The Ergosphere
Sunday, June 19, 2005

Post-oil airliners

One of the big issues of the peak-oil crowd is what will happen to air transport.  If there is no oil, they say, jet aircraft won't fly.  This prospect excites the pastoralists (who seem to find glee in the notion of no passenger vehicle moving faster than a stagecoach) and dismays or depresses most everyone else.  But is it realistic?

Despite neglect over the years, laboratories are simmering with activity in a dozen areas from airfoils to catalytic alkanes to quantum dots.  Human ingenuity being what it is, we can expect some form of relatively inexpensive renewable energy to come to market relatively soon.  The prospects for air transport depend on exactly what takes center stage.  If it turns out to be only wind and photovoltaics, it's a good bet that not many things will be flying; batteries are not up to the job of moving big, fast things through the air, and the production of chemical fuels from electricity is an inefficient and expensive process.  Such a development would likely mean a future of high-speed maglev, not soaring so high as the eagle but doing the job so long as the routes are purely terrestrial.

Things will be quite different if a significant part of the market winds up going to chemical fuels, either biochemical or wholly synthetic.  The University of Wisconsin process which turns biomass to alkanes would probably produce something suitable for jet fuel if it can be made cheaply and in sufficient quantity, but the limits of biological productivity of land plants and competition from other users might price it out of the market.  Aircraft look to be considerably better off if one of three possibilities comes to fruition:  algal biodiesel, methane, or photosynthetic or photolytic hydrogen.

Algal biodiesel or its feedstock would be nearly optimal.  If it could be selected for fractions which would not gel at stratospheric temperatures, it would be an almost exact replacement for jet fuel (at a slight weight penalty due to the oxygen).  An alternative would be to use the UWisc process to turn the raw algal oil into pure alkanes, which would be an exact replacement.  But if algal lipids don't come to market at reasonable prices, there are the simple molecules:  methane and hydrogen.  These have the disadvantage that there is no infrastructure for fueling with them, nor aircraft set up to use them.  But will they fly?

Methane is the easier of the two to obtain, handle and use.  We'll have a healthy supply of it for decades after natural gas wells lose their fizz.  It bubbles out of thousands of landfills nationwide, and isn't going to stop unless we stop dumping garbage (which may happen).  It can be liquefied at temperatures (99 K) where air is still a gas, and has a liquid density of 0.424.  Hydrogen is touchier stuff, not turning to liquid until the temperature gets down to twenty... Kelvin, and is extremely light even as a liquid with a specific gravity of about 0.070.  (Strangely, there's about 50% more hydrogen in a liter of liquid methane than there is in a liter of liquid hydrogen.)

Suppose we were going to fuel a 767 with this stuff, and the aircraft requires about the same amount of energy regardless of the specific fuel used.  A 767-200E carries 23,980 gallons (90,770 liters) of Jet-A, which is approximately the composition of kerosene.  The density, energy/weight and energy/volume ratios of kerosene, liquid methane and liquid hydrogen stack up as follows:

 Property / Fuel   Kerosene   Liquid CH4   Liquid H2 
 Density  0.825  0.424  0.070 
 Energy, MJ/kg  45.9  55.5  146.5 
 Energy, MJ/l  37.9  23.5  10.26 
 Fuel required, kg  74,890  61,930  23,460 
 Fuel required, l  90,770  146,400  335,300 

As we can see, liquid methane is a fairly well-behaved fuel, requiring only about 60% more volume than kerosene and weighing about 13 tons less for the same energy.  It might even fit largely inside insulated wing tanks; if the volume penalty for insulation was 15% and the balance of fuel was held in external tanks, they would only require 69,000 liters of volume.  This would fit in two tip tanks of 2 meters diameter and roughly 11 meters long; if they were made of two layers of 1.5mm aluminum with 5 cm of insulation between, the tanks would weigh about 600 kg each, or 1200 kg total.  Liquid methane fuel would allow the aircraft to weigh almost 12 tons less fully fuelled, allowing roughly another 12 tons of cargo to be carried (minus allowances for drag losses on the new tankage).  This is clearly within the realm of engineering feasibility.  An airliner running on liquid methane might be a better aircraft in some ways than one running on kerosene.

