So why not just do full manufacture of regular fuel? I've never understood where hydrogen fits between improving batteries/capacitors and synthetic hydrocarbons. It always seemed like an answer without a question outside of rockets (and even there the economics and practicalities of methalox may be better).
As I understand it, the production of complex hydrocarbons that can be used as a replacement for jet fuel is even more inefficient when compared to the production of hydrogen, as many of the known processes that start from CO2 actually use hydrogen in the process. So, although you could get energy-denser fuels, that would be at the cost of even more end-to-end efficiency.
Finally, hydrogen has a great energy / kg ratio, making it a very interesting fuel for aviation if the material science problems of hydrogen can be overcome, as it allows for longer distance flights / flights with more/heavier cargo.
Here's the thing though: we can efficiently store those complex hydrocarbons. We can't efficiently store hydrogen. It destroys containers, and can escape any container we have devised(unless in liquid form?)
Many of the proponents of electrolysis claim that renewables will make energy a non-factor(or they'll propose nuclear power, etc). If that's the case, then add another step to capture CO2 as well.
> Finally, hydrogen has a great energy / kg ratio
Yes but energy per unit of volume is terrible.
As for volume - there's a reason why they call them cryoplanes ;)
From Airbus - Liquid hydrogen is used as fuel for combustion with oxygen.
Volume on an aircraft is not expensive. Weight is expensive. Aerogel is very, very light. As is LH2.
Airports can make the LH2 directly on the spot from solar and wind power, so no need to transport or refine fuels.
Why would that route even be taken though? Biofuel for jet engines is proven technology in limited use for many years now. Let plants deal with the CO2, since they're good at it, then process the resultant vegetable oils to produce a suitable diesel or jet fuel.
They aren't good with it at all. Convert CO2 slowly, take space to do that, take other resources like fertilizers, require some shielding from pests and herbs, can produce massive fires... They have advantages, but it's not a silver bullet, especially when you're in a hurry.
That is not to say biofuel don't have a role to play. Agriculture and forestry wastes, municipal biowaste, etc. can all be used for energy. And yeah, it makes sense to use the available biofuel for sectors that are otherwise very hard to decarbonise, such as aviation.
Algae, well, that has been researched for decades with very little to show for it. It has great potential, but it seems very difficult to make it actually work.
I just don't see any way that synthesis of jet fuel from atmospheric carbon makes any sense for anybody but a nuclear Navy.
The other problem is that biofuel isn't actually all that green. It still needs lots of fertilizer, pesticides, farm equipment, etc., that ultimately gives it a relatively high GHG output.
At some point, we need to bite the bullet and go with hydrogen or one of its derivative fuels (e.g. ammonia, methanol, etc.). People are constantly trying to imagine a solution that avoids this outcome, but no one has come up with anything that looks like a viable alternative.
Even the worst electrolysis driven by cheap solar does better.
But that was before solar and wind became substantially cheaper than petroleum.
This is never going to happen at the necessary scale. Where do you want to put all those plants? The planet's land area that is suitable for agriculture is already being used for food production. Well, apart from a few areas with natural reserves but you really don't want to touch those if you care about the environment. And if you don't then the whole clean energy topic is not for you anyway.
Sure some areas can be made a bit more efficient, but that's never going to be enough.
You can manufacture hydrogen anywhere as long as you have water, an electrolyzer, and a power source. Oil can only be pumped out of the ground in certain places, water is technically _much_ more abundant everywhere.
Hydrogen storage tech has improved in the last twenty years. It's becoming affordable and "as safe" to store hydrogen at fuel densities similar to that of petroleum based fuels.
Obviously the devil's in the details, but shouldn't we attempt to engineer huge carbon emitters like container shipping or aircraft to run on hydrogen + solar?
I don't understand what you're arguing with here? You can manufacture synthetic hydrocarbons anywhere so long as you have water, carbon dioxide, and a power source too. So I'm not sure why you see this as an advantage for hydrogen.
>but shouldn't we attempt to engineer huge carbon emitters like container shipping or aircraft to run on hydrogen + solar?
Again, why hydrogen from solar vs hydrocarbons from solar? From a global warming perspective what matters is net GHG.
You need coal or some biomass to manufacture synthetic hydrocarbons. And tail emission is still a concern.
Perhaps the efficiency to produce methane is not so bad, but a chain with 6 or 7 carbons like gasoline is very complicated. It is not equivalent if they are aligned or they form a ramified structure like a tree, and it is very difficult to choose which one they will form unless you have a very complex process. There are some graphics in https://en.wikipedia.org/wiki/Octane_rating#Isooctane_as_a_r...
