Bjorn’s Corner: Sustainable Air Transport. Part 57. Summary

By Bjorn Fehrm

February 10, 2023, ©. Leeham News: We started the present series around Sustainable Air Transport on the 7th of January 2022, more than a year ago. During this time, we have covered a lot of subjects regarding the emissions from Air Transport.

It’s time to wrap up the series and summarize what we covered and learned.

Figure 1. Our energy consumption and where it’s coming from. Source: Our World in Data.

Do we have a problem?

The first area we covered was if there is a problem with our climate because of emissions of Green House Gases, GHG. The crazy weather in different parts of the Globe in recent years says unequivocally yes. There is something that has changed.

That something has changed is not surprising when you look at Figure 1. It’s the world’s consumption of energy since 1800. 85% of the energy comes from hydrocarbon-based fuels, emitting about three times its mass of CO2 to the atmosphere.

The problem with these fuels is that the CO2-absorbing growth of the plants that were the base material of the hydrocarbons happened millions of years ago, but we are releasing it back into the atmosphere now. This non-cyclic process increases the content of CO2 in the atmosphere, Figure 2.

Figure 2. The increase of CO2 in our atmosphere. Source; Wikipedia.

The results of all these CO2 emissions are that the heat radiating off the Globe is reduced, and our average temperature increases. Recent research around an increase in contrails from air transport has added to the reduction in heat radiation from the earth. To what extent the latter contributes to global warming is still under debate. The effects can be considerable, but the positive news is the contrails is a short-duration problem, in contrast to the CO2 emissions that stay in the atmosphere for a long time.

So what to do to reduce Global warming. Let’s first be clear that investments and changes in areas other than Air Transport are more urgent. Air Transport constitutes about 2% of CO2 emissions today, and if you include the non-CO2  effects, the contribution is still below 5%.

Actions for changes in the industrial production of Cement, Steel or how ground transport is done will have a larger effect at a lower cost. As Air Transport is in the public eye, it also has to change.

When we went through all the changes that could reduce emissions, one was the clear winner to reduce emissions until 2050, Sustainable Aviation Fuel, SAF. It can reduce emissions on all 25,000 airliners that fly each day. There is a caveat regarding how much SAF can be allowed in airplane tanks.

Fossile Jet fuel contains aromatic hydrocarbons that swell the seals in the fuel system. SAF, a cleaner fuel, does not contain aromatics and thus needs a new type of seals that don’t rely on the swelling effect.

Thus the authority that regulates Jef Fuel, ASTM, only allows a blend of 50% for older aircraft. It’s no problem, however, as we can’t produce SAF for a blend beyond 0.1% today and 8% by 2030. So until we reach a 50% possible blend ratio, we may have reached 2050 or beyond.

The production of SAF from energy, .called e-fuel, has hydrogen as the first product, and then the carbon atoms are added in a final process step. It would, therefore, makes sense in the long term to replace SAF for airliners where hydrogen works as fuel. Hydrogen is three times lighter than Jet fuel/SAF for the same energy content but takes four times more volume.

Hydrogen as a fuel is only practical up to midrange flights, like the domestic flights conducted by the airline’s single-aisle fleet. Long-range flights with Widebodies will have to rely on SAF as the hydrogen tanks would be difficult to accommodate.

So why do I only write about hydrogen-based airliners as a longer-term solution and not electric aircraft with batteries or hybrids? We will go through why in the next Corner, as it requires a thorough explanation.

16 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 57. Summary

  1. It would be interesting to hear about the impact of increased drag from aircraft volume to account for the lower volumetric energy density of hydrogen, and the extra weight of a pressure tank required to contain H2 at ~700 bar.

    Converting H2 to a liquid fuel (aka SAF) is requires extra energy, over time, new catalysts and processes might reduce this. Direct air capture, or other way of capturing CO2 as a feed stock also requires energy.

    A lot of engineering optimisation has been done over the last 50 years to make liquid fuels work well in transport aircraft; its convenient, lots of infrastructure exists and its familiar.

