Electric flight

louis · 2018-03-07 04:43:54

I am a bit behind on this technological development: electric motor flight.

https://www.siemens.com/press/en/featur … hp?content[]=Corp

Just thought others might appreciate reading about this if they aren't aware of developments.

Seems like it's restricted to smaller planes currently. I calculate you'd need 17 tonnes of motor to get enough power to get a 747 off the ground. Of course that's without factoring in the fuel/battery or any hybrid back-up. But you might envisage a system which uses a ground microwave beam to provide the power to the motor for the take-off. Not sure what you could get away with in flight in terms of a power source.

Antius · 2018-03-07 07:32:08

The range of a 747-400 is given as 13,450km on wiki.

The energy density of kerosene is ~43MJ/kg and turbojet efficiency is ~30%.

The energy density of lithium-ion batteries is 0.36-0.875MJ/kg – say 0.9MJ/kg for a scaled up, near term battery. DC motor efficiency is ~80%.

Crunching the numbers gives a range of 750km for a 747-400 airframe propelled by electric motors. However, a kerosene fuelled plane gets lighter as it consumes fuel. So maybe 700km is a more realistic estimate. With a 20% margin that puts maximum range at 500km.

That could have uses for internal flights. Especially if aircraft can function like buses, landing at an airport, dropping some people off and picking others up. On the ground, they could swap batteries. More streamlined airframes may push up the range of electric planes. Maybe one day a 1000km range is possible?

The economics of the prospect are marginal at present, but may improve as oil depletion accelerates. A kg of kerosene costs $0.5, which works out at 1.163c/MJ or 3.88c/MJ of engine power. One kWh of grid electric power presently costs about $0.1 in most countries, or 2.78c/MJ. One MJ of engine power therefore costs 3.74c.

The operational constraints and additional infrastructure imposed by an electric aircraft would appear to make it marginal at present. However, if fuel becomes more expensive and/or electricity becomes cheaper, the balance may shift in its favour. Most likely, both fuel and electricity will get more expensive, as fossil fuel depletion continues and we fail to find alternatives that deliver at the same cost.

Last edited by Antius (2018-03-07 07:34:40)

Terraformer · 2018-03-07 08:42:18

How much lithium are you going to need for that? How much will that cost, since you're competing against other uses such as grid storage for it?

If you want to electrify travel over a few hundred kilometres, use trains.

Antius · 2018-03-07 09:28:20

Terraformer wrote:

How much lithium are you going to need for that? How much will that cost, since you're competing against other uses such as grid storage for it?
If you want to electrify travel over a few hundred kilometres, use trains.

The 747-400 carries 216,840L (170 tonnes) of fuel. So a battery mass about the same, although it delivers a shorter range. That is 315 times the Tesla battery mass. About the same battery mass per passenger as the Tesla.

The electric aircraft is a much more efficient use of lithium, because a given battery mass will deliver far more passenger miles in its lifetime – likely 20 (or more) times greater. This is because: (1) Trips are longer – a car will typically make lots of trips <40km in its life and relatively few trips greater than 100km (incidentally, this is why hybrid vehicles make more sense than pure electric); (2) It is faster – my calcs are based on a Mach 0.8 cruising speed – the same as a 747.

That being said, I agree that trains are probably a more energy efficient means of transport per passenger km. But they aren't much good over water and they are slower.

The electric aeroplane could benefit from some significant efficiency advantages over the kerosene propelled counterpart. Firstly, the engines do not need to consume air to generate power. They can be tucked away inside the wing with no inlets. That is a huge reduction in drag and it opens up greater freedom in airframe design. Secondly, a turbojet needs to rotate at high speed to achieve necessary compression ratio. This results in relatively poor propulsive efficiency because even high-bypass turbofans tend to rotate too fast and produce exhaust velocity too high. Gear boxes are heavy and introduce failure modes. An electric motor speed can be tailored to the airspeed of the plane achieving much greater propulsive efficiency. So range could increase substantially above what I have calculated.

Ultimately, what at first appears to be a crazy idea could make a lot of sense.

Last edited by Antius (2018-03-07 09:30:13)

JoshNH4H · 2018-03-07 10:48:12

As far as electrically powered flight goes I've seen some really cool animations of people-carrying drones (is it really a drone if it has a passenger?) that could be used to carry people around cities at high speeds. Limited range of course but it gets you where you need to go faster than cars or trains.

I'd be all for the MTA starting a drone bus service.

Edit: I suppose the drone bus would have a driver and would therefore not be a drone.

Terraformer · 2018-03-07 11:06:49

Are there many routes over water that would be doable within the range of an electric plane? I suppose transatlantic flights could stop off in Iceland and Greenland for swapping out the batteries. At least in Iceland, they can power them using geothermal and hydropower.

