Electric aircraft: first hybrid, then fully electric?
Electric and hybrid propulsion systems are set to revolutionize aviation. But for now, large passenger aircraft will continue to rely on conventional engine technology.
11.2018 | Text: Dennis Dilba
Dennis Dilba holds a degree in mechatronics, is a graduate of the German School of Journalism, and founded the “Substanz” digital science magazine. He writes articles about a wide variety of technical and business themes.
Munich central station in the year 2037:
Below ground, the new, ultrafast regional trains come and go at five-minute intervals. Up on the station’s flight deck, electro-hybrid jets with a range of up to 1,000 kilometers serve destinations in Germany and neighboring countries. Meanwhile, at Munich’s airport to the northeast of the city, the only connections on offer are long-haul routes to European and transcontinental destinations. Is this science fiction, or something not far from reality?
Flying is getting more popular all the time. And according to virtually every forecast, this trend is set to continue in the years to come. For instance, in its Global Market Forecast 2018, Airbus expects there to be a demand for 37,390 new aircraft over the next 20 years. The European aviation heavyweight predicts that by the end of the year 2037, the current global fleet of 21,453 aircraft will grow to at least 48,800-in other words, it will more than double.
On the one hand, rapid growth is good for business; on the other it represents a massive challenge for the industry and a huge responsibility: without significantly more efficient technologies, aviation’s share of global CO2 emissions will quadruple from the current two percent to more than eight percent by the middle of the century. The electrification of aircraft is one answer to this development, says Dr. Frank Anton, head of the Siemens eAircraft division.
E-Fan XInitially, just one of this prototype’s four engines will be replaced with an electric motor.
E-Fan XInitially, just one of this prototype’s four engines will be replaced with an electric motor.
Hybrid-electric prototype already under construction
However, the 62-year-old physicist isn’t thinking about purely electric aircraft: together with partners such as Airbus, he wants to install a production hybrid-electric powertrain in a 100-seater regional aircraft and perform a test flight. Dubbed “E-Fan X,” this is the largest electro project for commercial aircraft to date. “This opens the door to a new era in aviation,” Anton is sure. In this “Toyota Prius of the skies,” a kerosene-powered gas turbine in the fuselage drives an electric generator. The turbine can continuously operate within its ideal speed range, which saves fuel. Electricity from the generator powers the electric motors for the rotors. In this way, the necessary thrust can be distributed among several small electric motors driving propellers on the wings or tail. This in turn dictates new forms in aircraft design and should lead to improved aerodynamics.
Depending on safety requirements and on where the aircraft are to be used, such hybrid-electric aircraft would be fitted with a larger or smaller additional battery, Anton says. “The turbine and the generator can be made large enough that they produce sufficient electricity for cruising flight. During the energy-intensive takeoff and ascent, electricity from the batteries provides extra thrust.” Fitting a slightly larger turbine would make it possible to recharge the virtually empty batteries in flight. The hybrid technology also makes an additional, elegant charging option possible: during descent, the air current can drive the propellers and hence the electric motors, which then operate as generators to charge the batteries-similar to how a hybrid car recuperates braking energy on downward gradients.
The world’s first production hybrid concept for aircraft was unveiled in 2011. Anton was one of the people who worked on the redesigned two-seater DA36 E-Star power glider made by Austrian manufacturer Diamond Aircraft. Now, the E-Fan X is to provide an opportunity to more closely examine the potential for using hybrid propulsion in larger aircraft. This starts with replacing just one of the four engines on the BAe 146 test aircraft with a two-megawatt electric motor. Anton says this is enough to test efficiency. A second hybrid unit will be added after a successful first test. “We expect to see significant fuel savings in the double-digit percentage range as well as a massive reduction in noise.” A passionate pilot and flying instructor, Anton predicts that by the year 2035, hybrid-electric aircraft will be transporting up to 100 passengers over distances of 500 to 1,000 kilometers.
Booming industry: New start-ups all around the world plan to take the aviation business to new heights with their electric aircraft prototypes.
AmpaireWith two versions of its TailWind™ model, the U.S. company is moving into electric flight.
EviationThe Israeli company focuses on fully electric aircraft and has already developed two prototypes.
SamadBased in the UK, the company develops prototypes for hybrid and fully electric aircraft. Starling Jet, its hybrid prototype, is scheduled to go into production in 2024.
Joby AviationEngineers at the U.S. company are developing the first prototype of an eVTOL—an electric aircraft that can take off and land vertically.
The need for at least a 20 percent drop in fuel consumption
It will be interesting to see whether aircraft with such propulsion systems can offer a genuine economic advantage, says Prof. Mirko Hornung, Executive Director Research and Technology at research institution Bauhaus Luftfahrt in Munich, who has been looking into the potential of hybrid aircraft for years. It’s not as if the aviation industry has been idle up until now, says Hornung: billions are invested every year in developing more efficient technologies. “On average, each new generation of aircraft uses 15 percent less kerosene than the previous one,” Hornung says. “Hybrid aircraft concepts must offer fuel savings of at least 20 percent, and ideally more.” Hornung’s experience tells him that the final efficiency payoff is always a few percentage points short of the initial estimate. And that’s the danger: “If hybrid aircraft can’t deliver at least 15 percent, no airline will buy them.”