    Current best solid storage estimates suggest 28kg H2 / m3 in film type storage. An A350-1000 with 158.8 m3 of fuel capacity could store 4446.4kg of H2 or at 3x energy vs liquid fuel ~13,339 kg equivalent. Actual capacity is 124,647 kg of liquid fuel.

    An A320, with 25.7 m3 fuel capacity – would require an extra 240m3 of space for similar energy – or about 20m extra length.

    Doing the same calc using 700 bar h2, 42 kg/m3 – so that is ~12m extra stretch, but that is the internal volume required and does not take into account the extra mass. 700 bar tanks weigh about 20x that of an equivalent plastic of metal tanks (

    Seems that SAF based on liquid fuels is the only practical alternative for aircraft; the extra mass and volume of H2 tanks seems insurmountable with current tech.

    • There are ideas of having the LH2 in long isolated pods under wing using its mass to limit bending moment from wing lift. The replacement with full “drop tanks” is pretty quick and the removed LH2 tanks would be trucked to the airports LH2 facility unless refueled with supercooled LH2 to limit “Gas-off”. So there is a tradeoff between drag of “drop tanks” vs. extending the fuselage and its drag including beefing up the wing and wingbox to take the load. The blended wingbody design might be suitable for LH2 tanks in the area between pax compartment and “clean outer wing”. We will see what Airbus comes up with in their LH2 blended wingbody design.

      • There is no doubt that H2 tanks can be added and there are many smart minds working on how to do this.

        What is the additional fuel burn for the extra wetted surface area and extra mass, and when does that penalty make it worthwhile to convert H2 to SAF using Sabatier and Fischer-Tropsch type reaction or a more efficient equivalents?

        Physics makes hydrogen hard to store, and right now, the most volume efficient, low mass option maybe 700 bar COPV tanks, but they are still relatively heavy and not exactly safe… we can contain fan blades, but containing a burst in such a tank is beyond engineering as far as I know.

    • You could use the very cold hydrogen to cool a second dry gas (e.g. Nitrogen) and bleed it out over the airframe. The colder the gas, the less skin friction – a major component of a transonic aircraft’s drag, helping to reduce the power and fuel volume requirement.

      On the other hand, a single meter cubed of dry ammonia weighs 900Kg and contains 3/17 that weight of hydrogen = 158Kg. To store 158Kg of liquid hydrogen you would need 3.76 metres cubed volume, and a vessel that operated at 700 bar rather than 4 bar. Storing hydrogen using nitrogen atoms takes just over a quarter of the volume and less weight than storing hydrogen as a liquid.

      If you could find a way to separate the nitrogen from the ammonia, a catalyst heated by a gas turbine say, a lot of the volume problems are solved. Spoiler – we can already crack ammonia that way. You wouldn’t need to crack very much, enough to run a hydrogen pilot light and burn the rest in its raw form.

      Why is no-one talking about this?

  2. The assumptions in this piece are simply in error. The reason “Global Warming” turned into “Climate Change” is because, although CO2 climbs, temperatures have not.

    Sea levels are NOT climbing at any threatening pace. And the pace has NOT accelerated AT ALL despite the growth in CO2. So the claim that CO2 is going to somehow prove to be a bad thing is entirely unfounded in the science.

    Look at the actual data. CO2 does not drive climate change. Don’t ruin this industry trying to fix a problem that is not there.

    • During November, when daylight in the northern hemisphere declines, simple math suggests the sun will be completely lost next January 🤔

      That’s what the sources in the article do: Simple predictions with data from the past.

      No word, I would love to see their predictions becoming true.

      Nevertheless it seems absurd to assume the other 95 % of scientists who predict an increasing danger of rising sea levels are all misleaded or part of a conspiracy. They just look beyond those simple ideas.

      • “Nevertheless it seems absurd to assume the other 95 % of scientists who predict an increasing danger of rising sea levels are all misleaded or part of a conspiracy. They just look beyond those simple ideas.”

        Could you provide a reference for this “fact” because I’ve heard this quoted many times but no one can ever provide any evidence of it.

  3. Boeing has an environmental group with a few atmospheric scientists who follow the IPCC work on aircraft contribution to global warming. If climate change were indeed a conspiracy, as the previous commenter claims, then I would think Boeing would have blown the lid off a long time ago.

