8 ms·

I never really thought the engine the limiting factor in electric flight, but rather the heavy batteries. Found this somewhere, is it still up to date? > Jet

I never really thought the engine the limiting factor in electric flight, but rather the heavy batteries. Found this somewhere, is it still up to date? > Jet fuel has an energy density of 9.6 kWh/L. What this means is that jet fuel, pound for pound, is nearly 50x more energy-dense than batteries. However, the inefficiency of internal combustion means that 1,000 pounds of jet fuel yields only about 14x more power than 1000 lbs of batteries.

quanto 5y ago
Yes, the 15x factor (14x in your quote) is what is often cited in discussions, given all else equal. What complicates is that fossil fuels can drive propellers, turbofans, and jets, where as electric motors can only drive propellers -- each technology with its own efficiency envelop.
- Robotbeat 5y ago
  That’s not true. You can drive a turbofan or jet with electric motors as well. For high bypass turbofans, a prop fan is functionally identical. For jets, the heat increase can be produced by multistage compression (although there’s little reason to do that versus a single stage turbofan/prop fan except for maybe supersonic, and even then single-stage makes a lot of sense).
  - guntars 5y ago
    Also there’s nothing that says a jet engine needs to use jet fuel. You could use electric heaters to heat the air, because ultimately it is a heat engine and it doesn’t care where the heat comes from. Surely, if you took an existing jet engine and put some electric heaters in it would be very inefficient, but if you played to the strengths of electric heat, you could potentially make something interesting. For example, there are no potentially corrosive combustion products and in fact you don't even need to have oxygen flowing through it - it might as well be CO2 (hello martian electric jets). The second benefit is that you can deliver electric heat into a closed system continuously without cooling it from fluid flow. It's more beneficial for furnaces and things, but maybe there's a design out there that can take advantage of it. Probably more relevant for some sort of closed cycle piston aircraft. You could reach higher internal temperatures as long as the materials can support it.
    - Robotbeat 5y ago
      The issue is that jets are super inefficient for thrust. That’s why all modern passenger jets use the jet core just as a power source pretty much and rely on the bypass fan for almost all of the thrust. There’s no good reason to replicate the jet core when you can just use an electric motor to drive the bypass fan directly.
  - nbadg 5y ago
    Technically that would be a different thermodynamic cycle. For a true jet engine, you need compression, heating, and then expansion. Using the heat of compression doesn't count; it's thermodynamically equivalent to a propeller (just less efficient because not all of the work is being converted directly into kinetic energy, but rather a bunch is converted into heat). One could however make an electric jet engine. You "just" need to replace the fuel-based heating element with an electric-based one. Then the front compressor would be electric, and you'd have no use for the turbine, since its sole purpose in conventional jet engines is to extract energy from the combustion to power the compressor. I have this recurring daydream of an electrodynamic jet engine. The inlet ionizes the airflow, the compressor is electromagnetic, the heating element inductive. Just one fancy-shaped tube with a bunch of complicated electronics around it. That would be really interesting from a whole lot of perspectives -- for example, noise reduction. It also would open up a lot more flexibility in terms of power density (which is more or less determined by the amount of heat you can dump into the engine, which is more or less limited by the melting point of the turbine blades). Something else that's interesting about that idea is that it doesn't depend on oxygen in the air, which means you could use it on Mars and Venus. I've never looked into the numbers for Venus, but as I remember, the local speed of sound is a significant challenge for rotorcraft on Mars, so having a rotorless, purely-electric jet engine there could open up a lot of flexibility in terms of aircraft performance.