Hydrogen is the outlier in all respects.  The energy equivalent of a 757 full of kerosene is a mere 23 and a half metric tons, more than fifty tons lighter than the dinosaur juice.  It's also close to four times the volume.  Insulating wing and fuselage tanks is probably impractical; it's likely that the full load would have to be carried in external tanks, either on the wing tips or on pylons like outboard engines.

Wherever you put them, they'd be monsters.  A pair of 3-meter diameter tanks would have to be 24 meters long each, or half the overall length of the aircraft.  Bulbous fairings below the fuselage wouldn't do it either; even if the lower cross-section was made square with extra volume, it would hold less than a third of the required fuel.  A massive forward delta-shaped wing root strake might hold a fair amount, but I can't even guesstimate how much.  Were the fuel to be divided among four separate 2.5 meter diameter tanks mounted at various points along the wing, each would have to be roughly 17 meters (56 feet) long.  This is a serious design headache, and would probably be best implemented starting from a clean sheet of paper.  The good news is weight.  Were the tanks made of the same 2 layers of 1.5mm aluminum (strength provided by internal pressure, like a blimp), they would weigh perhaps 5 tons total.  This would make the aircraft's fully-fuelled weight some 45 tons less than the conventional model, a large fraction of which could become extra cargo.  Another bit of good news is hydrogen's chemical and thermodynamic properties; a liquid hydrogen engine can pressurize its fuel, use it to recover energy from expanded exhaust and hot turbine blades, and expand the resulting high-pressure gas through a turbine to yield extra energy.

So what's the verdict on air travel in a post-oil world?  It depends, but if technology can make renewable fuel of any kind available at close to today's prices, we can bet that fleets will be scooting around the sky on it. 
I think ethanol has about the same energy density as liquid methane, and is maybe easier to contain. Surfing, I found a number of articles on small aircraft fueled by ethanol, but couldn't see any tests with large passenger jets.
batteries are not up to the job of moving big, fast things through the air

Maybe not today's batteries, but I'll bet there will be an energy storage solution eventually that will weigh far less, such as superconducting supercapacitors. A giant mass of electrons doesn't weigh that much. Electric motors weigh less for the equivalent output.
Ethanol: density 0.7893, heat of combustion 326.68 kcal/mol,
molecular weight = 46.07.

Energy available is 29.67 MJ/kg, 23.4 MJ/l.

It has roughly the energy density of liquid methane, but weighs more than kerosene.  That's not conducive to long-range air travel.
I can understand why it would give peace of mind to find a reasonable candidate for renewable jet fuel, but I just don't see a need. Once we stop using oil and coal for low value uses like heat, electrical generation and surface transportation I should think there would be 300 years worth for high value uses like jet fuel and plastics feedstock. We can be confident that in 300 years we'll have found substitutes.

So: how many Mbarrels per day do we use for jet fuel? Probably 2, which is what we can get easily from tar sands for 300 years. This may not satify those who look forward to apocalypse, but I would think it's sufficient answer for others..
C0unterp0int:  The best batteries are currently running ~150 Wh/kg.  This is around half a megajoule per kg; methane is literally two orders of magnitude better.  The prospect of any electric storage technology bridging that gap in the next century is very small; in the next half-century, roughly zilch.  It takes fifty years for physical discoveries to make it to products, and the physics is just not there.

"Electric motors weigh less for the equivalent output."

The electric motor in the AC-150 driverain puts out 200 HP peak and weighs 110 pounds; call it 1.8 HP/lb.  A quick perusal of turbine engine specs shows power as high as 2.6 hp/lb, and that's for turboprop engines which have the weight burden of the power turbines and reduction gearboxes.  Airliner engines do not need these things.

Sorry you lost, but thanks for playing.  You can increase your odds of winning by doing research before clicking the "Publish" button.
My point, Nick, is that you can't reserve (syn)petroleum for airliners; if supplies are scarce enough, they'll be bid up too high for any but a small elite to fly.  It makes far more sense to make aircraft run directly on something cheaper.

