Airbus and Rolls-Royce hit eject on hybrid-electric airliner testbed after E-Fan X project fails to get off the ground Airbus and Rolls-Royce have ended a joint venture to produce a hybrid-electric airliner testbed that could have paved the way for electric aircraft of the future. Airbus CTO Grazia Vittadini said in a statement late last week that "we need to re-focus all of our efforts on technology 'bricks' that will take us" to a low-CO2- …
Another point on power density.
For Lithium Ion batteries is the figure for full power delivery until the battery starts to become nearly fully discharged? That is, can you use all the energy at a constant supply rate until the battery is effectively discharged?
As far as I can see jet fuel retains the power supply characteristics until the tanks are dry.
As far as I can see jet fuel retains the power supply characteristics until the tanks are dry.
That's completely wrong. As fuel level goes down, you're spending more energy pumping it to the engines, dealing with vapor, and running risks of stalls due to the fuel not being over the pump inlet while the jet is turning, and there are dangers of inaccurate fuel gauges, etc.
Battery-electric vehicles are vastly superior on that front. Li-Ion batteries do maintain a very, very flat power curve until they are deeply discharged, and engineers prevent deep discharge to extend battery life, anyhow. At the low discharge rates used in vehicles, they are extremely consistent and predictable. I would NEVER consider driving my petrol car down to 5% of a tank of fuel, but with a battery electric car, it's quite safe to do and not uncommon.
I would NEVER consider driving my petrol car down to 5% of a tank of fuel, but with a battery electric car, it's quite safe to do and not uncommon.
Buit that wasn't the question. You don't get less energy from your petrol when the tank is 10% full than when it's 90% full, but does a battery have the same characteristics?
Buit that wasn't the question.
The "answer" was just before the part you quoted...
"Li-Ion batteries do maintain a very, very flat power curve until they are deeply discharged,"
You don't get less energy from your petrol when the tank is 10% full than when it's 90% full
Actually you do, though really quite minor at that scale.
But they're still there for the whole flight, unlike the liquid fuel that burns off during the journey.
Much of the fuel burns off, but the engines, storage tanks, pumps, etc., do not. Jet fuel can be as low as 20% of the overall weight. And the weight is of most concern at take-off, where both types of planes would be fully loaded in any case. This single effect you're focusing on is really just a footnote, for a theoretical jet that doesn't exist with as-yet unknown technology.
Jet fuel can be as low as 20% of the overall weight.
A 787 is 120 tonnes empty, and can carry 180 tonnes of fuel. A 747-800 weighs 220 tonnes empty and can hold 230 tonnes of fuel. It can't land at that weight.
the weight is of most concern at take-of
Landing is also a concern, a 747 can't land with full fuel tanks, it has to dump 100+ tonnes from a full load if it needs to land soon after a maximum-weight takeoff.
A 787 is 120 tonnes empty, and can carry 180 tonnes of fuel.
That may be true, but it's useless (and impossible) to fly an empty aircraft. Throw in the crew, passengers, baggage, or other cargo, and the fuel isn't such a large percentage. Plus, a significant fraction of the fuel is maintained as reserve capacity, which would be much less necessary without the vagaries of liquid fuels.
See: https://commons.wikimedia.org/wiki/File:Takeoff_weight_diagram.svg
"but it's useless (and impossible) to fly an empty aircraft."
Now I know you're talking out of your backside. Aircraft are routinely flown empty to reposition for route changes, for testing, for maintenance stops, and to maintain flight slots.
And that diagram shows loaded fuel being more than a third of the weight of a fully laden craft, not that it's much use without numbers.
I suspect you have no idea what your talking about.
Of course, on rereading, you were talking about flying a plane without fuel, which is a stupid argument on its face. Of course a plane can't fly without fuel. Fuel is a necessary part of the plane, but flight distances (and reserve fuel) are calculated on the back of the mass reduction from fuel expenditure. Fuel is a significant proportion of the mass of a fully laden plane, as yoir own otherwise meaningless diagram attests, but that mass is expectex to reduce over the course of the flight, extending its range significantly.
