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I want to do research for an alternative fuel source for a commercial aircraft that will reduce the cost for flights.

Will the hydrogen gas be ignited in the combustion chamber and produce enough thrust to power the aircraft for flying? As for the problem that people had stated before for storing the gasses in a high pressure gas tank, can an increase in the aircraft weight be reduced by replacing the tank with usage of a hydrogen generator instead of a whole bulky tank that will weigh more?

By using a generator which converts water into hydrogen gas, we can just use water as our main fuel source which is more cost efficient?

In a flame of pure hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy. If carried out in atmospheric air instead of pure oxygen, as is usually the case, hydrogen combustion may yield small amounts of nitrogen oxides, along with the water vapor.

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    $\begingroup$ Your energy to split the H2O still has to come from somewhere. Thus, you still have to store something that will provide you with that energy. Then there's a question of if converting that stored energy into a form that's useful for you to split H2O is the most efficient use of that energy, or can it be converted into thrust in another, more efficient, manner, rather than using it to generate hydrogen which you are going to burn. In general, the process you've described has significantly lower efficiency than other possibilities. $\endgroup$
    – Makyen
    Commented Feb 3, 2019 at 1:01
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    $\begingroup$ @Makyen +1 This is also known as No Free Lunch or the Second Principle of Thermodynamics. $\endgroup$
    – AEhere supports Monica
    Commented Feb 3, 2019 at 14:11
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    $\begingroup$ "By using a generator which converts water into hydrogen gas": Same as Is water a possible fuel for jet engines?. The other question has got a score of -4, yours +5, that's funny. Burning one kg of fuel creates 42 MJ. To do the same with water you need 3.5 kg of water, but also more than 42 MJ of energy to extract the 350 g of hydrogen they contain and burn it (source). This is not a good deal (neither for mass nor energy saving). $\endgroup$
    – mins
    Commented Feb 3, 2019 at 19:12
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    $\begingroup$ If you "want to do a research for an alternative fuel source for a commercial aircraft that will reduce the cost for flights", I strongly suggest that you take one or two university-level courses in Thermodynamics. No one has ever beaten the Second Law of Thermodynamics. $\endgroup$
    – Flydog57
    Commented Feb 3, 2019 at 22:38
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    $\begingroup$ @mins: In all fairness, that one has even more fundamental mistakes (sea water?!) while this one has at least a sane title. $\endgroup$
    – MSalters
    Commented Feb 4, 2019 at 9:21

9 Answers 9

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Yes, and it has been demonstrated 30 years ago on the Tupolev 155. This is/was a hydrogen-powered version of the Russian Tu-154B tri-jet. Only one has been built and has since been retired after demonstrating the use of liquid hydrogen in 5 experimental flights. In total, the Tu-155 performed about 100 flights with several fuels, among them hydrogen and liquefied natural gas.

Tu-155 (top), cut-out (center) and layout (bottom)

Tu-155 (top), cut-out (center) and layout (bottom). Source: https://www.aviaru.net/pr/?id=11633

Hydrogen is actually better for a jet engine: It is gaseous at normal temperature, so there is no delay for the evaporation step to happen as there is for liquid fuels before mixing and combustion can start. Also, hydrogen burns in a wide variety of mixing ratios with oxygen, so flame-outs are much less likely, making a smaller combustion chamber possible.

The downsides, of course, are storage and the small molecular size of hydrogen. It is very hard to contain, and it needs big volumes to store a given amount of chemical energy. Pressurized storage at 200 bars holds only 18 kg/m³ or 45 times less than kerosene. With 142 MJ/kg, hydrogen holds three times as much chemical energy than kerosene, but then the volumetric efficiency of kerosene is still a factor of 13.3 better.

Cryogenic storage swaps pressure for low temperature: Below 33 K and above 13 bar, hydrogen becomes a liquid and storage density increases to 30 kg/m³. Still, cryogenic hydrogen storage needs 4 times the volume of the same amount of energy stored as kerosene, plus the isolation and the energy to cool it down and compress it.

