Could we ever replace fossil fuels in aviation?

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Sleeper
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Could we ever replace fossil fuels in aviation?

Postby Sleeper » Thu Apr 10, 2014 8:04 am UTC

There's a lot of discussion of switching over to automobiles that use electricity or alternative fuels like hydrogen. It might even be feasible over time for the whole world to switch entirely to electric or alternative-fuel vehicles.

But what about flight? Flight is an essential part of the modern transportation infrastructure, and a very strong contributor to greenhouse gases. Is there any alternative clean fuel that could run a jumbo jet?

Could we replace gas-guzzling jets with slower, but much more efficient zeppelins?

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Re: Could we ever replace fossil fuels in aviation?

Postby Zamfir » Thu Apr 10, 2014 8:55 am UTC

I have seen serious preliminary designs for hydrogen-fueled airliners. As in, a department of a airliner manufacturer spent considerabke effort to identify the requirements, likely performance, cost, and bottlenecks. Not that they seriously thought about building one.

It seems possible, with acceptable bottle necks, but you have to accept significantly less bang for the buck. The great advantage is weight: hydrogen fuel weighs less than kerosene for the same energy. This seems to work out positively, even if you take the weight of the tank into account.

The trouble is volume. Kerosene is mostly stored in wings, in volumes that would be empty otherwise. A hydrogen tank would be large. You end up with something like an A380 where the entire top deck is hydrogen tanks, and if I remember correctly, that still implies less range and a lower cruise speed.

Obviously, there might be al kinds of safety issues that only show up in detailed design, and in experience. There would have to be a lots of other hydrogen planes before one tries an airliner.

Working from memory, I think there might be a size issue as well. The weight of a tank scales far less than its volume, so hydrogen aircraft make more sense for large aircraft than for small ones. That makes it hard to build experience, since you can't start small and build up to large. But there are some niches to experiment with. Loitering drones perhaps, or unmanned/military freighters. You should not expect hydrogen airliners for many decades yet.

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Re: Could we ever replace fossil fuels in aviation?

Postby wumpus » Thu Apr 10, 2014 3:47 pm UTC

Interesting. I would assume that the hydrogen would be liquid (yes, that is painful) and the primary issue would be hydrogen's nasty tendancy to eat fuel tanks. If it takes too much room, that will be a huge problem (although considering that the weight of the fuel can be nearly as much as the rest of the plane, I suspect there is a way to cram a hydrogen tank in those wings).

I had a coworker who was building a kit-based airplane, and had mentioned that people were trying to switch to electric. I found that hard to believe (battery powered electric makes plenty more sense for cars than just about any other fossil fuel use), but googling shows that there are more than a few electric planes out there (ignoring the obvious drones, they are roughly Cessna sized GA planes).

You will have a hard time replacing internal combustion engines in aircraft. They are pretty much the ideal case for turbines (high power, low wieght, constant high load(*?)). Jet engines are pretty hard to beat (although some of this is customer snob appeal, I understand you can likely get better efficiency with turboprops even at (commercial subsonic) jet speeds). Only GA planes really have the type of engines (ancient aviation certified piston engines) that have design issues that allow other motors to work better.

You might notice that wankel engines appeared to never catch on with aircraft, although they are powerful, lightweight, and reasonably efficient under high load (cars don't stay in the range wankles are efficient at). Aviation is more conservative than other fields due to just how dependant they are on the engine working until after landing. I would expect that non-fossil fuel systems would have to be proven in other fields (even more than tesla and volt have) before seeing them in aviation.

* I'm assuming that a plane's cruising speed requires roughly half the maximum rated horsepower out of the engine or more. Not that pilot/owners don't have the desire to put more powerful engines in the plane regardless of the max rated airspeed (exceed and die) of the aircraft, at least if the comments of said coworker's plane's websight are to be believed.

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Re: Could we ever replace fossil fuels in aviation?

Postby cyanyoshi » Thu Apr 10, 2014 6:41 pm UTC

I recall one of my professors talking about the possibility of nuclear-powered aircraft. The problem was that to make the thing fly it would either be totally unsafe for the people on board, or the amount of radiation shielding would be prohibitively heavy. There are other fuels that could be used, such as rocket fuels. But that doesn't really solve anything since if I remember correctly they are made from fossil fuels as well. A blimp could maybe get away with running off of another less powerful energy source such as solar. There's plenty of surface area to work with but those solar panels would likely be kinda heavy, so perhaps a really big blimp would be the best.

