Moteurs hybrides

Rappel: et pour le Pipistrel Pantera (à moteur hybride)

Source: The Economist

Science and technology-Commercial aviation
The electric-flight plan
Electrifying aircraft is tricky. But companies are getting serious about trying

ELECTRIC cars are clean, quiet and, it seems, the way of the future. Tesla, an American firm that has done much to help electric cars shed their museli-munching image, is struggling to meet demand for its mid-market Model 3 (though that has not stopped it announcing plans to build electric lorries as well). Volvo, a Swedish carmaker, has said that, from 2019, all its cars will be at least part-electric. Volkswagen has plans to offer battery options across all of its brands; General Motors has made similar noises. Some countries, including China, Britain and France, are mooting bans on internal-combustion vehicles, to take effect within a couple of decades.

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. Kilo-for-kilo, 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 of all shapes and sizes. 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 (Uber is keen on these). 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 on November 28th, when Airbus announced something more ambitious. 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 invention of the jet engine.

Insufficient, currently
The general view in the industry is that battery technology is not yet up to building fully-electric airliners. But just as hybrid arrangements help to extend the range of some electric cars, so hybrid systems will bring electric aircraft closer to take-off.

The Airbus team plans to modify a BAE 146, which is a 100-seat regional airliner powered by four conventional jet engines (see illustration above). 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 hope, will provide enough data to design a full-on hybrid-electric airliner with 50-100 seats from scratch. Such a plane might enter service in 2030 or so.

Other planes could be in the air before then. Zunum Aero, a startup based near Seattle, hopes to have its 12-seat hybrid-electric airliner ready to fly its first passengers by 2022, helped along by investment from Boeing, an American aerospace giant, and JetBlue, a successful airline.

Such aircraft will, their designers hope, serve as bridges to fully electric planes in the future. Overcoming the weight problem will be tricky. For big planes flying long-haul routes, full electrification may never happen, although hybrid systems would reduce their 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 very 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% efficiency during a steady cruise at the ideal altitude. But that number could fall by half or more 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% efficient, so the batteries that power them 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, which is a big cost in aviation.

Those are all reasons why Zunum plans to focus, at least at first, on relatively short routes, where the efficiency gains from electric motors are most significant. The idea, says Ashish Kumar, the firm’s chief executive, is to serve the hundreds of small American airports that have been left behind as flights have shifted to bigger hubs. The firm’s aircraft will cruise at around 550kph (340mph) and have a range of about 1,130km (700 miles). Like the Airbus machine, it would use a single small jet engine in the rear fuselage to run a generator that could power the plane’s two 500kW fans and top up the batteries, which will be mounted in the wings and designed to be swapped in for fresh ones after landing. This way, at some airports, the turnaround time could be as low as ten minutes.

The aircraft’s range, says Dr Kumar, should increase over time. For batteries have another advantage over fossil fuels: as a relatively underdeveloped technology, they still have plenty of room left for improvement. As production ramps up, led by the car and electronics industries, battery capacities are increasing and prices are falling.

This week, for instance, Samsung Electronics, a big South Korean firm, said that by incorporating graphene—an ultra-thin form of carbon—into a lithium-ion battery, it had managed to boost its energy capacity by 45% and greatly decrease the time needed for a recharge. Many other 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, thinks Dr Kumar, would allow his aircraft to increase its range to around 2,400km by 2035, and perhaps even ditch the on-board generator.

Combining all these benefits and drawbacks into a single figure is tricky. Paul Eremenko, Airbus’s chief technology officer, says a single-aisle hybrid electric airliner would be “safe, efficient and cost-effective”. Zunum’s Dr Kumar is prepared to go further and talk numbers. For airlines, the important figure is the CASM—cost per available seat mile. This is obtained by dividing operating costs by capacity, measured as the number of seats in an aircraft multiplied by miles flown. Zunum claims that its plane will have a CASM of 8 cents. Oliver Wyman, a firm of aviation analysts, reckons that the average for American airlines in 2016 was 11 cents.

And like electric cars, electric aircraft would offer other benefits that are worth having, but harder to quantify. They would be quieter than jet-powered planes, which may be attractive for airports near big cities. They would be cleaner too, and become more so as more electricity is produced from low-carbon sources. Sceptics doubt the weight problem can ever be properly overcome; cynics suspect that these projects are motivated by PR. But few people predicted the pace of electrification in other areas. If electric aircraft can offer faster door-to-door journeys avoiding crowded hub airports and provide cheaper fares at the same time, then air travellers will be happy for the sparks to fly.