Bjorn’s Corner: The challenges of Hydrogen. Part 11. Emissions

October 2, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners we look deeper at the emissions from a hydrogen airliner and compare it to the emissions from today’s carbon fueled aircraft.

Figure 1. The three Hydrogen concepts from Airbus. Source: Airbus.

Emissions of a Hydrogen aircraft

In Part 9 of the series, we wrote the emission from a hydrogen-fueled Turbofan or Turboprop take care of the CO2 problem (no CO2 emissions), it lowers NOx emissions and increases the emission of water, H2O, into the atmosphere.

Figure 2 gives a more detailed view of the emissions from a carbon fueled and hydrogen-fueled airliner.

Figure 2. Emissions from a kerosene-fueled turbofan and hydrogen turbofan. Source: Airbus Cryoplane study.

Today’s airliner that burns 1 kg of jet fuel emits 3.16kg of CO2, 1.24kg H20, Carbon Monoxide, Soot, Sulphuric Acid, 11.2kg of Nitrogen and air. This compares with no CO2, 2.6 times more water, one fifth the amount of NOx, and 9.4kg Nitrogen and air from a hydrogen-fueled engine (both burn the same amount of energy, producing the same thrust). A hydrogen-based airliner is a clear improvement in terms of emissions.

The only caveat is the increased amount of water vapor in the exhaust. Water vapor has a greenhouse effect in the atmosphere but it disappears 200 times faster than CO2, and studies show that water vapor in the atmosphere is not the key problem from the increase in water emissions.

It’s rather contrails (ice crystals that form from water vapor condensation on nuclei in the turbofan exhaust) that are contributing to an increase in the greenhouse effect. Though hydrogen-fueled engines put out more water vapor, the ice crystals formed when the conditions create contrails are larger. This changes the effects of the contrails so they are thinner and contribute less to the greenhouse effect than the same amount of water vapor from a carbon fueled engine.

The combined effect of the increase in water vapor and the formation of contrails, considering the different types of ice crystals formed, is a reduction in the greenhouse effect from hydrogen-fueled airliners by around 20%.

I have taken these results from both the Airbus Cryoplane study (from 2000) and the EU’s study, released in May 2020. Both documents say these results are according to the best knowledge but this subject needs more research.

Summary

To summarize these and other studies, hydrogen-fueled airliners, as Airbus’ ZEROe concepts in Figure 2, would:

  • Reduce CO2 emissions by 100%
  • Reduce NOx emission by 80%
  • Reduce the greenhouse effects from emitted water vapor by 20%

The above assumes the same efficiency aircraft and engines, transporting the same amount of passengers the same distance, fueled by Jet A1 kerosene alternatively hydrogen.

This assumes both aircraft fly the same trajectory, meaning the hydrogen airliner is not adapting it’s mission profile to avoid contrail creation (for instance, change flight level to one that does not produce contrails in areas where conditions predict contrail creation).

74 Comments on “Bjorn’s Corner: The challenges of Hydrogen. Part 11. Emissions

  1. Bjorn,
    Suppose today’s entire (pre-CoViD) commercial aviation fleet were powered by LH2. Have you any idea:
    – How many KWh of electricity would be needed to produce the annual LH2 requirement for that fleet, using electrolysis?
    – How the amount of O2 produced by that electrolysis compares with current worldwide consumption of bottled O2 for industrial/medical use?

    You can guess why I’m asking this. Most countries aren’t anywhere near 100% green electricity production for their current requirements…not to mind adding LH2 production to the equation. Uwe said previously that, in Germany, there’s a plan to use excess capacity to produce LH2 — which is a great idea, but probably won’t work on a large scale. In (nuclear) France, you have the advantage of currently generating a huge amount of electricity without emissions, but the Netherlands is currently at only 12%. A new wind farm was opened this week in The Netherlands to much fanfare…but local residents were disappointed to hear that most of its output will be sucked up by a new server farm located nearby. Can you imagine what an LH2 production plant would suck up?

    • I understand your question marks. We will get to this part in the series, right now I’m not prepared to give an answer as I want to cross-check more before commenting.

      In general terms though: Our present energy system is designed around local generation and consumption because we have no good way of storing energy. The best location for solar energy generation is our large deserts but we have no consumers there, same with wind power and the oceans. Nuclear power is best placed far away from habitats etc.

      Hydrogen can be the energy store and transport system to entangle this jam. We need to change the way we create, distribute and consume energy or big parts of Australia, California,…. burns down sooner or later and Bangladesh, the Caribbean, Florida,… gets more difficult to live in year by year (the list continues…).

      Now, is air transport the most pressing area of change to cleaner power and hydrogen energy distribution. No, but it’s in the public eye and it can be a poster child/catalyst for change.

      • Bjorn:

        I tend to go with where the most beneficial impact could be and develop the system and then see if it could be worked into aviation.

        That has been the US model via the EPA and while its had its ups and downs, its been overall very effective.

        I was deep into diesel engines for backup power and fire pumps and watched the moves of the US Tiers closely.

        They did not move to off road diesel regs until they had a solid tech base for on road (both emissions and the low sulfur fuel)

        I have lost track (no longer working) but the standby sets moved to Tier 2 before I quit.

        They were deemed to be such a small part of the picture as to not worth the forcing of replacements on everyone.

        They still allow 100 hours of standby ops for testing without having to upgrade. We ran about 55 hours a year mostly weekly testing.

        You could subtract an acualy outage (and fortunate never had any real run on the fire pumps)

        I had a huge laugh as the form they gave me did not have a column for subtraction real outages form the total but we never came close to going over but the management missed that and would not change it when I pointed it out.

    • Hi Bryce, here I would provide some numbers and cases you need to know to understand the hydrogen role in all this.
      Efficiency and cost competitive cases for current H2 state of the art technology:
      Electrolysis at ambient pressure: 90% efficiency
      700 Bar compression: 88% efficiency
      LH2 (depends on the scale of the plant): best today 70%, just increasing scale 85%
      Fuel cells: 50 to 60%
      Reversible solid oxide high temperature fuel cells: 90% electrolysis, 60% fuel cell mode.

      Transporting energy as hydrogen is around 1 order of magnitud cheaper than using power lines (this takes into account new infrastructure that would be needed, as pipelines, ships, or modifications to the current infrastructure).
      Hydrogen is more cost effective than any battery solution when you need to deliver or store power for longer than:
      Road Vehicles: 5 hours
      Flying vehicles: 2 hours
      House: 14 hours (it would be 5 hours when apps designed for this achieve higher scale production)
      Electric Grid: 6 to 8 hours.

