Bjorn’s Corner: Sustainable Air Transport. Part 11. Hydrogen and SAF.

By Bjorn Fehrm

March 18, 2022, ©. Leeham News: In our series, we have now seen the major limitations batteries as an energy source impose on an airliner and that hybrids work but don’t bring any advantages for an airliner.

The alternatives are to use an energy source with a higher energy density and combine it with an efficient propulsion system. Sustainable Aviation Fuel, SAF, has the same high energy density as today’s Jet fuel and hydrogen’s density is three times higher than Jet fuel.

Figure 1. The Volume and Mass densities of fuels. Source: Boeing.

Volume and Mass density for fuels

Figure 1 gives a graphical view of the energy volume and mass densities of the different fuels discussed for aviation. The graph is scientifically correct in using Joule and MegaJoule (MJ) as the measure for energy. Few have the feeling for a Joule or MegaJoule as it’s not a daily life term, but a Joule = a WattSecond. Divide a MegaJoule with 3.6 to get kWh.

We see the hopeless position of lithium-ion batteries both from an energy volume and mass density. Kerosene (Jet fuel) is very efficient from a volume perspective and good on mass density.

But hydrogen beats our Jet fuel on energy mass density three to one. Its problem is an almost four to one deficit in energy volume density for LH2 and six to one for H2 in 700bar bottles.

Costs of hydrogen and SAF

Sustainable Aviation Fuel (SAF) has the huge advantage its a “drop-in” substitute for our kerosene-based Jet fuel. The SAF can be produced in two ways;

  • From biomass or vegetable oils, where the biological base material has absorbed the same amount of CO2 as is later emitted in the aircraft engine combustion. The production, storage, and transport make the CO2 balance less than 100%. The drawback is the limited base material supply for this method.
  • Generated in a power to fuel process, called Electrofuel or Synfuel. A green method to produce Synfuel starts with electrolysis of water to produce hydrogen, which is then converted to carbon-based fuel via CO2 capture (to make it green) and synthesis to a hydrocarbon in a Fisher-Tropsch process.

Hydrogen is today produced by steam reforming of natural gas, but this is a process that produces CO2. The result is called Grey hydrogen.

Green hydrogen can be produced by the electrolysis of water. It requires electrical energy, but so does Synfuel-based SAF, Figure 2.

Both production methods start with producing hydrogen through electrolysis. We must add carbon atoms in a Fisher-Tropsch (FT) process to create SAF. To make the SAF green, these carbon atoms must come from the atmosphere as these will later be released as CO2 in the combustor of the aircraft engines.

Figure 2. Fuel supply with hydrogen or SAF. Source: Leeham Co.

Figure 2 also shows the energy efficiencies when generating fuel energy. As SAF is a product of further hydrogen processing, it’s more expensive to produce than gaseous hydrogen (H2 in the figure) or liquid hydrogen (LH2).

On the other hand, hydrogen is more complicated and energy-consuming to package for distribution and get to the aircraft in the desired form. It requires a whole new infrastructure for the production, distribution, and fueling of the airliner. It also requires a changed fuel and propulsive system for the aircraft.

SAF, being a “drop-in” hydrocarbon fuel like today’s Jet fuel, can use existing infrastructure with minor changes/adaptations.


The straightforward solution for greener air transport is Sustainable Aviation Fuel, SAF. Its problems are a limited biomass supply and a higher production cost for Synfuel-based SAF.

Hydrogen promises lower cost per kg but requires a new ecosystem, both on the production and distribution side and for the aircraft.

Which of these alternatives will prevail? The probable answer is both. The question then remains, in what proportions? It will be the subject of our next articles.

28 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 11. Hydrogen and SAF.

    • I’ve been following Synhelion for some time. It’s a very interesting technology, what must be proven is that it can scale so that interesting quantity can be produced. But it’s the kind of progress we need, and it can be placed where the Sun energy influx is at the max (the deserts).