Battery mass doesn't reduce as its energy is extracted, reducing the effective range of the plane as it has to continue to haul dead weight. This isn't a theoretical exercise either; this is well understood physics. Even with your mythical super batteries with their theoretical specific energy and a mass comparable to kerosene, an electric and will not have the same range as a comparable jet turbine plane, entirely because of that battery mass - a good third of the entire mass of the plane - remaining in place.
That remaining mass of empty batteries also means the cargo capacity has to be reduced, so that the plane can land safely, so on top of the reduced range, you have reduced carrying capacity.
Electric planes aren't viable. Simple as that.
Aircraft are routinely flown empty to reposition for route changes
Very difficult to fly a plane with zero fuel.
I suspect you have no idea what your talking about.
I'm the one supplying numbers and linking info. What you're accusing me of actually applies to everyone else but.
Are you sure your data is correct?
https://en.wikipedia.org/wiki/Energy_density (and other references) suggests Jet 1A has a specific energy density of around 43MJ/Kg whereas Lithium ion batteries are between 0.4 and 0.9 MJ/Kg... That is between 50-100 times worse.
Even lithium-air is only 9MJ/Kg, so 5 times worse, and they are unproven technology.
In addition, as a number of others have noted, fuel is burnt so not present at landing. For batteries this is not the case.
... solar panels ...
Max theoretical output about 1 kwh per square meter. In the tropics. At noon. On a cloudless day. Using far more efficient panels than we can make today.
Might be feasible on a 21st century zeppelin. On a conventional fixed wing airliner, probably not.
Maybe our great, great, ever so great, grandkids will indeed travel the world in gas filled bags propelled by solar electricity. But I doubt the groundspeed for lighter than air transport will ever be what air travelers today experience. Trains, however, may eventually go as fast as today's planes. Maybe faster. But probably not from Los Angeles to Sydney.
" diesel-electric trains replaced diesel a good while ago"
Indeed they did - back in the 1920s, however that was largely due to the great benefits of using an electric motor that can be co-mounted with the driving wheels rather than having to somehow transmit vast numbers of HP down to the wheels through a transmission that had to fit in quite a restricted space.
Nealry all Modern (post 1930) "Diesel" locomotives (Sprinters/Pacers and the like excepted cos they are just busses IIRC without bogies) are Diesel-Electrics.
The use case for electric fans on a plane is rather more obscure - and I can't pretend that I know what it is or even may be!
"
The use case for electric fans on a plane is rather more obscure
"
There are certainly advantages in mounting the heavy part of the power train in the fueselage rather than on the wings, and separating the power generator from the thrust convertor. Whether the advantages outweigh the disadvantages is a matter for the engineers to assess.
Also not an [aero]engineer, but I wonder if it's more of a pollution/exhaust thing? Put the power generation inside the fuselage and drive the exhaust through filters/catalytic converters/whatever, which slows the exhaust down significantly, not something you can do when the exhaust is your motive force in traditional jet engines/turbofans.
Yeahbut ... Remember this bit: "the generator – about the same size as a beer keg – but producing a staggering 2.5MW."
What kind of noise does it produce? How much sound insulation will be required to put it into the fuselage and yet still allow the passengers to hear each other scream? What kind of mass are we talking about? How much space will it take up (in paying customer ass-units[0])?
[0] If you're a resident of a small island off the coast of Yurp and incapable of translating for insular and/or holier-than-thou reasons, feel free to substitute "arse-units". The conversion factor is 1:1, even in teh metrics. You are quite welcome.
The electric fan used the same technology principals as an modern jet. This is where the burning jet fuel spins the shaft. This in turn drives the fan that pushes vast amounts of cold air through some very clever cones to provide the thrust. The jet fuel spinning the shaft is replaced with an electric motor.
As a principal it is great, it is just sorting out all the relative power delivery. This is where there generator will have come into the solution. I suspect that batteries simply cannot deliver the power required to drive the motor. It is not about the energy density of the batteries but how quickly you can get the power out without destroying them. This is where jet fuel (and ICE) have the advantage, you just keep making it bigger and as long as the pipe supplying the fuel is fat enough, power delivery is maintained. Obviously there are other constraints but it is not in the delivery of fuel to the engine.
