Bjorn’s Corner: Electric aircraft, Part 1

By Bjorn Fehrm

June 30, 2017, ©. Leeham Co: One of the areas I explored while at Paris Air Show last week was electric aircraft technology. With electric aircraft, we mean an aircraft where the propeller/fan is driven my an electrical motor.

We don’t talk about “more electrical aircraft systems” like for the Boeing 787.

One of the companies investing in technology for electric aircraft is Siemens. A Siemens-engined aerobatic aircraft, the Extra 330LE, made daily flight displays during the show.

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

Electric aircraft

When discussing electrically driven aircraft, one has to understand some basics:

  • While batteries can be used as energy stores, their efficiency compared with fuel is appalling. 1kg of Jet fuel stores 70 times as much energy as the best current battery using Lithium-Ion technology.
  • If one compare space efficiency, one liter of jet fuel stores 20 times as much energy as one liter of Lithium-Ion battery.

Weight is a larger problem for an aircraft than space. Therefore, the difference between jet fuel as an energy source for the engines compared with a battery will be compared on weight.

A kg of jet fuel stores 11,900 Watt Hours (Wh) of energy. A kg of Lithium-Ion battery stores 170 Watt Hours of energy.

The next important step is, with what efficiency can this stored energy be used by the engine to produce power to a propeller or a fan?

A modern gas turbine core has an efficiency of transferring Jet fuel energy into shaft work of around 55%.

The modern electrical motor as produced by Siemens for the Extra aircraft has an efficiency of 95%. Add to that a power converter (called an inverter) from battery DC power to electrical motor AC power of 90% efficiency.

Power chain efficiency

If we now put together the components we need from the energy store to a propeller or fan, we have:

  • Electric chain; our power source has 170 Watt-Hours per kg, driving a chain which has an efficiency of 0.95*0.90=86%. This gives us a shaft power of 145 Watts driving our propeller/fan for one hour. This equals 0.2hp of shaft power to our propeller/fan during one hour per kg battery.
  • Jet Fuel chain; our power source has 11,900 Watt-Hours per kg, feeding a gas turbine core, which has an efficiency of 55%. This gives us a shaft power of 6,545 Watts for one hour. This equals 8.8hp of shaft power to our propeller/fan per hour and kg fuel.

We can see that one kg of battery gives us a tiny power per hour. To get to the necessary power density to drive an aircraft for some time, we need to add thousands of kilos of battery.

The consequence is a battery-driven aircraft can only be used for short-range operations. The demonstration aircraft from Siemens at the Paris Air Show, the Extra 330LE, had an operational time of 30 minutes. The battery used replaced the forward pilots seat.

It will remain like this until more efficient battery technology has come onto the market. Research is ongoing, but it will take decades before batteries with, say, 1,000 Watt-hours per kg would be available. This then gives us a motor shaft power of 1.2hp during one hour for each kg of battery.

To fly longer, we need another solution.

Hybrid electric aircraft

By now, it’s clear battery powered aircraft will be special cases, used for local purposes. The more promising electric aircraft is the hybrid one, where a combustion engine and alternator is combined with a battery which cover top power needs. The combination drives our electrical motor.

This is the subject of our next Corner.

32 Comments on “Bjorn’s Corner: Electric aircraft, Part 1

  1. It’s a bit more complicated than just comparing power densities of fuel vs batteries. Fuel gets burned and dumped overboard (the exhaust), the aircraft gets a lot lighter, the fuel burn rate (power setting) comes down.

    With batteries, the aircraft weight stays the same, so the power setting needed towards the end of the cruise to maintain speed / altitude is the same as at the beginning.

    To give equal performance, batteries need to significantly exceed the power density of fuel.

    • So yes, a hybrid might very well be a better use. It’s worked reasonably well for cars.

      • A hybrid is a bad idea for aircraft. You still need a gas turbine with a high continuous power rating. Then you need a heavy generator coupled to a heavy electric motor with heavy cables. The whole thing works no better but weighs a whole lot more.

        In cars you can get regeneration instead of braking and size the gas motor for a much lower continuous power. Acceleration is aided by the electric motor(s). Aircraft don’t do a lot of starting and stopping. They need cruise power for the whole flight.

        • Aircrafts with two engines with 50% more thrust than needed in normal operation, just for the case of an engine failing in TakeOff/GoAround situation. And they carry these oversized engines for the whole flight under there wings, with all the drawbacks of drag and optimization for this seldom used operating point.

          A hybrid plane could provide a electrical foldaway prop, which is only operating during takeoff and landing. Therefore it would be enough to store the energy for about two GoArounds (50% because the other jet engine is assumed to be operating).
          Such a ToGA prop can be optimized for this operating point, because it will be never used in cruise flight. Usually it will stay in standby, because an electrical engine can provide 100% thrust within seconds. Because it’s only use in emergency situations, noise wouldn’t be an issue for this prop.