    - idiotsecant 5y ago
      That would be interesting! The only way I know to ionize air en masse is a strong electric arc. These planes would sound like science fair Tesla towers!
    - guntars 5y ago
      That's funny, I just wrote about the exact same idea! I literally had not heard about it until now, but clearly many people have had the thought. Do you know of anyone actually attempting to build one, toy scale or otherwise?
      - bityard 5y ago
        Someone get Colin Furze in here
- akira2501 5y ago
  People seem to be forgetting that the fuel is also a heat sink, and is often used to dump heat from the air conditioning systems. Planes can and do take off in +100 degF weather, I'm not entirely sure how you're going to manage all that heat on an electrical plane with no readily available sink like a wing full of fuel.
ginko 5y ago
I think for electric planes hydrogen fuel cells may be a real option since liquid hydrogen is so much lighter than pretty much any fuel or battery out there. The main problem is the density though.
- jillesvangurp 5y ago
  Liquid hydrogen or some other type of fuel cell is definitely a solution for flying big heavy jets. Another potential solution is simply burning the hydrogen in jet engines. Or methane. Both have been used as rocket fuel. Both could be sourced in a sustainable way. The technology to build jet engines that burn that type of fuel already exists. The reason that fuel cells are potentially interesting is getting better efficiencies in combination with light yet powerful electrical engines such as described in the article. The reason we still use kerosene is simply that the infrastructure to distribute that is there and it's a cheap fuel to create. Changing that industry is about ramping up the manufacturing and fuel infrastructure to the point where it is profitable. It's a very slow and expensive process. The right question to ask though is why big heavy jets are interesting to begin with. They are big, heavy, expensive, and complicated machines. Why do we go through the trouble of building those at a cost of around 100M per vehicle for a small one? This has to do with a lot of things but the two main ones that stand out are that fuel is expensive and pilots are expensive (and scarce). So the optimal conventional plane is one that moves around a lot of stuff with a minimum amount of fuel per kg of useful payload and the requisite two pilots. Hence the A380 briefly existed until it got to expensive to operate it. An autonomous small battery operated plane would fix both those pain points. Electricity is comparatively cheap and autonomous vehicles means no expensive pilots are needed anymore. People that are talking about making electrical airbuses or boeings are missing the point: these form factors become a lot less relevant when we can fly dozens/hundreds of autonomous vehicles for orders of magnitude less money. Mostly small planes are pretty cheap to manufacture. So, are electrical engines and batteries. And the electronics needed to run them. You could feasibly build hundreds of them for the price of a single A320. And they'd be comparatively dirt cheap to maintain and operate. And a lot more flexible in where they could go. You are not going to cross ocean in one any time soon. But short haul flight is a different matter. Neither of the best selling jets (737 & A320) is commonly used for intercontinental flights. They are used for short haul flights.
Robotbeat 5y ago
Why are they using volumetric energy density when speaking of weight? Anyway, jet fuel has a specific energy of 46MJ/kg (HHV) compared to 1MJ/kg for good lithium ion (conventional chemistry). Drop that 46 by a third for HHV efficiency of 33%, and you’ve got 15MJ/kg. But better chemistry lithium ion (lithium sulfur, lithium metal anode, etc) gets you to 2MJ/kg and the difference is only about a factor of 7. A factor of 7 sounds like a lot until you realize jet fuel is better than it needs to be and conventional aircraft still leave a huge amount of efficiency on the table. (This is the same reason why gasoline is ~15 times better than lithium ion, and typical gas car ranges are 400 miles but you can get a Model S with the same 400 miles range). The Virgin Globalflyer jet went over 40,000km on a single fueling of jet fuel at Mach ~0.5, and there have been aerodynamic and structural/material improvements since then. One seventh of that is 5700km, enough for long haul battery electric flight.
