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For anyone else like me wondering why hydrogen is difficult for airplanes (not mentioned in the OP article): from https://www.bbc.com/future/article/20210401-t

For anyone else like me wondering why hydrogen is difficult for airplanes (not mentioned in the OP article): from https://www.bbc.com/future/article/20210401-the-worlds-first-commercial-hydrogen-plane https://www.bbc.com/future/article/20210401-the-worlds-first... -------------- Hydrogen has higher energy by mass than jet fuel, but it has lower energy by volume. This lower energy density is because it is a gas at typical atmospheric pressure and temperature. The gas needs to be compressed or turned into a liquid by cooling it to extremely low temperatures (-253C) if it is to be stored in sufficient quantities. "Storage tanks for the compressed gas or liquid are complex and heavy," says Finlay Asher, a former aircraft engine designer at Rolls-Royce and founder of Green Sky Thinking, a platform exploring sustainable aviation. And there are other challenges. The energy density of liquid hydrogen is only about a quarter of that of jet fuel.

PostThisTooFast 5y ago
Not to mention that compressing or liquifying gas takes a shitload of energy to begin with.
ncmncm 5y ago
Probably you will want liquified H2. So, we need volume manufacturing of aerogel, for the tanks. If we don't go to wholly new airframes, we will need to sling the LH2 tanks in nacelles under the wings, like the engines, because the needed tankage volume won't fit inside normal wings. (With a new airframe with more internal space, the tanks could be inboard, but maybe you don't really want big tanks of LH2 inboard.) The LH2 would be electrolyzed on demand at the airport, using power delivered by transmission lines from all over, banking LH2 when the sun is out or the wind is blowing, when it is cheapest. Once these planes start flying, it will become impossible to compete with them, because the smaller mass of fuel needed translates to substantially more freight, both total capacity per flight, and per unit-cost of fuel. But there are still things to work out, so there will be an interim need to synthesize jet fuel from atmospheric CO2 and H2, which should soon get cheaper than extraction. It would be helpful if somebody were to come up with a good use for all the waste oxygen everyone is going to be producing.
- tuatoru 5y ago
  > Probably you will want liquified H2. More likely to use solid adsorbed hydrogen. Room temperature, room pressure, less mass in the tanks. Adsorption has been experimented with for several decades at least. But modern computer power and modern processes for controlling material surface properties make it more likely than ever before. https://en.wikipedia.org/wiki/Adsorption https://en.wikipedia.org/wiki/Adsorption - not specific to hydrogen Random PDF of study: https://www.cambridge.org/core/services/aop-cambridge-core/content/view/10F54D8F0E3E0ACB12D0083DF1C91672/S0883769400053458a.pdf/hydrogen_adsorption_in_carbon_materials.pdf https://www.cambridge.org/core/services/aop-cambridge-core/c... https://www.youtube.com/watch?v=U7CCq4oBgw4&t=464s https://www.youtube.com/watch?v=U7CCq4oBgw4&t=464s - latest "breakthrough". One day, one of these will be real.
  - ncmncm 5y ago
    Less mass than what? I promise you, adsorbed hydrogen along with whatever it is adsorbed onto will not weigh less than the same hydrogen, liquified. LH2 at atmospheric pressure does not need a heavy tank. If they can get it to adsorb to an aerogel, they might have something, but you still have to get the H2 back out.
    - ddek 5y ago
      The best solid adsorbed hydrogen is currently running about 5% the density of liquid hydrogen. Most solutions use either graphene or heavy metals too rare to meet mass demand. It's not a promising technology.
  - ddek 5y ago
    As you say, we've been working on solid adsorption for a long time now. Nothing has left the lab. The video presents an optimistic future, but we don't even have the basics yet. Firstly, there is a fundamental issue - H² doesn't really give you anything to work with. The vast majority of compounds have two protons, two neutrons (rare isotopes have more protons). [They're distributed in the most boring orbitals, just spheres with no real edge to gain leverage.](https://winter.group.shef.ac.uk/orbitron/atomic_orbitals/1s/index.html https://winter.group.shef.ac.uk/orbitron/atomic_orbitals/1s/...) This lack of leverage makes surface adsorption tricky. The molecule gives us nothing to grab. You won't