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Re: Could we ever replace fossil fuels in aviation?

Postby Nicias » Thu Apr 10, 2014 7:39 pm UTC

The main reason we use turbines in aircraft is the lower maintainance requirement. I don't know if wankel engines are as low maintainance. As to nuclear aircraft: http://en.wikipedia.org/wiki/Convair_NB-36H

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Re: Could we ever replace fossil fuels in aviation?

Postby thoughtfully » Fri Apr 11, 2014 12:53 am UTC

I don't know if this applies specifically to aviation turbines, but in general turbines only have one moving part. They are also pretty flexible with respect to what fuels can be used, although I'm not sure how relevant that is here. I'd expect a fossil fuel replacement for aviation to be some carbon-neutral hydrocarbon like biodeisel from soybeans or algae.
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Re: Could we ever replace fossil fuels in aviation?

Postby johnny_7713 » Fri Apr 11, 2014 8:09 am UTC

Sleeper wrote:There's a lot of discussion of switching over to automobiles that use electricity or alternative fuels like hydrogen. It might even be feasible over time for the whole world to switch entirely to electric or alternative-fuel vehicles.

But what about flight? Flight is an essential part of the modern transportation infrastructure, and a very strong contributor to greenhouse gases. Is there any alternative clean fuel that could run a jumbo jet?

Could we replace gas-guzzling jets with slower, but much more efficient zeppelins?


The much more efficient, at least in terms of fuel use, depends quite a bit on the size of the zeppelin. You need to have a 10^4 - 10^5 m^3 volume zeppelin before it becomes more efficient than a jet aircraft (even at the much lower speeds it goes). That's basically Hindenburg size. The Hindenburg had a passenger capacity of 72 passengers. A 737 (the smallish jets used for 2-3 hour flights) takes 189 in sardine configuration. Long-haul jets take about 300-400, so you would need about 5 Hindenburg size zeppelins per long-haul plane flying now, and about 2 per short-haul jet. Each of those airships is 245m long.
Additionally, although the Hindenburg fire got all the bad press, airships had an atrocious safety record, not so much because of fire, but because they don't handle bad weather well.

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Re: Could we ever replace fossil fuels in aviation?

Postby wumpus » Sun Apr 13, 2014 3:51 pm UTC

Nicias wrote:The main reason we use turbines in aircraft is the lower maintainance requirement. I don't know if wankel engines are as low maintainance. As to nuclear aircraft: http://en.wikipedia.org/wiki/Convair_NB-36H


Interesting. My initial thought about turbine efficiency was that they were developed to be more reliable and just as efficient as the (then efficiency kings) turbo-compound engines (which were a reliability/maintenance nightmare). Looks like aircraft fuel use is so tied to altitude that it is useless to think about things like brake specific horsepower and similar engine efficiency ideas.

As far as the nuclear plane, I think they were doing it wrong. Assuming the main issue was scaling down the reactor (too much shielding needed for the power used), they needed to build something the size of a cargo ship, and make it fly. No really, put wings on it and keep adding power (what's the point of a nuclear power plant if there is such thing as "not enough power"). You might need to design a nuclear engine pretty much from scratch (radiating away from crew, possibly away from cargo. Would there be a significant amount of high cost cargo that can be safely irradiated? (Maybe as a way of keeping beef/fish fresh instead of freezing, but I remember some cobalt isotope was used for that. People freaked and it never went anywhere). A huge issue of the plane would be weight balance/center of mass. [Fixed wing] planes need the center of mass under the "center of lift" (you can avoid this with a freewing plane. Unfortunately the only company working on such planes went out of business long ago, RIP freewing aerial robotics). This means you can either put the nuclear reactor (and presumably wings) at the end of the craft (leaving the crew to only be irradiated by the Sun), and essentially cantilever the rest of the plane (with the canard front lifting a bit). Note that this means you are pretty much stuck with a fixed weight of cargo (but can likely weigh down the front enough).