Jet fuel consumption has been running about 1.6 million barrels/day, or a bit under 8% of total US petroleum consumption.

I suggest you learn your way around the Energy Information Agency website, it is a tremendous resource.  It can help you deflate nonsense and make you look much smarter than you are... like me!
Hi, there will be several presentations on biokerosene research at the 14th European Biomass Conference which will be held in Paris at the end of this year.
by your numbers ethanol actually has more energy per kilogram than liquid methane, and can be held by lighter/simpler tanks than methane ... but it is less effective than methane? or are you saying methane is "not conducive to long-range air travel" either?
Well, yes EP, I use the EIA website, but I think you should be very impressed that my seat of the pants estimate was so close..

Anyway, I certainly agree that development of cheap direct replacements is very sensible. Where I was going was a simple rebuttal of the apocalyptic approach, which says that oil is indispensable for some things. You're working to show that oil is not indispensable. That's a great approach (and, indeed, would contribute to greater energy supplies, and lower prices for energy of all sorts), but I like the idea of showing that oil supplies are perfectly adequate for the things which currently appear to require oil.

Belt and suspenders.
odograph, let me run the numbers past you again:

Kerosene:  37.9 MJ/l
Liquid methane:  23.5 MJ/l
Ethanol:  23.4 MJ/l

Liquid methane:  55.5 MJ/kg
Kerosene:  45.9 MJ/kg
Ethanol:  29.67 MJ/kg

Ethanol is on the bottom of both the energy/mass and energy/volume lists.  Energy/mass is the most significant, as aircraft are limited more by their ability to lift weight than by volume; if you filled an airliner with ethanol it would either require 50% more fuel (and sacrifice of that much cargo capacity) or have only about 2/3 the range.
I think I went googly-eyed and was mixing up the weight/volume numbers. Yes, that is a big weight hit (on the fuel). On the other hand, given that it already works for general aviation, a little rutan-style private ethanol plane might work just fine.
Being an old guy engineer, I am prone to retooling things that history has deemed problematic.

Provided we are talking about aircraft and not necessarily jets, and that we are talking mass transit, wouldn't the dirigible concept be worth looking at with an eye to carbon fiber and alternate designs?

I say this because the flight from Houston to Dallas is 45 minutes, 30 of which is making/losing cruise altitude. The wait to board is about a half hour and ticketing and checkin is at least an hour. Deplaning is another half hour, and getting out of these big airports is another half hour. So to actually fly is 45 minutes, but the process makes it over 3 hours.

Driving it is 4 hours, airport to airport, 3 hours city to city. If one looks at the cost of flying including time, dirigibles lower speeds and altitudes might be acceptable as they wouldn't use the jet airports.

And only traditional design makes them look like they do - there might be a better way to build them..
Dirigibles are going to be horribly bulky things no matter what, and thus at the mercy of the weather to a far greater extent than airplanes.

Why not put in a TGV or Shinkansen line between Houston and DFW instead?  Yahoo says it's 239 miles from Houston to Dallas; you could make the trip in 2 hours from city center to city center.
EP -

We cannot even get a toy train built correctly here in Houston. I would gladly take a train if one were available! Even a fast Amtrak would be about the same, since the airport congestion could be bypassed.

I hit up my company about using their parking roof for PV - they will rent it to me for $100 per month. Boom - end of that idea.

My actual ride will be lead acid (I know this tech, and it is easy to service, and cheap) and DC power. 20 batteries and a fiberglass car, correctly balanced, should get me around 90-100 miles on a charge. My commute is 70 per day. I do have permission to plug in and recharge without cost during working hours, which will extend my range.

Lithium isn't available in large enough batteries without horrific costs. What is the point of doing it if it costs more than buying a new Prius or german JetCar?
Would your employer rent the whole roof for $100/month?  Someday, somebody might want to take them up on it.

At ~90 miles range and 70 miles used, you're going to be discharging your batteries pretty deep.  Take advantage of that free charging offer; it'll save you a lot of money and grief.

