ParisTech Review. Air traffic is expected to rise globally, particularly in Asia, but if it is to comply with the environmental objectives set out by the international community, the energy consumption of aircrafts has to be reduced. Are we on track?
Didier-François Godart. The whole aviation industry is focused on this ambition. Engine improvements, the electrification of planes and new solutions such as “green taxiing” (rolling on the tarmac with engines offline) will allow airline companies to save both CO2 and money. One should not underestimate the operational benefits: punctuality will increase and congestion at gates and parking areas will be reduced. As an indication, a major airline that adopts “green taxiing” on all its planes will save enough to buy a new plane each year. It is anything but a marginal factor.
The “electric plane” is all the rage in aeronautics. But what will truly be electric in it?
The only primary energy source on board of a commercial plane, in 2012, is kerosene. From that kerosene three other secondary energy sources are generated: hydraulics, pneumatics and electrical. These energies, also known as non-propellant, are used to power various items on board. About 5% to 6% of kerosene is consumed to power the non-propulsive secondary sources. This is an average. It depends on the phases of flight. The so-called “electric airplane” is a vehicle in which all three secondary energies will be replaced by electric-only systems. And believe me, this is quite a disruption already.
Given that kerosene is burned as primary energy anyways, what is the benefit of having electricity as the only secondary energy source?
Electrical power has many advantages over hydraulics and pneumatics. First, it consumes less, overall, because it consumes just what you need, whereas you need to maintain hydraulic pressure even when it is not in use, which involves an energy cost. Another significant gain revolves around maintenance: it is much easier to maintain and repair an electrical system than a hydraulic one. This improved maintenance will have important consequences on the availability of the aircraft, which is a key factor for operators. Finally, the electric plane will burn less fuel because the consumption of non-propulsive energy will be optimized. It won’t have to be grounded as often and will therefore be cheaper for airlines and thus, for passengers too.
Airplanes, excepting the most recent ones, have been designed in an era of cheap oil, without much attention to the secondary consumption. Where are we wasting the most?
For instance, take the design of the pressurization system, which is powered by collecting air on the engines. It can be streamlined, dramatically optimizing its overall energy balance… Today, when pressurizing the cabin, part of the compressed air which comes from the reactors is pumped in. But this bleed air is extracted at high temperature (over 300° C/570°F). So we must begin by cooling it with a “pre-cooler” or heat exchanger, hence incurring a loss of energy. In an electric-only airplane configuration, air will be taken from outside the fuselage and compressed as needed, therefore at a minimal energy cost. In general, as it is, switching to electric will allow an in-depth revamping of systems for an improved energy optimization.
The trend seems to be around electricity nowadays but with a single secondary energy source, what happens in case of failure?
In reality, the “reconfigurability” of the system, that is to say its ability to take over in case of local failure, will not only be maintained but even improved with electricity. Today, it is impossible to substitute with electric if pneumatic power or the hydraulics are faulty. With electric-only systems, we are going to much better detect and isolate breakdowns.
When will airplane electrification happen?
It already has begun. The A 380 and B 787 include electric systems that were formerly hydraulic or pneumatic. On the A 380, for example, reverse thrust and backup flight control actuators are electric. On the B 787, the brakes, the engine starter, and cabin pressurization have switched to electric power.
Taxiing will also be electric on the next generation of aircraft, which will benefit very unevenly to diverse planes.
“Green taxiing” will be a valuable asset, especially for short and medium haul: these planes spend about 30% of the time on the ground rolling on the tarmac. On a Paris-Toulouse flight, for example, time spent taxiing in Paris can take up to 20 minutes for a flight time of one hour. In that case green taxiing can save you 3% or 4% kerosene. Whereas a 0.5% gain in efficiency for a modern turbojet engine would be a dramatic success. We will have to wait until the next generation of Safran engines (LEAP) installed on the Airbus A320 NEO, the Boeing 737 Max and the COMAC C919, to truly step into a new era. The Leap engine will save 15% on average, but at the cost of an in-depth overhaul of engines.
Green taxiing seems a modest achievement, compared to what has already been done with the electric car…
Think again. Electric green taxiing requires considerable power which is difficult to mobilize. For a single-aisle A 320 type airplane, power of about 100 kilowatts must be applied at wheel level. This is a technical challenge. Some smaller aircraft include a motor in the nose wheels, but that does not guarantee the required performance of a large airplane as the front end typically supports only 8% of the weight of the aircraft and the front wheels would skid on a wet tarmac. Therefore, one must install electric motors on the main wheels but available space is scarce because of the brakes that are not only bulky but also heat up to several hundred degrees.