Not a chance of making one now, there should have been a window for making a Hindenburg-sized airplane again. I think all the engineers who thought big enough (and had the right skills for the job) were building the Orion.

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Re: Could we ever replace fossil fuels in aviation?

Postby Ormurinn » Sun Apr 13, 2014 6:21 pm UTC

Wenkel engines are actually very popular in kit planes like the Rutan long-EZ.

As for mass aviation, energy density is a killer.I believe Boeing has a concept for an all-electric short haul craft running on hypothetical future fuel cells.

If we get reliable direct-ethanol fuel cells in the pipeline it might well become a reality.
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Re: Could we ever replace fossil fuels in aviation?

Postby Tass » Mon Apr 14, 2014 7:52 am UTC

It seems like the US navy are actually about to start producing hydrocarbon fuel on their carriers from CO2 and water, using the nuclear power on the ship.

Even though it is probably not cheap it definitely shows that we could replace fossil fuels.

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Re: Could we ever replace fossil fuels in aviation?

Postby johnny_7713 » Mon Apr 14, 2014 3:03 pm UTC

Commercial (and military for that matter) flights have already been performed on a kerosene / bio-fuel blend. I think those bio-fuels are based on cooking oil, but other options are being researched, including algae-derived bio-fuels and bio-fuels derived from waste gasses from iron/steel production.

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Re: Could we ever replace fossil fuels in aviation?

Postby Xanthir » Mon Apr 14, 2014 3:23 pm UTC

Tass wrote:It seems like the US navy are actually about to start producing hydrocarbon fuel on their carriers from CO2 and water, using the nuclear power on the ship.

Even though it is probably not cheap it definitely shows that we could replace fossil fuels.

Ah, *using their nuclear power*. I was very confused by the article I read about that, which sounded a whole bunch like the "engine that runs on water!" crackpottery.

The article I had read cited production prices between $4 and $10 a gallon.
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Re: Could we ever replace fossil fuels in aviation?

Postby Futune » Tue Apr 15, 2014 4:39 pm UTC

Zeppelins could in principle power their engines with solar- or fuel cells, since their energy requirements are lower.

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Re: Could we ever replace fossil fuels in aviation?

Postby Neil_Boekend » Wed Apr 16, 2014 10:20 am UTC

Yes in the future I expect us to replace fossil fuels in aviation.
Jet engines burn just about anything. They can even get kerosene to burn. Seriously, kerosene is difficult.
We could run them on alcohol, biodiesel or charcoal powder if we wanted. Granted, these require some adjustments and new fuel nozzles (and burning charcoal would mean a new method of getting the fuel to the engine.) but the technology of a jet engine is thus that it will burn just about anything that can actually burn and can be aerosolized.

The main reason that it isn't done now is because of money. Kerosene carries no taxes (at least in most countries it doesn't). Fossil fuels are cheap. Biofuels are not cheap.

Edit to fix kerosene.
Last edited by Neil_Boekend on Thu Apr 17, 2014 7:08 am UTC, edited 1 time in total.
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Re: Could we ever replace fossil fuels in aviation?

Postby speising » Wed Apr 16, 2014 1:04 pm UTC

Neil_Boekend wrote:Yes in the future I expect us to replace fossil fuels in aviation.
Jet engines burn just about anything. They can even get kerosine to burn. Seriously, kerosine is difficult.
We could run them on alcohol, biodiesel or charcoal powder if we wanted. Granted, these require some adjustments and new fuel nozzles (and burning charcoal would mean a new method of getting the fuel to the engine.) but the technology of a jet engine is thus that it will burn just about anything that can actually burn and can be aerosolized.

The main reason that it isn't done now is because of money. Kerosine carries no taxes (at least in most countries it doesn't). Fossil fuels are cheap. Biofuels are not cheap.

are you sure you mean kerosine? because wikipedia suggests different. what some parts of the world call "Kerosin" is jet fuel and easily flammable, while kerosene is petroleum or paraffin.

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Re: Could we ever replace fossil fuels in aviation?