- JohnJamesRambo 5y ago
  The wheels on my Tesla hold up the batteries from gravity easily but in a plane you have to continuously lift them, does that change the dynamic?
  - dean177 5y ago
    Not as much as you might think. A huge amount of energy is required to gain the altitude for long distance flight. Cruising much less so.
  - GuB-42 5y ago
    It certainly does. To fly, an airplane has to push air down, it creates wing tip vortices, these vortices are wasted energy. The heavier the plane, the more air it has to push, the stronger the vortices and the more energy is wasted. Cars push on the ground, and the ground gives back all the force applied to it, so no losses here, planes that fly very close to the ground can also take advantage of the ground effect (that's the name) for more efficient lift. To how much extent, I don't know, but one thing for sure, manufacturers strive to make their planes as light as possible. As a side note, heavier gliders actually fly a bit better (faster) but unlike powered airplanes, they don't lift themselves. Edit: added ground effect
geon 5y ago
Jet fuel has the advantage that it is consumed as you fly. Some planes can start with full tanks, but not land. If they need to do an emergency landing, they have to dump the fuel. I imagine that would be difficult with batteries.
- pkdpic 5y ago
  Thats an excellent point. I wonder if flight batteries could be developed in small distributed cells with a weight / surface area that would allow them to hit a non-dangerous terminal velocity. And I wonder what the financial and environmental impact would be as compared to dumping jet fuel.
  - adrianN 5y ago
    Dumping solids along flight routes doesn't really sound like a good idea. People don't like getting hit by small pellets, so matter how non-dangerous their terminal velocity is. Just imagine all the broken windshields for example.
  - ehnto 5y ago
    We are making the assumption that dropping the batteries would actually be useful in an emergency situation. Considering that the plane needs to be designed to land fully loaded, since you're not dumping the batteries every flight, I think you would need to be way outside the operations planned safety envelopes to ever need to do that. I think building battery cells designed to fail safely in an accident is probably a more reasonable option than trying to dump them.
    - rich_sasha 5y ago
      Planes are safe to land fully loaded, but not designed for doing it repeatedly. The undercarriage, IIRC, needs a separate inspection after such landing. This is why, time permitting, planes will first burn off or drop fuel before doing an unplanned landing.
      - 5y ago
ZeroGravitas 5y ago
You can power an electric plane with jet fuel, just like diesel electric trains convert diesel to electricity to turn the wheels.
deleted 5y ago
[deleted]
ZeroGravitas 5y ago
It's the synergy of everything that matters. This same factoid is and was used by people doubting the feasability of battery electric cars and trucks because, especially in it's very misleading pre-combustion version, it's seems like an insurmountable difference. But the total impact is related to the full system, which in cars includes: - The useful power at the interface of road and wheel. - The weight of the engine you get rid of (including the safety precautions to stop this killing passengers). - The location of the weight. - high torque at low speeds feeling "peppy" - The type of journeys (stop'n'go traffic vs long distance trucking) - The ability to use regen to save brake weight and pollution. And so on. Some of these have unexpected impacts, in EV the low center of gravity, the ability to put the wheels closer to the corners, the quiter operation, the ease of home refueling, ease of building on a shared platform are among the things that emerged. For a while it seemed like hybrid cars might make the most sense, but a thousand small changes added up to basically make fully BEV the logical next step. In aircraft, it's all still a bit up in the air (pun totally intended) but things like operating in thinner air that give combustion engines trouble, the ability to have lots of propellers and rethink a bunch of design assumptions, quieter operation in urban areas are already being noted and explored.