Lithium-ion is expensive now; then again, a megaflop was expensive in 1980, too.  The raw materials (lithium, carbon, phosphorus, iron, plastic, fluorocarbon electrolyte) are relatively cheap, like silicon; the cost is going to come down, and as it comes down the batteries are going to wind up in more and more places.

It makes no sense to power a car with Li-ion batteries today, but I'll make a bet:

They'll cross the price/performance line with NiMH in a few years; at this point, the entire hybrid vehicle industry (which will be quite a bit bigger than it is today) will switch to Li-ion.  This increase in production volume will boost the economies of scale, and prices will fall even faster.  Within a few years after that (even if nothing else is done), you will see hybrids with "10 all-electric miles" as a standard feature.  Then it really takes off.
I'm not so sure that a change in fuel will be required en masse for airliners.

Sure, the price of oil will rise -- but this will (a) induce consumers to consume less, either directly (drive less, carpool) or indirectly (buy a higher MPG vehicle, (b) induce alternative transportation models to appear, competing with the rising cost of flying. The market moves incrementally, not suddenly.

In fact, I suspect the vision you alluded to where more domestic travel is via rail and airplanes are generally only used in cross-ocean flights may become more likely.

It's a shame that the US gov't can't figure out how to connect close major metro areas together by fast rail -- Boston-NYC-Philly-DC-RTP-Atlanta, Dallas-Houston, Sacramento-SF-SJose, Bakersfield-LA-SanDiego, Seattle-Portland, KC-StLouis, etc. It'd provide jobs, improve transportation quality, reduce oil consumption, and reduce airport traffic (both cars and planes!). Jeez, it'd be even better if (where appropriate) they lined the tracks with windmills generating some electricity to run the trains!
The question is a wee bit moot, as indeed even cheap oil is going away oil isn't. The question is how well these fuels work in a jet engine aircraft.

ANY pressurized fuel isn't going to pass muster by the FAA. One wee leak and you have a bomb that falls out of the sky. You need a fuel that's stable under all operating pressures. Sorry. Non-starter.

The Soviets experimented with methane and H2 powered aircraft in their heydey, which is unsurprising, they really were leaders in engine design back in the day. The problem was that turbine jet engines need to get their joules in a jiffy, and the reason jets like using kerosene is that it is a compact medium for energy, not so much for methane, and certainly not for H2. The pressure needed to pump will be substantial and likely too high. The potential for leakage was high, and the whole idea was shelved.

However, Diesel piston engines are already being certified for use in general aviation. Biokerosene is likely to augment jet fuel in short order.

Diesel piston aircraft engines have some advantages if you get their weight down, one is that the torque is especially helpful for STOL aircraft. Secondly they operate under high compression, which is better at altitude. Third they are 40% more efficient than AvGas powered engines. When you're burning 15-20 gph even in a smaller aircraft it begins to add up.

So there will be changes, but biokerosene is where it will be at.
Michael:  If oil-equivalents become too expensive, flying will become unaffordable no matter how available they are.  The existence of an airline industry with anything like its present form and reach depends on a fuel with high energy/mass and reasonable cost.

"The question is how well these fuels work in a jet engine aircraft."

Both methane and hydrogen work extremely well in gas-turbine engines.  Both liquid hydrogen and liquid methane could increase net engine efficiency and power by a.  intercooling between compressor stages to reduce compressor back-work, b.  cooling turbines more aggressively to increase combustion temperatures, and c.  recovering heat from hot surfaces in the exhaust path, recycling it to the combustion chamber (regeneration).

"ANY pressurized fuel isn't going to pass muster by the FAA."

You mean propane is prohibited in aircraft?  I'm so glad that hot-air balloons have been banned.

"One wee leak and you have a bomb that falls out of the sky. You need a fuel that's stable under all operating pressures."

One wee leak, like the Concorde that went down outside of Paris when debris punctured a fuel tank full of Jet-A?  Stable, like the conditions which allow Jet-A to form explosive mixtures in the air space in tanks (which appears to have brought down TWA flight 800 outside of NYC)?

Neither liquid methane nor liquid hydrogen would allow enough oxygen into a tank to form combustible mixtures.  As for pressurization, many combustible fluids in a modern aircraft are pressurized.  Hydraulic fluid is a big one.

