I think the real innovation will come up from small sizes, drone to urban air taxi to regional island hopper.
Those seem to have a bit more room to throw out the rulebook and try some crazy stuff to take full advantage of electric motors.
Meanwhile the long range jets are likely to use a blend of e-kerosene made from green hydrogen in the short to medium term as it lets you gently reduce carbon without going too crazy.
Worth noting that for the medium range, hydrogen burning jets and e-kerosene have non-carbon GHG issues too, so scaled up small electric with hydrogen range extenders seems like a contender for the medium range because they avoid that.
Some of the interesting side effects of this transition are from moving from a hub-spoke model to a short hop network.
Exactly. The main reasons for the way aviation works today are economical. Large planes are more fuel efficient and fuel is very expensive. When fuel becomes very cheap (for example because of dramatically lower electricity prices), this changes dramatically. Large, complicated, expensive planes stop making sense.
For short hops, battery electric is going to dominate. It's the simplest and the cheapest to implement. And unlike jets, smaller is more efficient for those anyway. Instead of carrying people in batches of a few hundred, transporting them in small groups would be both cheaper and more flexible. More runways / pads that you can fly to. Cheap manufacturing cost and mass production. Low complexity. And if we automate the need for human pilots away, these things will basically fly at a very low marginal cost (kwh of electricity + inspections/maintenance).
IMHO for longer ranges, there may need to be some fuel cell or combustion engine based alternatives. Hydrogen is one option. A more logical option would be methane. Much easier to handle, only slightly worse energy density (and well offset by the lack of a need for heavy tanks), easy to source both the traditional way or by synthesizing it. Can work in both fuel cells, piston engines or jets/turbo props. Even synthetic kerosene might be better (there are companies working on that).
The key challenges all of these share currently is 1) availability: lots of prototypes but no volume production of anything yet 2) logistics: most airports are not equipped for this 3) access to cheap renewable energy to synthesize the fuel or power the batteries.
To me, liquid hydrogen does not make any sense in aviation (except maybe for rockets), because it's so difficult to store and handle on the ground, and even more onerous in the air.
Also flying wings will be like riding in a rollercoaster, since the window seats are far from the axis of rotation when banking, etc.
I've seen concepts for small (2-6pax) hydrogen fuel-cell electric planes using Toyota mirai fuel tanks, which make some sense.
But synthetic hydrocarbons (eg butanol), and the process to make them sustainably, make much more sense to me as future aviation jet fuel.
From an engineering point of view, planes would be close to the bottom of the list of fossil fuel uses to transition to hydrogen/other "X-fuel" [0].
Still, my cynical part expects that we will see renewable-powered flight as a relatively early (premium) use of power-to-X for the sole reason that it will allow the affluent and/or powerful to fly while claiming to be climate neutral.
[0] Our society uses insane amounts of hydrogen. One fundamental use is for synthetic fertilizer which supplies around half the nitrogen input for agriculture, without which we would be hard pressed to feed the world. This hydrogen is currently produced from fossil fuels.
I don't think any of these green fuels are viable, because the thermodynamics don't make sense. Kerosene makes aviation viable because it has a high energy return on investment and is readily available because it is produced by fractional distillation that would occur regardless. Conversely, hydrogen and e-fuels involve substantial energy losses even in ideal circumstances, and require dedicated supply chains that would not otherwise exist, which adds additional energetic costs. In a world where available energy is going to become scarcer, I don't think using these fuels is going to make sense.
Hydrogen has been used to get rockets to orbit. The thermodynamics make lots of sense. It's the pressures and temperatures at which you need to store the hydrogen that make it hard. Synthetic methane might be a much more practical fuel. That too is used for rockets and storing it is much easier. The reason it is a preferred fuel for lots of new rocket companies is that it is easy to handle and still has excellent energy density. Which is important if you are trying to get a few tonnes of useful payload to orbit.