If you look on a Dymaxion map, you'll see that there isn't actually that much water to cross to get from one point on land to another. The biggest gap to cross is probably the North Atlantic. We might be able to build a tunnel from Scotland to Iceland to Greenland to Canada, and power the whole thing from Iceland. But that's a megaproject beyond anything humans have done before, and I'm not optimistic we're going to actually do that. An electric plane could close the loop, perhaps.

I hope something comes up. I have family on both sides of the Atlantic, and I'd like to be able to move between the US and UK.

JoshNH4H · 2018-03-07 11:18:37

I'm all for an arctic hyperloop.

Here's an interesting one: Below is a map of historic human migrations from Wikipedia that could also double as a route map for a global hyperloop (or hyperhyperloop) system:

GW Johnson · 2018-03-07 12:22:20

So far, electric flight is limited to motors spinning propellers. That's appropriate for an aircraft cruising under 300 mph (490 kph) true airspeed at altitudes under (or well under) 20,000 feet (6 km). Jet airliner aircraft cruise much faster, typically about 530 mph (860 kph) true airspeed at altitudes nearer 30,000 feet (9 km).

For an electric airliner, you are probably looking at a multiple-propeller airplane flying around 150-200 mph (240-320 kph) at something like 10,000 feet (3 km). What is a 10 hr trip transatlantic today would be about 30 hour ride in an electric airliner, no better than the DC-3 back in the late 1930's, and then ONLY if you can achieve transoceanic ranges, which you cannot. Not yet.

I wouldn't hold my breath for this kind of commercial air travel. Its first application will be light aircraft and private pilot training for under 120 mph (190 kph), under 10,000 feet (3 km), and within 10 miles (16 km) of an airport. It may prove less expensive than conventional flying, for that kind of an application. But it cannot entirely supplant the use of conventionally-powered aircraft for private pilot training, because fresh pilots must be able to fly those, too.

This technology (along with hybrid propulsion) is already flying in a few experimental homebuilts.

GW

Antius · 2018-03-07 13:07:29

Short haul flights are classed in Europe as distances shorter than 1500km. With modest improvements in battery technology and innovative airframes, the electric aircraft might stretch to the upper limit of that without sacrificing payload.

https://en.m.wikipedia.org/wiki/Flight_length

London-Berlin = 931.5km
Edinburgh-Dublin = 350km
London-Madrid = 1263km
London-Barcelona = 1139km

So this could take care of most air travel within Europe.

Achievable with 1 stop:

Paris-Moscow = 2486km (stop at Warsaw).
Glasgow-Helsinki = 1768km (stop at Stockholm).

Transatlantic is much more difficult. As Terraformer says, you would need to hop from Scotland to Iceland, from Iceland to Greenland and from there to Canada. You probably wouldn't use an electric plane for transatlantic flights as all those stops would be logistically expensive and it is better to fly in a straight line.

JoshNH4H · 2018-03-07 17:07:46

This seems like exactly the kind of case where you might want to use a fuel cell.

H2 has a much higher energy content than kerosene (143 MJ/kg vs 40 MJ/kg), and fuel cells are also roughly twice as efficient as thermodynamic cycles.

I don't know if the power density is high enough but it seems like you could actually fly pretty far with it.

I'm not sure what the benefit here really is though. I'm all for making air travel more climate friendly but this seems like one of the industries where there's actually really good reasons to be using hydrocarbons, and maybe they just need to be synthetic/biofuels.

Ethanol seems like a genuinely viable alternative to kerosene. Slightly lower energy density than kerosene but it has a pretty rich history as a fuel, including its use on the V-2 (the Nazis called it B-stoff), and in Mercury Redstone.

EROI considerations don't matter as much in a niche application like air travel.

louis · 2018-03-07 18:12:55

I think the Hyperloop is v. exciting but I've never got a fix on the possible passenger load. Could it handle numbers similar to trains and planes.

JoshNH4H wrote:

I'm all for an arctic hyperloop.
Here's an interesting one: Below is a map of historic human migrations from Wikipedia that could also double as a route map for a global hyperloop (or hyperhyperloop) system:
https://upload.wikimedia.org/wikipedia/ … ations.jpg

louis · 2018-03-07 18:16:46

Thinking about it, if we had an efficient and effective microwave beam/laser transfer of energy, I guess you could have ground or sea stations every 50 miles or so that that would beam the energy to the planes which could then power their electric motors.

JoshNH4H · 2018-03-07 18:49:31

Hey Louis,

So far as I can tell, the hyperloop would be able to carry passenger loads intermediate between planes and trains.

Terraformer · 2018-03-08 04:39:49

I would happily accept a 2 day trip across the Atlantic, if I could travel in style and it was cheaper than a plane. Airship or transatlantic train, though I'd prefer the former given the views (no use having a window when you're in an undersea tunnel). Passengers who have to get there quickly can take the far more expensive jets.