          • I wrote a very nice response as to why this couldn’t possibly work, then thought that I’d be jumping the gun on your next article so I’ll wait.)

            I will say that energy storage for propulsion using batteries is so terrible (as you state in your article) that it doesn’t make sense for airliners using today’s technology. Since battery weight must be carried even after the power is used, the multiplier is actually twice the 44x jetA power chain differential from the article. (The average weight of jetA is only half the initial weight) I could never make the calculations work for any phase of flight, so I am looking forward to some education.)

          • Rather than having a fold away prop, it might be better to integrate motors into the turbo fan engines. Need a big boost of thrust all of a sudden? Then dump some Coulombs through the motor, and the fan spins a lot harder. That’d be much simpler than a pop out fan.

            It might also act as the starter. It’s on the wrong shaft; normally it’s the HP shaft that’s driven by the starter. But I suspect it could be made to work.

            It could also act to recover energy. During phases of flight requiring drag (descent?), flip it into generator mode; now you’re tapping flight idle power, the fan is now producing drag, so you’re also tapping potential energy, and your batteries are recharging.

            This closely follows what hybrid cars do. Bursts of electrical boost when required, rob the energy back instead of heating up brakes.

    • The only realistic option for long haul using hybrid would be something like the air batteries eg. Al-air. But then youd be dumping hydrated aluminium out of the plane to save weight so you’re still polluting – no point. Its a long way off technologically.

      • Talking of Aluminium, there is an idea to use metallic fuels. There’s been a fair amount of work put into developing internal combustion engines that run on metallic nano particles:

        Basically it involves turning metal into metal oxide, which is a solid. No CO2. The idea involves using hydrogen to turn the metal oxide back into metal, so it’s like a proxy hydrogen cycle.

        You could probably do the same thing with a gas turbine. A long time ago RR demonstrated gas turbines running on sugar, powdered wood, coal, petrol; basically if it can flow and burn you can use it in a turbine engine, including powdered metal.

        Metal has a much higher energy density than carbon fuels, so you need less of it. That’d be another way of making efficiencies.

  2. Bjorn great writing as usual!

    Might fuel cells be a bridge technology until batteries can catch up? What is their energy storage like?

    • Green Ammonia (NH3) has an energy density less than but comparable to jet fuel, and can be split into Nitrogen and Hydrogen – in fact you can get more Hydrogen in a litre of NH3 than you can by compressing pure Hydrogen, and at far less pressure.

      Ammonia can be burned (it was used in the X-15 rocket plane flown to M6.7) and can be also in a fuel cell, either directly or just using the Hydrogen component.

      NH3 is already widely produced and handled, its much safer than compressed hydrogen (though still toxic).

      The best bit is that Green Ammonia really can be green – if you use renewable power to make it (wind, solar, hydroelectric, geo-thermal, nuclear) according to the well known Haber-Bosch process, then there are no greenhouse gas emissions associated with it – no CO2 at all!

      The fuel cell technology isn’t advanced or ready enough for it yet, but I’d do my best to leave batteries on the ground and concentrate on Green Ammonia hybrid powered aircraft.

      • Ammonia. Methanol or Hydrogen can fuel a fuel cell. Hydrogen has the highest gravimetric energy density.

  3. A powered sailplane might be the first practical electric craft…

    • “Electric powered sailplane”

      On the market for quite a while now ( 2003, using proven Saft Li-Cells. you find a related product from Saft in the A350 batteries :-):

      55% is for large gas turbines. Here you would have to compare to the standard recip engine used on the Extra. Surprised if it exceeds 35% efficiency. Smallish gas turbines are even worse afaik. Their attraction is in specific weight and reliability.

      • Even more, 55% is for 100MW+ electric generation gas turbines with a steam recuperator on the exhaust, not a turbofan. GE’s 9HA attained 62.2% and Mitsubishi-Hitachi claims 63% on ~500MW but with sub 43% cores (as is the GE90 derived 65MW LM9000).

        Turbofan efficiency in cruise is 30.5% (CFM56-2) to 36.1% (GE90) [Ilan Kroo, Aircraft Design, Stanford]. The 15% more efficient LEAP should be at 35%, the 10% better GE9X at 40%. This is already impressive, and this is thrust efficiency, so to compare to a propeller engine we should include the propeller efficiency, typically 85% in cruise. (McCormick, 1979). A piston gas engine or a turboprop is 30% efficient (25.5% thrust eff.) but a small diesel piston can attain 40% (34% thrust eff.).