quanto 5y ago

Yes, the 15x factor (14x in your quote) is what is often cited in discussions, given all else equal. What complicates is that fossil fuels can drive propellers, turbofans, and jets, where as electric motors can only drive propellers -- each technology with its own efficiency envelop.

Robotbeat 5y ago
That’s not true. You can drive a turbofan or jet with electric motors as well. For high bypass turbofans, a prop fan is functionally identical. For jets, the heat increase can be produced by multistage compression (although there’s little reason to do that versus a single stage turbofan/prop fan except for maybe supersonic, and even then single-stage makes a lot of sense).
- guntars 5y ago
  Also there’s nothing that says a jet engine needs to use jet fuel. You could use electric heaters to heat the air, because ultimately it is a heat engine and it doesn’t care where the heat comes from. Surely, if you took an existing jet engine and put some electric heaters in it would be very inefficient, but if you played to the strengths of electric heat, you could potentially make something interesting. For example, there are no potentially corrosive combustion products and in fact you don't even need to have oxygen flowing through it - it might as well be CO2 (hello martian electric jets). The second benefit is that you can deliver electric heat into a closed system continuously without cooling it from fluid flow. It's more beneficial for furnaces and things, but maybe there's a design out there that can take advantage of it. Probably more relevant for some sort of closed cycle piston aircraft. You could reach higher internal temperatures as long as the materials can support it.
  - Robotbeat 5y ago
    The issue is that jets are super inefficient for thrust. That’s why all modern passenger jets use the jet core just as a power source pretty much and rely on the bypass fan for almost all of the thrust. There’s no good reason to replicate the jet core when you can just use an electric motor to drive the bypass fan directly.
- nbadg 5y ago
  Technically that would be a different thermodynamic cycle. For a true jet engine, you need compression, heating, and then expansion. Using the heat of compression doesn't count; it's thermodynamically equivalent to a propeller (just less efficient because not all of the work is being converted directly into kinetic energy, but rather a bunch is converted into heat). One could however make an electric jet engine. You "just" need to replace the fuel-based heating element with an electric-based one. Then the front compressor would be electric, and you'd have no use for the turbine, since its sole purpose in conventional jet engines is to extract energy from the combustion to power the compressor. I have this recurring daydream of an electrodynamic jet engine. The inlet ionizes the airflow, the compressor is electromagnetic, the heating element inductive. Just one fancy-shaped tube with a bunch of complicated electronics around it. That would be really interesting from a whole lot of perspectives -- for example, noise reduction. It also would open up a lot more flexibility in terms of power density (which is more or less determined by the amount of heat you can dump into the engine, which is more or less limited by the melting point of the turbine blades). Something else that's interesting about that idea is that it doesn't depend on oxygen in the air, which means you could use it on Mars and Venus. I've never looked into the numbers for Venus, but as I remember, the local speed of sound is a significant challenge for rotorcraft on Mars, so having a rotorless, purely-electric jet engine there could open up a lot of flexibility in terms of aircraft performance.
  - idiotsecant 5y ago
    That would be interesting! The only way I know to ionize air en masse is a strong electric arc. These planes would sound like science fair Tesla towers!
  - guntars 5y ago
    That's funny, I just wrote about the exact same idea! I literally had not heard about it until now, but clearly many people have had the thought. Do you know of anyone actually attempting to build one, toy scale or otherwise?
    - bityard 5y ago
      Someone get Colin Furze in here
akira2501 5y ago
People seem to be forgetting that the fuel is also a heat sink, and is often used to dump heat from the air conditioning systems. Planes can and do take off in +100 degF weather, I'm not entirely sure how you're going to manage all that heat on an electrical plane with no readily available sink like a wing full of fuel.

ginko 5y ago

I think for electric planes hydrogen fuel cells may be a real option since liquid hydrogen is so much lighter than pretty much any fuel or battery out there. The main problem is the density though.

jillesvangurp 5y ago
Liquid hydrogen or some other type of fuel cell is definitely a solution for flying big heavy jets. Another potential solution is simply burning the hydrogen in jet engines. Or methane. Both have been used as rocket fuel. Both could be sourced in a sustainable way. The technology to build jet engines that burn that type of fuel already exists. The reason that fuel cells are potentially interesting is getting better efficiencies in combination with light yet powerful electrical engines such as described in the article. The reason we still use kerosene is simply that the infrastructure to distribute that is there and it's a cheap fuel to create. Changing that industry is about ramping up the manufacturing and fuel infrastructure to the point where it is profitable. It's a very slow and expensive process. The right question to ask though is why big heavy jets are interesting to begin with. They are big, heavy, expensive, and complicated machines. Why do we go through the trouble of building those at a cost of around 100M per vehicle for a small one? This has to do with a lot of things but the two main ones that stand out are that fuel is expensive and pilots are expensive (and scarce). So the optimal conventional plane is one that moves around a lot of stuff with a minimum amount of fuel per kg of useful payload and the requisite two pilots. Hence the A380 briefly existed until it got to expensive to operate it. An autonomous small battery operated plane would fix both those pain points. Electricity is comparatively cheap and autonomous vehicles means no expensive pilots are needed anymore. People that are talking about making electrical airbuses or boeings are missing the point: these form factors become a lot less relevant when we can fly dozens/hundreds of autonomous vehicles for orders of magnitude less money. Mostly small planes are pretty cheap to manufacture. So, are electrical engines and batteries. And the electronics needed to run them. You could feasibly build hundreds of them for the price of a single A320. And they'd be comparatively dirt cheap to maintain and operate. And a lot more flexible in where they could go. You are not going to cross ocean in one any time soon. But short haul flight is a different matter. Neither of the best selling jets (737 & A320) is commonly used for intercontinental flights. They are used for short haul flights.

