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

Text:
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?

“This opens the door to a new era in aviation.”

Dr. Frank Anton, Head eAircraft Siemens

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.

(strich:E-Fan X)Initially, just one of this proto­type’s four engines will be replaced with an electric motor. Hover over the image for a bigger view

E-Fan XInitially, just one of this proto­type’s four engines will be replaced with an electric motor.

aeroreport_e-fanx-3d-graphic

E-Fan XInitially, just one of this proto­type’s four engines will be replaced with an electric motor.

(strich:Starling Jet)This proto­­type from start-up Samad Aerospace has a range of up to 2,400 kilo­meters. Hover over the image for a bigger view

Starling JetThis proto­­type from start-up Samad Aerospace has a range of up to 2,400 kilo­meters.

aeroreport_london_starling_jet

Starling JetThis proto­­type from start-up Samad Aerospace has a range of up to 2,400 kilo­meters.

(strich:Zunum Aero)The start-up expects to launch its ­nearly 16-meter hybrid jet on the market in four years’ time. Hover over the image for a bigger view

Zunum AeroThe start-up expects to launch its ­nearly 16-meter hybrid jet on the market in four years’ time.

aeroreport_za10-over-mountains

Zunum AeroThe start-up expects to launch its ­nearly 16-meter hybrid jet on the market in four years’ time.

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.

Slideshow

Booming industry: New start-ups all around the world plan to take the avi­ation busi­ness to new heights with their electric air­craft proto­types.

aeroreport_01_Corey-Combs-Electric

AmpaireWith two versions of its TailWind™ model, the U.S. company is moving into electric flight.

aeroreport_02_eviation-aircraft-alice-prototyp-2017

EviationThe Israeli company focuses on fully electric aircraft and has already developed two prototypes.

aeroreport_03_Samad-10

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.

aeroreport_04_joby

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.

Video: Answers from MTU experts: Will we soon fly using electrical power only? Article with video

Answers from MTU experts: Will we soon fly using electrical power only?

Dr. Jörg Sieber provides his opinion on the matter: Will we soon fly using electrical power only? He furthermore touches upon hybrid-electric flying and the possibilities to further develop conventional gas turbines. To the video ...

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.

“We won’t be able to meet any climate goals, without additional and far-reaching invest­ment in more efficient engines and air­craft systems and also sustainable fuels.”

Dr. Jörg Sieber, Head of Innovation Management, MTU Aero Engines

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.”

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