      Now, you need to know that Solar and Wind are the cheapest sources of power today, up to 4 times more cheap than Nuclear, so we can produce cheap power, but without a way to store it, it does not help us much.
      We also need to know, that the electric grid just represents half of the world wide emissions, the second half is the natural gas grid and the utility transport sector (ships, airplanes, trucks, or any application that can not be solved with batteries).
      You can start to blend hydrogen in the natural gas grid without the need of compression up to 20% in some places without any modification to the grid (which represent Terawatts hour of energy), and you can increase that % with minor modifications and new products that would start to come out (as fuel cells for electricity and hot water, or heaters that allow a higher % of hydrogen in the mix), there is time and several countries already started to do this.

      So.. as you can see, the cost of hydrogen depends on the amount of solar and wind energy on the grid.
      We need to generate on excess always, to deal with seasonal disparities and with the other 50% of co2 emissions.
      So hydrogen is the only solution for a cost effective co2 reduction, in 20 years, hydrogen would be cheaper than any fossil fuel. But even today, we have several applications in which they already are the most cost effective solution.

      Current 22 technologies being developed in labs, already achieves much better numbers that the ones I mentioned in the state of the art.

      • AP:

        Some good information there.

        I have a couple of questions.

        1. How much realistic Wind Power can we get?

        2. The best solar of course is dessert areas, which lacks water.

        Right now only 4% of H2 is from Electrolysis and what is the future prospect of that improving? And where?

        You have gotten an interesting mental shift and reality as to moving energy sources and what the cost of that is vs others.

        Still in the mix is the environmental impact of both wind and solar as both have impacts and how much bare dessert for solar is there that odes not have a cascade environmental affect?

        It would be good to develop models that you can input data into that takes the various aspects into account and spits out a relevant comparison .

        • I am researching energy technologies, physics related and economies over the last 10 years, I am creating a website with the hope to resume such complex topic in ways that are easy to visualize and understand.
          But I do it on my free time and as hobby, so it may pass few months more before I have the initial version.

          1-There are many studies about how much wind energy could be exploited on surface level without an important impact to the local
          climate characteristics.
          Some caution figures talk about around 30 TW, others more.
          There is a nice graphic here in page 2 that break down all the solar energy, how much of that is converted into kinetic energy (wind) taking into account different atmosphere regions.
          https://esd.copernicus.org/articles/2/1/2011/esd-2-1-2011.pdf

          To have a comparison, the instant power energy consumption of the world is around 18 TW, this mean 18 x 24 x 365 = 157000 TWh, if I remember right, this value includes all energy consumption, from the power grid, all transport fuels, industrial, etc).

          And if we include the new promising airborne wind energy technologies in development, that can reduce a lot the cost and take power from high altitude winds (the day they solve the algorithm problem to pilot kites 24×365 in autonomous mode), then we can said that there is not close limit to how much we can exploit wind power.

          But even in that case, is not even the need, because Solar power is reducing cost faster than wind and only a 2% of the Sahara desert can power the world.
          Of course, solar would become so cheap, (printed in thin films on rolls), that wasting money in structure with optimum tilt, location, or cleaning, would be a waste of money. Any surface would be fine and cost effective.

          2-As I said, with the constant price reduction on solar cells (92% in the last 10 years), location would matter less and less with the pass of the years, even today is proved that you can benefits many types of crops under solar panels, and these crops would also help to keep them cool rising their efficiency.
          A big part of the cost of solar panels today is structure, protective glass, instalation, etc.

          The cost to transfer power long distances is quite high too, in my personal opinion in the future we would start to see power generation more close to consumption, all interconnected by superconductive underground wires working on DC low voltage cooled by the same cryo hydrogen that would be embedded in the same cable with the purpose to transport hydrogen too.
          This way, you only need to transform power from DC to AC before consumption, instead of several multiples transformations steps from power source to house, without the need of an extra natural gas grid.
          There are already super conductive wires prototypes in testing transporting power using a liquid nitrogen loop (the loop in my opinion is a waste of energy), but even in that case the study finds that could be quite cost competitive with current methods.

        • “Still in the mix is the environmental impact of both wind and solar as both have impacts”

          Thank you for raising that point.
          Do we want to live in a world in which wind turbines and solar panels cover every available patch of ground? There is such a thing as landscape pollution. We have experience with this in The Netherlands, where large areas of the western provinces are covered by greenhouses…there’s no landscape any more, just oceans of glass.
          And where would a country like Singapore have room for such generation sources?
          Plus: Turbines make noise, and birds collide with them.

          Energy transition is a really nasty problem.

          • AP:

            Thank you for the insight. It boggles the mind as it is a system and how to factor all the variables in.

            Very good to see someone looking at it like that.

            Bryce/AP:

            The solar part is interesting. We are seeing it up here now, not something even 5 years ago would that have happened.

            Ironically they built a cluster of houses a couple years back that they had to de-tune the solar heating part as at the low sun angles (solar gain) was so high they could not handle it.

            It is all South facing though.

            While not a complete answer, its interesting that inverter technology is such they have built some power line systems that used DC in between the AC generation and AC conversion at the other end (sadly the one was coal powered but that was done for a mix of reasons that made more sense 20 years ago)

            So it is possible to generate solar power in a dessert and put electrolysis on the closest coast.

            H2 then could be the storage mechanism we lack and it does not have to run 24 hours a day (though that has its problems that would need looked at for a constant startup and shutdown in a plant)

  2. Bjorn, Thanks for the interesting series.

    To achieve zero emissions from H2 aircraft, production, chilling, liquefaction and transport chain needs not emit carbon.

    Chilling and liquefaction of H2 requires ~11.5 kWh/kg LH2. Specific energy in H2 is ~39 kWh / kg. Therefore for each kg of H2, about 30% additional energy is needed for chilling and liquefaction.

    US pricing for wholesale electricity is ~$40 / kwh, electrolysis process for H2 is ~80% best case, so 1kg of H2 requires about 50kWh plus 14.75kWh to liquefy – total ~75kWh / Kg or $3 plus transport and storage.

    Bjorn has helpfully shown that 0.36 kg of liquid H2 is required so cost equivalence is about US$1.10 per kg compared to JetA at about US$0.65 / kg.

