Bjorn’s Corner: Electric aircraft, Part 2

By Bjorn Fehrm

July 07, 2017, ©. Leeham Co: In last Corner we could see that battery driven electric aircraft will be for the extreme short-haul.

The stored energy per kg battery is 70 times lower than for fuel. In addition the weight is constant. Fuel gets burned off during the flight.

A more useful configuration is the hybrid, which adds a combustion driven generator as energy source.

Figure 1. The Siemens electrically engined Extra 330LE aircraft. Source: Siemens.

Electric hybrids

An hybrid electric aircraft adds an extra energy source on top of the battery we discussed in the last Corner. A generator driven by a combustion engine (running on fuel) is added. The components in a normal hybrid drive chain compared to a Turbofan engine is shown in Figure 2.

Figure 2. A Turbofan engine compared with a hybrid electric propulsion system. Source: Leeham Co.

There are electric hybrids for cars which mixes the combustion engines shaft output (core in the figure) to the drive of the vehicle. We will only discuss hybrids where the combustion engine is driving a generator. This in turn feeds energy to the aircraft’s electrical motor (driving a propeller/fan) and battery.

To convert the batteries DC output to the AC waveform, needed by the electrical motor, we need an Inverter. The generator creates energy as AC current, but the frequency of the waveform needs adaptation to control the speed of the AC motor. Therefore, we also need the inverter between the generator and the motor.

Drive chain efficiency

Today’s airliners use Turboprops or Turbofans to generate propulsive power. In both cases the main propulsive power comes from the air mass being accelerated by the propeller or fan. The jet power from the engine’s core exhaust is 10% or less in modern aircraft engines.

From Figure 2 it’s obvious we introduce more steps between the core and the fan in a hybrid case. Each step involves energy conversions and conversions means efficiency losses.

The challenge for a hybrid electric aircraft is to minimize such losses. Otherwise an electric propulsion systems cannot compete with Turboprop or Turbofan driven aircraft.

Component efficiencies

The hybrid will use a propeller or fan accelerating the air mass, the same as today’s solutions. So we can skip over the efficiencies of these at first. The assumption is, these are unchanged for the two cases.

The Cores that will be used when electric airliners can be realized will have efficiencies around 55%. Comments in last week’s Corners said present cores have lower efficiencies, more like 40%. This is correct. But we are here examining power chains which will be realized after 2025.

The projected core for the Rolls-Royce Ultrafan Turbofan has a cruise efficiency of around 55%. Its pressure ratio is over 50 at cruise and it uses advanced turbine cooling techniques to reach such efficiency levels. Other engine OEMs are working on similar cores. We must therefore assume hybrid cores with efficiencies in the 55% bracket.

The next step is the Generator. Superconducting generators suitable for aircraft use are planned which will reach 98% efficiency. As the electrical Motor is in principle the inverse of a generator, we can assume an efficiency of 98% there as well. Siemens and Airbus joint venture for electrical aircraft is working on such designs, as are others.

Batteries we discussed in last Corner. This leaves the Inverter.

We assumed inverters of 90% efficiency last time. These use present technology switching semiconductors to shape the AC current that suit the motor, from the battery or generator power. They also feed the battery when the generator produces excess power that can fill the battery. And the inverter reverses this process to feed the motor from battery DC power when needed.

New switching semiconductors are developed (based on Silicon Carbide technology) which can raise the inverter efficiency to 97-98%. We will assume the availability of such inverters in our discussions. 

Efficiency of the hybrid chain

We have now described the efficiencies of the extra components in a hybrid chain. As these are put in serial we would have a chain efficiency of 0.98*0.98*0.98=94%.

If we introduce a generator, an inverter and an electrical motor between the core and the fan we loose 6% efficiency compared to the direct drive Turbofan.

This assumes everything else being the same. In the next Corner we discuss why this might not be the case.

19 Comments on “Bjorn’s Corner: Electric aircraft, Part 2

  1. 55% is still a stretch for 2025 (in only 7.5 years!). The gain over a generation can be ~10%, so from 40% it could climb to 43-44%. To get over 50% it would need a recuperator on the exhaust, but I don’t know if it could be feasible in aerospace applications needing low weight and volume. On the other hand the propulsive efficiency could be much higher and other gains could be made through aerodynamic enhancements (boundary layer ingestion, etc.). Next weeks corner!

    • huh, but the 40% is including the propulsive loss, so the cores are currently at ~47% and should be indeed at 50% in 2025

  2. Hello Bjorn

    There’s a mechanical ink between Motor – Fan and Core – Generator,
    No mechanical link between the other parts. Maybe with some relevant colors, you can show it in the figure 2?