Postby Zamfir » Wed Apr 16, 2014 4:04 pm UTC

Kerosene (including other spellings) is the category of refinery products heavier than gasoline. Depending on who you ask, diesel is the next heavier category, or the heaviest part of kerosene. It's not a very formalized category.

There are specifications that determine whether a hydrocarbon mixture can be sold as jet fuel. Those specifications are basically the same range as the stuff called kerosene, or the lighter part of the kerosene spectrum. Two supllies of 'jet fuel' can be rather different, as long as fall within the specs.

Jets don't have to fly on kerosene, in practice they run on anything that vaguely has the same properties, and with some modification they fly on a wider range yet, as Neil says. I think you have to fly an airliner jet on the 'jet fuel' kerosene, but that's a safery precaution. It's the stuff with the most experience behind it, and it's the cheapest anyway.

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Re: Could we ever replace fossil fuels in aviation?

Postby Robotbeat » Thu Apr 17, 2014 6:01 am UTC

Of course we can. Already for short flights, you could power regional air transports with the 250 Whr/kg batteries we have now. Longer flights can use methane which can be synthesized or ammonia which is also easy to synthesize. Both are likely more practical than hydrogen. And of course you can make gasoline from carbon dioxide and water and electricity. But really, Lithium-sulfur batteries can get reasonably high cycle life and have twice the specific energy of lithium ion and could do much more than that even. And Lithium-air, starting in about a decade or so, has energy density comparable to chemical fuels, so you could electrify everything. Also, further down the line, if we start getting much stronger materials and make steady progress with superconductors like now, then you will eventually get superconducting coils with energy density comparable to lithium-ion but with incredibly fast charge and discharge rates and ultra-high efficiency... There are more advanced schemes like beaming power via solid-state laser (already demonstrated with drones achieving essentially unlimited flying times).

So yeah, no problem. If we're talking 20-30 years out, then yeah, we can certainly do away with fossil fuels for aviation. "Ever"? Well, then absolutely!!! We'll even start seeing passenger service with electric aircraft within a decade.

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Re: Could we ever replace fossil fuels in aviation?

Postby Neil_Boekend » Thu Apr 17, 2014 7:08 am UTC

speising wrote:
Neil_Boekend wrote:Yes in the future I expect us to replace fossil fuels in aviation.
Jet engines burn just about anything. They can even get kerosine to burn. Seriously, kerosine is difficult.
We could run them on alcohol, biodiesel or charcoal powder if we wanted. Granted, these require some adjustments and new fuel nozzles (and burning charcoal would mean a new method of getting the fuel to the engine.) but the technology of a jet engine is thus that it will burn just about anything that can actually burn and can be aerosolized.

The main reason that it isn't done now is because of money. Kerosine carries no taxes (at least in most countries it doesn't). Fossil fuels are cheap. Biofuels are not cheap.

are you sure you mean kerosine? because wikipedia suggests different. what some parts of the world call "Kerosin" is jet fuel and easily flammable, while kerosene is petroleum or paraffin.

Nope. I mean kerosene. Didn't know there was a difference. Fixed naow
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Re: Could we ever replace fossil fuels in aviation?

Postby Minerva » Mon May 26, 2014 2:20 pm UTC

Replacing fossil fuels in aviation seems to be a relatively difficult "high-hanging fruit" problem that accounts for a small portion of greenhouse gas emissions, compared with other areas like power plants on the ground that account for a large portion of greenhouse gas emissions and fossil fuel use.

As the USAF Aircraft Nuclear Propulsion program showed, nuclear power is possible, especially for large cargo aircraft or bombers with minimal people on board. Contrary to popular misconceptions, the ANP program was not cancelled because of any engineering failures, or because it was impossible to keep the crew radiological dose down at safe levels, but just because the development of ICBMs made the long-range, long-endurance nuclear powered bomber aircraft obsolete.

The only other way to do it is to use clean thermal energy and electricity generated on the ground from environmentally friendly energy sources to synthesize liquid chemical fuels in a way that is neutral with regards to fossil CO2.
Ammonia, methanol, liquid hydrogen, dimethyl ether, and petroleum-like hydrocarbon mixtures are all possible.

Liquid hydrogen has the best stored energy per mass ratio of any chemical fuel, which is why it's preferred in spacecraft. But in terms of stored energy per unit volume it's pretty average.
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Re: Could we ever replace fossil fuels in aviation?