The reason the aviation industry is dragging its feet with hydrogen is that they are currently making money with selling kerosene burning jets. The transition away from that is going to be very disruptive for them as they'd be killing the thing that makes them money now. So, like some ICE car manufacturers, they study hydrogen endlessly without ever really committing to doing anything with it. They are not in a hurry. Methane, which I mentioned before, works fine in jets. There have been flying prototypes using that as a fuel. It might be much more practical than hydrogen even. And it can be synthesized just like hydrogen. The latter is often actually synthesized from methane. That's currently the cheapest way to produce it.
Energy is not going to be scarce but abundant and dirt cheap long term. You are confusing the short term shortage of fossil fuels with the long term growth of renewables. We're not running out of wind and solar basically. And we are long past the point where exploiting either economically is a problem, even when you consider necessary storage mechanisms. The main thing we are short of here is production capacity, which is growing very rapidly.
I'm not so sure. For very long haul flights, I see hydrogen making sense.
There are 2 things going for liquid hydrogen: the square-cube law and the rocket equation.
The square-cube law: the mass of the hydrogen in the tank goes up with the cube of the size, while the heat loss rate by the square of the size. Which means for bigger tanks, the refrigeration overhead goes down.
The rocket equation: the actual equation doesn't matter, but the idea is that you need fuel to carry the plane, then you need fuel to carry the fuel, than fuel to carry that one, etc. It all converges, but the lowest the density energy of the fuel, the more fuel you need.
Hydrogen has an energy density 3 times as high as kerosene, by mass. By volume, it's about 8 times lower. But mass matters more than volume.
A long range airplane, like the Dreamliner, starts with a lot of fuel at takeoff, about 45% of its total mass. If you replace that fuel with a fuel that's 3 times as energy dense, then you don't need only 3 times less fuel, but about 3.7 times less fuel. Which means the takeoff weight gets reduced by about a third. Which means you can reduce the structural weight of the airplane by a lot. Maybe not a third, but by maybe 20%.
The compounding benefits of having a fuel that's 3 times more energy dense are considerable. Will they be enough to overcome the need for cryogenic tanks and much larger volume of fuel? I don't know, I think it's a close call. But it doesn't sound absolutely crazy to me.
> Kerosene makes aviation viable because it has a high energy return on investment and is readily available because it is produced by fractional distillation that would occur regardless.
Kerosene (and liquid petroleum-derived fuel generally) works well as an aviation fuel because it is relatively stable and energy-dense in both volumetric and mass terms. That it often has a high EROI means it can be dollar-cheap as well, which doesn't hurt, but it's still used even when the EROI is terrible (eg. military aviation - delivering fuel to a war zone costs a lot of energy) because the other advantages are still overwhelming relative to the energy cost.
> Hydrogen and e-fuels involve substantial energy losses even in ideal circumstances
Absolutely, though this is true of every industrial activity.
> and require dedicated supply chains that would not otherwise exist, which adds additional energetic costs.
While this is certainly true for hydrogen, e-fuels (which I take to mean synthetic electrochemically-produced hydrocarbon fuels, though perhaps you mean something different) can be produced and distributed using the much of the same infrastructure that exists today - for example, if you're making Sabatier methane, there's no reason you need to build new pipelines and tanks for it, though I'll grant that you do need new production facilities.
Of course, the rub is that the energy to drive it all has to come from non-carbon-producing (in operation) sources - solar, wind, hydro, or nuclear - and there's no denying that building out that infrastructure is going to be a massive project.
> In a world where available energy is going to become scarcer, I don't think using these fuels is going to make sense.
Available fossil energy is indeed going to decrease, but what makes you think that's true for renewables too?
As I was writing this comment, I took a small detour to do a bit of rough figuring here:
- Assume $77/MWh (Q1 2021) for utility-scale solar+storage [1]
- Assume 75 cents/L (today) for wholesale gasoline [2]
- Assume 105 L of gasoline contains 1 MWh (roughly) [3]
Then: $77 of solar energy can, if 100% efficiency were possible, produce 105 L of gasoline having a wholesale value of $78.75 at today's prices.
Obviously, 100% energy efficiency (or anything close to it) is impossible. But if solar energy instead cost $30/MWh, it suggests that an industrial process with an overall efficiency of 38% in energy terms (i.e. 2.625 MWh is needed to make 1 MWh of fuel) would be able to produce gasoline that matches the current market wholesale price.