    Rotation speed of an electrical engine can be set from 0 to 100%, rotation speed from an classical turbomachine can be set between 70-100 % is that it ?

    The figure 2 hybrid chain should be reversible, with the fan being used as generator for the batteries, to slow down the aircraft, the provide a degree of reverse

    Waiting for part 3 !

  3. My thoughts relating to electrification of aircraft are that it is a nonstarter. It will also become increasingly irrelevant. We are on the cusp of decarboning (?) the world economy as renewables become more and more significant and cars and other ground based vehicles move electric. As a result the economic reasoning for the same on aircraft falls away. We have reached peak oil, but from a demand sense and not supply. The other issue is pollution, be it noise or emissions. Noise has been to a greater extent licked (don’t you miss the screech of the VC10) and taking out the core pollution from other sources (vehicles, power stations etc) gives the airline industry more head room.

    So in summary other factors away from the industry itself will have considerable benefits to the future use of kerosene based power. I really do not see any alternative. The caveat being that efficiency in terms of fossil fuel usage will dictate the move to alternative turbine
    engine types

    • Apologies for repeating the late comment I made in the last Bjorn Corner, but I think relevant to your comment and also why I think electric propulsion without massive battery sets is going to be the next major development for aircraft.

      The wins I see for electric propulsion are:

      1. Propulsive efficiency. It is more efficient to push larger quantities of air slowly than to small amounts quickly. Fan sizes have been getting bigger but there is a physical point where this becomes counterproductive. We are at that point. By separating the motor from the fan and using the first to drive the second electrically we can have more fans than generators.

      2. Aerodynamic efficiency. By separating fans from the engines you can place both in more aerodynamically effective positions. One idea is to place the fans above the tailing edge of the wings to pull air across the wings and improve lift.

      3. Better power management. You need the most power on takeoff, you are generating surplus power on descent and you need minimal power on the ground. By using batteries for backup on takeoff and on the ground and feeding surplus power on descent back into the batteries you can reduce fuel use and at the same time have a less overengineered engine for engine out situations on takeoff..

      4. Redundancy/enhanced safety. With big enough batteries you can eliminate a second engine for engine out situations, or if it is a single engine plane you get redundancy. You can also save on networks and APU while still getting the same safety level.

      Now, you don’t need any batteries to get benefits (1) and (2). Benefit (3) is not all or nothing. Smaller batteries will give you better power management; larger batteries give you more. You only need the full battery set for some of the benefits of (4).

      • @FF: I think your points are very valid indeed. By allowing for the freedom to co-integrate airfoils and propulsion, some major aerodynamic advances becomes attainable. Potential gains relate to both induced drag (more efficient lift) and parasitic drag (laminar flow).

      • @FF

        Precisely what I meant by alternative turbine type, but so much more eloquently explained, thanks

    • “We have reached peak oil”

      We reached peak oil in the 70s.
      Then we reached peak oil in the 80s.
      Then we reached peak oil in the 90s.
      Then we reached peak oil in the 2000s…

    • If you ditch the battery in favour of a fuel cell, and power both that and the core with anhydrous ammonia, then you could realise similar performance to turbo-prop engines and completely eliminate CO2 emissions, the proviso being that the ammonia is produced using renewable power (wind, solar, hydro-electric, nuclear or geo-thermal).

      Getting rid of aviation’s CO2 footprint should be a priority.

      The fuel cell technology does not exit yet.

  4. Besides the loss of efficiency in the chain described above, there is another obvious issue of concern – weight. Adding a battery, inverter, generator and electric motor all add weight to what would presumably be a similar core and fan… unless this design somehow allows for a far lighter core.

    • The key point is that it’s only worth doing if you improve fuel consumption relative to standard jet engines. In that case you balance the extra weight of the items you list against the lower weight of fuel you need to carry to go a certain distance.

  5. What about transmission losses between the various components? With all the talk about using a larger number of smaller propulsors for aerodynamic/redundancy gains and possibly utilizing a single engine/generator set at a different location, large amounts of electrical power will have to be transferred. The system voltage will need to be high but even then the line losses can be significant when compared to ultra high efficiency motors and generators, especially if the system needs connectors in order to be serviceable.

    Relying on superconducting motors and generators seems a bit sci-fi-ish. Sort of like the way fusion energy has been only 20 years away for the last 60 years… and I’m a fan of fusion energy since I spent the better part of a decade working in that field.

    I do think electric propulsion could be the next greatest thing for aircraft, but it is important to be realistic about it.