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  • $\begingroup$ They used the Tu154 because its center of gravity is very aft, due to the engines being aft. They need to place the hydrogen tank at the center of gravity (so fuel burnup doesn't affect trim). With the Tu154 CG being so far aft, it allowed a single cabin space instead of two half-cabins fore and aft of the tank, which would be a big mess. $\endgroup$
    – Harper - Reinstate Monica
    Commented Feb 3, 2019 at 7:19
  • $\begingroup$ Evaporation of kerosene is a complete non issue at the temperature of the air exiting the compressor. The issue with liquid fuels is atomisation. That is, getting a fine spray of small droplets. When fuel nozzles become dirty with deposits, the spray pattern can deteriorate. $\endgroup$
    – Penguin
    Commented Feb 3, 2019 at 10:27
  • $\begingroup$ @Harper: Right, and a smaller type like the Tu-134 would had less volumetric efficiency, so the larger one offered the better choice. An Il-62 would had worked, too. $\endgroup$
    – Peter Kämpf
    Commented Feb 3, 2019 at 13:51
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    $\begingroup$ Re "cryogenic hydrogen storage needs 8 times the volume of the same amount of energy stored as kerosene": How did you come to 8 times? The energy density of jet fuel is 3.7 times that of liquid hydrogen (37 MJ/l vs. 10 MJ/l) $\endgroup$
    – Peter Mortensen
    Commented Feb 4, 2019 at 10:43
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    $\begingroup$ @cat: I have no inside information. I think the main reason is really the fall of the Soviet Union which had all Russian (and Ukrainian) aircraft design bureaus enter a dark period. However, while some designs were good enough to weather that storm, the Tu-156 showed no commercial promise and could not secure any private funding. In a way, that would be the other limitation. $\endgroup$
    – Peter Kämpf
    Commented Feb 12, 2019 at 18:48
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Hydrogen works just fine on rockets. However "just fine" on rockets doesn't mean it is practical on an aircraft.

The only way you can utilize $\mathrm{H}_2$ is storing it cryogenically. This is because $\mathrm{H}_2$ goes supercritical at $-240\,{}^\circ\mathrm{C}$, and no matter how hard you squeeze it beyond this temperature it would refuse to liquify and thus remain a low number density fluid. Of course, you could store it as a supercritical fluid, but that would require an incredibly heavy pressure vessel.

If you agree that you must store $\mathrm{H}_2$ cryogenically, then take a look at this. What do you think are your odds of carrying all this hardware onboard an airplane and still have usable payload?

Your trouble doesn't end there. To burn $\mathrm{H}_2$ as a fuel, you must move it out of the tank and into the engine. And to do that, you must use some sort of pump, and a pump must have some moving parts that are immersed in the liquid that it's supposed to pump. And here comes the trouble: you are pumping a liquid that boils at $-240\,{}^\circ\mathrm{C}$, and even the tiniest surface imperfection, the tiniest burr, the tiniest machining mark, the tiniest grooves and troughs on the surface of the immersed moving pump creates minuscule surface vortices, and these vortices heats up the liquid $\mathrm{H}_2$ near the part's surface so that it boils, forming bubbles, which merge, split and collapse thousands of times per second, and the minuscule pressure pulses sent out by these events impacts on the already extremely cold thus brittle moving parts of your pump, chipping it almost instantly, and after chipping the damaged pump will stir the entire stream of liquid $\mathrm{H}_2$ to a violent boil and blow itself off.

Liquid $\mathrm{H}_2$ is the most difficult fuel to handle, and even rockets steer away from it whenever possible. It is one thing to use it on something that only lasts for a few hundreds of seconds, quite another on something that lasts tens of thousands of hours.

EDIT: I almost forgot. Hydrogen, its molecule being so small, diffuses like crazy, even within the "solid" objects to the naked eyes like steel, titanium, copper, and aluminum. So all metal parts are inevitably impregnated with hydrogen with use and form hydrides with it, causing it to decrease in strength. So good luck with the whole fuel system! The entire aircraft will be a literal ticking time bomb.

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    $\begingroup$ @meatball-princess there's another option. You can use electrostatic forces from other atoms to bind the hydrogen in chains. There's a class of these that works quite well: hydrocarbons. But enough of me being a smartass - thanks for the boil off info, that's fascinating! $\endgroup$
    – Peter Wone
    Commented Feb 3, 2019 at 10:25
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    $\begingroup$ @StephenS bad idea – gaseous hydrogen diffuses extremely quickly in metals. the only way you can get hydrogen under control is when it's a cold liquid. that's the only way. and remember even then it's got almost no viscosity, a needle hole in a barrel, and the entire barrel of LH2 will be gone in less than a minute! $\endgroup$
    – Meatball Princess
    Commented Feb 3, 2019 at 13:40
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    $\begingroup$ @MeatballPrincess A car is refueled once every week or two; an airliner is refueled before every flight, so it only needs to keep the fuel contained for a few hours. $\endgroup$
    – StephenS
    Commented Feb 3, 2019 at 21:17
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    $\begingroup$ I think the odds of planes being able to carry "all that hardware" are close to 100%. The rest of the plane is hardware, you know, the engines are hardware, the kerosene fuel systems are hardware, the flight control systems are hardware, why do you think planes can't carry lots of hardware? $\endgroup$
    – Criticizing Israel not allowed
    Commented Feb 4, 2019 at 2:39
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    $\begingroup$ @MSalters hydrogen flame is completely smokeless and sootless so no use for smoke detectors, LH2 has very low density so LH2 tanks are very bulky, and more tank area=more chance of leakage, LH2 tanks must allow LH2 to constantly boil off, unlike rocket which only accelerates longitudinaly airplanes roll, so LH2 sloshing is something never encountered and solved before. A deeply cryogenic fuel is the last thing you want to live through. A small mistake, then you have a Challenger of a plane. $\endgroup$
    – Meatball Princess
    Commented May 8, 2019 at 3:59
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Using water as the carrier and splitting it on-board to hydrogen and oxygen is a nonstarter. Electrolysis takes vast amounts of electric power, so instead of just water you need to carry water plus (big, heavy) batteries. If you use batteries, you're better off just using electric motors to drive the turbines, that would be lighter than an electrolysis setup.

