Bjorn’s Corner: Electric aircraft, Part 12

By Bjorn Fehrm

September 15, 2017, ©. Leeham Co: Last week we compared the energy economics of our 10-seater electric commuter to an equivalent commuter with gas turbine power.

Now we dig a bit deeper in the operational costs of the two aircraft. Is the electric commuter cheaper to operate over short sectors than a gas turbine driven variant? For the energy costs, it could be the fact. What about other operational costs?

The commuter designs we discuss would be similar to the Zunum Aero 10-seater commuter in Figure 1.

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

Operating economics of commuters

In the last Corner, we calculated the energy cost for the 150nm trip for the commuters. The electric variant would costs us $34, paying 8 cents per kWh for the needed 430kWh. The 8 cents per kWh is a best price for energy from the power grid, paid by transport category customers (railways and others).

The energy needed to propel the gas turbine variant the same distance would cost us $42. The inefficiency of the gas turbine (40% of the energy in the fuel would be converted to shaft hp for the fans) meant we needed 24 gallons of fuel, holding 864kWh of energy.

Besides the energy costs, there are other costs to operate the commuter. If we assume the crew and landing fees would be the same (landing fees would be slightly higher for the electric variant at 7 tonnes MTOW instead of 5 tonnes, but not a lot), the key area of cost difference would be the maintenance costs for the aircraft.

Once again we focus on the key cost drivers in the maintenance costs. The key part for the electric aircraft is the battery costs. The battery type we use needs replacing after about 1,500 charges. A replacement battery would cost ~$500 per kWh, in total $580,000. With the battery lasting 1,500 charges, we count a trip battery cost of $390.

The equivalent cost for the gas turbine propulsion would be the overhaul costs of the engines. At the overhaul, several parts are renovated while others are replaced, mainly because of fatigue limits.

Such overhauls cost around $500,000 for the turbines in question. The typical time on wing for such turbines are 10,000 trips (the wear is coupled to the take-offs, when the turbine temperatures are the highest). This gives us a cost for gas turbine overhauls of $100 per trip.

The other parts of the aircraft would be similar in maintenance costs between the two aircraft (we assume similar systems). By focusing the battery and the gas turbines, we have found the key cost drivers.

Summary

The electric commuter has competitive energy costs compared with the gas turbine variant. The replacement costs for the batteries makes it costlier to operate, though.

There is a lot happening on the battery front, with the car industry driving the development of more powerful batteries, with lower costs for line fit and replacement. The costs we have given are for an aircraft introduced in about five-to-seven years. The cost disadvantages will gradually reduce, driven by car industry advancements.

In our next Corner, we will summarize the series and discuss future developments for electric aircraft.

16 Comments on “Bjorn’s Corner: Electric aircraft, Part 12

  1. It is easy to assume electric is the solution. When the dust settles after all the numbers are discerned, you still have a huge problem. You will never be able to carry enough energy in batteries. The only solution is reducing energy demands and using a charging system. Sea-Kite.com has solved the big hurtles and is now ready to prove how to use electric power.

  2. What about turn around times?

    I doubt the batteries could be recharged fast enough for such a small aircraft. So how much aircraft would we need to replace one fuel powered jet?

    • presume quick change batteries and you could have turn times comparable to or less than refueling. there is a cost factor there, but you can do all kinds of good things for battery life if you don’t have to quick charge them to hit a turn time number.

  3. Do not know what kind of battery you are using, the auto industry is currently paying about $120/kWh.

    • They’d send you away with a stick if you offered to install those on a plane. Though $500 may be a bit high.
      Twice the auto rate i.e. $250/kWh ?

    • Let us see an affordable drone battery from the shelf: 5.8 Ah, 3 cell, 40c discharge rate. This pack costs 53$. If to assume a constant amperage on discharge (cruise flight), the stored energy in this pack computes: E= 5.8 A*h x 3.7 V x 4 = 85.84 VAh = 85.84 Wh = 0.0858 kWh for 53 bucks. From here follows: 1kWh battery costs 53/0.858 = 618 dollars.
      If you cell me 1VAh pack for 120$ – I’ll buy instantly.