Diesel-electric trains have been around for a while, and they make sense because of the engineering benefits of:-
A: being able to keep the diesel engine running at the most efficient speed whilst moving the train at a range of velocities
B: regenerative braking: there's a lot of energy in a moving train
C: distributed power to the wheels:- You can power as many axles you want on a train allowing a better use of traction.
D: If you design the train right, you can swap between diesel electric and pure electric running, making use of the better power conversion efficiency of a static power plant when overhead cables are available.
Diesel electric ships have been a thing for a long time. There are two benefits that I can think of from the top of my head:-
1. Power redundacy; Useful on a ship going to remote locations, where you can have "spare" diesel generators in case of failures. (e.g. icebreakers)
2. Mass distribution: You aren't constrained to straight lines from the propellor locations to where you can position the ships engines.
With regard to electric aeroplanes, given the efficiency of a large, modern, turbofan engine, I can't see how a generator or fuel cell and electric motor combination can beat that. In fact, I can't see how any engineer could seriously consider an electric aircraft for long distance freight or passenger transport without some magic breakthrough in battery energy density or a breakthrough in materials science that allows really lightweight high pressure hydrogen storage.
> Diesel-electric trains have been around for a while, and they make sense because of the engineering benefits of:-
I thought the main advantage was the torque characteristics of the electric motor and the way that gearboxes break with the sort of loading that trains have.
Diesel-electric ships have some other pretty big benefits:
3. The engine can run at its maximum efficiency for the desired power output at all times, regardless of the needed propeller RPM
4. The thrusters can use the same generator
5. You can have "Azi-pods", which mean you don't need a rudder or thrusters at the aft of the ship.
On an aircraft, (3) is still an advantage, though I don't think fan RPM varies very much.
A: being able to keep the diesel engine running at the most efficient speed whilst moving the train at a range of velocities
No. Locomotives don't have a huge bank of batteries. The engine has to throttle up and down as power needs vary, as it would with a mechanical transmission. Although in the case of multiple locomotives you can turn some of the engines on and off as needed to try and improve fuel efficiency to a small degree.
B: regenerative braking: there's a lot of energy in a moving train
No. Again, diesel electric trains do not have a huge bank of batteries to dump that energy into. Regenerative braking only works for electrical grid-connected electric trains, while it's a benefit there, it has nothing to do with why trains became diesel electic.
I suspect, from talking to a few commercial pilot friends, that you'd get a significant fuel saving simply by putting electric motors in the wheels; it seems that a significant portion of the fuel load is burned simply getting from the terminal to the end of the runway. Jet engines aren't terribly efficient at low speeds, it seems.
It's economic when oil prices are high but abandoned as soon as oil prices drop.
Main problem is that you can't tow at very high speeds (tugs would be massive and you would have to strengthen front landing gear) so it really slows down your airport operations.
Unless you can guarantee such tugs are available at all airports that the aircraft type would ever serve you still need the engines to be able to run in taxi state so don't gain as much
Virgin tested this at Heathrow as a green program.
It wasn't capable of towing at anything like taxi speed (max 30knts for the 737, 25knts for a 747).
IIRC it was abandoned because it was limited to little more than walking pace, a combination of a safety requirement (eg. stopping distance) and nose gear strength.
It would make a lot of sense from noise, cost, pollution, safety etc - but I suspect it's one of those things where the aircraft makers, airlines, airports, aviation authorities all have to align to enable the changes.
As far as I know, all aircraft are designed to be manoeuvred by tugs, I suspect there is a relatively standard nose oleo design to accommodate this. I worked on light aircraft in the seventies and used to earn extra money at weekends pulling and taxiing aircraft in and out of the gangars and parking and orpping them for owners. The different models of which there were many only needed a few adapters for the hand operated tug to connect to them, the tug had a 5HP Honda mower engine. That would pull. Cessna Golden Eagle with ease at about 2MPH.
Aircraft are designed to be moved by tugs very slowly and carefully
If you wanted to tow a fully loaded and fuelled 747 at 30mph you would need a much stronger towing point - especially if you wanted the ability to stop quickly. F=ma is a bugger.
