Electric cars are clean, quiet and, it seems, the future.
But not all forms of transport are so easy to electrify. One of the hardest is aviation, where battery power runs up against a serious problem: weight.
By weight, fossil fuels contain roughly 100 times as much energy as a lithium-ion battery. On the road, that is a problem which can be designed around. For a machine that must lift itself into the sky, it is much harder to solve.
But it is not impossible. Dozens of firms are working on electrically powered planes. Some resemble flying cars, such as those which Larry Page, one of Google's founders, is backing. Others are hovering, drone-like machines that could operate as autonomous aerial taxis. Pipistrel, a Slovenian company, already makes a two-seater electric training plane. Another two-seater, the E-fan, has been flown by Airbus, a European aviation giant, although it recently abandoned the project.
The reason for that became clear Nov. 28, when Airbus announced it has teamed up with Rolls-Royce, a British jet-engine producer, and Siemens, a German electricals group, to convert a small airliner into a "flying test bed" to prove the feasibility of hybrid-electric propulsion.
"We are entering a new world of aviation," said Frank Anton, head of Siemens eAircraft. Electric power, he predicted, would prove to be as significant to commercial aviation as the jet engine.
The general industry view is that battery technology is not yet up to building fully electric airliners. But just as hybrids help to extend the range of some electric cars, so hybrid systems will bring electric aircraft closer to takeoff.
The Airbus team plans to modify a BAE 146, which is a 100-seat regional airliner powered by four conventional jet engines. The first step will be to replace one of those engines with a 2MW electric unit, consisting of a fan contained in a shroud. As with a hybrid car, the fan will be powered by a combination of a battery and a range-extender, in the form of a small jet engine mounted in the rear of the fuselage and hooked up to a generator. This range extender can be switched on during parts of the flight to power the fans or to top up the battery. Because it can be run at its most efficient speed all the time, unlike a jet directly propelling a plane, it would be highly fuel-efficient.
Flight tests are due to begin in 2020. If they are successful, a second engine on the aircraft will be replaced. The results, the team hopes, will provide enough data to design a full-on hybrid-electric airliner with 50-100 seats. Such a plane might enter service in 2030 or so.
Such aircraft will, their designers hope, serve as bridges to fully electric planes. Overcoming the weight problem will be tricky. For big planes flying long-haul routes, full electrification may never happen, although hybrid systems would reduce fuel consumption.
But design changes can help. Airbus, for instance, thinks it can blend its electric motors into the aircraft's fuselage to reduce drag.
And electric power offers some advantages that offset its big drawback. One is that combustion engines are not efficient at turning the energy in their fuel into motion. Instead, a great deal of it ends up wasted as heat. A jet engine might manage around 55 percent efficiency during a steady cruise at the ideal altitude. But that number could fall by half when taking off, climbing, landing and taxiing on the ground, which is what aircraft that fly short routes spend much of their time doing.
An electric motor can do much better. The latest models are more than 95 percent efficient, so the batteries would not need to match the energy density of jet fuel. Electric motors are also lighter than jet engines, which helps offset some of the weight disadvantage. And they contain far fewer parts, which means they require less maintenance.
Those are all reasons why Zunum plans to focus, at least at first, on shorter routes, where the efficiency gains from electric motors are most significant.
The aircraft's range, said Zunum CEO Ashish Kumar, should increase over time. For batteries have another advantage over fossil fuels: They still have plenty of room for improvement. As production ramps up, led by the car and electronics industries, battery capacities are increasing and prices are falling.
Many new battery chemistries are being developed. One promising idea is a solid-state lithium battery, which replaces the liquid electrolyte of current cells with a solid substitute. Besides offering much higher energy densities, such batteries should also be cheap to mass produce. Those trends, Kumar thinks, would allow his aircraft to increase its range to around 1,500 miles by 2035.