Hydrogen storage has the same problem as batteries: Batteries store far less energy per kg, hydrogen stores far less energy per m3 than jet fuel.

Hydrogen also requires heavy, high-pressure tanks.

A better alternative would be converting hydrogen and carbon monoxide into liquid fuel on the ground (using the Fischer-Tropsch process) and burning that in your engines.

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    $\begingroup$ Re: "Hydrogen storage has the same problem as batteries: both store far less energy per kg than jet fuel. No, that is not true. Per kg hydrogen has 3.3 times more energy (142 MJ/kg for hydrogen and 43 MJ/kg for jet fuel). (Per volume is a different story, 10 MJ/l and 37 MJ/l, respectively.) $\endgroup$
    – Peter Mortensen
    Commented Feb 4, 2019 at 10:50
  • $\begingroup$ Yes, I meant per volume for hydrogen. $\endgroup$
    – Hobbes
    Commented Feb 4, 2019 at 18:24
  • $\begingroup$ @Hobbes as you stated "you're better off just using electric motors to drive the turbines." Is there actually a motor powerful enough to produce the torque and speed needed to turn the heavy turbine? not trying to sound mean or sarcastic, i'm just really curious about your statement and genuinely want to know if such motor exist. thanks for your answer btw $\endgroup$
    – cat
    Commented Feb 12, 2019 at 19:06
  • $\begingroup$ Electric motors produce up to 10 kW/kg, so 3.5 tons to produce 35 MW (which is in the range of what you need for a large airliner). You'll have to add a fan to that, cowling etc. but (and this surprises me) 10 kW/kg is as good as a modern turbofan. $\endgroup$
    – Hobbes
    Commented Feb 12, 2019 at 19:27
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Hydrogen works just fine as turbine fuel, and does so in space launch turbopumps. Achieving full efficiency, power, and engine life on hydrogen will require tweaks to a pre-existing engine, of course.

On the environmental side, H2 normally burns hotter than hydrocarbons, which produces more N2O, but combustion temperatures can be regulated, and have to be to match turbine life specs.

Storage is the problem. All generators based on storing hydrogen in room-temperature liquids/solids share the problem of considerably worse net:tare ratios than liquid hydrogen tanks. LH2 adds cost and maintenance with cryogenics and still falls short of hydrocarbon energy density.

You can't get net energy by extracting H2 out of water, as that consumes all of the same energy that H2 produces in combustion, plus the losses. Energy cannot be created, only converted. For heat to be created to drive the engine, energy has to be lost elsewhere. In this case it's lost in joining two chemicals, which store potential energy (combustibility) in their state of separation.

If you had a different energy source (actually energy converter) on board, e.g. nuclear, you could use its output to propel the aircraft without the need for combustion.

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  • $\begingroup$ Re "still falls short of hydrocarbon energy density": in volume yes, but not in mass (though technically specific energy). Hydrogen has 142 MJ/kg whereas jet fuel has 43 MJ/kg (yes, 3.3 times more). Per volume it is 10 MJ/l and 37 MJ/l, respectively. $\endgroup$
    – Peter Mortensen
    Commented Feb 4, 2019 at 10:36
  • $\begingroup$ @PeterMortensen True, I was addressing volumetric density. As for gravimetric, you'd have to compare entire systems complete with storage, insulation, and extra fuselage volume to get a practical estimate of the effects. $\endgroup$
    – Therac
    Commented Feb 4, 2019 at 10:53
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Your plan is to not store the hydrogen, but generate the hydrogen inflight.

The problem is that generating hydrogen inflight is even more awkward than storing it. The component chemicals you combine to make hydrogen are much, much heavier than the hydrogen itself (hardly a surprise since hydrogen is by far the lightest atom)... and both heavier and more expensive than getting the same energy out of petroleum fuel.