        An inverter could be nearly 99% efficient, if not then DC motors would be used to avoid wasting 10%.

        I think electric aircraft will be used on short commuter routes within 15 years.

        • Correct, current SOA turbofan total efficiency is below 40%, current in-service legacy engines below 35%. Furthermore, the theoretically possible limit is below 60% total efficiency (Carnot cycle). See (p.29)
          General aviation engines are even worse, except for the modern turbo-Diesels.
          Electric aircraft are significantly more efficient, they easily reach 85% total efficiency, including battery and propeller. If designed cleverly, they can make use of synergies which are impossible to use for combustion engines. The better total efficiency, together with clever use of the motors can make it possible to mitigate the significant drawback of the gravimetric energy density of petrol-based fuel versus SOA battery technology. See e.g. what NASA are doing with MAXWELL.

  4. One option is electrical assisted Take-off where ground Power helps the Take-Off run up to a certian altitude & speed before battery Power kicks in, some modern gliders have surprising L/D ratios at pretty heavy loads and normal sunny day thermals allows you to fly straight to your destination.

    • Most attractive option IMHO is a (parallel like Prius ) hybrid power train for helos.
      You can underengine ( potentially using a high efficiency diesel ) the craft while still having the multi engine fall back advantages.

      For pure electric:
      No idea how “real” this project is:

      • You can use a high strength light weight Power cord that is dropped as on sailplanes wiched to altitude.

  5. There is also a significant difference in the respective weights of an electrical engine and a combustion engine.
    A lycoming 360 engine weights 117kg empty, and an electric drive of same power would weight about 25kg and associated electronics 10kg max.
    This would help make electric viable for smaller aircraft.

    • Not that you can’t make them lighter, but a the lightest 100 hp AC motor I can find weighs about 600 lbs.

  6. I feel that a fundamental aspect of electrical usage must be about providing the taxiing and initial take off thrust for aircraft. In this manner the aircraft can reduce considerably on waste fuel.

    Effectively if we could have assisted taxiing with autonomous vehicle technology we cut down on the most inefficient and unnecessary use of aircraft fuel. Beyond that use of an electromagnetic catapult can provide substantial impetus to initial roll .

    This is all marginal stuff but would have a significant impact on both fuel usage and unnecessary pollution.

    • Sounds good. What’s the purpose of electric versus fuel? If it’s not cheaper, the only purpose is to reduce pollution. From that perspective, electric taxi and takeoff seem like a great idea.

      If the aircraft can be built lighter with a catapult takeoff, that is a huge economic benefit, not related to electricity. The catapult could be fuel powered. The weight savings in flight either justifies the concept or doesn’t.

      • The electric taxing doesnt even need to takeoff with the plane, electric aircraft tugs.
        But that might defeat on of the purposes of engine start up at the gate, to see if everything is working ok. That might be a rare enough problem to get around.

        • Autonomous taxi bots could give the crew quite some time while the plane is moved to the runway. ( cue automatic/autonomous carrier vans in box carrier loading/unloading/freight staging)

  7. Hello Bjorn

    Some of the wasted weight in the battery system can be regain by lighter engines?
    A 160 kW lycoming is 140 kg
    A 260 kW electrical moteur is lighter. I should check my photos…

    Best regards

  8. Could someone please tell me where I can see a dynamic graph of the aircraft? I am interested in IR. I will be taking a course, Digital Game-Based Learning and Design:
    This course provides an overview of the learning theories, best practices, and classroom application models involved with incorporating educational games and simulations into learning environments. The use of current and emerging technologies found in the gaming arena will be explored and documented for classroom application. This course brings together cultural, business, government and technical perspectives on developing and integrating electronic gaming techniques and technologies to enhance and enrich learning. Course participants will develop an understanding of the current trends (technical and sociological) in computer and console gaming, and what can be learned and applied from the world of gaming to positively affect teaching and learning. They will also experience an authentic creative process when they explore the game design process. As you can see, this is not remote stuff. My project would like to see the diagram of the graft accepting the parts after being checked for cohesiveness. They will create the visualization tool. I can go on and on, but it isn’t in the future you know. It’s just safety concern. Got some?

  9. What about Methanol Direct Fuel Cells?
    Methanol has only half the energy of jet fuel but a high conversion efficiency.

    Another advantage of electric drives is the distribution of propulsion systems.

  10. 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 variable. Smaller batteries will give you better power management; larger batteries give you more. You only need the full battery set for benefit (4).

    I think electric hybrid planes will be the next major development

  11. To whom it may concern
    I do Need kWh /p-km consumption of an electric plane.
    Please, if you have any data, share with us.
    Many Thanks

  12. Pingback: Is the reign of turbo-engines over? The future of the Gas Turbine – Science ramblings with Matt

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