Robotbeat 5y ago

Why are they using volumetric energy density when speaking of weight? Anyway, jet fuel has a specific energy of 46MJ/kg (HHV) compared to 1MJ/kg for good lithium ion (conventional chemistry). Drop that 46 by a third for HHV efficiency of 33%, and you’ve got 15MJ/kg. But better chemistry lithium ion (lithium sulfur, lithium metal anode, etc) gets you to 2MJ/kg and the difference is only about a factor of 7. A factor of 7 sounds like a lot until you realize jet fuel is better than it needs to be and conventional aircraft still leave a huge amount of efficiency on the table. (This is the same reason why gasoline is ~15 times better than lithium ion, and typical gas car ranges are 400 miles but you can get a Model S with the same 400 miles range). The Virgin Globalflyer jet went over 40,000km on a single fueling of jet fuel at Mach ~0.5, and there have been aerodynamic and structural/material improvements since then. One seventh of that is 5700km, enough for long haul battery electric flight.

JohnJamesRambo 5y ago
The wheels on my Tesla hold up the batteries from gravity easily but in a plane you have to continuously lift them, does that change the dynamic?
- dean177 5y ago
  Not as much as you might think. A huge amount of energy is required to gain the altitude for long distance flight. Cruising much less so.
- GuB-42 5y ago
  It certainly does. To fly, an airplane has to push air down, it creates wing tip vortices, these vortices are wasted energy. The heavier the plane, the more air it has to push, the stronger the vortices and the more energy is wasted. Cars push on the ground, and the ground gives back all the force applied to it, so no losses here, planes that fly very close to the ground can also take advantage of the ground effect (that's the name) for more efficient lift. To how much extent, I don't know, but one thing for sure, manufacturers strive to make their planes as light as possible. As a side note, heavier gliders actually fly a bit better (faster) but unlike powered airplanes, they don't lift themselves. Edit: added ground effect

geon 5y ago

Jet fuel has the advantage that it is consumed as you fly. Some planes can start with full tanks, but not land. If they need to do an emergency landing, they have to dump the fuel. I imagine that would be difficult with batteries.

pkdpic 5y ago
Thats an excellent point. I wonder if flight batteries could be developed in small distributed cells with a weight / surface area that would allow them to hit a non-dangerous terminal velocity. And I wonder what the financial and environmental impact would be as compared to dumping jet fuel.
- adrianN 5y ago
  Dumping solids along flight routes doesn't really sound like a good idea. People don't like getting hit by small pellets, so matter how non-dangerous their terminal velocity is. Just imagine all the broken windshields for example.
- ehnto 5y ago
  We are making the assumption that dropping the batteries would actually be useful in an emergency situation. Considering that the plane needs to be designed to land fully loaded, since you're not dumping the batteries every flight, I think you would need to be way outside the operations planned safety envelopes to ever need to do that. I think building battery cells designed to fail safely in an accident is probably a more reasonable option than trying to dump them.
  - rich_sasha 5y ago
    Planes are safe to land fully loaded, but not designed for doing it repeatedly. The undercarriage, IIRC, needs a separate inspection after such landing. This is why, time permitting, planes will first burn off or drop fuel before doing an unplanned landing.
    - 5y ago

ZeroGravitas 5y ago

You can power an electric plane with jet fuel, just like diesel electric trains convert diesel to electricity to turn the wheels.

deleted 5y ago

[deleted]

ZeroGravitas 5y ago

It's the synergy of everything that matters. This same factoid is and was used by people doubting the feasability of battery electric cars and trucks because, especially in it's very misleading pre-combustion version, it's seems like an insurmountable difference. But the total impact is related to the full system, which in cars includes: - The useful power at the interface of road and wheel. - The weight of the engine you get rid of (including the safety precautions to stop this killing passengers). - The location of the weight. - high torque at low speeds feeling "peppy" - The type of journeys (stop'n'go traffic vs long distance trucking) - The ability to use regen to save brake weight and pollution. And so on. Some of these have unexpected impacts, in EV the low center of gravity, the ability to put the wheels closer to the corners, the quiter operation, the ease of home refueling, ease of building on a shared platform are among the things that emerged. For a while it seemed like hybrid cars might make the most sense, but a thousand small changes added up to basically make fully BEV the logical next step. In aircraft, it's all still a bit up in the air (pun totally intended) but things like operating in thinner air that give combustion engines trouble, the ability to have lots of propellers and rethink a bunch of design assumptions, quieter operation in urban areas are already being noted and explored.