    • Interesting calculations.
      And whereas the infrastructure for production and distribution of JetA is already present, the equivalent infrastructure for LH2 has yet to be built…together with the “green” electricity generation infrastructure to power it.

    • Please excuse any errors in calculations – note there are several missing energy consuming steps including transport and storage in these numbers.

      “How many KWh of electricity would be needed to produce the annual LH2 requirement for that fleet, using electrolysis?”

      https://www.statista.com/statistics/655057/fuel-consumption-of-airlines-worldwide/ “The global fuel consumption by commercial airlines increased each year since 2009 and reached an all-time high of 96 billion gallons in 2019.”

      https://mye6b.com/Fuel/ – 96BGal / 96 E+9, is 292 E+9 kg. Based on 0.36 kg H2 matching 1kg JetA, we would need 105 E+9 kg H2, which will need about 7.9 TWh to produce at 75kWh per kg.

      7.9 TWh pa requires generation of about 1GW capacity. World Hydro production was about 7,000 TWh in 2019.

      • All errors/assumptions are fully excused: we’re just doing some rough calculations here.
        So, if we consider a 1,5 MW-rated wind turbine, and assume a 25% efficiency (wind isn’t always blowing at optimal speed), then we arrive at about 2700 wind turbines.

        Alternatively: current global wind-generated power is about 600 GW.
        https://en.wikipedia.org/wiki/Wind_power_by_country

        Sounds very do-able indeed.
        Also a very neat calculation to show the relatively low amount of energy consumed by commercial aviation as a slice of the total worldwide energy pie.

        • “current global wind-generated power is about 600 GW”

          To be exact, Wiki source does not say “generated power” but “capacity”… so we should apply the 25% on the 651GW as well…

          • True.
            But the rough 1GW figure that Mark produced is still less than 1% of (651*0.25) = 163 GW.
            At this juncture, a rough/ballpark figure is indicative enough. Of course, losses and other inefficiencies are going to influence the final figure…but not by orders of magnitude.

      • Having had time to check my calculations, apologies for an error due the starting unit per Kg LH2 being in KWh not Wh. Please note, there are other production methods like thermochemical splitting not considered.

        The updated figures are:

        96 billion gallons JetA 96 E+9 US gallons annual consumption is
        363 E+9 Litres JetA @0.804 Kg/Litre is
        292 E+9 Kg JetA is equivalent to
        105 E+9 Kg LH2 at 0.36 H2 Kg / Kg JetA

        to produce 105 E+9 Kg LH2 by electrolysis, compression and liquefaction, not including transport energy: 67 KWh / Kg LH2 / 59% eff.

        7.1 E+15 Wh of energy
        7.1 E+12 KWh of energy
        7,100 TWh of energy to produce the equivalent fuel for annual JetA use.

        World all renewable: 7,027 TWh

        1 GW plant for 1 year produces
        1E9 W x 8766 hours x 0.9 (90% capacity factor)
        7.9 E+15 Wh per annum
        7.9 TWh

        Wind and Hydro have capacity factors around 25-40% per annum. A 1GW plant with CF of 40% will produce 3.16 TWh

        Switching to CO2 free energy in the form of LH2 produced from electrical generation is a significant challenge.

        In other transport scenarios, compressed H2 and LH2 production is a less efficient energy storage cycle than batteries if we need to store electricity. Lithium batteries have a storage efficiency of around 99.5%.

        Losses in H2 production, compression, transport and storage result in an efficiency of around 50%, and a fuel cell to produce electricity from H2 is around 60%, or about 30% recovery. This means an H2 > fuel cell system have higher losses than a battery system and explains why major vehicle manufacturers are focusing on batteries.

        • That sounds more in line with expectation: trying to use electricity to emulate the calorific value of millions of years of dead plants and animals is a tough call.
          This is a slap in the face for LH2. Bearing in mind that we also need a transition for other forms of transport, the amount of green electricity required becomes about 6 times higher…essentially untenable until fusion power comes (if ever).

          It looks like algal fuel is a better alternative, if it can be produced at a sufficient industrial scale.

        • With regard to what you say about batteries, the elephant in the room is the limited global reserve of Lithium — which will run out very quickly is there is a massive transition to battery-powered propulsion.

          https://www.greentechmedia.com/articles/read/Is-There-Enough-Lithium-to-Maintain-the-Growth-of-the-Lithium-Ion-Battery-M

          Moreover, acceptable/viable solutions still have to be put forward for EOL use/recycling of spent batteries. Where that’s concerned, the EV industry has its head stuck in the sand.

          • I fully agree on ballpark. Its the best that can be done and drill down would need exact systems to do better.

            I am reading moves on what they refer to as solid state batteries.

            Also Capacitors are an interesting area. I was looking at shifting to those for battery starting replacements (batteries are a major weak link in reliability of starting as they can fail with no notice)

            I could not use them as there are regs on standby power that means you can use them in parallel to batteries but not replace.

            I did have one system that had 3 and only one required for the standby function, the other two were for production resumption and staying on until the sort was done (no more interruptions)

            That said that a capacitors setup can replace a starting battery is major advancement and they are talking super capacitors for other uses.

            Solar is also interesting as its free, conceptually you could use it to both crack H2 and store it in pressure containers.

            It might not work for aviation but if it worked to replace other aspects in the system that is all to the good.

        • Just check the grade of growth for solar and wind:
          https://www.iea.org/reports/electricity-information-overview

          When you start to understand how cheap has become wind and solar in the last 10 years vs all the other power sources, this is not even a question of how challenging would be, just of time.
          At this exponential rate, in 10 years, we would have enough capacity to generate the equivalent to the whole electricity consumption (even if is used on that sector or not), add 5 years more and you would have enough for the aviation industry, 10 years more and you have enough for all energy sectors.

          The world is adding around 200 GW by year of solar and wind, next year would be 240, 300, 400, etc..
          The only thing that limits how much we can add, is short grid balance (batteries) and long term and other uses (hydrogen).

          One correction, from what I know, the efficiency of lithium batteries (charge and discharge) is around 85%, not 99.5%
          Now sure what you wanted to mean by that.
          But is not about efficiency. Imagine this case:
          You have 2 days of strong wind.. you need to storage 48 hours of certain power output. With the same capital cost you can storage 6 hours with batteries, or an almost inlimited amount with hydrogen.
          If you want to double your capacity with batteries, you need to double your capital cost, with hydrogen.. you just need to inject more in the natural gas grid, or increase your tank dimensions by a 25% (which is nothing in comparison to the system cost).

