Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 13. Serial Hybrid.

March 13, 2020, ©. Leeham News: In this week’s Corner, we address an often forgotten aspect of Electric and Electric-Hybrid aircraft design.

The battery as an energy source, as the only or assisting source, has the same weight during the whole flight. A fuel (alternate, fossil, or hydrogen) consumes during the flight. You gradually fly a lighter aircraft. Let’s see how this affects the aircraft’s efficiency.

Figure 1. Embraer’s E175-E2, a latest-generation 88 seater jet used for our example. Source: Embraer.

What the constant battery weight means for Electric and Electric-Hybrid

My first round of Corner’s about Electric airliners showed these aircraft have fundamental problems. Different from a battery-driven Electric airliner, you can make a Hybrid work. The question is, will it use less fuel than the aircraft it replaces and by it reduce our environmental CO2 load.

The first round of this analysis was made with back of the envelope calculations. No more sophistication was needed to see the problems. Now we go a step deeper by using our airliner performance model.

To test the Hybrid case for plausibility, we will do a simple experiment. We use our airliner performance model that can calculate an airliner’s mission fuel consumption within the odd percent of the true value.

We pick the most fuel-efficient small regional jet to do our check, the 88 seat Embraer E175-E2, Figure 1. It’s just started it’s certification flights and will be available in the market next year.

Figure 2 shows the two configurations of the E175-E2 we compare. The first variant uses the engines it was designed with, the modern Pratt & Whitney PW1700 GTF.

Figure 2. Components of a Turbofan versus Electric-Hybrid powerplant. Source: Leeham Co.

The second variant has hybrid engines with the same key characteristics as the PW1700 GTF (the battery is an energy source so it’s not included in the powerplant). The battery alleviates the takeoff fuel energy consumption from the gas turbine so we can make the gas turbine smaller and thus less energy-hungry. We assume we can fill the battery with less polluting electric energy from the grid before takeoff.

We further assume the Turbofan variant and the Hybrid have the same installed weight for the complete powerplant. I have before argued this is not the case, the complex hybrid installation plus gas turbine will be heavier than Turbofans. But we assume them equal for the experiment; we are running a best-case test for the Hybrid.

We fly the Turbofans from jet fuel and the Hybrid from jet fuel + battery energy.

Instead of arguing the increased efficiency of the Hybrid upfront, we run the E175-E2s over a 300nm still air mission and look at the improvement in efficiency required for the Hybrid to consume the same fuel as the Turbofan version. We need equality before we discuss any improvement.

Modeling wise, this means we have one aircraft with the mission’s fixed weight as empty weight + passengers and the second as empty weight + passengers + battery weight. To the fixed weight we add the needed fuel for the missions to reach takeoff weight.

We assume our certifiable battery system (not cell, not battery alone) has an energy density of 0.15kWh/kg and a weight of 6,000kg. It gives us an energy store of 900kWh, the equivalent of 75kg of jet fuel.

For the 300nm ESAD trip, our Turbofan E175-E2 with 88 passengers has a takeoff weight of 39 tonnes and fuel consumption of 1180kg.

For our hybrid, the takeoff weight is 47.7 tonnes as it flies the mission with a constant 6t higher fixed weight. Now the fuel consumption is 1290kg if we assume the same efficiencies for the two powerplants (everything else being equal) and that all battery energy is used during the flight.

We see we can store the equivalent of 75kg of fuel in the batteries, but these cause the equivalent of a 110kg increase in energy consumption for the flight. This is when we assume this 6t Battery system has no influence on the structure of the aircraft (which it has, MZFW and MLW have to increase which affects the structure) and that the Hybrid powerplant weighs the same as the Turbofans (which it doesn’t).

You can argue a higher efficiency hybrid powerplant reduces the amount of jet fuel that would be consumed but we are losing 9.3% in fuel efficiency due to the constant 6t higher weight caused by the batteries. A Hybrid aircraft has its work cut out to reach equality, let alone lower fuel consumption (increasing the precharged energy from the grid doesn’t help, if we do this the Hybrid efficiency gets worse).


The example has several simplifications. It’s a first-order check to see the challenges a hybrid airliner faces, this time narrowed down to the increase in fixed weight from the batteries. A detailed analysis would model a complete Hybrid airliner with its fixed weight and powerplant efficiency against the best Turbofan aircraft at the time of introduction.

