re-designing airplanes to work with the strengths of electric motors, which are lighter and more reliable than conventional jet engines.

For example, Clarke plans to move the propellers on the X-57 out to the edges of the wings. "This may improve aircraft efficiency by reducing the drag caused by the vortex that forms at most wingtips. This isn't really feasible with gas burning engines because they would be too heavy mount at the wingtip," he says.

Also, he said "aircraft are required to operate even with a failed engine, which would not be feasible if the only remaining engine is at the wingtip. Electric motors may become so reliable that it isn't credible to have a motor fail entirely."

Hybrid systems can bridge the gap

Jeff Engler is the CEO of Wright Electric, the startup EasyJet says will provide it with electric passenger planes within the next decade. In a Thursday phone interview with Ars, Engler readily acknowledged that the projected range of 335 miles was beyond the capabilities of today's lithium-ion batteries and that the lithium-ion battery technology may not improve quickly enough to achieve the goal within the next decade.

The company is still in the early phases of designing its aircraft, and Engler says the company is considering a number of alternative approaches. "We're looking at other battery technologies: lithium sulphur, aluminum air, and fuel cells," he told Ars.

Wright Electric is also looking into the hybrid approach, Engler said, using "turbine engines as a range extender." One advantage of this approach, he pointed out, is that airplanes always need extra fuel to provide a margin for safety. Extra fuel is much lighter than extra batteries, and the spare fuel doesn't get burned on most flights, minimizing its environmental impact.

Zunum Aero is planning to build a hybrid plane, and that allows it to offer longer range than EasyJet is touting.

"Our plane should fly a 700-mile range in the early 2020s, out to 1000 miles by 2030," Zunum chief marketer Sandi Hwang Adam told Ars in a Friday phone interview.

Zunum plans to start small by building airplanes designed for 10 to 50 passengers. Adam said the electric propulsion system should reduce noise by 75 percent compared with conventional airplanes. And carbon emissions could be reduced by as much as 80 percent.

"We also designed the aircraft so it's battery technology agnostic," she told Ars. In the early years, the aircraft will depend heavily on supplementary power from the onboard generator. However, she said, "you can easily swap out batteries." As batteries with higher energy densities become available, airlines will be able to swap them into existing airplanes, allowing them to draw more power from the batteries and less power from burning fossil fuels.

Electric airplanes won't replace conventional airplanes any time soon. The huge gap in energy density means that long-range flights will remain the domain of conventional airplanes for the foreseeable future. But Adams, Clarke, and Engler all envision a future when short-range flights—those measured in hundreds rather than thousands of miles—are handled by hybrid airplanes and eventually by purely electric ones. Those short flights account for a significant share of overall air travel. Electric airplanes have the potential to make these flights quieter, more efficient, and better for the environment.

Update: A reader pointed us to this 13-part series on electric airplanes by industry analyst Bjorn Fehrm. He was a bit more pessimistic than the NASA expert I talked to for this story. You can read his conclusions in the final installment of the series. Fehrm finds that batteries are simply too heavy for all-battery airliners to be practical within the next decade. Hybrids are more promising, he says, but they're unlikely to be economical within the next decade.

However, much depends on the pace of improvement in battery energy density. As battery densities increase, hybrid systems become more efficient because the battery can supply more power during takeoff, allowing the aircraft to get by with a smaller and lighter generator.So if battery technology improves faster than expected—and the billions pouring into the electric car business might speed up battery development—then hybrid airplanes might become feasible relatively soon.

What seems clear is that electric power will start with the shortest routes and gradually move up-market as battery technology improves. Electric power might open up opportunities for small vertical take-off airplanes flying intra-city routes before they start becoming viable for the shortest conventional commercial routes. It will be a long time before electric airplanes will be able to compete with conventional airplanes on longer routes.