          So with the same capital cost, you are wasting 42 hours of power with batteries, around a 90% including loses. And you are just wasting a 20 or 30% with hydrogen (depending how you storage that).
          If you would use that hydrogen in the natural gas grid or in the utility transport sector, then you can not add the extra efficiency loss, because batteries would not work anyway.
          In that case, you always generate on excess, and anything that is not used by the power grid, is converted to hydrogen.

          Major vehicle manufacturers are focus on batteries, because they need to deliver power for less than 5 hours, in that range, batteries are more cost efficient.
          But Honda, Toyota and Hyundai already did huge investments announcements for the next 5 years on hydrogen cars.
          Because even if BEV had an edge in that sector, not everyone has that much time to recharge, or no everyone lives in a house with garage.

          Stadistics 2012:
          120 PWh of energy was consumed by year.
          From those, 20 PWh was in form of electricity and 3 PWh in form of jetfuel.

        • Hi Mark,
          1. Liquefaction process for H2 is ripe for improvements. There are credible scientists who are adamant they’ll be able to do it at 6KWh/kg H2.
          2. Given the time frame, 2035+, more relevant power prices for renewables start with Los Angeles DWP’s deal for $19.97/MWh and down from there. $10-15/MWh is probably a good guess, imho. Maybe even lower.
          3. There is an electrolyser start up claiming 93% efficiency called HydrogenPro. They have impressive resumés and investors, lead by Mitsubishi Power. But it’s still a startup, so we shall see. Price of the electrolyser is however a much more relevant factor given how cheap curtailed renewable energy can get. (Negative pricing, even)
          4. Just because energy is spent doesn’t mean it’s lost. F.ex. electrolysis plants can (and do currently) distribute heat to homes or industries near by, and sell the oxygen byproduct for industrial use, f.ex. fish farms or medical use. Likewise energy spent on liquefaction can be used to compress H2 and it can be used for cooling. An airplane needs quite a lot of cooling. Point being: Can the energy ‘losses’ be used for other purposes and increase utilization of energy source?
          5. A simpler way to look at this is using California as an example. Historical electricity peak in the grid is about 50GW. You could hit that peak in July, August & September, but the other 9 months out of the year you’ll only peak at half that. To have enough for the big peak you have to overbuild renewables + use storage. But what to do with all that excess electricity 9 months out of the year? This is where you’ll get your electricity for H2 production from.
          6. There are many more ways to produce green H2 than through electrolysis. F.ex. pyrolysis of methane (captured from land fills) or plasma gasification of garbage + many more. Point being electrolysis will play a big role, but not 100% of green H2 will come from electrolysis.
          7. The Economist estimates the world needs about 500-700 Million Tons of green H2 per year to combat climate change. Up from today’s production of about 60 Million Tons (a lot of which is not green). Your estimate of 105 Million Tons for Aviation alone is a lot, but it’s still not that much compared to steel and ammonia H2 needs. The airline industry will benefit from that massive scale.

    • They are working hard around the World to develop cathalysis to reduce Power Required for the water catalyst process, still many areas at sea and in deserts will see 12-20MW windmills making up farms of 100-1000 units. GE is among the leaders together with Siemens and Vestas but I would expect P&W and RR to join this race as well as the volumes will be there to make Money and the creation of many green jobs at sea and in the deserts.

    • Well, not completely true. Averages are nice, but can give you an wrong impression of the true ramifications to costs. A lot of the time there is an overproduction on the net, especially with solar and wind. During those times the costs of electricity can be dirt cheap or even be negative. As Bjorn mentions above, if we would produce electricity in the deserts and store the energy as hydrogen, the costs “can” be extremely low. There are already projects of $23-29 Mwh.
      https://www.pv-magazine.com/2020/04/30/lcoe-from-large-scale-pv-fell-4-to-50-per-megawatt-hour-in-six-months/

      • ” A lot of the time there is an overproduction on the net, especially with solar and wind.”
        Most countries energy production from those sources is tiny.

        Electricity is never dirt cheap, if it was aluminium production, a big consumer of electricity as there is no alternative , would be a lot cheaper.
        And dont forget there is bigger wave of electricity use coming with cars and some trucks.

        In my view it doesnt all add up, you cant shift air travel from fossil fuels ( and along with surface transport an industrial processes) at the same time as moving rapidly away from fossil fuel power production.
        But thats not what the discussion is about, but beware of spurious thinking about ‘cheap prices’

        • The problem with the so called overproduction is that as much as possible is taken off line when not in use.

          So, unless it is Hydro (or Nuke) , you fire it up and you have more emissions.

          The real term is Capacity Availability, not production at any given time.

          Any solution has to be from clean power.

    • Current prices, that reflect real-world costs and losses, are between $3 and $4 per delivered kg of H2, when the source is reformed methane. For electrolysis-sourced H2, the delivered cost is between $5 and $7 per kg.

      Also as NASA has found, it’s typical to have a 30% to 50% loss rate after delivery, due to handling and onload/offload losses. This is for sporadic rocket launches, the airline industry will doubtless do much better than this in commercial practice.

      So the $1/kg price is a highly idealized value. It might be possible to get there eventually, with extremely cheap renewable energy to offset the losses. But we still have a long way to go to get there.

    • Energy efficiency of electrolysis is closer to 50%. Two largest manufacturers of electrolyzers show these numbers in their spec sheets:
      NEL Electrolyzer: 59 kWh/kg at 1 bar pressure
      Hydrogenics: 65 kWh/kg at 350 bar and 68 at 700 bar.
      The capital cost is also very high and amortization can be comparable to the cost of energy consumed

      • Hi Victor,
        The price of electrolysers have dropped approximately 75% in the last 2-3 years. Efficiency is not 50%! See my answer to Mark higher up in thread.

        Rob,
        Those are relevant numbers, but they’re set to drop sharply. See above. Mass manufacturing of electrolysers is going straight from 747-8 volume production to Toyota Prius volume & automation in one swoop. This is happening right now and obviously: It’s quite a dramatic change! This combined with continued drop in prices for solar pv (and batteries) should allow for cheaper green H2 from renewables than the prices you quote.

  3. How do I have to understand the creation of N2. After all normal air already includes about 80% N2. The figure as shown implies new N2 is being created. If that would be the case, wouldn’t it be converted into some kind of nitrogen compound due to the high combustion temperature of the engine, or is this what is meant by the N2 emission from the engine? I would think actually that is covered by the NOx.