The different checks in previous Corners show it will be difficult to make a hybrid-powered airliner that is more efficient than a turbofan-powered plane. The added weight of generators, distribution networks, inverters, motors, and battery kills the Hybrid. The most likely outcome is the first generation of Hybrids will consume more fuel than the aircraft they want to replace.

Re. battery driven airliners. I don’t need to make an example of such an aircraft. It doesn’t matter what fantasy figures I apply for battery energy density; it still produces non-viable airliners.

Different to battery-electric airliners, Hybrids are feasible. It’s just difficult to make them reach the fuel consumption of the aircraft they shall replace.

37 Comments on “Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 13. Serial Hybrid.

  1. So, as with an all electric design as previously discussed, it remains important to focus siginificantly on structural battery technology, to mitigate the mass penalty.

  2. How about a ground effect hybrid Aircraft like a Beriev flying slower on the 300nm route?

    • Essentially could only fly that low over water, the ground effect was at around half of wingspan.
      “In any collection of ekranoplan photographs and illustrations, the two phrases most frequently seen in captions are “artist’s impression” and “computer-generated image.”

      • WIG “Wing In Ground” effect seems to increase lift by 2.3 without an commensurate increase in drag. The “Reverse Delta” developed by Alexander Lappish is far more effective, even useful up to an altitude 50% of wingspan, than the straight winged WIG developed in the Soviet era. Nevertheless Russian makers persist with the straight winged Ekranoplane though there are no more “Caspian Sea Monsters” anymore, sadly.

        The Widgetworks Airfish 8 seems to easily meet the theoretical ability of Lippisch forward swept WIG deltas to cruise at 1kW per 20kg, hence a mass fraction of 20% battery with 250 Watt hours/kg energy density should provide for 1 hour of cruise flight at about 80 knots.

        Airfish 8 has made it on to Lloyds register and it looks like it will be certified as a boat.

  3. It is large assumption that the battery can be filled with less polluting energy from the grid. The thermal efficiency of a modern, large steam turbine plant is less than 50%. Add to that generator, transmission, transformer and battery charger losses and you would be lucky to have 40% efficiency overall at the point of battery charge.

    • The best supercritical steam plants, mostly from Germany because of Brown coal is perhaps 48% efficient. Transmission losses are nowadays only about 10% whereas the lithium battery is almost 99% efficient though the final drive motor/invertor only 90% or so. You do end up with about 44% efficiency for a Tesla or similar. This compares to 22% for a piston engine car. However this isn’t a fair comparison because if the oil that was used to produce petrol was directly burned in a combine cycle plant (gas turbine with steam turbine) the mechanical efficiency could be 64% best case and electrical 63%. Over 55% of the power could get to the wheels of an electric car and maybe 60% if sumitomos superconducting electric motors work out. I suspect a hybrid cars might be 40% efficient. The Great attraction of the electric car is that 9 sqm of photo voltaics (ie 3m x 3m) could produce 14.4kW.Hr day on a sunny day which would drive a Tesla model 3 about 100km. Batteries will drop 70% in the next 5 years and electric cars should match petrol in price within 10.

      E-fuels will be needed for earth moving machinery, ploughing and long range aviation and truck haulage.

      However her is Elon Musk talking of building a supersonic eVTOL transcontinental aircraft.

      Note this man has utterly left established aerospace firms such as Boeing and Lockheed-Martin looking like losers that wasted the lead they inherited. His Tesla cars (Model S, Model 3 and Model Y) are the three safest cars ever tested in the world. They can realistically drive 400 miles / 620km. Full auto pilot just dropped and now the Teslas can self drive through traffic with traffic lights, pedestrians etc. The hardware 2 processor executes 72 Trillion operations/second. Compared that accomplihment to the 80286 used on Boeings MCAS that couldn’t even recognise it was in a 40 degree death dive.

      • A Tesla devotee that believes every bit of hype … anyway Graphics processors like that mentioned can go to 320 TOPS. Not comparable to the 737 processor and software which does different operations to analyzing video
        As for autonomous driving
        “Tesla, for all its promises of a self-driving future, still only provides what’s known as Level 2 autonomy: a driver-assisted mode where vehicles can accelerate, brake, and steer on their own but only in controlled settings with drivers ready to take control.”

        Back in 2016 Musk claimed a Tesla could be able to drive automously from LA to NY by 2017-18 . Still hasnt done so.
        However there seems to be some always ready to pump Musk’s tyres for him.