    • We are talking about a battery system developed and qualified for flight up to 30,000ft and -60°C. And we talk about production quantities after an expensive development of perhaps 100-200 per year.

      Compare to the car industry which develops a system for quantities of 1000 times that. You can’t compare these markets or replacement prices. You’d be lucky to get it for $500/kWh.

  4. There’s also the likely reduced manpower costs of maintenance as technologies, and therefore knowledge and tools to maintain them, converge across different markets. Batteries, electric motors, power systems initially, with a good chance of structures and electronics in the not too distant future.

    Re the number of charges before replacement, although an order or magnitude improvement (making replacement an exceptional rather than frequent issue) seems very likely by the 2030s at the latest, right now doesn’t the capacity loss in suitable tech still reach 20% (ie a significant amount) in the low hundreds of cycles and so perhaps lead to a shorter usable life? To which needs to be added the fact that aircraft are very restricted in when they can actually recharge compared with other users of the technology, meaning route choice could have a further limiting effect

  5. You didn’t include any estimate for electric motor maintenance costs, which would differ in the comparison. These are assumed to be reliable as they are widely used in continuous duty in all kinds of industry.

    Industrial motors are not what would be used here as they get reliability from size. A single 500kw industrial electric motor would weigh 3 tons, half the projected take off weight of the electric commuter. Your earlier weight estimations size this motor at about 100kg, so it is a great deal more highly stressed. Given aircraft weight constraints, it’s materials would be pushed to their limits.

    Maintenance for this motor would surely be less than for the turbine since it has nowhere near the same thermal stress, but a turbine is also very simple mechanically so I think it wouldn’t be fair to say it is zero. I know of no weight constrained, safety critical, continuous duty electric motor applications with any history of cost. Maybe someone else can help.

      • I have seen several of these prototypes that qualify as “weight constrained, safety critical, continuous duty electric motor applications”. The problem is the last part of my concern “with any history of cost” and the Siemens link does not mention this. They don’t have the data even if they were willing to share. Simulation, test stand work and a couple of flights don’t tell you enough. Just ask Pratt.

        On a separate topic, quoted high performance electric motor weights seem optimistic. I doubt the 50kg includes coolant, radiator or pumps. And did you see the cables coming off that thing? Thick as your finger, which in copper would likely weigh about a kg/m, and there are six of them. Put the batteries in the wings where lift is made and you probably have 20kg just in cabling.

        The Siemens aircraft does a good job of summarizing this whole series. The battery powered aircraft weighs 50% more than the gas powered equivalent, but has a 15 minute endurance as compared to 4 hours for the original from which it was derived.

        • One other thing on the Siemens demonstrator: the additional weight has compromised the aircraft structurally.

          I wouldn’t be surprised if Siemens picked the Extra because it is designed to 4.5g acrobatic standards, which allowed them to add 50% more weight and still have a 3g utility standard aircraft. The modified aircraft is no longer capable of flying to its original purpose, let alone the 94% reduction in mission length.

  6. Brushless as in DC

    However, to get that AC motor to work you need an inverter (actually a rectifier /inverter aka UPS, VFD, Electronic GPU

    That too is a “stressed” element as weight is always going to be an issue with an aircraft.

    And just some thoughts:

    How much fuel does the aircraft burn in weight?

    Ergo, landing fees will be less still as the electric never looses any weight in flight.

    Your 2 tons becomes different and you need more power for that two tons.

    How does that affect tires and strut wear?

  7. I suspect that a brand new electrical aircraft as the Zunum would have lower maintenace costs as no APU/GPU required, flight controls fully electrical, brakes electrical and maybe regenerative to reduce brake wear, electrical air condition and cabin pressurization. I agree the electrical permanent magnet motors will be highly loaded and require dry ice cleaning every quarter. Would be nice to see the Zunum systems description and assumed maintenace cost as the Boeing engineers modify the systems architecture. Can a power cord give electrical juice and take off assist it will change the numbers.

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