You would probably also need an interface to allow the tug to control the brakes on the main gear
So the airline would have to pay for heavier nose gear and carry the weight on every flight for the benefit of the airport. So unless a major continent made this mandatory at all airports you have a catch-22.
I think the idea is that if they could run it on electricity then they'd have the option of alternative fuel options.
Also not being left behind if someone does develop a good fuel cell/battery/nuclear fusion/fusion energy source, maybe there's some efficiency to an internal generator, but I doubt it was ever the objective, just good enough for powering the test engine
You need far more power for take off and landing than during cruise.
So much so that cruise is less efficient than it could be, if you could use a couple of smaller jets and then add in electric fans for extra power when you need it you could improve fuel economy by a couple of percent.
NASA are trying another approach, using a smaller wing which is fine for cruise but then adding electric fans to increase the airspeed over the wing during take off and landing to gain the extra lift required. - https://www.nasa.gov/specials/X57/index.html
"You need far more power for take off and landing than during cruise."
Steam catapult for take-off and a tail hook for the landings. It's a tried and true technology that actually works. Lots of retired 35ish year olds with the requisite skill-set out there, too.
A not insignificant side affect is reclamation of all that newly unneeded runway space nearish to city centers that could be used to house the homeless.
As I understand it, the electric motors would not be powered from rechargeable batteries, but by a fossil fuel engine & generator. There are some advantages in doing it that way despite the power loss in the double conversion.
Quite a few ships are nowadays driven by electric motors & fuel-oil powered generators. Electric "azipods" remove the need to put propellor shafts through the hull (which inevitably has at least some water leakage), the heavy engine can be positioned in the best place for C of G rather than needing to be placed where the geometry allows them to drive the propellors, and having azipods that are able to rotate through 360 degrees gives the ship far better manoevourability. Other advantages are: Instant response to engine commands. Servicing & repair is far easier. The main engines can also serve to supply the ship's electrical needs, so separate generators are not required. In the event of an engine failure, both props can still be driven (at reduced power) so you don't have asymmetric thrust.
> Can anyone explain why an electric plane would be a good idea?
That was my first thought too, but then I thought it's possible that some different way of thinking might come from such a project.
The only one my non-aerospace-engineer brain could think of is to reduce necessary onboard power, along with associated weight etc, for the critical take-off phase of flight. Most of the flight is spent in cruise mode, with far less power being required, so if this can be supplied from a new design of generator, while the extra power required for take-off could be supplied by supplementary power, eg batteries, maybe that's worth exploring.
Just a thought, and may be far off, of course.
... reduce necessary onboard power, along with associated weight etc, for the critical take-off phase of flight
I have an idea for that, somewhat tongue in cheek ...
You have a socket at the end of the runway, and a cable about the length of the runway. You plug in the plane, it takes off using (in addition to it's own power) power from the grid for that energy sapping "accelerate a couple of hundred tons of stuff up to flying speed" bit, then the plug pulls out and the cable is wound back in ready for the next plane.
Actually I'm not alone in having had a similar energy saving idea some years ago, but I think all the manufacturers rejected it on the basis that they couldn't get elastic bands big enough.
OK, I'll get my coat.
"It's a genuine question, I'm not an aero engineer but I struggle to see the benefits of electrical propulsion beyond the "look how clever my engine is" bragging rights."
There is ONE thing in which that a theoretical electric plane would be superior than a normal one: noise.
One big problem to airports is the noise of the planes. A quieter plane could operate closer to the city - or with far less restrictions.
>"It's a genuine question, I'm not an aero engineer but I struggle to see the benefits of electrical propulsion beyond the "look how clever my engine is" bragging rights."
For super small private planes it might be worthwhile. ie a Piper Telsa. Mainly if you can reduce the service costs over a regular engine.
I strongly suspect this project is a combination of Greenwashing and a way for RR to score some Eco grants/tax-credits to do R&D for their small turbine on-site generator business.
If you can have 2.5MW on a pallet, compared to their current container size powerplant then a lot of on-site diesels could be replaced.
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