carefully craft a surface that can hold H². I think right now there's a focus on activated carbon. That's become a bit of a synonym for 'graphene', with it's well known limitations. When I studied the subject, metal-organic complexes were the focus. These would be extremely customisable, but if they worked we'd probably have seen by now. They tended to use heavy metals. I wasn't optimistic at the time, I thought anything using materials past the 4th row of the periodic table was unrealistic. As the metals get heavier they get expensive and sometimes rare. If we want to solve a common problem, we need common components. If your solution has lanthanoid, it won't scale. I was right btw. The research group closed when funding was reallocated. Each experiment ran the research budget of another group. MoC's don't really get much love any more. Anyway, we should probably think about what makes a good hydrogen adsorption material. I think it's pretty simple, you want a material that: stores a high density of hydrogen, is reusable, quickly picks up hydrogen, and quickly releases hydrogen. The latter 2 categories is the 'kinetics'. Of the materials we know, all (literally all) that have 'good' kinetics are either single use or low density. Conversely, all that are both multi use and relatively high density have poor kinetics. Additionally, most of those with good kinetics have unrealistic conditions, like requiring 400˚C temperature to release the hydrogen.
jessaustin 5y ago
When hydrogen is used to power aircraft, it will be in the form of liquid anhydrous ammonia. Safer than elemental hydrogen, already mass-produced for its regular use in agriculture, more energy-dense, doesn't embrittle, etc.
- ncmncm 5y ago
  Anhydrous ammonia makes a better fuel for ships. They can use the existing engines, and just replace tankage and plumbing. ("Safer" is hard to quantify. Less explosive, but overwhelmingly more toxic if vented.) Tankers may carry ammonia synthesized in the tropics to northern ports where local solar is impractical, to burn in existing gas plants in competition with long-distance transmission lines. Or, synthetic CH4 from captured CO2 might win. Either way will need massive H2 production.
  - jessaustin 5y ago
    Well, sure, it's good for ships too. In general powering a large ship has a much more generous performance envelope. Thus, "bunker oil". Ammonia is better than bunker oil and it's also better than compressed elemental hydrogen, for powering any machine that moves.
    - ncmncm 5y ago
      I don't know of anyplace where compressed elemental hydrogen would be a good choice.
coderenegade 5y ago
You'd use LH2 or ammonia rather than compressed H2. All else being equal, cruise power for an aircraft is linear with mass, and LH2 has 1/10th the mass for the same volume. For a 40% fuel fraction, an aircraft with liquid hydrogen comes in at 65% of the weight and 25% of the energy content for the same volume of fuel. This will get you a bit under 40% of the range of the kerosene jet, if both jets are equally efficient. Ammonia is fairly similar, maybe a few percentage points higher. You could probably get more than 50% of the range of existing designs using fuel cells that are more efficient than jet engines. More likely we're looking at a rethink of how we design aircraft to accommodate larger volumes of fuel. If we did this, we could approach currently used fuel mass fractions with LH2, and potentially go further than we do with current aircraft. Keep in mind the effective energy density of ammonia after taking efficiency into account is still ten times the energy density of today's state-of-the-art lithium ion batteries (by mass). LH2 is of course much higher than that. I find it hard to see batteries replacing anything other than short hop flights for the foreseeable future, if that. Fuel reserve requirements alone make it a tough proposition.
- ncmncm 5y ago
  You cannot use in-wing tankage for LH2, so calculating with the same fuel volume is a pointless exercise. Keeping similar (or retrofitted) airframes, the tankage would probably need to be slung in nacelles under the wings. There, the extra volume is no problem, and being closer to spherical minimizes tank wall insulation. You might guess the extra nacelles would be a huge drag problem, but fluid dynamics is a deeply unintuitive science. The mass efficiency is amplified by not needing to loft as much fuel mass, which capacity can be used for extra payload, offsetting any drag losses. And, LH2 produced from solar will be a lot cheaper per joule delivered to the engines.

