Bjorn’s Corner: Electric aircraft, Part 9

By Bjorn Fehrm

August 25, 2017, ©. Leeham Co: We finished the weight sizing of the hybrid propulsion system for our 50-seat regional turboprop in the last Corner. Now we go a bit more into detail. There are additional parts needed in the system that we skipped over last time.

We discuss the impact of these additional components and any gains we can make with the aircraft configuration.

Figure 1. ATR42-600 serves as a template for our 50-seat regional turboprop. Source: ATR.


Details of the hybrid power chain

The power chain in the lower part of Figure 2 need more components to function.  If we use the high efficiency generators, inverters and motors as outlined, they all need cooling. The superconducting components (so-called High Temperature Superconducting (HTS) motors/generators) need cryogenic (liquid nitrogen) cooling. Such cooling systems need power and add weight.

Figure 2. Hybrid chain with APU backup compared with Turboprop chain. Source: Leeham Co.

Additionally, we need to switch generators, motors, inverters (we need two for redundancy) and the battery in and out of the system. This requires solid-state switches, which adds losses and weight.

Transmitting our 3.200kW of power through the system at takeoff forces high currents. If we assume the system to operate at 1,000V, we are pumping 3,200A through the wires during take-off and 3,000A during cruise. That’s a lot!

To avoid the high currents and by it, losses, one increases the system Voltage. This can’t be driven too far as insulation from arching increases with Voltage and altitude. At present, the designs seems to stop at around 2,000V, halving the current to 1,500-1,600A.

At such high voltages and currents, the wiring will be hefty and the wire’s tiny resistance will cause heating losses. To reduce wiring losses, the distribution of power is based on DC and not AC current. We then need AC-to-DC converters after each generator and a DC-to-AC converter before each motor.

Hybrid system summary

In the last Corner, we concluded the hybrid chain for our 50-seat turboprop would weigh 2.4 times as much as the original turboprop propulsion. We now see that this is not enough; it will be more with cooling, switching and power distribution.

One could argue that electrical aircraft would give new freedom to improve aerodynamics with distributed propulsors and boundary layer suction aft fans.

These are all possible solutions. But they bring improvements on the order 10-20% and we need 100%-200% or more. The electric hybrid handicaps are an order of magnitude larger than the possible gains we can make. It will take time before an electrical aircraft can compete for the propulsion of a regional aircraft.

Next Corner

During the series, I was contacted by Zunum Aero. We had a good discussion around electric aircraft. Zunum’s first product would be a 10-seater, all-battery airliner for short-range transport. In our final Corners on electric aircraft, we look at how such a concept could work.

8 Comments on “Bjorn’s Corner: Electric aircraft, Part 9

  1. Hello Bjorn,

    Thanks learned something on the advantage of DC lines vs AC lines…

    Then : Core- Generator – Rectifier – Transmission – inverter (phase variation / voltage…)

    Why is 400Hz used in aviation ? Can the 400Hz bus be used ? First TGV in France that was gas turbine powered had a Twin Turmo XII in at each end running 400Hz alternator (70’s tech) in the 3000 kw range

    Best regards

  2. Hi Bjorn,
    thanks again for an interesting Corners. Getting back to my thinking of last week and your clarifications; one can see how “the coin drops” at my side slowly that you just showing a different way of converting the energy stored within the fuel into propulsion energy:
    Speaking of your “…One could argue that electrical aircraft would give new freedom to improve…” I have a suggestion for improvement that may (it is to be seen) bring a bid more than 10-20%; Despite having there own issues, what about getting the electricity out of Hydrogen driven fuel-cells? With an energy density of 120MJ/kg, LH2 may have some weight advantage against 42.8MJ/kg(?) of carbon propellant? Well there issues like are leakages/evaporation looses, weight of tanks etc. However, I would be curious in a discussion.
    Thanks!

    • Hydrogen of the liquid or gaseous variety will leak in a crash. Think Hindenburg.

      • Hydrogen is hard to handle and causes hydrogen embrittlement on some alloys. In free air hydrogen rises very quickly and in the Hindeburg case most combustion happened above the ship. However in constrained areas when mixed with oxygen it has a extremly quick combustion speed. Fuel cells works with other fuels as well.

  3. Only way to avoid all that weight seems to me to use some kind of Fuel cell. German Navy seems quite happy with they fuel cell powered submarines, and since fuel can storage more energy per unit of mass, it could theoretically be more weight efficient.
    http://edition.cnn.com/2011/TECH/innovation/02/22/hybrid.submarine/index.html
    Also, electricity is generated directly, no need for large APU or Core needed, only some batteries. Maybe that could work better for your proposed model.

    • The next German submarines will still run on Hydrogen power but the Hydrogen will be stored as Methanol. That could also be the future for aviation.

      Maybe we should forget the turbine engine.
      Today a 135 kW modul weights 900 kg. So about 24 would be needed for an regional airliner as mentioned above (3.200kW) or 21.6 t of fuel cells (MTOW of ATR 42 is 18 t).

      I like KERS. This system has about the same energy density as Lithium batteries but can discharge and recharge far better. You can’t suck out all the energy out of a battery within 5 minutes. Depending on the wires KERS could be sucked out within seconds. Some fuel cells for take off could be replaced by KERS but still fuel cells are not quite light for an aircraft.

      https://en.wikipedia.org/wiki/Kinetic_energy_recovery_system

  4. I enjoyed the article very much as it describes the hybrid chain which is a very important factor for the aircraft. This article helps me to gain the knowledge of the hybrid chain. Thanks for sharing this informative article with us.
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