    • Here my take on why;
      The engine takes in a lot of air, whereof about 10% (new engines) to 20% (1990s engines) pass through the core and take part in the combustion process. The air has 78% N2 and 21% O2 (and a lot of small gas fractions).

      Some of the N2 forms NOx due to the combustion heats and pressures, the rest is leaving the core through the turbines and the nozzle. I think it’s this which is shown in the graph from Airbus.

      Edit; see also Alban’s response below.

    • @Rob,
      Good to read I am not the only one feeling confused with Airbus way to presents it’s numbers!

  4. There might be a problem with mixed operation where the water vapor from a LH2 Aircraft then attaches to the soot emissions of the following JET-A1 Aircraft 60-180 seconds later on the same GPS controlled route. Hence you might want to have the LH2 Aircraft fly i.e. 100m below the JET-A1 to reduce the amount of contrials.

  5. Bjorn,

    Thanks for the focus on contrails that close my question from part 9 (and thanks Rob for it’s inputs too btw).

    According to you analyse part 9 article, 0,36kg NH2 vs 1kg fuel seems already taking into account an optimised LH2 engine with turbine cooling by heat exchanger (5% efficiency improvement vs straight convertion).
    Is that correct?

    My old chemistry teacher used to say “nothing is created, nothing disappears, everything transforms”.
    Looking that way to figure 2 numbers tells me:
    – JetA: 15,6kg of emissions =1kg of fuel consumption + 14,6kg of air intake (which is ~ 78% N2 + 21% O2 at cruise altitude)
    – LH2: 12,61kg of emissions = 0,36kg NH2 + 12,25kg air intake.

    If this approach is not scientifically correct, do not hesitate to correct me!
    Otherwise, where does the difference 2kg of air intake comes from?

    Side question: would you minf to add in your articles links to the documents you refer too in your analyses (If they are open sources?)

    • Alban, if I understand your question, I think the answer is in the equivalency ratio. All fuels have an ideal air/fuel ratio based on chemistry (stoichiometric). But real-world engines run well away from that value in order to deal with practical problems such as material temperature limits.

      For carbon fuels, the engines tend to run with excess air because excess fuel produces soot and particulate matter pollution. With hydrogen, that problem is not present so the engine can run with excess fuel, or at least closer to the ideal value. So less intake air is needed.

      • You need to run with more air than stochiometric due to turbin inlet temperature limits, hence normally only 20% of compressor outlet air get into the buring zone, hence 80% is for cooling and only 20% oxygen of the 20% airflow reacts with the fuel in the combustion process. All these numbers are for JET-A1.

      • Thanks Rob and Claes for your answers.

        So (theoretically at least) for the same flight conditions (speed, altitude) and the same thrust, Airbus numbers show that a LH2 engine should be smaller than Jet-1A one (less air intake).

  6. Hi Bjorn,
    short comment on the climate impact evaluation of Airbus’ Cryoplane project: I would not take those numbers as reference any more, since knowledge in atmospheric physics with regard to aviation’s impact on climate has so much advanced since that time. There are tons of papers out there on the climate effects of contrails and contrail-cirrus, e.g. from DLR, the Universities of Reading or Manchester. But admittedly, they focus on jet fuel. The Cryoplane estimation is just not on the same level of scientific understanding – thus it is difficult to compare values.
    I appreciate your article on the subject, since it supports the need for substantiation of understanding of the underlying physical effects, not only for H2 combustion engines, but also for fuel cells, of which the water emissions would be much colder.
    Let’s hope that Airbus fosters research in this regard while working on the design of their zeroE aircraft.

    • Thanks, Regina.

      This is why I cross-checked the results from 2000 with the same section of the EU report which was made over the last year. As there was no significant change in the analysis I wrote down the synopsis of the two studies results. But as you say, this is a cagey area requiring more research. That’s why I was careful in stating my sources and their results.

      • Thanks, Bjorn. Which EU study are you referring to? The one conducted by McKinsey (Hydrogen-powered aviation
        A fact-based study of hydrogen technology,
        economics, and climate impact by 2050)?
        I could imagine they even took (amongst others) values from the Cryoplane project. I doubt there are many similar projects out there with similar depth. Cryoplane was impressive. If you find something else on the Climate impact of water droplets of non-Jet Fuel, I would be more than interested.
        Thanks again for your article.

  7. The NOX emissions are one of the major factors in ending the hydrogen-fueled [1] MHD electric power projects in the 1970s. Lower temperatures in a GT than an MHD, but still something to keep an eye on.

    [1] hydrogen source was to be electrolysis from baseload nuclear plants running at night

    • Todays designed jet engines has much reduced NOX emissions thanks to better modelling and testing, hence the NOX emissions are just a fraction of allowed levels. I assume they can do almost as well burning hydrogen that is hotter but with higher flame velocity giving a much shorter time at high temperatures before the mix in of cooling air. The increase speed of combustion can allow for shorter burners lowering engine length and mass while increasing engine stiffness.

  8. This is a good summary of the next technology.
    https://www.greenaironline.com/news.php?viewStory=2733

    It takes 7 to 8 years to develop a new airliner… We can’t just keep going the same way we are doing now. Don’t push one technology away because we are not ready to receive it ‘today’. Aviation did not get where it is today overnight… Let’s make a plan.

  9. Energy efficiency of H2 production for your next installment. Two largest manufacturers of electrolyzers and H2 fueling stations show these energy consumptions in their spec sheets:
    NEL electrolyzer: 59 kWh/kg at 1 Atm pressure, and
    Hydrogenics: 65 kWh/kg at 350 bar or 68 kWh/kg at 700 bar. (Nearly twice the energy content of produced H2).
    I have studied the issue of FCVs and have collected large amt of relevant info, including capital costs etc. Some of it was published in Green Car Reports (Google my name).

    • Hi Victor,

      thanks for the offer, please mail me for hooking up. You find my mail address on the contact page of the site.

      • Green Car Reports has been on a war path against Hydrogen Fuel Cell Electric Vehicles for a very long time. That doesn’t preclude data Victor is presenting from being accurate, but using GCR as a reference should set off a lot of alarm bells when it comes to Hydrogen for transport. I trust Bjorn will see right through it if Victor tries to present nonsense.