So green taxiing, which seems to be a local issue, is somehow also a systemic problem for planes?
Yes, because wheel motors are managed by “power electronics” (the one driving the different actuators, that is to say, the electric motors that power air compressors or control surfaces like flaps). Power electronics constitutes a critical issue for aircraft electrification. It must be reliable enough, dense enough and light enough to be embedded. In 2012, we are able to embed electronic power of up to 120 kW. The catch is they have a tendency to heat. We require them to be at once effective, light and dense. They are therefore inevitably compact and hot spots are very punitive, and very difficult to reverse. You want to evacuate these calories out of the plane. All current research revolves around this one goal: cooling down power electronics.
Can an electric airplane be manufactured with the same teams, the same processes, and the same management as a conventional airplane?
No, an entirely new approach is needed. For generations, the aviation industry has been organized around silos, the famous “ATA chapters” which cut a plane into several almost independent segments: landing gear, wheels and brakes; cabin conditioning, engines; etc. As we further electrify systems, we will have to build bridges between these different systems, notably by pooling electronic power between different uses and users. This is a major technological revolution. Take the case of a plane about to take off. In the future, the same power electronics elements will successively feed wheel motors, then the electric starter for the aircraft’s main engines, then landing gear retraction actuators, then slats and flaps retraction actuators, and finally, the electric compressors for cabin pressurization.
Specifically, what are the changes commanded by electrification in the management of aeronautical groups?
To achieve the pooling of power electronics, it will be necessary to break down the walls between the old silos so to speak, and to build technical gateways but also gateways for business models and management. This will have a deep impact on the industrial organization of major aerospace OEMs, worldwide.
In the old silo model, each division of a group would probably have come up with its own power supply, with many redundancies and wastage. The new model streamlines it all. At Safran, we have created two entities named Safran Power and Safran Electronics, whose respective missions are to design future power electronics and command/control electronics, to the benefit of the firm’s diverse users. The transversality demanded by electrification rests with those entities. Safran Power and Safran Electronics provide solutions to all companies within the conglomerate and this cross-cutting dynamic, that is to say, the confrontation with all the specific requirements of the former, is enriching the skills of these two poles.
Are corporate groups in the sector going to change their scope of business?
The strategic specialty is now electrical distribution. The best positioned corporate groups will be those able to offer a wide range of electrical systems and to conduct transverse cross-system optimizations, but also engine optimization. Regarding the latter, their design will have to take into account the specificities and contributions of electrification. Thus we see the emergence of engine and equipment manufacturers groups capable of offering complete electrical solutions to aircraft manufacturers. The number of such groups is likely to be very limited.
General Electric has bought Smiths Aerospace, a British company. UTC, owner of Hamilton Sundstrand, has just acquired the supplier Goodrich. For our purposes, Safran has partnered with Esterline, a world leader in on-board electrical distribution systems. This partnership has allowed our subsidiary Hispano-Suiza to be selected by the Brazilian aircraft manufacturer Embraer to supply the electrical distribution system and full aircraft electrical system integration for the KC-390, its future military transport aircraft.
A 100% electric airliner, propulsion included: conceivable or science fiction?
For now, this is science fiction. One can consider creating electric motors to propel small crafts with limited capacity, but electric propulsion for airliners is not coming anytime soon. To imagine an onboard electric motor delivering the power of a CFM56 is just not an option today. And anyway, the issue of massive generation or storage of electricity would have to be resolved first. Storing large amounts of electrical energy in a plane is impossible with the technology available in 2012.
Be it the case, where would the primary energy able to deliver propulsion electricity come from?
Hydrogen could well be the energy of the future. One would therefore have to embark large amounts of hydrogen. Safran is a world-class leader in hydrogen utilization in its rocket engines. We have also been working in the field of fuel cells for several years. However that was on a rather modest power scale, in the order of a few tens of kilowatts. A major challenge remaining is the complexity and the mass ratio (about 10 to 1) of hydrogen storage. It still meets the same reality constraint: kerosene is an extraordinarily effective fuel and its storage constraints are relatively easy… making it quite difficult to replace. Again, electricity as a total energy solution is not coming anytime soon.
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