    • The process is explained here:

      -The original synhelion process was to use CO2 and H2O to produce syngas by solar concentration over a cerium catalyst. This is still the long term objective for 2039 but an intermediate process where methane and CO2 is solar reformed to produce syngas and a jet fuel with about 1/2 the emissions of conventional jet fuel. The intention is to use bio methane for a genuine carbon neutral SAF that is genuinely competitive.
      -There are other approaches such as thermochemical water splitting to obtain the hydrogen followed by direct syntheses of jet fuel over H2 and CO2. One by sunfire that performs coelectrilyisis of CO2 and water to get syngas.
      -This is I think the 3rd order Lufthansa (in this case via Swissair) has for a synthetic SAF. An order was placed with one of Lufthansa’s suppliers of certified offsets Airfair who have a small plant that takes CO2 from a combination of direct air capture and bio methane co2 using wind and solar. That should be producing a barrel per day now. There is also a 10 million order for electro fuel from a Canadian supplier.

      • Slight correction, Synhelion wants to produce fuels via CO2/Water solar thermochemistry by 2030 not 2039. The methane route is ready now for industrialisation.

  1. Does (any member of) the airline industry have any plans to produce any SAF/LH2 itself rather than buying it all from third parties?
    Delta Airlines owns an oil refinery, so the idea isn’t that alien.

    The gulf carriers could certainly build/procure large solar farms (in the sunny Gulf) to produce fuel in this manner, but they currently have no incentive to do so because the Gulf is an oil producing region.

    • -Using the search term “small scale hydrogen liquefaction” comes up with plant as small as 1 Liter per hour liquefaction capacity with 200 L storage vessels.
      -Under US DOE contract Praxair made 30,000 Litre/day plant and studies for 60,000 and 90,000. Production of 90,000L is equal to about 36,000 Litters Jet fuel or 2 x A320neo fuel loads.
      -So it is possible for an Arline to make its own. My feeling is that refuelling is something that will be provided by the airport who will subcontract to a suitable company.
      -Airbus is singing airlines up with certain airlines to collaborate an ZEROe such as Delta in the US and Air New Zealand (generally those with capable maintenance ability)
      -Airbus and CFM have just signed up on a collaboration for a test bed hydrogen engine which is part of GE RISE program.
      -Small scale electro SAF should also be possible probably by the methanol route. The US Navy once looked at producing it on board aircraft carriers.

  2. The Gulf states with Saudi are watching the LH2 transition and have opportunities to make Blue LH2 from natural gas and Green LH2 from solar farms in the deserts. Small investments so far but eventually there will be a race to install equipment in deserts areas that have no sandstorms and complement with cheap wind power as it are windy locations in deserts as well and not that many to disturb with windmills. There will be a big trading market for the best locations eventually.

    • -There are plenty of solar isolation maps on line. The good news is sunlight is well distributed beyond the Middle East. Africa, Australia, parts of the USA and Sth America, Central Asia and China are near 7kwHr/day per square meter average of which 20% can be harvested by present technology.
      -Northern and central Europeans should not think they do not have resources though only 3kwhr ie 40% as much with photovoltaics optimised it is still economical to harvest. Rooftop solars should be enough to supply all of Euopes heating and electrical needs with the appropriate architecture changes. Costs not withstanding.
      -Blue hydrogen I think is going to be a big factor. It enhances oil recovery.
      -The extraction of CO2 from sea water is surprisingly efficient, more so than Direct Air Capture. This raises the possibility of giant oceanic floating wind turbines producing liquid fuels at sea and offloading to tankers as required.
      -One of those skyscraper sized 15MW wind turbines will only generate enough SAF for one A321 so effectively will be critical.

  3. On Sustainable Aviation Fuels, SAF, we also need to become bluntly honest on the bottom line environmental impact of the total SAF production chain. And not turn blind eyes, point to others and/ or incorporate knowingly unrealistic assumptions.

    • The definition and certification requirements for SAF “Sustainable Aviation Fuel” have been very thorough by past standards in order to avoid criticism. The disastrous effects of EU biodiesel subsidisation leading to rainforrest clearing has been learned. To be frank I don’t know who is the certifying body.

  4. Bjorn –

    BP boasts of converting food waste to fuel. They are also looking at converting “poultry litter” to fuel. Could ideas like these solve the “material supply” problem for SAF?