Hornung warns against too much hype for hybrid aircraft: giving them electric motors, batteries and gas turbines with generators effectively means giving them the power they need three times over, which will make them heavier. “Can the aerodynamic advantages of distributed electric motors really offset a weight drawback of this scale? There is as yet virtually no reliable data on this,” Hornung says. A project such as E-Fan X, which lays bare the fundamental problems of hybrid propulsion and brings greater certainty to the discussion, is in his view the right way to go.
Inside MTUDevelopment of hybrid-electric powertrain
Together with Siemens, Swiss aerospace company RUAG and the German Aerospace Center (DLR), MTU Aero Engines intends to venture into development of hybrid-electric powertrains. A Dornier Do 228 is to serve as tester for exploring the potential; a hybrid-electric version is to take to the skies in 2021. The four partners are hoping to test the electric propulsion systems with a short purely electric flight in 2020, and together they have the necessary industrial and scientific expertise for the job.
Hybrid jet for regional routes by 2022?
Ask Zunum Aero, however, and it seems that all questions have essentially been answered: the U.S. start-up from Kirkland, a suburb of Seattle, is aiming to get its hybrid jet for 12 passengers onto the market by 2022. Measuring almost 16 meters in length, this small aircraft’s two 500-kilowatt electric motors should allow it to reach a speed of 550 kilometers per hour and give it a range of 1,130 kilometers.
Through fuel savings and lower maintenance costs for the more straightforwardly installed electric motors, Zunum expects a 40 to 80 percent reduction in operating costs and a 75 percent drop in noise compared to conventional aircraft. “This would get around the ban on night flights, which would make operation more profitable,” says Zunum’s head of marketing Sandi Adam. The start-up’s big break is the significant cost advantage it hopes to achieve. “In the United States, many of the aircraft in use for regional routes still use entirely inefficient technology from the 1960s,” Adam says. The old planes are burning money, which is why the airlines are open to the notion of replacing their fleets. California-based charter airline JetSuite actually announced at the end of May that it intends to purchase up to 100 of Zunum’s small hybrid aircraft. Adam also sees good sales opportunities in many other countries, estimating the overall size of the global regional aircraft market at a trillion USD.
No doubt this figure is also what won over Boeing’s venture capital division, Horizon X: together with JetBlue Technology Ventures, Boeing bought a stake in the start-up almost a year ago. But no details have been released as to the extent of the investment or the battery technology being used. The latter in particular gives experts like Hornung cause to doubt that the version that is scheduled to take off four years from now can be anything more than a prototype. Zunum’s CEO Ashish Kumar is very optimistic and also sure that we can expect great advances in batteries in the future. He foresees that by the year 2035, his hybrid jet will have a range of some 2,400 kilometers. “Perhaps we will be able to do away with the gas turbines and the power generator altogether,” Kumar says. Dr. Jörg Sieber, who is in charge of innovation management at MTU Aero Engines, is skeptical: although batteries are getting better bit by bit, there’s been no discernable breakthrough in the technology.
The industry is changing
“For flight operations, batteries must be at least five to ten times more powerful than they are today,” Sieber says. He is currently working with partners including Bauhaus Luftfahrt and Airbus to test the pros and cons of electric and hybrid-electric flight. “Of course we want to know when the technologies are ready for market, so that we are then in a position to offer the relevant propulsion systems.” Sieber agrees that electrification offers new freedoms in aircraft design, but he feels that at present, purely electric propulsion is an option only for lightweight motor gliders, sport aircraft or short-range air taxis. As for an electro-jet for 180 passengers with a range of 540 kilometers, which the California start-up Wright Electric has promised low-cost airline easyJet for 2027, Sieber doesn’t think it will happen. He also considers the schedule set by Norway’s state-owned airport operator Avinor to be vastly ambitious: from the year 2040, all domestic flights are to be purely electric. Nevertheless, the coming year will see the first electric aircraft prototypes take to the skies, Sieber says.
This has been announced by a host of start-ups, including Eviation from Israel, Ampaire and Joby Aviation from the United States and Samad Aerospace from the UK. Exactly when their small electric aircraft get off the ground and how far they then fly remains to be seen, says Sieber. “But we already know today that the road to commercial applications is a very long one-even for hybrid-electric flight. When it comes to aircraft the size of the Airbus A320, we’ll still be seeing traditional gas turbines even in the year 2050,” Sieber says. But they will be much more efficient: “For the geared turbofan, which we have just launched on the market, further refinements are bound to yield improvements in fuel consumption of 10 to 15 percent,” Sieber says. Further efficiency improvements for air-breathing engines put new kinds of cycle processes on the horizon, such as the composite cycle. This pioneering concept uses piston machinery and allows for superb pressure ratios of over 300, compared to a mere 60 for the latest turbofan engines. The result: 15 percent lower fuel consumption and a 10 percent drop in NOx emissions.
Sieber says that everyone in the industry knows increasing electrification will change aviation over the coming decades. “That’s exactly why we’re closely monitoring the developments and preparing for them.”
But there’s no reason to expect the aviation industry’s challenges to be resolved overnight. “We won’t be able to meet any climate goals,” Sieber says, “without additional and far-reaching investment in more efficient engines and aircraft systems and also sustainable fuels.”