  6. Fuel cells might do the combustion to electrical DC power at higher efficency.

  7. Comparing current aircraft power and energy requirements with battery capacity may not be the most relevant approach. Electric systems are excellent at providing short duration high power, without extra weight.

    NASA are testing “DEP” wings which could significantly reduce drag during cruise because the wing can be perhaps 1/3 of the width. Elon Musk has suggested gimbled fans to eliminate control surfaces – established aviation may reject this, however long term the economics may become compelling.

    In flight refueling could be a challenge however!

  8. I am unconvinced by the architecture outlined in the article. It is using a generator / motor in place of the gearbox a GTF has.

    Having a core driving a full power generator driving a full power inverter driving a full power motor means that the generator, inverter and motor are all large and heavy. Plus with the only linkage between the core and the fan being electrical, the electronic component reliablity would have to be very high, a real challenge for power electrics running in a harsh environment.

    Super conducting generators and motors sound unconvincing to me; super conductors really, really don’t like high magnetic flux densities, which tend to cause quencing (whereupon they stop being super conductors). With, say, 30,000hp being transferred magnetically between rotors and stators, the flux densities are going to be large. That’s going to be a real design difficulty. This is especially so with the high temperature liquid nitrogen ceramic super conductors. So it’ll probably require low temperature superconductors, which require liquid helium, a major logistical nightmare to handle, acquire, etc. Qatar’s boss is already grumpy about the cool down time of the P&W GTFs…

    Also one doesn’t use soft iron cores (or anything like them) with super conducting magnetics; you have just the superconducting coils, and that’s it. That means that there’s little mechanical support for the coils in a super conducting motor / generator, so then they have to be mechanically strong as they take the full load (I suppose you could have plastic stators and rotors for mechanical support). In a normal motor / generator, you have flimsy pieces of wire set into a nice, strong soft iron rotor / stator.

    Another aspect is the cold. If an engine is shut down in flight, once that inverter has gone down to -70C there’s a real question mark over whether it can be re-started, or whether it has even survived intact. Electronics really, really doesn’t like the cold, and the characteristics of semiconductor junctions are significantly temperature dependent. It’s most likely going to be slipstream cooled when operating; it’ll have to dissapate several hundred kilowatts at 98% efficiency. When it’s no longer operating it’s going to be chilled down really quickly. It’s going to be hard work for heaters to keep up, so they probably won’t be an option, espcially as you’d rather use that energy for propulsion at the time. Plus at take off, with maximum power, the cooling air is going to be ground ambient temperature and minimum airflow; that’s a tougher cooling challenge. All this will multiply the design difficulties for the inverter.

    The inverter could be brought inboard, in the nice warm cosey fuselage, but then that’s less efficient (all that heavy duty cabling) and you’d be having to dissapate several hundred kilowatts of waste heat from inside the fuselage, which sounds like a bad idea. If out there in the engine pod, there could be moveable flaps over its cooling ducts, but then that’s another place to worry about icing, etc.

    The efficiency of a good gearbox is going to be something near the 94% of that electrical drive train, if not better. But I think it stands a good chance of weighing less overall. Surely it would be better to have a core driving a fan through a gearbox, and then have a single smaller motor-generator on the shaft? With the gearbox cruise optimised it would be very efficient. It would be the only thing that has to be big enough to handle the full power of the core. Meanwhile the motor-generator could be far smaller than in the article’s architecture. And there’d only be one of them, not two. And if you put it on the fan side of the gearbox, it reduces the take-off loadings on the gearbox. Plus there’d be less need to get all exotic with superconductors, etc. It could be much more conventional. And as the electrical coupling of core and fan would be supplementary to the mechanical coupling via the gearbox, there would be no particular need to achieve the same reliability. If it breaks, there’s still the mechanical coupling through the gearbox.

    To me that sounds like the easiest way to incorporate hybridicity into a turbofan. Whether it brings enough benefit, I don’t know.

  9. Bjorn: Something of a technical point, we need both a rectifier and an inverter.

    Rectifier to turn the AC into DC, then the Inverter to change it back to DC.

    Batteries sit in between on the DC link and get charged and or feed back into the drive as needed.

    I know the common name is Inverter, but its actually more a UPS with the battery string involved.

    VFD is another name.

    Electronic GPU as well.

  10. Dear Björn,

    In case you don’t yet have similar information available, here is a bit about the total power train efficiency of Solar Impulse: http://www.fzt.haw-hamburg.de/pers/Scholz/ewade/2009/EWADE2009_Ross.pdf
    Note that their motor is slightly below of what’s currently available, while the quoted battery efficiency is quite outstanding. Summarized, from battery to prop shaft, an electric power train will achieve 85%-90% of efficiency with today’s COTS components.

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