    • Various companies in The Netherlands are “converting” biowaste (kitchen waste, garden waste) to biogas via a fermentation process. The procedure produces ca. 75Nm3 (Normal cubic meters) of gas per metric ton of waste, and the post-fermentation product can be used as high-quality compost. A win-win situation.

      • Most waste water plants can also produce biogas and a growing number does so. Lots of trails with sorted municipal waste to syngas after separation and filtration then on to SAF. Still no easy processes to be stable and to get a high yield as the F-T process also consumes energy. The easiest way right now is mass production of the offshore 15MW wind turbines. They can be places in formation of lagoons with foundations customized for fish breeding. Desert solar power is also easy but long distance transmission is an issue unless they produce H2 into pipelines with water of the right quality fed in the outer tube and high pressure H2 in the inner one to a costal harbour. Still someone needs to pay. The small scale housing solar panels work pretty well and is a growing industry.

      • The CO2 produced along with the biogas (about 50% by weight I suspect) itself is a resource. Removing CO2 from biogas makes it biomethane, a higher grade product. The CO2 itself is a resource for electro fuels. The US is setting up a network of pipelines to collect CO2 from ethanol production. Used for soft drink production at the moment!

  5. It would be handy to see figure 2 include “Power to Bio-SAF” and “Power to JetA plus direct carbon capture” if that makes sense

    • -There is something wrong with that table. The production efficiency of electro fuels is about 45% using present technology not 26%. I’ve never seen figures that low. They usually start at 40% and 55% should be practically possible and maybe 60% using direct air capture and 65% using concentrated sources.
      -It to an extent gets down to how energy is recycled. The Fischer-Tropsch reaction is exothermic. Energy can be recovered using a steam engine, supercritical carbon dioxide cycle or use the waste heat to regenerate the amine absorbers used for CO2 capture. The amine absorbers use low grade waste heat so even though they might use a lot of energy it is low grade heat that is often waste energy. There are also electrochemical absorption systems using hydroxides and a new US system which electro galvanically absorbed onto metals.

  6. For hydrogen 150 MJ/kg should be the energy content with condensation of water vapour (high).
    Without condensation should be 120 MJ/kg (low).

  7. It would be a good start if the author (and his editor) could learn to spell “Fischer-Tropsch” correctly:

    “Fisher Tropf,” as used in this article, more than once, is pretty pathetic. The Fischer-Tropsch process has been around for nearly 100 years, probably enough time to get the spelling right.

    (That being said, it has always been uneconomical, it still is today, and it probably always will be. A century of failure is a pretty good sign. But SAF will be made by other means, so it’s really only relevant for those who build their hydrogen fantasy world on a Fischer-Tropsch foundation.)

    (See! I spelled it correctly THREE times, without an Editor in sight.)

    As for content, Figure 2 is clearly intended as hydrogen-hype misinformation. The SAF column, for example, would score 0% in “Required replacement of existing aircraft,” while both hydrogen columns would score 100%. The global commercial jet fleet is over 32,000 aircraft today, expected to grow to 35,000 long before H2 fueling can be made available ( Those planes last for DECADES, and new ones cost from $100 to over $400 million. Take $200 million as an average, 35,000 planes is $7 TRILLION headed to the junkyard if hydrogen fantasies come true.

    And of course every single one of the 15,000 commercial airports in the world would need to be retrofitted for H2 fueling, since no one is going to spend >$100 million on a jet that can only refueled at SOME commercial airports.

    EVs win on light-duty (cars) and short-haul freight (delivery, commercial trucks). SAF and Renewable Diesel win on air and marine transport. Long-haul freight (trucks, trains) are a gray area where EVs (maybe), liquid fuels (probably), and H2 (maybe, but probably not) will fight it out.

    H2 for air travel is sheer idiocy. Might as well call it: “Project Hindenburg.”

    • “…$7 TRILLION headed to the junkyard if hydrogen fantasies come true.”

      Planes head to the junkyard all the time; for example, premium carriers such as Qatar and SIA typically rid themselves of aircraft at about the 10 year mark, and even a third-rate airline like Allegiant eventually dumped its MDs. Just as with the slow transition from 4-engine to 2-engine, nobody is saying that future transitions have to occur overnight.

