Fuel economics of battery-powered airplanes

Factual Questions
21

Quick-charge tech (as exemplified by the Tesla superchargers) can recharge a battery in an hour or less. It’s a lucky airline that can have an aircraft pull up to the gate, unload, service, and reload and pull out in less than an hour. I don’t think charge time is an issue. i would be more concerned about aircraft sitting in a lineup waiting to take off, as some US airports seem to be notorious for - but then, an electric aircraft just sitting needs only a small amount of power for instruments and AC. The props only need to turn when it’s moving. Perhaps that would be seen as a bonus - cheap idling.

On top of the issues of needing an aircraft mechanic to supervise any battery changes, take note of what Tesla also determined - battery swaps are not worth the hassle. The battery unit has to be encased in a solid structural package; then the vehicle has to have the structure to accommodate that battery pack. This results in redundant structures, each contributing weight - and the structure to allow easy access and change of batteries also complicates and adds to weight and complexity. Instead, the battery compartment is an integral part of the vehicle, structure as well as containment.

Another good example is cellphones. In the good old days, you could change batteries (and sometimes needed to). Modern smartphones are a sealed unit, so can be thinner, lighter, and as a bonus many are waterproof to a certain level of immersion.

The trick in both cases (sorry) is ensuring the battery lifetime and reliability is such that changing the batteries should only happen on a rare basis. With heavy use and fast charging. perhaps an aircraft using current battery tech would need new batteries every 2 years or so. (Based on the story of a Tesla used as a taxi between LA and Vegas being supercharged several times a day.)

As for fire issues, no doubt the aircraft battery pack would be better wired to detect hot spots (battery cell malfunctions) long before they become a safety issue.

As to whether passengers get off or stay on - the stated purpose is short commuter hops on feeder routes, for now, which is no different than what airlines do now. Passengers get on at small regional airports, get their luggage checked and security and printed boarding passes, thus reducing the strain on the main airports - plus using the feeder routes probably funnels them into the same airline’s longer routes, thus increasing their passenger count.

22

It may not be an issue, but comparison to a car with a relatively small battery is not the best model. A better model will be large trucks: how long will it take them to recharge? I don’t think we know the answer right now. At least I don’t.

Thank you for laying out the weight issue with swapable batteries. I tried to make that point upthread, but obviously didn’t use enough words. You spelled it out much better than I could.

23

But aren’t they designed to only charge to 80% on the road? The rate of charge slows way down as you approach 100%. An airplane wouldn’t be allowed to do that second, or third flight of the day if it only started out with 80% range.

True, the energy required to keep the aircraft taxiing is minimal when empty & obviously goes up when sitting with a load of passengers & the HVAC requirements to keep them comfortable & their devices charged. This could be an issue in summer thunderstorm season where waiting times to take off can be greatly increased, possibly to the point of them having to go back & top off the charge.

Vehicle & aircraft have different safety requirements. Vehicles routinely get into collisions & I bet that some part of Tesla’s model accounts for that fact whereas commercial aviation very rarely is involved in a collision.

Some of that is because of monopolistic reasons that will go away if Right-to-repair becomes law.

24

The 80% rule is to prevent battery degradation (some suggest 90% is OK). So whatever is optimal, most planes will do the same level of charge for battery longevity. You can charge to 100%, which is a bit slower than the initial charging. Frequent supercharging is also not good for the battery long term. This is another problem that will have to be dealt with for commercial aircraft.

Presumably, aircraft electronics will monitor battery power levels and an aircraft will not depart without sufficient range and reserve. The smallest Dash-8 (to pick a commuter turboprop out of the blue) does 289kn (~300mph) so a 45-min range would be 225mi. Lets say our ePlane only does half that, 150mph, then 45-min range is about 115mi. Any slower and we’d be using a Cessna instead. So our plane would be OK to fly about a 130-mi leg. (A Dash-8 Q200 has 2 turboprops rated at 2150hp/1600kW each, if you want to figure battery pack requirements)

Like the Tesla Semi, I presume if necessary a electric aircraft would charge multiple battery packs in parallel. Whether it’s cars, semi’s, or aircraft - the amount of power being fed into those battery is immense by our common household experience; a supercharge will charge at 450V, and charge a 70kWh pack in 40 minutes or less -. if my math is right, 233A. Few houses have more than 100A service That’s the equivalent of 4 houses pulling enough at 240V, 100A to pop the main breaker. For a simple 4-door sedan. And your airport will need dozens of these to feed a terminal of many parked electric aircraft at once.

My other point was - aircraft waiting for takeoff are wasting gas turning propellers while just sitting there. There is no need to “idle” and electric motor. There are no engine warmup issues. The power required to air condition or heat a cabin for 19 people is probably almost nothing compared to turning a 10-foot-diameter prop. The waiting will probably be more efficient for an electric plane. (I wonder if it would be simpler to put a small electric motor in one of the landing gear instead of using the props to move around the tarmac?)

