Bjorn’s Corner: Sustainable Air Transport. Part 8. Serial Hybrids.

February 25, 2022, ©. Leeham News: This is a summary of the article Part 8P. Serial Hybrid, the deeper discussion.

We take an ES-19 type of battery aircraft and add a range extender to avoid the inadequate range we found in Part 6 and 6P.

Initially, it seems a good idea. We can use the benefits of the battery and then complement it with energy from the range extender. As you systematically work through the concept, the problems surface.

Figure 1. Serial Hybrid works for cars (Toyota Prius pictured), but not for aircraft. Picture: Toyota.

The Serial Hybrid

When battery-based projects add a “range extender” to fix the inadequate range and reserve problems we saw with the Alice and ES-19 in Part 6 and 6P, they create a Serial Hybrid. This configuration is harder to make work than it seems.

In the deeper discussion article, we build a 19 seater with a range extender using an ES-19 type of aircraft and compare it to the Beech 1900D, the most produced 19 seat Commuter.

As we build the propulsion system, our initial configuration (ex. battery) is 50% heavier than the twin-turboprop system of the 1900D. By utilizing the whole 19,000lb Maximum Takeoff Weight (MTOW) limit of CS23/Part 23 (the Beech 1900 uses 90% of the limit), we can get 150kg more Useful load (payload plus fuel/battery) for the ES-19RE than for the 1900D.

The problem starts when we size the battery. We only have 1,118kg for fuel plus battery if we transport 19 passengers (the hybrid shaves of 450kg from this figure compared with a pure battery aircraft).

As we saw in Part 6 and 6P we need much more battery weight if we want to fly anywhere on battery before shunting in Turbogenerator power. So the idea of a “range extender” is moot, we haven’t even battery for the 45nm climb after takeoff.

Top power assist hybrid

The next idea is for a takeoff and climb top power assist to a Turbogenerator from battery stored energy, tanked from the electric grid before taxi. The combined electric power feeds the motors driving the propellers.

Our model tells us we need a 1,360kg battery if we want to assist a Turbogenerator sized for cruise. The battery assist gives us extra electrical power for our electric motors for takeoff and climb to cruise altitude. Once there, we could recharge the battery during cruise and descent. As we only have 1,118kg for battery and fuel, it doesn’t work.

Even if it did, we now have a single point of failure commuter (we rely on the Turbogenerator to recharge the battery for reserves), and it’s questionable if the market would accept such an aircraft. The single-engine Pilatus PC-12 and Cessna Caravan are only accepted as 9-seat Commuters because of 40 years of outstanding reliability from their Pratt and Whitney Canada PT6 engines. A new turbogenerator doesn’t have this pedigree.

Further, to recharge the battery on the short flights, the Turbogenerator now has to be at the same power level as the two PT6s for the Beech 1900D. Consequently, we are now at a lower useful load, and the fuel consumption is only marginally lower than for the 1900D.

When we add the cost for the grid electricity, the energy cost for the hybrid is about the same as for the commuter it shall replace.

In the end, we have a heavier, more complex, costly to maintain Commuter with a shorter range and less useful load, and with the same energy cost as a 40-year-old Beech 1900D. All projects that have ventured this path have skipped the idea for the above reasons.

Does a parallel hybrid change the above? This we check next week.

35 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 8. Serial Hybrids.

  1. How will the numbers chance if the ES-19 gets wire catapult TO to max speed at 50ft and then just climb and cruise on battery power. I understand it is not easy to certify but removing the T-O energy consumption to Vmax would extend the range xx nm.

    • Hi Claes,

      you save 17kWh on the aircraft level, i.e. 22kWh on the system level. This is a few percent of your Taxi+To+Climb consumption. The climb is the problem as it goes on for 15 minutes.

      • If the release speed is 3 x V2 it will help the climb a bit, in a pretty short range parabolic flightpath with constantly reduced speed up to top of climb and then a shallow constant speed dive to target. (Nothing todays ATC likes and more like sailplane competitions).