The Fischer-Tropsch process is almost a hundred years old, and has been used on an industrial scale. E.g. Nazi Germany used it to produce a non-trivial fraction of their liquid fuels during WWII. Similarly South Africa used it during Apartheid. Now FT itself is better suited for producing mostly straight hydrocarbon chains like diesel or jet fuel rather than the branched ones seen in high octane gasoline.
The efficiency of traditional FT is around 40%, but by adding extra hydrogen to the process it can be increased by around a factor of 3 (the efficiency meaning the energy content of the carbon carrying feedstock vs the output, so over 100% is possible as hydrogen provides some of the energy in the final product).
The problem is to get the CO needed for FT in a carbon-neutral manner. Biomass is limited, capturing and reducing CO2 is expensive and requires a lot of energy.
I know it's not an argument against, but the Nazis lost WW2 because they ran out of oil. They invaded Russia at the last moment they could before they wouldn't have the oil to invade, and when they invaded they made a beeline for Russia's oil fields.
They were desperate for fuel from anywhere, because their tanks are useless and they either have to fight to the death wherever their tanks run out of fuel, or abandon their tanks.
...also, the main non-fossil fuel the nazis used was probably hay. For their horses. It's just a super weird example.
Well, they would most likely have lost anyway, but failing to capture the Caucasian oil fields as well as the Allies attacking their fuel infrastructure certainly hastened their demise.
One can make a good argument that the strategic bombing on the western front in WWII was largely ineffective, but there were parts that were wildly successful, like attacking the fuel infrastructure.
As for why they didn't just use synthetic fuel all the way instead of just a fraction of the total use, I don't know. I guess they just couldn't scale it up fast enough. And certainly getting oil from the ground is much cheaper than creating synthetic fuel from coal in huge industrial parks.
> ...also, the main non-fossil fuel the nazis used was probably hay. For their horses. It's just a super weird example.
Sure. Blitzkrieg captured the public imagination, but in reality only a small fraction of the German army were these mechanized shock troops. Most were infantry that moved by train or marched with horses carrying equipment.
Have we solved hydrogen embrittlement issues? What about hydrogen leaking out of containers?
Unless you plan on throwing away the planes after one or two flights, those tanks will be filled with hydrogen for tens of thousands of hours over their lifespan. Perhaps they have light weight alloys that can cope with this though, I'm no metallurgist..
Frankly hydrogen planes are nonsense. Even if it works on paper, it's not realistic to expect all the extant planes to be rapidly phased out with brand new planes with exotic designs that aircraft manufacturers, pilots, and airlines don't have any experience with. On the other hand, old planes can be retrofitted to burn biofuel and new production planes of old proven designs can be made to burn biofuel from the get-go.
'Concept planes' like the ZEROe are basically just science fiction meant to wow shareholders and give the public the impression that legislative reform isn't necessary because corporations are already taking the problem seriously. I don't think these are serious proposals, it's just corporate propaganda. Car companies pushed hydrogen cars for years for the same reason. Hydrogen fuel cell cars were never going to happen for a myriad of practical reasons and car manufacturers knew it but promoted the concept anyway for PR reasons. ZEROe is Airbus doing the same.
Liquid per liquid, why not liquid methane that's far easier to work with and can be manufactured at zero carbon emissions.
The thing is, no matter how far you are overshooting or undershooting that ratio, as soon as you are doing both there'll be a boundary where it's matching.
The boundary, if there is any, also has to coincide with an ignition source. Furthermore, the ignited bit needs enough fuel around it to build a chain reaction.
Systems will be designed to maintain positive airflow to keep concentration, in the event of a leak, below the threshold. That is engineering.
Why hydrogen at all then? The source is solar so let's keep it at that, why the middle state?
Aircraft need concentrated energy. Batteries are nowhere near enough for anything big. Liquid H2 is the best possible fuel for aircraft, once certain practicalities are worked out, and can be produced from solar and wind.
H2 doesn't need solar, it needs power, it doesn't care where that power came from. It's essentially a disconnected system, to sell it as if H2 somehow means solar (which I get from earlier threads) is a little bit disingenuous.
Unless you think it would be natural to use the more expensive power instead?
You need hydrogen for steel production, ammonia, and synthetic hydrocarbon production. Likely cement, glass, and plastic production too.
> So why not just do full manufacture of regular fuel? I've never understood where hydrogen fits between improving batteries/capacitors and synthetic hydrocarbons.