Yes, the collision-proofing for aircraft battery packs is probably less than for automobiles, so lighter. But the main weight is the batteries, and one consideration will be where they are located so typical crash scenarios don’t have the passengers stoned to death by flying batteries, or a scrambled battery pack bursting into flames underneath the passengers. One suspects putting them in the wings is an answer, but that has implications for the strength of the wing spars.

Right-to-repair or not, it’s the cellphone makers who have to design the phones - and I don’t see them seeing a need to allow for easy change of batteries. You can change a battery right now; just if a third party does it, they’ve opened the case and likely the manufacturer won’t warranty the waterproofing after that.

Bu the same applies for electric aircraft, and Tesla automobiles. By the time the batteries need replacement, the cost and difficulty of performing significant mechanical surgery to do the replacement are far outweighed by the cost of the battery pack itself.

25

I’ve heard the idea of electric planes having such a motor(s), not only to taxi, but also to help accelerate down the runway at takeoff. The idea is that they can also do regenerative braking when landing and, just as in cars, it would greatly reduce wear on the brakes as well as help recharge for the next flight. I don’t know who, if anyone, is actually planning on doing this, but it sounds like an interesting idea.

26

Feel free to bookmark this prediction :slight_smile:

20 years after new car sales pass 90% EV, commercial aviation will still be 90%+ fuel-based.

27

My prediction is that large jets will be hydrogen-powered. For all the drawbacks, big jets at least tend to fly from a limited number of central facilities, so a small number of sites can service a large number of aircraft. .

Unless there’s a massive breakthrough in lighter battery tech, I suspect we won’t see anything better than the short-hop commuter e-planes described above. I suspect that jet fuel for turboprops and commuter jets will be the norm - fortunately, that’s not a lot of carbon emissions compared to automobiles or long haul jets.

28

Yes, I agree with this prediction. I think we are far more likely to see widespread adoption of carbon-neutral electrofuels (whether hydrogen or just synthetic Jet-A) than we are to see widespread battery-electrics in aviation.

29

Physics Girl (see video below) makes a case for hydrogen powered cars. The video was sponsored by Toyota which is pushing its hydrogen powered vehicle so I think Physics Girl was definitely being generous towards the concept as a result.

But, she does note some good points which are worth considering. I would guess if planes ever go electric they will use hydrogen fuel cells rather than batteries.

Take with a few grains of salt (not least because the video is about cars and not planes but we can extrapolate from that):

30

Missed the edit window:

Of note is the point she makes at 6:34 in the video. (tl;dw: Hydrogen is better for faster speeds and heavier loads and going further).

It is worth noting that hydrogen powered cars (or whatever) are still electric cars. It is just a question of where the electricity is coming from. Batteries or a hydrogen fuel cell.

31

Personally, I think that by the time cars go almost-completely electric, “commercial aviation” will be mostly ground-bound and track-based.

What heavier-than-air flying vehicles still exist will be powered by a chemical reaction between a fuel and atmospheric oxygen, but most of the travel that’s currently done by planes will instead be by trains. Or not done at all, being replaced by virtual options.

32

Moving to electric aviation will be incredibly hard for all but a few niche applications. Replacing the fleet of current fommercial aircraft will be incredibly expensive if you are trying to do it fast, as the average commercial jet stays in service for 35 years, and in many applications commercial aircraft can be 80 years old or more. There are still DC-3’s in service.

It might be cheaper and faster to just build ground-based carbon sequestration into the system. Imagine a startup that you can buy sequestered CO2 from for $50/ton. It might be way cheaper and better to use traditional aviation fuel, but have a reg that says you have to sequester every kg of CO2 your plane emits. So airlines either lease sequestration facilities, or the avfuel companies do and bake the cost into avgas and jet fuel, or third parties do it and sell sequestered carbon to the airlines.

33

The problem I see is one of convenience.

My car has a 300-mi nominal range, and I can recharge it overnight in my garage, and for that it uses about the same current as running the oven steady for a few hours. (Not a fair comparison because a oven is not full power all the time you are cooking) This is comparable to my gas vehicle’s range. For either, I’d stop for a break after about 4 hours or less anyway.

The argument for EV’s is the grid has the capacity for this, if it can also handle everyone cooking dinner at 6PM. It is in fact more efficient, because the grid can make more use of it’s peak capacity, thus generating more revenue for the existing infrastructure. (Which will change as EV’s become more plentiful - more power will be needed) Downside is many people live in rentals and multi-family dwellings not wired to support a large number of chargers.