  2. I thought when I saw the picture of the Alice full size mock up/prototype that those looked like ‘turbines’ on the rear pylons

    • It’s the electric motors with the cooling inlets for the electric motor cooling. At takeoff you have 650,000W*6%=38,000W losses to cool for the motor plus about half that for the inverters on each side.

      • Another reason for superconducting motors: weight, efficiency & no cooling drag?

        • Another blue sky solution. Just need to have ultra low temperatures in the motor , no problem

          • Superconductivity works and is in service on both motors and generators. Even on wind turbines. It will work well with cryogenic hydrogen.

    • -The PC12 glide ratio is about 16:1, the Caravan series seem to vary from 14:1 to 12:1. They would need to be within 34km of a suitable airfield assuming cruise at 3000m and a 20% margin. Interestingly a motorised sail plane like the Stemme S12 with a glide ratio of 53:1 could operate 150km from an suitable airfield if cruising at 3000m under IFR (assuming instruments were fitted)

      • The PC-12 is pressurized and has a 30,000 ft ceiling, for a 146km max radius from an (sea-level) airfield, and its short landing distance of 660 m gives it a large coverage of airfield-dense areas like europe or north america.

        • you’d need engine independent cabin pressurization.
          (i.e. electric compressor and a capable power source
          or longtime oxygen supply for all seats ?)
          Otherwise you would have to descent to save pressure altitudes fast.

          • 15 minutes of oxygen per occupant seems easy to provide to gain the operational flexibility and efficiency of FL300 cruise

  3. I am left with the impression that the Walther RII.203 hydrogen peroxide monopropellant rocket motor used on the Me 163A would produce a more environmentally friendly, longer ranged aircraft than a battery powered aircraft. Indeed hydrogen peroxide has been proposed as an energy carrier.

    • Environmentally friendly maybe, passenger-friendly in a crash not so much (see pilots being dissolved/rapidly turned to ashes)

      • -I was of course being somewhat ‘tongue in cheek’ with my suggestion.
        -Batteries are showing themselves fire prone as well. After all what would one expect in a battery that is effectively two reagents, like a bipropellant rocket, separated by a slim membrane.
        -Me 163B pilots did almost always survived crash landings and over turned aircraft without harm from the propellants. The poor sink rate absorption capability of the landing skid made for a harsh landing and back injuries. (To be remedied in the Me 163D with a proper undercarriage). The dissolving pilot case does have a sad basis. The pilot involved was popular and affable and had the day before the incident just married with the squadron in attendance. Upon his take-off the aircraft suffered an engine failure. He was able to turn around but the wing hit a radio tower that was in the way. It’s likely he would have perished from the impact but the propellants did attack his remains.
        -One day there will be an accident with batteries (uncontrolled fire, melting through a spar) and or hydrogen and measures will need to be in place. It’s hard to imagine a hydrogen explosion (maybe a small fire) with so many sensors detecting leakages and the gas venting upward.

      • But a Thrust to Mass ratio of 100:1 which is why it can perform so well. If we use bipropellant chemical reagents to drive a fan or propeller aren’t we achieving the same as a battery? Apparently sugar syrup reacted with 40% Hydrogen Peroxide is quite energy dense.

  4. The only reasonable electric airplanes now are small ones, and their advantage is cheap operation.
    Velis Electro has very low operating cost, and likely Tecnam P-Volt will be the same (with all the related limitations).
    Scaling down a turbine for range extender is not going to work; much better would be scaling up an ICE.
    And not having it as the sole source of power should allow to relax some of the certification issues that are keeping alive 50 years old powerplants.

    • Self launching electric sail planes are somewhat of a success. They don’t annoy the neighbours with engine sounds (a big issue in Switzerland & Germany).
      They probably give you 3-4 launches, probably more these days, to several thousand feet. They also provide a safety if needed.
      High glide ratio is clearly something that is desirable in an electric aircraft. If it can be achieved with minimal weight penalty range will be extended.