Antius · 2018-03-08 04:53:20

Terraformer wrote:

I would happily accept a 2 day trip across the Atlantic, if I could travel in style and it was cheaper than a plane. Airship or transatlantic train, though I'd prefer the former given the views (no use having a window when you're in an undersea tunnel). Passengers who have to get there quickly can take the far more expensive jets.

Problem with airships is that although they could ultimately be more fuel efficient than jets, they are limited to speed of 60-100mph. That is economically difficult because the vehicle delivers fewer passenger-miles within the investment window. That would make an airship transatlantic flight expensive. For shorter journeys of up to 1000miles, they may compete better with jets and land transport. Energy efficiency is a function of size for an airship, so bigger is better.

As mass transport, i.e. more than just a technological curiosity, there is really no alternative to hydrogen as a lifting gas. Bringing back the airship would require that we mitigate any hazards involved in the use of hydrogen lifting gas to at least tolerable levels. That would be easier today than it was in the 1930s, but it may not be possible to reduce risks to levels that satisfy aviation authorities.

Last edited by Antius (2018-03-08 05:00:59)

Terraformer · 2018-03-08 05:57:45

Aviation authorities are fine with passengers sitting between two large fuel tanks filled with highly flammable jet fuel...

The hydrogen isn't the problem. It's above the passengers, and significantly lighter than air. As long as the envelope containing it is fireproof, a fire should result in a (slow) crash as the airship loses lift, rather than people burning to death. Best make the gondola seaworthy...

JoshNH4H · 2018-03-08 12:12:37

I believe hydrogen balloons were traditionally spiked with some amount of H2S, so that in the event of a leak everyone would become immediately aware by smell of rotten eggs.

You might try to do one better, and also spike the balloon with some gas or another that will emit visible light if any combustion happens.

SpaceNut · 2018-03-08 15:02:08

Or you could use helium....but still add in the smell detection as we are trying to remove the mass penalty of batteries and of the motor to be able make the flight possible.

elderflower · 2018-03-09 05:49:28

There's a problem with a proposal for a trans Atlantic tunnel. The distance gets a few centimetres greater every year due to sea floor spreading. You might have to come up in Iceland and then down again so that the expansion would occur in the only on-land section of the mid Atlantic rift. Alternatively you could go the long way round via the Bering Strait. You would have to be extra nice to Putin for that!
In the deep sea a buoyant, tethered tube might be used to limit the external pressure by keeping it at a few hundred metres depth. This would restrict submarine activity. The aforesaid gentleman might have something to say about that as well.

Terraformer · 2018-03-09 11:30:32

The distance between Greenland and Iceland is only a few hundred kilometres, I think. That would keep the tunnel with the EEZs of Iceland and Denmark.

Alternatively, people could fly that distance, or take a ferry. Take the train from Manchester to Iceland, fly/ over to Greenland, and take the train to Boston via Canada. Not as simple as a single journey, but perhaps worth it if liquid fuels become a lot more expensive. It probably makes more sense for cargo than passengers. Would an electric ship manage that distance?

Antius · 2018-03-10 16:51:29

Terraformer wrote:

The distance between Greenland and Iceland is only a few hundred kilometres, I think. That would keep the tunnel with the EEZs of Iceland and Denmark.
Alternatively, people could fly that distance, or take a ferry. Take the train from Manchester to Iceland, fly/ over to Greenland, and take the train to Boston via Canada. Not as simple as a single journey, but perhaps worth it if liquid fuels become a lot more expensive. It probably makes more sense for cargo than passengers. Would an electric ship manage that distance?

Maybe. Ships are in many ways much easier to adapt to non-fossil fuel energy sources than aircraft, because they are nowhere near so limited in terms of power to weight. If they work on stored energy, the mass of the energy store can be part of the ballast weight of the ship, provided it is within or close to the keel.

A ship could be nuclear powered with a pressurised water reactor. This has been done before and it would be easy in principle to adapt commercial PWRs to provide propulsion. Or it could be powered by batteries or maybe even stored thermal energy in a molten salt with an S-CO2 generation cycle. Cryogenic energy storage is another possibility, as the ship is surrounded by a huge heat sink. Ships could even burn coal, liquefy the CO2 and dump it in the deep ocean with a long pipe. Far more options using ships. Provided people are willing to slow down a bit. It would take a ship about 4 days to get from the British Isles to New York. The journey could be a lot more comfortable than the cramped standard class seats on a transatlantic jet. But there is no way it could possibly be so fast.

Last edited by Antius (2018-03-10 17:29:26)

Terraformer · 2018-03-11 03:51:27

Well, we've known how to do nuclear powered ships for quite a long time. To get a civilian nuclear ship industry going, though, I think governments would have to allow them to use the facilities used to handle the navies nuclear ships, until there's enough civilian ones to justify new facilities. It would also mean they wouldn't be able to hire cheap crews from the Philippines, and a constant police presence. All in all, more expensive, but perhaps justifiable as oil becomes more expensive.

The parallels to space travel, of course, should be obvious...

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Electric flight

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