"The pressure needed to pump will be substantial and likely too high."

Oh, really?  Is the combustor pressure going to be radically different just because of the fuel, and why is a fuel which is fully gaseous at compressor outlet temperature/pressure going to require higher fuel pressure than one which has to be atomized to burn?

"Diesel piston engines are already being certified for use in general aviation."

I've been following this for years, especially Delta Hawk.  I wish them well and hope that they take over the piston light-aircraft market.

However, no airliner of the last 40 years has been even capable of using a diesel engine.  They require too much power/weight out of their engines.

"Biokerosene is likely to augment jet fuel in short order."

If biokerosene costs even $3/gallon, there won't be many airliners flying on it.
methane can be easily synthesized from Hydrogen and Carbon Dioxide with the Sabatier reaction:

4H2 + CO2 -> CH4 + 2H2O

(about 20% of the energy of the H2 is lost in the slightly exothermic reaction)

As engineer-poet has said, methane has more favorable carrying properties than hydrogen. (so there will never be a H2 economy....)

If renewable electric can get low enough (3 cents per kwhr or less) then there will be enough methane to fuel aircraft and everything else. The cost to liquefy is also reasonable.

If you hydrogenate your ethanol to ethane, that would be a nice fuel too. Liquefies easier, etc.
One issue that may have been overlooked may be a very strange-sounding safety issue: if I remember correctly from 8th-grade chemistry, the flames generated by burning ethanol and liquid methane are invisible in daylight. In the event of a crash, this could present a significant dilemma for crews trying to fight fires and rescue survivors.
No, wait, clearly on meth myself... I was thinking methanol, not methane. Forget that. Methane flames--clearly visible, just look at any oil refinary.
I don't know if this design has any chance of being built, or if the concept even works, but I found this interesting.

It looks like a cross between a hang glider and a blimp. It uses helium to become lighter than air and float up to altitude, then dips its nose down and glides forward under power of gravity. No fuel needed.
Oh, it takes energy all right.  If you consider the process as a cycle, you have to compress some of the (very rarefied) lifting gas into a smaller tank to reduce the buoyancy so you can come down again.  The semi-aerostatic nature cuts induced drag a great deal, but you're also going to suffer on the speed; bulky craft do not fly quickly without lots and lots of power.
This comment has been removed by a blog administrator.
The gravity plane is a no go: you need huge quantities of helium, which is already scarce. And helium prices go up with oil prices.
As a concept, it might work, but when it comes to building hundreds, even thousands of it, we're stuck.
I'm more with Michael: diesel engines for aircraft will make a comeback, and you just use biodiesel instead of regular. There's already one company in Austria that has created great lightweight diesels for small aircraft.The efficiency gains are huge, even for such small craft.
Check it out here:
Check under Twinstar. They made the first non-stop transatlantic flight on diesel engines.
Aerodiesels are getting big.  I'm also aware of:

Delta Hawk

Still, they are not going to run airliners and they depend upon supplies of liquid fuel being cheap enough.  Depending on the supply/demand situation, that may not be the case in 20 years.
True, these diesels will only replace a few airplanes here and there, certainly not airliners.

How about the simplicity of hydrogen airplanes? The EU + Airbus is working on the Cryoplane.

It's just a matter of time before these things fly.
Yup, that's about how I would expect such a thing to look - lots of extra volume for the hydrogen tankage.
That thing looks almost exactly like an A380 with tankage on the upper deck.

Strange, A380 production just began...
Really interesting post, and equally interesting and enlightening comments...

I too find it hard to believe that diesel engines could become commonplace in jet airliners...perhaps in a few niche airplanes diesel engines could figure, but I do not see how they can find a place in an A380 anytime soon...

Yes, if airplanes could fly on diesel engines (oh well, I might as well say if pigs can fly?, guess biodiesel from algae ( see for more inputs on algael biodiesel) and other useful feedstock might be able to take care of our precious airline fuel...for now, it appears that we have to look to the good old oil refineries to run our airlines

NS from eIT
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