Unfortunately I wasn't able to find anything quickly that describes industrial process efficiency in quantitative terms, so I don't know if 38% overall efficiency is reasonable to expect; personally, I think that probably would be considered high efficiency, so solar would need to get even cheaper (which it will) and/or gasoline will need to get more expensive (which it will) to improve the economics.
[+] [-] ZeroGravitas|3 years ago|reply
Those seem to have a bit more room to throw out the rulebook and try some crazy stuff to take full advantage of electric motors.
Meanwhile the long range jets are likely to use a blend of e-kerosene made from green hydrogen in the short to medium term as it lets you gently reduce carbon without going too crazy.
Worth noting that for the medium range, hydrogen burning jets and e-kerosene have non-carbon GHG issues too, so scaled up small electric with hydrogen range extenders seems like a contender for the medium range because they avoid that.
Some of the interesting side effects of this transition are from moving from a hub-spoke model to a short hop network.
[+] [-] jillesvangurp|3 years ago|reply
For short hops, battery electric is going to dominate. It's the simplest and the cheapest to implement. And unlike jets, smaller is more efficient for those anyway. Instead of carrying people in batches of a few hundred, transporting them in small groups would be both cheaper and more flexible. More runways / pads that you can fly to. Cheap manufacturing cost and mass production. Low complexity. And if we automate the need for human pilots away, these things will basically fly at a very low marginal cost (kwh of electricity + inspections/maintenance).
IMHO for longer ranges, there may need to be some fuel cell or combustion engine based alternatives. Hydrogen is one option. A more logical option would be methane. Much easier to handle, only slightly worse energy density (and well offset by the lack of a need for heavy tanks), easy to source both the traditional way or by synthesizing it. Can work in both fuel cells, piston engines or jets/turbo props. Even synthetic kerosene might be better (there are companies working on that).
The key challenges all of these share currently is 1) availability: lots of prototypes but no volume production of anything yet 2) logistics: most airports are not equipped for this 3) access to cheap renewable energy to synthesize the fuel or power the batteries.
[+] [-] jacknews|3 years ago|reply
Also flying wings will be like riding in a rollercoaster, since the window seats are far from the axis of rotation when banking, etc.
I've seen concepts for small (2-6pax) hydrogen fuel-cell electric planes using Toyota mirai fuel tanks, which make some sense.
But synthetic hydrocarbons (eg butanol), and the process to make them sustainably, make much more sense to me as future aviation jet fuel.
[+] [-] japanuspus|3 years ago|reply
Still, my cynical part expects that we will see renewable-powered flight as a relatively early (premium) use of power-to-X for the sole reason that it will allow the affluent and/or powerful to fly while claiming to be climate neutral.
[0] Our society uses insane amounts of hydrogen. One fundamental use is for synthetic fertilizer which supplies around half the nitrogen input for agriculture, without which we would be hard pressed to feed the world. This hydrogen is currently produced from fossil fuels.
[+] [-] janef0421|3 years ago|reply
[+] [-] jillesvangurp|3 years ago|reply
The reason the aviation industry is dragging its feet with hydrogen is that they are currently making money with selling kerosene burning jets. The transition away from that is going to be very disruptive for them as they'd be killing the thing that makes them money now. So, like some ICE car manufacturers, they study hydrogen endlessly without ever really committing to doing anything with it. They are not in a hurry. Methane, which I mentioned before, works fine in jets. There have been flying prototypes using that as a fuel. It might be much more practical than hydrogen even. And it can be synthesized just like hydrogen. The latter is often actually synthesized from methane. That's currently the cheapest way to produce it.
Energy is not going to be scarce but abundant and dirt cheap long term. You are confusing the short term shortage of fossil fuels with the long term growth of renewables. We're not running out of wind and solar basically. And we are long past the point where exploiting either economically is a problem, even when you consider necessary storage mechanisms. The main thing we are short of here is production capacity, which is growing very rapidly.
[+] [-] credit_guy|3 years ago|reply
There are 2 things going for liquid hydrogen: the square-cube law and the rocket equation.