  4. The examined configuration is a ‘series’ hybrid: a gas turbine mechanically driving a generator which then drives an electric motor via an invertor with an electric battery float that drives the propeller. This configuration does save a gearbox, a starter motor and generator. In the case of a highly efficient new generation of superconducting motor generators pairs we can avoid the gearbox with little penalty. Sumitomo is in fact developing superconducting electric motors for automobiles. These are the people you go to get a small heat pump to achieve superconducting temperatures.

    The series hybrid is considered inefficient in the automotive world however the latest Honda hybrid cars use this configuration with the very critical refinement of a mechanical lockup clutch between the generator and motor which will be engaged for all but the lowest or highest speeds. Usually this configuration is only useful when the heat engine is a supplement such as the ‘range extender’ used in the BMW i3 used to overcome range anxiety.

    But there is also the “parallel” hybrid in which the propeller is driven both via combined motor/generator in parallel with a gas turbine either through a combiner gearbox or through the rear of the electric motor. Again a battery float can be provided. This avoids the weight of separate motors and generator and also the power conversion losses. The motor generator also replaces the standard alternator/starter but some gearing will still be needed.

    Advantages are that the combined gas turbine and electric motor provides take-off power and the safety of multiple engines. The question then is does one switch of the electric motor for cruise or the gas turbine? The nice thing about this is that an unavoidable ancillary system such as a solid oxide fuel cell ((60%-80% efficient) could be kept going to power the electric motors.
    If electricity is used for cruise the propellers can regenerate energy in decent and act as powerful air brakes even in landing. Allowances for headwinds, divert and holed are easily dealt with.

    Of course I am speculating and I see both electric aircraft and hybrids as a niche but they might open up truly massive niches such as pilotless eVTOL and eSTOL. If drones are flying and delivering parcels then it is only a matter of time.

    I believe that the long development cycles and awkwardness of hydrogen will make it a niche product suitable for only certain routes.

    Carbon neutral ‘e-fuel’ or PtL “power to liquid fuels’ which allow CO2 and Hydrogen from renewable sources (hydro, geothermal, solar, nuclear) to be produced.

    The German firm ‘Sunfire’ has developed a combined steam and CO2 electrolysis system whose output is syngas (Carbon Monoxide and Hydrogen) which can be reacted over fischer-tropsch catalysts to produce a very clean burning jet fuel which is already certified. The Norwegian company Nordic Blue will use this technology at one of its hydro ammonia plants to produce 8000 tons of carbon neutral fuel per anum from hydro power. The Swiss firm climeworks is suppling the amine/ceramic pebbles absorbers to remove CO2 (from the air or concentrated sources).

    This is likely the jet fuel of the future. Cryogenic for certain routes only.

    • We will run the case with a parallel hybrid next week and see where we land.

  5. The hybrid design can be simplyfied.
    ( Take the Toyota Prius setup as guidance.)

    Fan – e-engine – Core

    E-engine – 4qadrant inverter – battery

    Fan and Core are mechanically coupled.
    Design power for the E-part can be much less than
    the full power core output.
    System addons are now much lighter.

    During controlled descent the core can be switched off.
    power recuperated via the windmilling fan.

    • Uwe, I think Bjorn used the case of series hybrid as it gives the best power-to-weight ratio for the electrics. Changing to parallel lowers the power requirement but the weight drops less rapidly than the power for that case. So it only gets worse for lower power levels.

      In a ground vehicle with stop & go driving, you only need a fraction of the internal combustion power to start or continue motion, and that cycle is repeated many times in transit. In an aircraft, you need full power at takeoff and there is only 1 stop/go cycle in the flight. Also the aircraft is much more sensitive to extra weight.

      For hybrid to work in aircraft, would require a significant fraction of takeoff power to be supplied by the electrics, then recovered and stored on descent and landing, for use on the next takeoff. The series configuration assumes the best case for that scenario, parallel would be a lesser case. But the weight and losses required to do that impose a larger penalty than the benefit.

      • Not quite sure what I should make of your combo of condescending tone and repeating lack of text understanding of what you reply to.
        This is not the debate club.

        • Bjorn mentioned he would run the parallel case next week, so we will let the facts speak for themselves.

  6. This example does not address the benefit of distributed propulsion, the reduced impact of gas generator failure (less asymmetric thrust, thus smaller vertical tail rudder, etc…). Electric or hybrid-electric offers new design solutions that will bring benefit. We just need time to come up with ideas.