  10. Hi Bjorn, love the article, now I have a more clear understanding of the pollution comparison.
    Sadly, I still do not understand the key problem that you remark for LH2 airliners, which is the tank mass required to store LH2.
    As this is the key issue, I would love if in the future, you extend a little more your findings to explain why LH2 tanks need to be so massive vs the current tanks.
    Why the sweet spot for storage pressure needs to be 1.5 bar.
    What would happen if they keep it always at ambient pressure? Even when they fly at higher altitudes, taking into account that the only heat input is over the tank walls, and the engines would consume the fuel much faster anyway.

    I tried to read papers to find an answer, but I can not find it. clearly there is a key physics notion that I am ignoring, or some legal regulations that compromises cryo tanks performance.

    My regards.

    • This paper gives a review of the technologies, the tank design parameters, and the reasons for selection of the best alternatives (as of 2006).

      https://ntrs.nasa.gov/citations/20060056194

      The reason given for 1.5 bar is that it minimizes the required tank strength & mass while also providing a safety buffer to maintain LH2 in a liquid state. This was based on research & experience with the space program and terrestrial applications.

      The best aircraft tank structure was thought to be a double-wall with vacuum, covered by external insulation, made of either metal alloys or PMC composites. It must be strong enough to endure flight loads, aircraft flexing, and internal free-surface effects, with a substantial safety factor since located with passengers in the fuselage. All without loss of vacuum which is catastrophic. Those factors account for the weight of the tank.

    • Hi Ariel,

      thanks for the good info you give above. Read the series from the start (part 1 to 9), we go through the tank questions there (write “challenges of hydrogen” in the search box above, right).

      The comment section is also an asset with many knowledgeable commenters chipping in. If you want to did deep, go to Amazon and look for Brewer’s book “Hydrogen aircraft technology”. It’s very good, even though research (NASA) was done in the 1980-1990s. It goes very deep in the tank questions and all other areas.

      • I was reading yesterday the paper that @Rob provide me, in which that same book is quoted when they claim the 170 kpa sweet spot, but I search the book on google yesterday, but is not free.
        https://books.google.com.ar/books?redir_esc=y&id=hf-iyU2R7eIC&q=170+kpa#v=snippet&q=psi&f=false

        I could not buy each piece of paper or book everytime I need to check the source of some values, not on my personal economy of my third world country 🙁
        But I would read as you said more carefully all yours articles again and comments in those to see if I miss something.

        The thing that keeps confusing me, is that if we check hydrogen boiling point at lower pressures than 1 bar, it does not seem to change much, maybe, maybe 3 celcius degree at 1/10 of the pressure (maybe my data is wrong or there are other aspect that ignore that power the hydrogen boiling beyond the energy input of the tank surface).
        Another aspect is that the airplanes fuselage already needs to be strong enough to resist a pressure differential and different structural forces, how few extra m2 of surface could scale so much the mass to counter all the huge fuel mass reduction we get? I know, if they use 1.5 bar plus a 2 or 3x safety margins, it becomes quite heavy.
        It puzzle me… And all papers that I read, all takes their pressure value from others papers or books that I can not read.

        But well, I will keep reading to see if I understand.
        Thanks for the reply.

        • OV-099 had located the NASA final report prepared by Brewer, which contains a summary of much of the work that appears in the book. This is free to download in pdf form.

          https://ntrs.nasa.gov/citations/19760012056

          If you consider the phase diagram for hydrogen, 1.5 to 2 or 3 bar is in the center of the relatively small liquid phase range, which is why it’s valued for safety.

          https://www.researchgate.net/figure/Primitive-phase-diagram-for-hydrogen-Liquid-hydrogen-only-exists-between-the-solid-line_fig1_8618603

          Also by maintaining a higher H2 vapor pressure above the liquid, evaporation and losses are lessened. Just as water evaporates more slowly in a humid space than a dry space. Same basic principle applies.

          One thing I neglected to mention in my earlier post about tank strength, is the thermal stress that the tank must also withstand. This is greatest during filling and draining.

          So one of the challenges is to match the thermal expansion coefficients of the materials so that the tank doesn’t split from differential contraction when suddenly cooled to 20 K. Or if exposed to much higher temperatures after being cooled. There are two equilibrium states (empty and warm, full and cooled) but the tank must handle transitions as well.

          This is why perhaps integrating the tank with the fuselage structure might not be practical, as the thermal stress would be added to the structural loads for that section. A separate tank allows the loads to be isolated.

          • Yes. The early fuel tanks for the most volume in aircraft were bag types. That was because integral tanks were leak prone from flight and thermal stresses. There was no need for insulation. That challenge was solved by special coatings so that in essence the flexible material that was previously a bag surface now became integral with structure that was surrounding it. The use of carbon fibre as continuous shells and skins shows promise for lightweight containers for LH2

          • Thanks Rob for the Brewer´s report, it does not comment on the pressure, but even in that time, their assumptions for long and mid range LH2 aicrafts, showed some benefits vs jet fuel on overall aircraft mass.

            Your phase diagram is not linear, those units are exponential, to better visualize the liquid phase, check this:
            https://www.engineeringtoolbox.com/docs/documents/1419/Hydrogen%20phase%20diagram.jpg

            As you can see, there is almost no difference in boiling off temperature between 0.5 bar and 3 bar.
            So, this return to my initial question? why is needed to have 1.5 bar of working pressure which also demands 2 or 3 times higher structural limit which is translated to 3 or 4 bar?
            The biggest benefit of liquid hydrogen, is not only its higher density vs 700 bar compression, the problem with compression is that your tank mass increase linear with its volume, instead with its surface, as it happens when you only need to deal with a thermal loss.

            Lets imagine that the fuel inertia moving inside the structural requirements of the fuselage and other variables force you to have 4 bar, because even if you dont need it to reduce boil off, it would be needed for the other issues.

            But why jet fuel tanks does not have the same issues in that case? we are not talking on a huge surface difference, around a 50% more tank surface or less.

            You mention thermal stresses, yeah I read many times on several papers all the different points that are needed to be considered on material selection to deal with all those issues, but none of those issues change the tank mass in a significant way. They dont neglect the scale benefit from the surface-volume ratio either.
            Is just a material choice, not related to thickness.

            All those things are mentioned in the papers, from what I remember, the only thing that they limit, is the type of materials that you can choice.

            But well, lets imagine that you need that working pressure for things that clearly I could not figure out yet..
            The spacex starship already achieved 8 bar test, its emptly weight is 20 times less heavy than with fuel, this taking into account that the empty mass includes rocket engines, wings, etc.
            So.. from any perspective that I see this, I can not find any reason on why an hydrogen aircraft should have similar weight than a jet fuel version.