    • Who said it was a conspiracy? The Climate Change debacle is nothing more than a combination of scientists following the funding, those who view the planet as Mother Earth, and politicians who see Climate Change as an excuse to accrue more power.

      Businesses that oppose the “concensus” are shamed and cancelled. The latest is the head of Toyota, who spoke truth on electric cars, and was kicked out. Nobody wants to end their career that way.

      Why not view the link I shared? Look at the actual data on sea levels. Look at the satellite data, which has proven 95% of models wrong.

      • Thank you iWe,
        Not only sea level give a very strong proof of the climate change debacle!
        Please give a look at
        You will see plenty of other proofs…
        Also the book “unsettled” by Dr Koonin (ex Obama adviser, not precisely a republican climate denier!) is worth a careful reading.
        Where i (and Dr Koonin) disagree with you is when you say that 95% of models are wrong.
        Actually 100% of models are wrong!
        And they are not improving from one IPCC report to the next.
        The fight against CO² is totally useless , it will ruin not only the aircraft industry but most advanced economies if they keep committed in this useless fight.
        Green Hydrogen is fashionable, but the awful efficiency of its process makes it hopeless and a non starter.
        Billions of € are being wasted on H2

  4. Thanks again Sir for all of your efforts and helping us to get better educated. Some of the deniers and activists need to glue themselves to the library, as usual. The oceans are warmer and warming…the glaciers are 50% less in summer and may go to zero sooner than expected since it is complex and non-linear. The Younger Dryas may show what will happen if we do shut down the conveyor. I’ve stood in the higher sea levels in Delray, and the king tides are famously higher than in 1985…not just in Florida. Nobody mentions Turkey Point nuclear power station (close to Homestead AFB)…or NOx…or methane, which seemed to go up during the lock-downs. I think it retains heat in the atmosphere 84 times more than CO2. Average Joe may not like it, but the US military was convinced years ago given the airbases and ports that will be so badly affected. Patrick AFB is only a few feet away from swimming already. Wonder if global military emissions (off the scale) are included in the denominator…or is this left out in general.

    If anyone in European aviation cared, they would have fixed the ATC silliness decades ago. And the focus for Luisa would be more on BASF & Co., and less on telling retirees to turn the heat down…after they worked like a dog and paid 50% tax rates the whole time. Planet of the Humans (on YT) is an education too. Best wishes…

  5. Climate change isn’t a conspiracy, it’s a part of the natural order of things from what all the evidence tells us.

    “If climate change were indeed a conspiracy, as the previous commenter claims, then I would think Boeing would have blown the lid off a long time ago.”

    This is naive beyond belief! The climate change business is estimated by the UN to be worth US$26 Trillion by 2030 and this will mostly come from government subsidies (aka tax payers).

    Also using the IPCC as a source when they are mandated to show consensus (not science) is a little like using the government as a source when discussing integrity.

    • “Naive beyond belief?” I’m sorry SI but I worked in Boeing’s environmental group. Our mission was to “enable unconstrained aviation growth” and to “keep our customers healthy”. We would have been only so happy to disprove climate change science as Emissions regulations and possible future carbon constraints
      harms airline’s profit margin. as stated earlier. Our paychecks came from Boeing, not government funding, and we couldn’t disprove aviation’s impact on the environment. The way forward is innovation.

  6. During a hard landing if the H2 tank cracks, quite a few guys on board will freeze to death, just like your finger freeze to the CO2 cartridge if the rubber seal leaks. Expansion of compressed H2 gas will lower significantly the ambient air temperature, depends on the leak rate. Similar like mid-air decompression fog, but several magnitudes worse.

    Bulkhead insulation may or may not stop the heat extraction from the cabin.

    • I am not sure that a cracked tank with 700 bar of pressure will leak in a hard landing scenario. A study published on NASA described it this way:

      “…the consequence of a COPV rupture can be catastrophic to surrounding spacecraft structure and components, equivalent in energy to the release of several pounds of trinitrotoluene (TNT) depending on the size of the COPV.”

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