Postby johnny_7713 » Tue May 27, 2014 7:31 am UTC

Minerva wrote:Replacing fossil fuels in aviation seems to be a relatively difficult "high-hanging fruit" problem that accounts for a small portion of greenhouse gas emissions, compared with other areas like power plants on the ground that account for a large portion of greenhouse gas emissions and fossil fuel use.

As the USAF Aircraft Nuclear Propulsion program showed, nuclear power is possible, especially for large cargo aircraft or bombers with minimal people on board. Contrary to popular misconceptions, the ANP program was not cancelled because of any engineering failures, or because it was impossible to keep the crew radiological dose down at safe levels, but just because the development of ICBMs made the long-range, long-endurance nuclear powered bomber aircraft obsolete.

The only other way to do it is to use clean thermal energy and electricity generated on the ground from environmentally friendly energy sources to synthesize liquid chemical fuels in a way that is neutral with regards to fossil CO2.
Ammonia, methanol, liquid hydrogen, dimethyl ether, and petroleum-like hydrocarbon mixtures are all possible.

Liquid hydrogen has the best stored energy per mass ratio of any chemical fuel, which is why it's preferred in spacecraft. But in terms of stored energy per unit volume it's pretty average.


Technically the USAF ANP program only showed you could run a nuclear reactor semi-safely (did the radiation dosage meet modern standards of safety?) while it was flying on a plane, not that you could also use it to provide thrust.

In other news, an Australian tree might be a possibility for producing aviation-grade biofuel: http://blogs.crikey.com.au/planetalking ... potential/

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Re: Could we ever replace fossil fuels in aviation?

Postby stoppedcaring » Tue May 27, 2014 4:59 pm UTC

Minerva wrote:Liquid hydrogen has the best stored energy per mass ratio of any chemical fuel, which is why it's preferred in spacecraft. But in terms of stored energy per unit volume it's pretty average.

Yeah, there's more hydrogen in a liter of gasoline than there is in a liter of liquid hydrogen.

The greater weight (but much higher density) of liquid methane and kerosene mean they're much better for a compact reusable spacecraft. They also offer much higher thrust-to-weight ratios despite their proportionally lower specific impulse.

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Re: Could we ever replace fossil fuels in aviation?

Postby Frenetic Pony » Tue May 27, 2014 10:01 pm UTC

I've been trying to figure out the relative power density of jet fuel as compared to todays best capacitors. But while capacitors have convenient figures I can't google up the figures for jet fuel.

Regardless, over time we could of course just run a jet on electricity or some other far more convenient and efficient method of power storage versus "thing what explodes". I just don't know when and can't gin up the figures (at least conveniently) to try and figure out what we could do today. I suppose I could go to a lot of trouble for efficiency of movement and power and burn rate of jet fuel, but I'm not that kind of person.

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Re: Could we ever replace fossil fuels in aviation?

Postby speising » Tue May 27, 2014 10:40 pm UTC

isn't this from the jet fuel wikipedia page what you seek?

Energy density 34.7 MJ/L

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Re: Could we ever replace fossil fuels in aviation?

Postby PM 2Ring » Wed May 28, 2014 6:14 am UTC

And using Wikipedia's figure of .81 kg/L for the density of jet fuel, Google Calculator says

(34.7 (MJ / L)) / (.81 (kg / L)) =
11 899.8628 watt hours per kilogram

From http://en.wikipedia.org/wiki/Supercapac ... er_density
As of 2013 commercial gravimetric energy densities range from around 0.5 to 15 Wh/kg. For comparison, an aluminum electrolytic capacitor stores typically 0.01 to 0.3 Wh/kg, while a conventional lead-acid battery stores typically 30 to 40 Wh/kg and modern lithium-ion batteries 100 to 265 Wh/kg.


So capacitors & batteries have a long way to go before they get close to the energy density of chemical fuels.

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Re: Could we ever replace fossil fuels in aviation?

Postby EdgarJPublius » Wed May 28, 2014 5:42 pm UTC

Airbus just demoed a fully electric aircraft at a Berlin Airshow.
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