Because fuel cells are batteries themselves, and synthetic hydrocarbon needs hydrogen.
Always fascinating when people use "just" in sentences where they propose something that hasn't been done before (at least not at any scale that would matter) and that is a huge challenge.
As for the reason, my understading is that producing fuels from electricity is just very inefficient. You still use hydrogen as the first step and then add other steps to produce hydrocarbons. Using hydrogen spares a few conversion steps and you'll need less electricity (but likely still a lot).
Large scale energy storage is probably handled better using either redox flow batteries , hydro storage systems or energy storage vaults using concrete blocks which are lifted when there is too much energy available, lowered when energy is needed.
Hydrogen is usually considered a very simple conversation (e.g. compared to synthetic hydrocarbons) that would be very open to opportunistic transformation utilization.
"Only fire up the converters when there's an open order for synfuel and the grid has excess power", that's the dream. I'm not convinced that this dream is significantly more viable for simple hydrogen than for more complex synfuels (particularly factoring in the many ways hydrogen isn't easy to store), but I can easily see how some people might want to believe that.
E.g. liquid methane would be a good alternative for hydrogen. The appeal of both hydrogen and methane would be that converting engines to burn it seems doable and given that rockets can use either as propellant there is no question about their suitability as a fuel from an efficiency point of view. The difference of course is that with methane it would end up coming from fossil reserves which from the point of view going carbon neutral kind of defeats the purpose of making these planes carbon neutral. Swapping out kerosene for methane kind of just moves that problem. IMHO it's still worth exploring (and some companies have actually done that). Also, I imagine synthetic methane is eventually going to happen and kill the market for natural gas. Just not in the next decades.
The key here is a combination of becoming carbon neutral and doing it in a cost effective and timely way. Synthetic methane may or may not actually become a thing but the simple reality is that it's an industry that simply does not currently exist. Clean hydrogen production on the other hand exists today (at small scale) and seems to be on a feasible path to scaling in the next 15 years or so. Additionally hydrogen infrastructure is something that is attracting lots of investment currently. Airbus is targeting a launch in the mid to late 2030's. So they need to be targeting a fuel source that they can reasonably expect to have some supply for by then so they can actually fuel the planes where-ever they need to fly. So the supply and infrastructure for hydrogen are not a certainty but at least on a path where it could be there in 15 years. The rest is just solving engineering problems. They seem to believe they can do it.
I agree hydrogen isn't the best fuel, but the round trip efficiency of any other synthesised hydrocarbon must be an order of magnitude lower - and even if solar energy is abundant, it's not so abundant we can waste 90%+ of it on process inefficiencies.
Hydrogen solves long term storage for electricity generation and heating (winter time)
High altitude water vapor is just as big of a problem as CO2 as the lifetime of water vapor in the stratosphere is measured in years, not days or hours like it is in the lower troposphere. Water vapor is, after all, a much stronger greenhouse gas than CO2, it's just that normally its lifetime in the atmosphere is so short so as not to matter.
So unless we can be certain these planes will fly solely in the lower troposphere (unlikely, considering the aircraft presented), then hydrogen is not actually necessarily a silver bullet and in some situations (i.e. very high altitude flight, ala Concorde) could actually be worse than kerosene.
On Table 1 of your study, it’s measuring residence time of stratospheric water vapor in days. How are you getting years from that?
> f Residence time = accumulated H2O divided by emitted H2O (days). Corresponding values for the stratosphere only are given in parenthesis.
That seems like it is in days. Unless there's something that I missed, I'm not seeing the part that claims water vapor lasts for years in the stratosphere.
Due to the fuel's low density, the conventional fuselage designs lose a lot of load capacity.
The blended wing body looks practical, with plenty of interior volume. Hearing people complain about passengers not wanting to sit in a stadium, or that their inner ear will be unmitigatedly upset by roll changes remind me of historical folks who said cars would never take off because they'd scare the horses.
The process is straightforward:
1. A lot of cheap solar energy, from the best location.
2. Electrolyse to produce hydrogen.
3. Capture CO2. Either from air or from carbon intensive processes (e.g. producing cement).
4. Use Sabatier reaction to bond CO2 and hydrogen to produce carbohydrate fuel.
5. Since we use same amount of CO2 to produce as there will be burn this carbon neutral fuel.
It's much easier to do it at scale, than redesign, get approved new planes, replace all existing fleets and the infrastructure. This could easily take 60-120 years, when we need to act much sooner to prevent worst consequences of global warming.
