Bjorn’s Corner: Electric aircraft, Part 11

By Bjorn Fehrm

September 8, 2017, ©. Leeham Co: After our definition of an-all electric 10-seater for Ultra short-haul flying in last Corner, we now compare its economics to a gas turbine propelled design.

Our designs have the Zunum Aero 10-seater commuter in Figure 1 as reference.

Figure 1. Zunum Aero’s short-haul turbofan commuter. Source: Zunum Aero.

Economics of a battery powered commuter

Our 10-seat commuter is used for short-haul routes of 100-450nm. As batteries reach higher power density (we calculate 0.3kWh/kg for the first version), its range will increase.

The architectures we compare are shown in Figure 2. A gas turbine engine 10-seat commuter is normally a turboprop design. Turbofans are suited for higher speed and longer-range designs (the specific thrust is higher, which makes them more efficient than turboprops at higher speeds).

Figure 2. The commuter’s battery based electric propulsion compared with a classical turbofan concept. Source: Leeham.

To make the comparison simpler, we will assume both designs to be a shrouded fan design with a very high bypass ratio. This gives a low specific thrust, by it optimizing the low speed range of the commuter and lowering its noise level.

The electric variant will use propulsors driven by a motor with a maximum power of 500kW. Normal take-off power is 450kW per propulsor with cruise at 350kW.

Aircraft drag

Before we dimension the gas turbine variant, let’s look at the drag situation. There are two principal drag components for an airliner. Drag due to size (parasitic drag, with air friction against the aircraft wetted area as main component) and drag due to weight (induced drag).

As described, our electric commuter has an empty weight of 6 tonnes with a Max Take-Off Weight (MTOW) of 7 tonnes (one tonne of passengers with bags is added). The 7 tonnes remains constant during our 150nm trip, in fact during all trips. Consumed electrical energy in the batteries doesn’t lower our weight. We will land with 7 tonnes as well.

Our gas turbine design has three tonnes empty weight. To this we add one tonne of passengers with bags and 500kg of fuel. Take-Off Weight (TOW) is 4.5 tonnes. During the trip ~100kg of fuel is used. We land with 4.4 tonnes landing weight. Different trips will have adapted weight profiles, dependent on trip distance and weather situation.

On average during the 150nm trip, the gas turbine variant will have 80% of the drag of the electric variant, as the induced drag is half of the heavier electric variant for much of the trip. We have assumed a wing with a high aspect ratio of 13 for the aircraft, to give the electric variant best conditions.

Economics

In last Corner, we assumed a consumption for the electric variant of 45kWh during take-off, 160kWh during climb, 250kWh during cruise and 20kWh during descent and landing. This gives a total energy consumption of 430kWh.

If we buy the 430kWh from the Power grid in the US and we assume we pay 8 cents per kWh, our trip energy cost is $34.

For the lighter gar turbine variant, we reduce these values with 20%. The fan shafts then consume 345kWh.

Our fuel for the gas turbine contains 12kWh per kg of Jet fuel. This translates to 36kWh per US Gallon of fuel, with a present price of $1.75 per Gallon. Our gas turbine core is only 40% efficient however, which means we will only get 14.4kWh on the fan shafts per consumed gallon of fuel.

Our needed 345kWh of shaft energy to drive the fans for the trip then consumes 24 gallons. The 24 gallons cost us $42.

Summary

The electric commuter is consuming more energy than our gas turbine driven one, as it’s heavier and doesn’t get lighter during the trip. The high efficiency of the electric power chain means our total consumed battery energy is 430kWh, which at Power Grid prices costs us $34.

The lower efficiency of the gas turbine core spoils the lower drag advantage for the gas turbine driven aircraft. It needs 24 gallons of fuel for the trip, holding 864kWh of energy. This costs us $42 to buy, meaning the trip energy costs are 24% higher than for the electric aircraft.

In the final Corner on electric airliners, we will summarize the series and discuss about possible development trends.

8 Comments on “Bjorn’s Corner: Electric aircraft, Part 11

  1. Good Article, however one can discuss if the approx 10pax turboprop will be anything but the Beech King Air in different forms and the electrical Aircraft will be a brand new composite design.
    The total cost of operation includes purchasing price, engine, components and APU Power by the hour rates of 2ea PWC PT6’s or equivalent. So most likely will the battery powered Aircraft have lower maintenance costs until the two battery packs bolted to the wing undersides need replacement.

  2. Electric energy cost should include wear on the battery, since degradation is directly linked to the energy used. There is no corresponding wear on the turbine model’s fuel tank.

    If you use Uber Elevate’s near term cost/capacity/cycle numbers, which I think are optimistic, every kwh of energy used costs $0.20 in battery wear. The 8 cent/kwh energy cost now becomes 28 cents, and the $34 energy cost becomes $120. This means energy cost for the electric is three times higher than for the turbine, not 20% lower.

    • Thanks Peter and Claes,

      this is what I planned to include in the wrap-up article. I will research a bit more on the battery costs but you have given me one estimate. The gas turbine has a similar cost, the Life Limited Parts (LLPs). They are not cheap as a set but they last for 10,000 to 30,000 cycles on the mature engine types.

      We’ll discuss the total operating costs more in next Corner. Today we only covered the energy costs.

  3. High density batteries have 2000 (?) cycles before serious detoriation. And then they are serious chemical waste needing a solution.

    As said, if the energy to charge the batteries keeps coming from conventional coal / natural gas plants, we are totally fooling ourselves.

    • That doesnt make sense. Should we shut down electric trains because we are using conventional thermal power stations to supply the electricity ?
      Wooden shoes arent the ultimate answer

  4. Most likely will the aircraft manufacturer guarantee a batteri life combined with a charging fee that works as a maintenance reserve for an exchange battery. That price should be cheaper than corresponding turboprop LLP’s maintenance reserve. The rest of the electrical aircraft components should be pretty low maintenace. The electrical anti icing system, air conditioning with cabin pressurization and battery cooling system in addition to some solid state transformers might cost more than expected in maintenance but overall the maintenance reserve per cycle (except batteries) should be much lower and the yearly checkup be done quickly and at lower cost if they design it all electrical with minimal pneumatic and hydraulics.

  5. you state :
    “The electric commuter is consuming more energy than our gas turbine driven one….”

    but then state :
    “..the electric power chain means our total consumed battery energy is 430kWh,…”

    and

    “The lower efficiency of the gas turbine core spoils the lower drag advantage for the gas turbine driven aircraft. It needs 24 gallons of fuel for the trip, holding 864kWh of energy.”

    I think 430kwh is less than 864kwh, which is the energy you need to load onto the A/C

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