          • Ariel, your question has been asked and answered here, several times now. In rational explanatory terms, in terms of established practice, and in terms of the research and the facts involved. All of which point to the same conclusion & consensus.

            The scale of the phase diagram doesn’t alter the thermodynamics of phase change. Hydrogen still has a small liquid range, under a narrow range of conditions that must be maintained for an LH2 tank in flight. Along with substantial margins of safety.

            So there is nothing more we can say, either you accept this reasoning or you don’t. That’s up to you to decide. But you can be assured that others who are pursuing this topic, will accept that information and build upon it.

          • If you are annoyed by my poor quality of English or some phrases that I repeat because I forget to delete them, then I apology for that.

            If you are annoyed because you were not able to answer why the tank venting pressure should be at 1.5 or 1.7 bar or why should have so low gravimetric efficiency, then let me tell you that you never were force to do it, nobody here is being paid to share knowledge, I do it if someone is asking about a topic that I know.. Bjorn and everyone in this comment section do it. I am not forcing anyone to provide me an answer.

            But do not come and tell me “books quote that 1.5 pressure is the sweet spot, so they may have a holy reason why is like that, accept that as all the community and stop asking for that”
            What’s wrong with wanting to understand?

            It does not help either when you tell me that the tanks are heavy because:
            -the liquid phase goes from 1.5 bar to 3 bar
            No, is wrong.. it goes from 0 bar almost to 15 bar, and between 0.5 bar to 3 bar the boiling point is almost the same.
            -materials need to deal with thermal stresses
            No, just change a bit the material selection, no the thickness or overall mass.
            Neither other possible causes that you mentioned, because I also provide several real examples and logic exercises which contradicts those cases.

            “The scale of the phase diagram doesn’t alter the thermodynamics of phase change”
            Yeah, I already hinted in my old answers that it may be something that I ignore about entropy or phase change behavior who is not related to the heat input over the tank walls. (remember?)
            But I would like to know what it is exactly, or find out a paper that talks about that with the special case of LH2.

            If the spaceship that works with super chilled fuels, with an structure that deals with 8 bar and still accomplish a tank structure that is 25 times less heavy than its fuel with huge thermal stress.
            Why is so crazy to wonder why we can not have at least a tank that would weight as much as 1/3 of its LH2 capacity.
            Instead the same or half of the whole LH2 mass.
            Is crazy.

            So yeah.. do not blame me for wonder.. even less when I am not forcing you to provide an answer.
            My regards Bob.

  11. There is no question that the use of hydrogen eliminates pollutants (except NOx), especially the greenhouse gas, CO2, at locations of its use (pollution is transferred instead to places of generation and transport, unless electricity is used – even then there is some carbon footprint). But the problem is that hydrogen is an energy carrier and not an energy source. Nature took millions of years to produce the dense energy source of petroleum (heating value of jet fuel is 11.9 kWh/kg) that can be “readily” mined, refined and used. According to the Comments above, it takes about 60 kWh/kg to produce H2 and another 11.5 kWh/kg to liquefy and chill it, and therefore even ignoring transportation costs and inevitable losses during transport and storage, we are looking at something like say 75 kWh/kg of hydrogen delivered. This number must be compared to its heating value of 33.6 kWh/kg. This says that the energy costs of delivering H2 for use is more than twice its heating value! Granted cheap electricity could make its costs reasonable, but …
    It would be nice to know these numbers for jet fuel. How much energy does it take to mine and refine petroleum and deliver 1 kg of jet fuel for aviation use? Bjorn, Can you help?

    • Thanks, Kant.

      I’ll see what I can find. Though one could also pose the question, is it rational to consume the petroleum energy source for the simple purpose of releasing its stored energy when it has the drawbacks we now see? Shouldn’t this asset, that took millions of years to emerge, be used smarter?

      • On that point, I am in complete agreement with you. We should reserve petroleum for its various other human needs like petrochemicals, rather than just burn it and cause global warming to boot! I made that point back in 1973, during the oil embargo. A useful analogy is: It is like burning your valuable antique furniture in the fireplace to heat your house!

        • On the other hand: sometimes, rather than trying to prevent a process, it’s more efficient to allow the process to proceed but to mitigate its effects.
          Today’s climate problems are not being caused by tomorrow’s emissions, so reducing emissions between now and 2035 doesn’t address the (runaway) problem that we have right now. Serious attention needs to be paid to industrial CO2 extraction/storage, which is hundreds of times more efficient than absorption by trees, and far more desirable than absorption by oceans (with attendant acidification). This small pilot project in Switzerland illustrates that it can be done, but it needs to intensified and rolled out on a much greater scale.

          https://www.dezeen.com/2017/06/06/world-first-commercial-carbon-capture-plant-switzerland-pollution-technology-news/

    • When I calculated the amount of energy to produce LH2 I collected data from papers including the 2009 paper “9013_energy_requirements_for_hydrogen_gas_compression.pdf” as the source for the 14.42kWh/Kg for liquefaction and compression. It was the current actual number, although there are novel approaches in the pipeline to reduce this. Over all, I used 67.32 kWh per Kg which included the 39kWh for the electrolysis of H2 at 77% efficiency which was the average of the ranges suggested in other papers. (72-80%). I did not include losses or transport.

      Your last point, about the energy used to produce JetA; modern refineries are reportedly about 90% efficient. JetA has around 12kWh/Kg of stored energy, so about 1.2 kWh/Kg to produce. In a jet turbine, only part of the fuels stored energy is extracted to do useful work – possibly 60%

      I know Bjorn will get to the impacts of increasing the fuselage length to provide volume for fuel storage tank increasing drag. LH2 requires a specialised insulated tank and LH2 is ~11x less dense than JetA and weighs less, but offset by greater energy by mass. A larger volume tank with a specific shape will be required.

      The paper hyd_economy_bossel_eliasson is interesting and has tables of the stored energy value by weight and volume in various fuels including hydrogen, methane and ethanol. The H2 electrolyser efficiencies quoted in the bossel_eliasson paper are from ~1999.