Of course, today this would not work as individual steps are too expensive. Though all of the steps could be much cheaper as solar energy panel shows.
It gives possibility for oil rich countries to transform to post carbon economy when they resources will be depleted and consumers would prefer carbon neutral fuels.
I guess all of this sovereign funds would be much better of investing in this tech than pouring money in SoftBank to do WeWork style opportunities.
Google crowd oil or solar kerosene for papers about it.
I recommend that you watch the "Planet of the Humans" documentary to understand why there is no cheap solar energy, no so-called "green" energies: https://www.youtube.com/watch?v=Zk11vI-7czE
So there will never be any "green" planes, no matter if they use hydrogen or kerosen. Since the minimum required amount of energy to fly from point A to point B with N passengers will always be the same.
There are some truths in the movie (e.g. biomass would not work), but many times he is plain wrong. E.g. 8% solar efficiency, when my neighbours installed 21% one and 15% is common on utility scale. No solar panel on the market has just 10 years+ lifespan, many come with 20+ year warranty and could still produce energy afterwards with tiny decrease in efficiency.
Not necessarily, it depends on the byproducts - hypothetically, suppose when you burned the fuel the emissions were SOMEHOW 100% methane. You're essentially turning CO2 into methane in this farcical hypothetical, which is probably worse than burning normal fossil-kerosene.
Obviously the emissions won't be methane, but you'll need to track the CO2e of every type of emission it produces other than CO2.
Battery planes will eat the market from below, step by step.
: It uses fuel cells too, but looks like most of the propulsion comes from the turbines.
But actually, batteries are better than people think. 1000km battery-electric range is feasible with current chemistries, and ~3000-5000km is feasible with metal-anode, maybe lithium-sulfur or other advanced chemistries plus extremely high performance structures and aerodynamics. Lithium-air batteries with these improvements could enable >10,000km flights.
You do realize a fuel cell is a hydrogen-air battery? If you’re going to bring up metal-air batteries, you might as well skip to the end and try to figure out fuel cells.
And sure, I guess you could argue lithium-air is a type of fuel cell (or that hydrogen fuel cells are a kind of inefficient battery), however lithium-air is much more energy dense (volumetrically) than hydrogen-air and both the "fuel" and the "exhaust" are solid and stay in the battery, which has operational advantages. (Although to be completely fair, hydrogen fuel cells are high TRL whereas lithium-air is still lab scale at best.)
I addressed this in my other reply. Table 1 of the study you cited stated that the residence time of stratospheric water vapor is measured in days. I'm not seeing a lifetime of years in that study.
> however lithium-air is much more energy dense (volumetrically)
Maybe. Lithium-air absorbs oxygen when it is operating. Not sure how to calculated this though.
Conversion losses may be lower, but a battery-powered plane doesn’t lose weight during flight. That makes the impact on landing harder, so planes will have to be made stronger and, thus, heavier.
And yes, that should make quite a difference. Reading https://en.wikipedia.org/wiki/Fuel_fraction, 40% of the take-off weight of long-haul flights is fuel. Not all of that will be gone at landing, but if 10% of fuel is reserve, such an airliner still will be a third lighter at landing time.
Also, commercial planes have turnaround times of about an hour, sometimes half of that (https://simpleflying.com/turnaround-time-importance/). I would think pumping in hydrogen would be easier to do in that time than charging batteries. Batteries could be replaceable, but that would add more weight.
I think they'll just handwave it and release it periodically.
Furthermore, without internal combustion, you are limited to ~350 knots.
Further, since air resistance scales with the square of the velocity, the drag forces on an aircraft is so high that you typically never reduce thrust to zero until to just before touching down. An aircraft never idle-glide any substantial part of the flight. I.e, it's like trying to recuperate energy while driving slowly in a car up a very steep hill. Just releasing the throttle will make you stop quite rapidly...
I have no good sources but don't articles I have read said it's potentially worse then the CO2 emission of current air plainness.
So additional care must be taken to not condense the water from the hydrogen burning into larger potions/prices before emitting it.
These concept renderings omit the basic physical facts: that hydrogen is less energy dense and therefore you need more of it. Tanks will be bigger and distances will be shorter.
Energy per kg is much higher for liquid hydrogen (122.8 MJ/kg) than for kerosine (42.8 MJ/kg): https://www.fzt.haw-hamburg.de/pers/Scholz/GF/GF_Paper_DLRK_...
So planes would look bulkier, but could actually be lighter than before.