PostThisTooFast 5y ago

Not to mention that compressing or liquifying gas takes a shitload of energy to begin with.

ncmncm 5y ago

Probably you will want liquified H2. So, we need volume manufacturing of aerogel, for the tanks. If we don't go to wholly new airframes, we will need to sling the LH2 tanks in nacelles under the wings, like the engines, because the needed tankage volume won't fit inside normal wings. (With a new airframe with more internal space, the tanks could be inboard, but maybe you don't really want big tanks of LH2 inboard.) The LH2 would be electrolyzed on demand at the airport, using power delivered by transmission lines from all over, banking LH2 when the sun is out or the wind is blowing, when it is cheapest. Once these planes start flying, it will become impossible to compete with them, because the smaller mass of fuel needed translates to substantially more freight, both total capacity per flight, and per unit-cost of fuel. But there are still things to work out, so there will be an interim need to synthesize jet fuel from atmospheric CO2 and H2, which should soon get cheaper than extraction. It would be helpful if somebody were to come up with a good use for all the waste oxygen everyone is going to be producing.

tuatoru 5y ago
> Probably you will want liquified H2. More likely to use solid adsorbed hydrogen. Room temperature, room pressure, less mass in the tanks. Adsorption has been experimented with for several decades at least. But modern computer power and modern processes for controlling material surface properties make it more likely than ever before. https://en.wikipedia.org/wiki/Adsorption https://en.wikipedia.org/wiki/Adsorption - not specific to hydrogen Random PDF of study: https://www.cambridge.org/core/services/aop-cambridge-core/content/view/10F54D8F0E3E0ACB12D0083DF1C91672/S0883769400053458a.pdf/hydrogen_adsorption_in_carbon_materials.pdf https://www.cambridge.org/core/services/aop-cambridge-core/c... https://www.youtube.com/watch?v=U7CCq4oBgw4&t=464s https://www.youtube.com/watch?v=U7CCq4oBgw4&t=464s - latest "breakthrough". One day, one of these will be real.
- ncmncm 5y ago
  Less mass than what? I promise you, adsorbed hydrogen along with whatever it is adsorbed onto will not weigh less than the same hydrogen, liquified. LH2 at atmospheric pressure does not need a heavy tank. If they can get it to adsorb to an aerogel, they might have something, but you still have to get the H2 back out.
  - ddek 5y ago
    The best solid adsorbed hydrogen is currently running about 5% the density of liquid hydrogen. Most solutions use either graphene or heavy metals too rare to meet mass demand. It's not a promising technology.
- ddek 5y ago
  As you say, we've been working on solid adsorption for a long time now. Nothing has left the lab. The video presents an optimistic future, but we don't even have the basics yet. Firstly, there is a fundamental issue - H² doesn't really give you anything to work with. The vast majority of compounds have two protons, two neutrons (rare isotopes have more protons). [They're distributed in the most boring orbitals, just spheres with no real edge to gain leverage.](https://winter.group.shef.ac.uk/orbitron/atomic_orbitals/1s/index.html https://winter.group.shef.ac.uk/orbitron/atomic_orbitals/1s/...) This lack of leverage makes surface adsorption tricky. The molecule gives us nothing to grab. You won't carefully craft a surface that can hold H². I think right now there's a focus on activated carbon. That's become a bit of a synonym for 'graphene', with it's well known limitations. When I studied the subject, metal-organic complexes were the focus. These would be extremely customisable, but if they worked we'd probably have seen by now. They tended to use heavy metals. I wasn't optimistic at the time, I thought anything using materials past the 4th row of the periodic table was unrealistic. As the metals get heavier they get expensive and sometimes rare. If we want to solve a common problem, we need common components. If your solution has lanthanoid, it won't scale. I was right btw. The research group closed when funding was reallocated. Each experiment ran the research budget of another group. MoC's don't really get much love any more. Anyway, we should probably think about what makes a good hydrogen adsorption material. I think it's pretty simple, you want a material that: stores a high density of hydrogen, is reusable, quickly picks up hydrogen, and quickly releases hydrogen. The latter 2 categories is the 'kinetics'. Of the materials we know, all (literally all) that have 'good' kinetics are either single use or low density. Conversely, all that are both multi use and relatively high density have poor kinetics. Additionally, most of those with good kinetics have unrealistic conditions, like requiring 400˚C temperature to release the hydrogen.

jessaustin 5y ago

When hydrogen is used to power aircraft, it will be in the form of liquid anhydrous ammonia. Safer than elemental hydrogen, already mass-produced for its regular use in agriculture, more energy-dense, doesn't embrittle, etc.

ncmncm 5y ago
Anhydrous ammonia makes a better fuel for ships. They can use the existing engines, and just replace tankage and plumbing. ("Safer" is hard to quantify. Less explosive, but overwhelmingly more toxic if vented.) Tankers may carry ammonia synthesized in the tropics to northern ports where local solar is impractical, to burn in existing gas plants in competition with long-distance transmission lines. Or, synthetic CH4 from captured CO2 might win. Either way will need massive H2 production.
- jessaustin 5y ago
  Well, sure, it's good for ships too. In general powering a large ship has a much more generous performance envelope. Thus, "bunker oil". Ammonia is better than bunker oil and it's also better than compressed elemental hydrogen, for powering any machine that moves.
  - ncmncm 5y ago
    I don't know of anyplace where compressed elemental hydrogen would be a good choice.

coderenegade 5y ago

You'd use LH2 or ammonia rather than compressed H2. All else being equal, cruise power for an aircraft is linear with mass, and LH2 has 1/10th the mass for the same volume. For a 40% fuel fraction, an aircraft with liquid hydrogen comes in at 65% of the weight and 25% of the energy content for the same volume of fuel. This will get you a bit under 40% of the range of the kerosene jet, if both jets are equally efficient. Ammonia is fairly similar, maybe a few percentage points higher. You could probably get more than 50% of the range of existing designs using fuel cells that are more efficient than jet engines. More likely we're looking at a rethink of how we design aircraft to accommodate larger volumes of fuel. If we did this, we could approach currently used fuel mass fractions with LH2, and potentially go further than we do with current aircraft. Keep in mind the effective energy density of ammonia after taking efficiency into account is still ten times the energy density of today's state-of-the-art lithium ion batteries (by mass). LH2 is of course much higher than that. I find it hard to see batteries replacing anything other than short hop flights for the foreseeable future, if that. Fuel reserve requirements alone make it a tough proposition.

ncmncm 5y ago
You cannot use in-wing tankage for LH2, so calculating with the same fuel volume is a pointless exercise. Keeping similar (or retrofitted) airframes, the tankage would probably need to be slung in nacelles under the wings. There, the extra volume is no problem, and being closer to spherical minimizes tank wall insulation. You might guess the extra nacelles would be a huge drag problem, but fluid dynamics is a deeply unintuitive science. The mass efficiency is amplified by not needing to loft as much fuel mass, which capacity can be used for extra payload, offsetting any drag losses. And, LH2 produced from solar will be a lot cheaper per joule delivered to the engines.