Hydrogen-electric drives for aviation

Effi­cient, safe and thus com­mer­cial­ly viable hybrid-elec­tric fly­ing : This is the goal of the EnaBle research net­work, which is being fund­ed by the Ger­man Fed­er­al Min­istry for Eco­nom­ic Affairs and Ener­gy with a total of 8 mil­lion euros. The aim is to fur­ther devel­op and opti­mize a high­ly inno­v­a­tive hybrid propul­sion sys­tem for avi­a­tion that com­bines fuel cells and bat­tery sys­tems. At its heart is a 250 kW elec­tric pow­er­train mod­ule using com­pressed air-fed fuel cells. The Uni­ver­si­ty of Ulm is also involved in the project, which is being led by Diehl Aerospace.

Air traf­fic must become clean­er and qui­eter in the future. To achieve this, high­ly inno­v­a­tive solu­tions for envi­ron­men­tal­ly friend­ly air­craft engines are need­ed. Hybrid sys­tems that com­bine fuel cells and bat­ter­ies are par­tic­u­lar­ly promis­ing here. Not only do they achieve sig­nif­i­cant­ly high­er ranges than pure elec­tric air­craft, they also offer the tech­no­log­i­cal poten­tial for upscal­ing to larg­er pow­er class­es. To accel­er­ate the path to indus­tri­al pro­duc­tion and com­mer­cial exploita­tion of this sophis­ti­cat­ed tech­nol­o­gy, the Ger­man Fed­er­al Min­istry for Eco­nom­ic Affairs and Ener­gy (BMWi) is fund­ing the EnaBle research con­sor­tium with 8 mil­lion euros. The con­sor­tium includes Diehl Aero­space and MTU Aero Engines, two lead­ing indus­tri­al com­pa­nies from the aero­space sec­tor, as well as the Ger­man Aero­space Cen­ter (DLR), the DLR spin-off H2Fly and the Uni­ver­si­ty of Ulm.

“We are work­ing togeth­er to devel­op a hybrid elec­tric propul­sion sys­tem con­sist­ing of a fuel cell, bat­tery, pow­er elec­tron­ics and pow­er man­age­ment sys­tem. The spe­cif­ic goal we want to achieve in the process is time­ly indus­tri­al imple­men­ta­tion for light motor­ized air­craft with up to 19 seats.”

- Ron­ny A. Knepple

The engi­neer is respon­si­ble for the pow­er sys­tems divi­sion at Diehl Aero­space. The com­pa­ny, which coor­di­nates the EnaBle research alliance, is a tech­nol­o­gy leader for avion­ics sys­tems and a spe­cial­ist for cock­pit equipment.
How do such hybrid sys­tems actu­al­ly work? “The fuel cell pro­duces elec­tric­i­ty from hydro­gen and thus pro­vides the ener­gy basis for the pro­peller dri­ve. Lithi­um-ion bat­ter­ies pro­vide addi­tion­al pow­er need­ed to reach cruis­ing alti­tude dur­ing take­off or climb,” says Dr. Car­o­line Willich, a sci­en­tist at the Insti­tute for Ener­gy Con­ver­sion and Stor­age at Ulm University.

The engi­neer is lead­ing the Ulm sub­pro­jects togeth­er with her insti­tute col­league Dr. Chris­tiane Bauer. Among oth­er things, the air sup­ply mod­ule for the fuel cells will be devel­oped at Ulm Uni­ver­si­ty. “The fuel cells that are used here are oper­at­ed with com­pressed air. Pres­sur­ized charg­ing makes the fuel cells more effi­cient and enables high­er per­for­mance. This is of par­tic­u­lar inter­est in air­craft, because they move at high alti­tudes and thus in the neg­a­tive pres­sure range,” Willich explains.