    • Battery flight will be cost effective for very short flights where the fuel fraction (of hydrocarbon fuels) would be very little anyway and therefore the battery fraction not to large either. An aircraft with a fuel fraction of 5% would translate to an battery fraction of about 10 times as great ie 50% (( use 10x rather than 30x because of the higher efficiency of electric motors). That is a very short flight, maybe 50-70NM. In a world of self driving autonomous automobiles and autonomous delivery drones we may come to accept pilotless eSTOL or eVTOL (in part due to the redundancy you mention). If you wanted to go from say Dunkirk France to Eastbourne England, why would you put yourself through a drive to Ostend Belgium to catch a flight to London Gatwick and a further drive to Eastbourne. You may just be able to catch a Lilium jet or some kind of eSTOL capable of operating from 100 meter long strips that offer pilotless direct flights. Even if a stop on the way and change over is required it will be more acceptable. I can’t see electric aircraft competing with fuel aircraft, I see them making new markets that will encroach on normal aviation.

    • Please go back through this series and you will find answers to your questions/propositions.

  7. Ignoring the weight and efficiency of electric motors and inverters is a significant omission. The E175-E2 has 30k lb thrust installed. At takeoff you need about .5hp for each lb of thrust or 15k hp.

    The Magnix aircraft motor weighs 300lb and produces 750hp, which scaled would be 6000lb without the inverter. You also need a generator which even if sized half as big for cruise would still weigh 3000lb. This is 9000 lb/4000 kg, almost as heavy as the battery. This extra weight would cost 6.2% in fuel efficiency using your numbers.

    Inverter and motor efficiencies are on the order of 95% and you have to go through three: the generator, the inverter and then the motor. You lose 15%, or another 200kg in fuel burn. Airlines would kill to get a 15% improvement in fuel efficiency.

    The hybrid penalty is roughly twice what you state, even without considering that your lighter weight version already exceeds the 45 tonne E175-E2 MTOW.

      • He specifically said that he doesn’t believe that installed power plant weights are similar, and I agree that the difference is significant.

        • I see what you’re saying – I was referring to “but we assume them equal for the experiment.” Agreed that in all actuality the electronics weights are too low (with current tech).

  8. Any justification for a 900 kWh battery? Seems to be off by an order of magnitude if only used for takeoff.

    • Because it weighs 6t. Any larger battery with higher capacity would necessitate a completely new design of the aircraft. So to run the check a 6t battery was used. If I would have increased the battery capacity further the results would have been worse for the Hybrid, not better.

      Essentially, Hybrids are not viable with the present battery technology, not now, not in five years, not in ten years. We have to reassess the situation after 2030 if we get a completely new battery technology.

      We are not 30% or 50% off where we have to be as the extra weight is fixed. We are 10,000%! off.

      • For a typical takeoff profile, it seems the battery is oversized by an order of magnitude, not undersized.

        • This aircraft has 30,000lb thrust which requires about 12,000kw of power. At this rate a 900kwh battery lasts 4.5min if completely drained, less than 3min if the battery charge/discharge limits are 85/25%. Double if the battery only supplies half the takeoff power.

          Both will get the aircraft off the runway and partway through the climb, but neither of these gets to a normal cruising altitude. One tenth this capacity might not make it off the runway.

          • I get 8177kw on the basis of the schuebeler DS-215


            Specifications DS-215-DIA HST® with DSM10066-290

            Inner shroud diameter: 195 mm
            Fan swept area: 215 cm²
            Weight incl. motor, wiring,
            Plug and Secure Fan Fix: 3400 g
            Thrust range: 215-250 N
            Exhaust Speed Range: 84-98 m/s
            RPM range: 12.000-14.000 U/min
            Input power: 9,8-15,6 kW
            Allowed battery: 12-14S 20000 mAh
            Overall efficiency: 78 %

        • Albert, for a 90 kWh battery, the equivalent fuel weight would be 7.5 kg (about 10 liters), or less than 1% of the 1200 kg required fuel. So the 900 kWh battery was probably an estimate, but in the ballpark, to have a significant reduction effect on required engine power and fuel consumption.

          • you are mixing thermal energy and mechanically useable energy. ( Think Carnot Factor )
            You’ll probably not exceed a Carnot factor of .3 .. something. 90kWh ~25kg kerosene @ ~11kWh/kg thermal.
            900kWh ~= 250kg. ( a jet engine can burn fuel faster than you can get it out of a battery though.)

          • Uwe, these figures are taken from Bjorn’s article, no further assumptions by me.. The point was that the battery was not undersized.