What's more, back on the ground, you would need to spend energy to create the component chemicals (to "charge them up") would require electricity or heat, which would require burning other fuel, due to efficiency far more fuel again than the airplane is able to harness.

If the public policy motivation is to reduce fuel spent by airplanes, the greenest way to do that is effective high-speed-rail systems. Downtown London to downtown Paris is so stacked in favor of rail that I can't believe there are any airflights at all. High speed rail powers directly off electric, efficiently making the most out of green electric sources such as wind or solar.

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  • $\begingroup$ Alternately, find ways to cleanly synthesize hydrocarbons instead of digging them up. $\endgroup$
    – StephenS
    Commented Feb 3, 2019 at 8:14
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    $\begingroup$ We can create jetfuel from hydrogen and carbon dioxide, in the Fisher-Tropsch process. However, its very inefficient. Until a vast majority of the world's electricity is produced by renewable or nuclear, it is unlikely that artifically produced hydrocarbons will be used for jet fuel $\endgroup$
    – CSM
    Commented Feb 3, 2019 at 9:50
  • $\begingroup$ @CSM: Carbon monoxide, actually. But I agree with the premise. Biofuels are much easier to manufacture, and plants are pretty good at turning carbon dioxide into hydrocarbons. $\endgroup$
    – MSalters
    Commented Feb 4, 2019 at 9:32
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By using a generator which converts water into hydrogen gas, we can just use water as our main fuel source which is more cost efficient?

NO!

Splitting water into hydrogen and oxygen, done at 100% efficiency, takes exactly as much energy as you get back from burning that hydrogen in the oxygen, at 100% efficiency. Where are you getting all that energy from to split water? Your plane would be lighter if you just used that energy source directly to propel the plane and didn't waste time converting water to hydrogen and oxygen and back to water.

In reality, you don't have 100% efficiency, so the water-to-hydrogen-and-oxygen-to-water equipment isn't just dead weight: it's weight that's actively using up some of your power.

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It may turn out that compressed hydrogen will be excellent for fixed structures where storage volume is not as great an issue. Transportation such as trucks or aircraft greater favor higher energy density, or liquid fuels. Exotic applications in rocketry favor the higher specific impulse per gram of hydrogen compared with hydrocarbon (more bonds to oxidize per unit weight).

Amazingly, conversion of water to hydrogen may make it easier to transport water over long distances as a convertible gas rather than energy intensive pumping and/or building canals. A 5 psi hydrogen pipeline will cross any mountain range, and, at its destination, provide both fuel for heating/electricity and one gallon of water for every pound of hydrogen burned.

However, issues of volume and extremely low boiling point may limit its applications for large scale transportation. Liquid natural gas may be a better bet.

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    $\begingroup$ "Liquid natural gas may be a better bet." - Jet engine cores are already run on natural gas with no significant modifications, for gas pipeline pumping. And they happily run continuously and maintenance-free for months at a stretch, in locations like Alaska which are inaccessible in winter. $\endgroup$
    – alephzero
    Commented Feb 3, 2019 at 21:26
  • $\begingroup$ Yes, but would such pipelines have the capacity for the required amount of (equivalent) water? (What would the water be for? Tap water? Irrigation? Pepsi Cola?) $\endgroup$
    – Peter Mortensen
    Commented Feb 4, 2019 at 11:22
  • $\begingroup$ My application is the American Southwest. Water would be for irrigation and for household use. $\endgroup$
    – Robert DiGiovanni
    Commented Feb 4, 2019 at 15:01
  • $\begingroup$ Converting water to hydrogen seems a very Rube Goldberg-esque method. Conversion efficiency is on the order of 50%, pumping losses would have to be huge for the conversion to be more efficient. $\endgroup$
    – Hobbes
    Commented Feb 4, 2019 at 18:22
  • $\begingroup$ @Hobbes yes, but it is better than leaving a windmill idle when the power grid demand is low. It is a form of energy storage, enabling power companies to design for peak load (more windmills) rather than average. The H2 goes right onto the pipeline. In the Southwest, any water is precious. $\endgroup$
    – Robert DiGiovanni
    Commented Feb 4, 2019 at 19:48
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Google "cryoplane final report" or just cryoplane for a full answer. Jet liners using cryogenic hydrogen are feasible but won't be cheap unless hydrogen becomes very cheap. It might do that as renewable energy sources like wind and solar could generate hydrogen by electrolysis when their electricity output is greater than demand the time.

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I guess the first Aircraft flying with a Turbine, the Heinkel-178, with the Turbine of Hans Joachim Pabst von Ohain, patent US2256198, inventor stated M Hahn, had the jet engine tested with Hydrogen as fuel, the issue with Hydrogen may be storage. www.SAE.org has documents about Hydrogen as Automotive Fuel, no need being member to purchase.

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