The Ulm-based com­pa­ny is also respon­si­ble for devel­op­ing and opti­miz­ing the pow­er man­age­ment sys­tem. This must ensure pre­cise­ly, quick­ly and fail-safe­ly that the bat­tery pro­vides addi­tion­al ener­gy for the propul­sion sys­tem when pow­er require­ments are high and can be recharged dur­ing the flight. The pow­er man­age­ment sys­tem should be able to respond pre­cise­ly and appli­ca­tion-ori­ent­ed to the require­ments of dif­fer­ent flight pro­files. A very spe­cial unique sell­ing point at the fuel cell research site in Ulm is a test stand that is inte­grat­ed into a cli­mate-con­trolled neg­a­tive pres­sure cham­ber. This allows entire pow­er­train sys­tems to be char­ac­ter­ized and test­ed under real­is­tic, flight-rel­e­vant conditions.

Modularization increases scalability and facilitates maintenance and repair

A very cen­tral aspect of pow­er­train devel­op­ment is mod­u­lar­iza­tion. On the one hand, the col­lab­o­ra­tive part­ners want to use it to increase the scal­a­bil­i­ty of the sys­tem, which is ulti­mate­ly cru­cial for a pro­to­type to be able to go into series pro­duc­tion on an indus­tri­al scale. On the oth­er hand, a mod­u­lar devel­op­ment con­cept also favors fault detec­tion and rec­ti­fi­ca­tion, thus mak­ing main­te­nance and repair eas­i­er, which in turn leads to greater safe­ty. Hard­ware and soft­ware must be opti­mal­ly coor­di­nat­ed for this.

A deci­sive fac­tor for the suc­cess of the project is not least the gener­ic com­put­er plat­form to be devel­oped and deployed with­in the scope of EnaBle, includ­ing com­pre­hen­sive con­trol and reg­u­la­tion algo­rithms that are to ensure effi­cient and smooth oper­a­tion of the pow­er­train. For this pur­pose, Diehl Aero­space pro­vides a so-called Inte­grat­ed Mod­u­lar Avion­ics (IMA). The abbre­vi­a­tion refers to a mod­u­lar com­put­er-based elec­tron­ic unit con­sist­ing of stan­dard­ized com­po­nents and inter­faces, which ensures that the var­i­ous sys­tems in the air­craft can com­mu­ni­cate with each other.

The Insti­tute of Tech­ni­cal Ther­mo­dy­nam­ics at DLR is specif­i­cal­ly respon­si­ble for devel­op­ing the fuel cell and bat­tery sys­tem. At the Uni­ver­si­ty of Ulm — as already described — the focus is on the air sup­ply mod­ule for the com­pressed-air fuel cell, fail-safe pow­er man­age­ment and test­ing of the new hybrid pow­er­train in the uni­ver­si­ty’s own test facil­i­ty with a cli­mate-con­trolled vac­u­um cham­ber. MTU Aero Engines, Ger­many’s lead­ing engine man­u­fac­tur­er, is work­ing on the over­all inte­gra­tion of the devel­op­ment con­cept for air­craft in the 19- to 80-seater class. As part of EnaBle, DLR spin-off H2Fly is focus­ing on clar­i­fy­ing safe­ty require­ments and cer­ti­fi­ca­tion issues.

“Indus­tri­al com­pa­nies, research insti­tu­tions and spin-offs are work­ing hand in hand in EnaBle. Ulti­mate­ly, the aim is to build up over­all sys­tem exper­tise for fuel cell-bat­tery hybrids, which will help strength­en Ger­many as a tech­nol­o­gy loca­tion and cre­ate new jobs,” the project part­ners are con­vinced. But the Uni­ver­si­ty of Ulm and its stu­dents also ben­e­fit from this indus­try-ori­ent­ed joint project: “EnaBle gives our young sci­en­tists the oppor­tu­ni­ty to con­duct appli­ca­tion-ori­ent­ed research in a high­ly inno­v­a­tive envi­ron­ment. Our engi­neers learn to work accord­ing to stan­dards and qual­i­ty guide­lines, to pre­pare for the indus­tri­al­iza­tion of prod­ucts and, at the same time, to dri­ve for­ward future tech­nolo­gies,” empha­sizes Dr. Chris­tiane Bauer.