        • The battery is not oversized. Our airliner model gives the necessary thrust for the takeoff, climb and cruise. The necessary TopOfClimb thrust value is 9% higher than the cruise thrust required at that altitude (33kft). The takeoff and climb fuel to 33kft is 1070kg (this includes the battery energy).

          If we resize the core we could shave off 9% of 1070kg which is 96kg. But the Pilot/Autopilot needs thrust margin to fly the aircraft and with the battery weight causing all the problems this margin is designed into the gas turbine. A matching of 75kg battery equivalent is thus reasonable, any smaller and you have sized the core incorrectly.

          • Bjorn, the technical details of your analysis are appreciated. My comment was based on the energy required during takeoff and initial climb. I was assuming battery power would not be needed past initial climb, the generator supplements the battery, and the battery was not power limited (sufficient specific power for a 0.15kWh/kg battery). If operating for more of the mission, then that’s the weight difference.

  9. Mar. 03 2020,
    e-plane will ONLY stand for the environment if we supplied it with power generated only from safe generated energy such as WIND, TIDE, WATER FALLS sources.
    e-plane will be no be friendly with the environment if we make use of energy generated by power stations driven by fossil, fuel, diesel sources and located outside the central of the cities .

    • I find myself not mentioning nuclear even though I believe in it. Nuclear is however the only way to make the iron, aluminium, fertiliser, concrete and synthetic fuel we need. I’m not prepared to accept the medieval dystopia of an low energy world. There are Generation IV reactors, Thorium Molten Salt and SMR “Small Modular Reactor”. The inventor of the pressurised water reactor developed them for submarines but said they were unsafe for power ratings above 60MW. Unfortunately we didn’t follow his advice and scaled up rather than using other types of reactor or multiple small ones.

  10. “… our certifiable battery system (not cell, not battery alone) has an energy density of 0.15kWh/kg and a weight of 6,000kg. It gives us an energy store of 900kWh, the equivalent of 75kg of jet fuel.”

    — There you have it. Despite all the hype and wishful thinking, it takes 6,000kg of battery to store as much energy as 75kg of jet fuel. There is no way to overcome this handicap.

    — Even in the automobile world, pure electric propulsion is facing serious challenges:

  11. It looks like this generation of Li-ion batteries does not cut it for aircrafts. Next generation batteries with glass electrolyte looks to be more powerful, lighter and safer. It will take some time before being certified for aircraft use. Present aircraft battery project risk of being the “bleeding edge” still many parts of the electrical system for todays electrical aircraft systems will help for the next generation of electrical aircrafts. Both the Toyota Prius and Mirai had first generation models thar was not that impressive but were important first steps.

  12. The first poster ( Woody) nailed it.As of today the issue is totally about the present energy density and mass fraction of today’s lithium ion batteries.Its simply not good enough for commercial flight whether it be all electric or hybrid – he’ll they are not good enough when you don’t have to fight gravity so hard ( cars).

    This will change.First with solid state lithium metal batteries ( doubling density) then later with technologies such as Aluminium air batteries. (X5 energy density).These types of batteries are also lighter ( no heavy liquid electrolytes ).

    Until then ( as above) only real alternative is green ‘conventional ‘ fuels.For burning hydrogen ( with oxygen) my personal favourite ( won’t happen mind you) is green ammonia carried under low pressure in a liquid state.

  13. Did you not check the Aluminium-air batteries chemistry to see that they are not rechargeable ?
    I dont think liquid electrolytes would be ‘heavy’ in terms of a fuel replacement for planes.
    As for the other sorts of new discoveries, they are often like medical ‘breakthroughs’, are interesting but hyped research that dont deliver

  14. Bjorn,

    Excellent analysis. I agree completely. Electric and hybrids are just not competitive with turbofans for the foreseeable future, the only advantage as I see it being reduced or no CO2 emissions during the flight, but that does not necessarily mean reduced carbon footprint, since energy for batteries has to come from somewhere.

    One more point – safety. Even the small battery a hybrid carries has to be a high energy density battery and here we are looking for a potential source of intense fire that cannot be put out, and there is no place to park (as you said in one of your articles) to disembark passengers and take care of the problem. Even a small percentage of hull losses of hybrids will make people not fly in them and so no business case can be made at all for hybrids, even for short flights.

    Best to leave aviation alone to turbofan power and concentrate on reducing CO2 emissions from 1. power plants burning fossil fuel, 2. cars and trucks.

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