The trouble with hydrogen cars is - then the number of hydrogen sales points will need to be about what we have now for gasoline sales; and will need to be the same format, with tanks, delivery systems, attendants, etc. Electric can use slow chargers installed anywhere - work, corner of the mall or restaurant parking lot, etc. There is less hassle with the delivery system - just need handshaking to enable charging and payments which can be done unattended. Electric supply need far less supervision.

They have a point that for large haulers, hydrogen may make more sense - but here, the trucks go from point to point for a limited number of points, or run out of a home base. So a limited number of places need to supply fuel. (or power)

This is the same argument as with jet travel. There are only so many airports that handle jets bigger than a 737. Each jet uses a tremendous amount of fuel, so if/when jet fuel becomes expensive, adding hydrogen fueling capabilities can be done incrementally in a small number of places and still be effective - unlike long haul trucking or private cars, where the infrastructure for H^2 or electricity charging needs to be installed all over.

The other problem is handling hydrogen. Yes it has much higher energy density per kilogram, but how large is that kilogram? It’s either compressed - implying an incredibly strong (i.e. heavy) container, or it’s liquified which means thermal containment issues, venting explosive gas to prevent pressure buildup, etc. And… don’t be fooled by liquid oxygen or nitrogen - the latter two are liquid at -183 to -196°C while H2 is liquid below -252°C; that’s a much bigger leap in refrigeration, whether for the fuel depot or the aircraft/vehicle. Hydrogen has its own handling issues which make it not consumer friendly.

I agree that for intermediate range transport, surface will possibly beat out air travel - at least outside North America. The USA seems to have a political problem with investing in the infrastructure necessary to build for example, high speed rail let along mag-lev, while Europe and Japan seem able to build this and countries like China are not far behind. But then, that leads to discussions about population densities and distance… Another possibility is when electric self-driving cars are a thing, you will be able to grab a vehicle in your driveway (yours or a taxi) and get to the international airport in good time. You may not travel as fast a commuter aircraft, but without having to personally deal with traffic, and avoiding the need to get to the commuter airport,check in early, and then change flights in plenty of time in the hub… The convenience may be worth the time for a drive of maybe 3 hours or less.

Except the example flight in Sweden is from an island to Stockholm. Or, if the flight goes over mountain terrain… then a flight using a short-range electric plane will still have an advantage.

34

The advantage that long-haul trucking has when it comes to hydrogen is you need only put a relatively few hydrogen re-fueling stations in place. Just plop them along highways. No need for them to be sprinkled throughout communities.

Then traditional battery EVs or fossil fuel trucks can take the cargo the last mile.

When it comes to planes you would only need a hydrogen station at major airports (hell…they could even produce the hydrogen where they are…no need to ship it in).

35

Whether electric or hydrogen, using that to power aircraft is simpler because the infrastructure can be grown incrementally. Equip the hub and a couple of nearby feeder airports, and now those routes can be electric. Add to the feeder airports, cover more routes.

Similarly for long haul. Add the hydrogen capability in only a dozen airports and now a large number of routes can use it. (I.e. O’Hare, JFK, LAX, Denver, Atlanta, Heathrow, Schipol, de Gaulle, Frankfurt, Honolulu, Narita, Sydney, Hong Kong, Dubai, Johannesburg, Delhi… You’ve covered a lot of the long haul flights.

There are two issues with hydrogen for long haul trucking - unless routes are planned carefully, you need a really long range between stations. Perusing a map of the USA, Denver to Vegas seems to be about 600mi without a lot of population centers in between. Meanwhile, going north Salt Lake City to Spokane, for example - would require detouring via the Pacific coast for a much longer trip. The installation and handling and refilling of hydrogen fuel facilities is more complex than running power lines to any particular spot to charge vehicles. The sweet spot for long haul trucking either power source would be about 600mi per refueling stop.

The same argument would apply to putting hydrogen at regional airports. Some of these may only get one or two jet flights a day; is it worth it, compared to running a bigger power line?

36

I think it is important to remember that none of this is expected to happen overnight. A switch to alternate fuels for planes and trucks and cars will take many, many years. It will mostly sort itself out with (maybe) a nudge from the government.

37

That’s the easy part. Simply install a windmill on the plane. In flight, the windmill will turn a generator to recharge the batteries. :wink:

38

Right up there with pointing a fan at a sail on sail boat.

I think you are on to something here!

39

This Nova from earlier in the year is very relevant to this discussion.

40

Is it that silly? I don’t know enough about the details of flight, but could the prop motors regenerate during descent? I mean, I’m sure they could, as long as the plane could stay above stall speed.

Piston planes engine braking isn’t something I hear about; is that something a pilot would ever do? If nobody does it because flaps, spoilers, air brakes, and such are more effective at slowing down, then propeller based regen might work. If prop braking is avoided because it makes the plane fly poorly, then it probably isn’t a good idea to try and use the propeller to charge the batteries.