  5. What happens to our electric aircraft airliner or transport when we configure our electric aircraft (or hybrid) as an ultra high performance sailplane with say a glide ratio of 50:1? Would that not produces a viable electric aircraft, albeit a slow one? For instance if it can get to an altitude of 7500m/25000ft it can literally glide 375km.

    • Assuming a 40% battery fraction with advanced batteries at 200Watt Hours/Kg (720kJ/kg) results in 288kj/kg for the aircraft. Equating 288kj = mgh leads to an altitude of 30,000m. Assuming 80% propellor efficiency and that 50% of this is used for sustaining level flight at minimum flying speed and the other 50% for gaining altitude we end up at an altitude of 15,000m from which a L/D of 50 produces a drift range of 750km. This would be after a very large distance covered during the climb. What am I missing?

      • Björn pointed to the climb energy consumption in his first response. You are right that L/D as per Breguet eq. is directly proportional to range. With limited energy available and a good L/D together with short range flying up to 500km I think optimal lowest engergy consumption routes can be calculated. The climb is pretty slow as you gain altitude and distance to target, then as you point the nose down at top of climb you accelerate getting help from a heavy aircraft and just a few degrees sink alfa. Just like sailplanes load up with a water to gain speed as they point the nose down a little. Those routes are different from todays routes for regular Beach/Pilatus/Saab’s and ATC need to define those before battery pax planes can get traction as commercial airliners. See motorgliders today https://en.wikipedia.org/wiki/List_of_motor_gliders

        • It was a though experiment. I failed to take into account propeller efficiency of 80% I’d noted but the aircraft should still reach 12000m/40,000ft and glide 600km. Factoring in the distance travelled during climb (could be another 600km) it would seem ultra high glide ratios might be the key to long range in electric aircraft. The problem might be in obtaining a battery mass fraction of 40% along with the ultra high aspect ratio.

      • I would say what you are missing is the obvious. Glide range is only for complete engine loss and no regulatory agency will approve climb and glide as normal operations. It’s an indication of ..we could say …a rather detached from reality approach of the enthusiasts who more like to do a disservice to the technology

        • Friend of mine works in the domain and reported from a DE ATC prsentation:
          Future for air traffic control will be individual flightpath, time of arrival managed AND a longish glidelike phase after reaching top of climb. ( at least for the EU airspace.
          transocean is a different domain.)

          • It makes sense to have these aircrafts “book” 4D low altitude shallow tracks in the air with the help of ATC computers and automation. These will include preprogrammed alternatives in case of weather, failures of systems, communication, government/military flights, or medical/firefighting/police in its programmed way

  6. A Prius type “side-arm” hybrid setup would make more sense.
    combine generator and motor and a gearbox into regular geared turbofan.

    Your next installment, Bjorn?

    • Thats An-225 a 6 engine derivative of the An-124
      An ‘A225’ could be mistaken for the A220 series member

  7. In 3-4 years time, when the A220-500 might be launched that could become a real hypsographical problem.

  8. Prius is not a serial hybrid. it is a parallel hybrid where the gas motor directly drives the transmission, and the motor/generator unit is on the shaft connecting the engine to the transmission.

    the two most commonly known serial hybrid production cars are the BMW i3 with the range extender (which was not capable of fully powering the car) and the original Fisker Karma.

    Nissan has a line of serial hybrids, but they are not sold in the US.

    • I think it has aspects of both. The ICE engine and the small motor/generator are connected on the same shaft and are therefore in parallel, these are then connected via the PSD (Power Split Device; essentially an differential) to the larger electric motor. The ICE+small motor/generator and the larger motor/generator sum their shafts in to the PSD (differential) to the output shaft.

      It all seems in parallel unless you consider that in some modes the small motor/generator (in generator mode being driven by the ICE) can transfer power to the larger motor along with the battery. This effectively allows the Hybrid Synergy to also work as an infinitely variable gear box. So it does seem to have a mode where it is effectively both in parallel and in series simultaneously.

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