      To cater to ground-based EVs, cities and service stations have had to provide extensive charging infrastructure…so why wouldn’t airports be similarly able to adapt their fuel infrastructure to a changing world?

      H2 for ground vehicles is coming one way or another: it’s logical to see to what extent air travel can avail of this transition.

    • -One aspect of decarbonisation will inevitably be shorter life cycles of aircraft. That will simply be a cost that needs to be factored in. It will simply be cheaper to upgrade to new aircraft than pay more more of an expensive fuel whether SAF or hydrogen which will be twice todays prices unless produced by nuclear.
      -Electric Vehicles are compelling because they are so efficient at absorbing renewable electricity, essentially 80% efficient from generation, transmission, charging, discharging, invertor to motor. They make sense if plugged in for most of the daylight hours when both solar and wind power are at their peak availability,
      -Hydrogen vehicles and aircraft are compelling because in a realistic 100% renewables economy around 50% of the time some kind of storage will be needed and it may make sense to burn the hydrogen directly in a vehicle. Fuel Cells, Spark Ignition and Diesel all work well with hydrogen (ignore archaic Info on this). In addition hydrogen vehicles don’t reduce battery life if deep and fast charged. The average BEV commuter will use only 8% of his/her BEV charge 90% of the days.
      -Hydrogen and BEV will work well together and I suspect most hydrogen vehicles will be pluggable hybrids allowing optimal use of either hydrogen or electric.

      I see renewables going through 5 stages
      -Stage 1 Solar/Wind supplements fossil fuel power. The fossil fuel plants must be run inefficiently to backup the renewables and stabilise the network.
      -Stage 2 Approximately 1/2 an hour battery backup is added to greatly increase efficiency and stability of the network.
      -Stage 3 Approximately 4 hours of battery backup is added allowing renewables to handle peak loads without peaking plant allowing thermal plant to be a more efficient combined cycle type.
      -Stage 4 Renewable capacity is increased greatly and the frequent excess power is converted to green hydrogen for use in either direct heating, cogeneration or thermal power generation. Blue hydrogen is added in to make up the difference and allow scaling to a hydrogen economy.
      -Stage 5 Average daily Renewable production capacity is set to about 2 to 2.5- times more than actual electrical needs which means that on some days 4 times more energy than needed is produced while it would be rare for there to be a shortfall of energy. Approximately 16 hours of battery storage capability combined with the ‘over sizing’ ensures that even on relatively overcast and wind free days all electrical needs are met by efficient battery storage. The vast excess of power on these days is converted to hydrogen. A small amount of hydrogen is used to generate electrical power on rare days that there is insufficient production but most is available for use in synthetic fuels, vehicle propulsion ammonia production, heating needs.

      • “Stage 4” contains numerous misconceptions common in the hydrogen world.

        But as long as it is acknowledged that Stage 4 must FOLLOW Stages 1-3, it clarifies the point that hydrogen is at best a topic for further R&D at this point, not something that is an effective way to combat climate change in the next decade.

  8. Interesting article regarding the (slow) energy transition in the USA:
    “Renewables Will Generate 44% Of U.S. Electricity In 2050”

    “Power generation from fossil fuels, on the other hand, is expected to decline from 60 percent in 2021 to 44 percent in 2050, according to EIA’s estimates in the Reference case of its Annual Energy Outlook 2022 (AEO2022), which assumes current laws and regulations.”

    • What it surprising to me is that it is 21% right now. That is no longer a bit player. King coal is only about 20%!

  9. According to ICCT’s study on H2 aviation potential ( required (green) energy input to produce e-fuels is about ~20% higher vs. LH2. That is a vastly different ratio as compared to the conversion efficiency table in the article. If true (figure is based on sientific papers), e-fuels based on hydrogen and CO2 synthesis could be extremely competitive vs. LH2. Maybe worthwhile to add some more research in the assumptions used.

    • Thanks for the link, I hadn’t seen this one. The study is well made with some low points. One of these is; the researchers omit taking advantage of the cold sink in an LH2 variant. It’s treated as a negative in the study as it introduces a heat exchanger in the fuel system. You do it in another way where you use the cryo fuel heat sink to your advantage, not a disadvantage. We will cover it in the series. I’m not sure we will reach the same result.

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