The square-cube law: the mass of the hydrogen in the tank goes up with the cube of the size, while the heat loss rate by the square of the size. Which means for bigger tanks, the refrigeration overhead goes down.
The rocket equation: the actual equation doesn't matter, but the idea is that you need fuel to carry the plane, then you need fuel to carry the fuel, than fuel to carry that one, etc. It all converges, but the lowest the density energy of the fuel, the more fuel you need.
Hydrogen has an energy density 3 times as high as kerosene, by mass. By volume, it's about 8 times lower. But mass matters more than volume.
A long range airplane, like the Dreamliner, starts with a lot of fuel at takeoff, about 45% of its total mass. If you replace that fuel with a fuel that's 3 times as energy dense, then you don't need only 3 times less fuel, but about 3.7 times less fuel. Which means the takeoff weight gets reduced by about a third. Which means you can reduce the structural weight of the airplane by a lot. Maybe not a third, but by maybe 20%.
The compounding benefits of having a fuel that's 3 times more energy dense are considerable. Will they be enough to overcome the need for cryogenic tanks and much larger volume of fuel? I don't know, I think it's a close call. But it doesn't sound absolutely crazy to me.
[+] [-] adrianN|3 years ago|reply
[+] [-] carbonguy|3 years ago|reply
Kerosene (and liquid petroleum-derived fuel generally) works well as an aviation fuel because it is relatively stable and energy-dense in both volumetric and mass terms. That it often has a high EROI means it can be dollar-cheap as well, which doesn't hurt, but it's still used even when the EROI is terrible (eg. military aviation - delivering fuel to a war zone costs a lot of energy) because the other advantages are still overwhelming relative to the energy cost.
> Hydrogen and e-fuels involve substantial energy losses even in ideal circumstances
Absolutely, though this is true of every industrial activity.
> and require dedicated supply chains that would not otherwise exist, which adds additional energetic costs.
While this is certainly true for hydrogen, e-fuels (which I take to mean synthetic electrochemically-produced hydrocarbon fuels, though perhaps you mean something different) can be produced and distributed using the much of the same infrastructure that exists today - for example, if you're making Sabatier methane, there's no reason you need to build new pipelines and tanks for it, though I'll grant that you do need new production facilities.
Of course, the rub is that the energy to drive it all has to come from non-carbon-producing (in operation) sources - solar, wind, hydro, or nuclear - and there's no denying that building out that infrastructure is going to be a massive project.
> In a world where available energy is going to become scarcer, I don't think using these fuels is going to make sense.
Available fossil energy is indeed going to decrease, but what makes you think that's true for renewables too?
As I was writing this comment, I took a small detour to do a bit of rough figuring here:
- Assume $77/MWh (Q1 2021) for utility-scale solar+storage [1]
- Assume 75 cents/L (today) for wholesale gasoline [2]
- Assume 105 L of gasoline contains 1 MWh (roughly) [3]
Then: $77 of solar energy can, if 100% efficiency were possible, produce 105 L of gasoline having a wholesale value of $78.75 at today's prices.
Obviously, 100% energy efficiency (or anything close to it) is impossible. But if solar energy instead cost $30/MWh, it suggests that an industrial process with an overall efficiency of 38% in energy terms (i.e. 2.625 MWh is needed to make 1 MWh of fuel) would be able to produce gasoline that matches the current market wholesale price.
Unfortunately I wasn't able to find anything quickly that describes industrial process efficiency in quantitative terms, so I don't know if 38% overall efficiency is reasonable to expect; personally, I think that probably would be considered high efficiency, so solar would need to get even cheaper (which it will) and/or gasoline will need to get more expensive (which it will) to improve the economics.
[1] https://www.nrel.gov/docs/fy22osti/81325.pdf
[2] https://markets.businessinsider.com/commodities/rbob-gasolin... price quoted in gallons and converted
[3] https://hextobinary.com/unit/energy/from/gasoline/to/megawat...
EDITED because I can never remember to double-space when I want newlines
[+] [-] avmich|3 years ago|reply
Factual error here. Sputnik rocket - both stages - used liquid oxygen and rocket-grade kerosene.