      For anyone interested in energy production costs: https://www.lazard.com/media/451086/lazards-levelized-cost-of-energy-version-130-vf.pdf

      Opportunity to generate solar based fuels in deserts. NREL produced a 2013 report (https://www.nrel.gov/docs/fy13osti/56290.pdf) suggesting 30MW of PV per km2 (8.1 acres / MW) and a capacity factor of ~26%. PV efficiencies have improved and deserts may have better capacity factors, but to take these numbers, each sq km could produce 68 GWh pa. Not accounting for production facilities, power lines, transformers etc, producing ~7000 TWh requires an array of about 100,000 sq km or 320km by 320km (200 miles). At $1 per watt to build this (Lazard) its about 3 trillion US$. Deserts are often ecologically protected areas; nuclear has a land use per MWh of about 1/4th of solar – no solutions are without some downsides. At 116GW annual world-wide PV manufacturing capacity (2018), an array of this size would take about 26 years to build. Wind annual manufacturing is ~60GW per annum.

      • If you wanna have a good estimate about the cost to produce fuels from clean sources, then always quote studies from the last 2 years top.. because some of your reports date from 2013, at that year the cost of solar was 7 times higher than now.
        So those numbers are already totally outdated.
        The same for hydrogen technologies, you need to quote system from the last year, because they improve fast on performance and mostly cost.

        Second: your yearly assumption for fixed installed capaciy of 116 gw for solar, and 60 gw for wind does not follow the trend, because if you check power installed on the previous years, you would notice an exponential rate, so the next year would not be 116 for solar, and 60 gw for wind, it would be more similar to 130 gw for solar and 75 gw for wind, rising year by year with an exponential capacity growth.

        My regards.

        • Hi Ariel, thanks for the comments and appreciate you raising these points. It provides an opportunity to re-consider posts and its easy to make mistakes. My goal was to highlight the rough size of the challenge to create the zero CO2 energy to provide LH2 production requirement for aircraft.

          The cost figures are from Lazard 2019 Levelised Cost Of Energy (LCOE) report – Lazard are an investment bank, and are quoting $900 – $1100 per MWh generation.

          The build out capacity in Solar and Wind was from 2019 manufacturing figures, and land use from 2017 (THE FOOTPRINT OF ENERGY: LAND
          USE OF U.S. ELECTRICITY PRODUCTION). I agree solar panel efficiency has improved slightly.

          The older articles were used for specific energy of H2 and the energy required to chill and liquefy LH2. These are physics constants, although novel approaches to chilling are noted such as Magnetic Liquefaction, in 2020 these are theoretical and not generally available.

          With respect to increasing solar cell manufacturing, agree I did not factor in increasing rates in the timeline. The annual rate of increase is 10-11% in 2018/19 vs 19-25% in 2016/17. The rate of increase is decreasing possibly due to subsidy policy. At 15% pa increase, 26 years becomes 16 years, not including wind energy. There are 25,000 aircraft to replace; it will take a lot longer than 16 years to build 25,000 new aircraft to replace the equipment in service.

          • Mark: Ohh sorry, my bad, this happen to me for read your comment by parts.
            On energy topics, I develop the habit to pay attention first to dates from the sources that are being used to reach certain conclusion before wasting time on understanding each calculation made base on numbers that may be outdated.
            My eye focus were from electrolysis efficiencies of 1999 to the 2013 NREL report, to later see that you were making calculations in which I assume were based on those values.
            my mistake!
            Yeah, the Lazard report in my opinion is the best well crafted taking into account how all the data is arrange with details that other reports does not take into account. Around this month every year, we have a new Lazard report, this time would be the 14.
            I would love a report like that for the whole world.

            If I need to make a calculation base on technologies like thermal power plants, hydro, etc. I usually dont care if my sources has 5 to 10 years old, numbers remain on point, but with super fast growing technologies like Wind, Solar and Storage, I usually set my search engine for the last (2 years).

            About Hydrogen Liquefaction: There are already many studies that agree that is not hard to improve current plant efficiencies that are between 10 to 15 kwh/kg to 5 kwh/kg just for a plant scale of what an Airport would need, without using new technology at lower capital cost by ton of hydrogen.

            On magnetocaloric methods, yeah, they still need more development, mostly to find cheaper material alternatives for each one of the temperature reduction steps.
            I guess this technology would be better suited on the future for small applications who need low LH2 production, without requiring much space and with an energy lost as low as 12.5 %

            The impact on land required, it does not seem a concern, as I said in my previous comment, only a 2% of the sahara desert can power the world, even without such good locations, is not a bit problem either.
            Everyone seem very concern about the land area, but what about agriculture? Which already requires 50% of the whole habitable land area.
            https://ourworldindata.org/land-use

            Plants are only 0.5 to 2% energy efficient.
            Your calculations seem on point, but I guess taking into account the commitment that so many countries already show for an hydrogen economy and the future technology and capacity increase, all that combined with the super need to store and exploit the low cost of solar and wind. For me is clear that in 20 years we would have a lot of capacity, of course, this would be splited between all energy sectors, not just the aviation industry.
            Yes, it takes a lot of time to remplace all current airplanes, ships, trucks, etc.. That is what it marks the progression rate.. not so much the hydrogen production.

  12. @Bjorn

    I would really like to see some discussion of the very real challenges regarding the reliability and maintainability aspects of a hydrogen fuel system.

    if you look at the experiences of the space industry, which has similar requirements for light weight structures, getting the hydrogen system to maintain good sealing for even 1 flight is a challenge (although SpaceX has made progress in this area) requiring days of checking fuel system integrity for each flight.

    hydrogen, due to the small atomic size, is much more leak prone than long chain hydrocarbons and the extreme temperature cycling associated with cryogenic fuels creates material fatigue issues, problems with seal materials, fasteners working loose etc. you can’t just spray the inside of the fuel tank with a sealant and call it good. weld integrity QA takes on a whole new meaning when dealing with cryogenic fuels and small molecules. a pinhole that would not leak in a kerosene fuel system will create massive issues in a hydrogen system.

    Engineering a flight weight cryogenic fuel system that is going to be reliable for 10,000 cycles between major maintenance events is beyond today’s material science.

    is it technically possible to build a hydrogen powered aircraft? sure. is it technically possible to build one that can fly 7 times a day, every day, for a year without major maintenance? absolutely not. if the space industry is anything to go by, you _might_ get through a single day before you have to check every single connector in the fuel system.

    • Bilbo, these are all good points and illustrate the challenges of moving to a hydrogen economy. I think it’s safe to say that much of what we now take for granted in fuel use, handling, and delivery, reliability, and convenience, will no longer be relevant. The costs will also be higher, but as Bjorn says, what is the cost of continuing to use fossil fuels?

      So that is the balance that must be struck. How it all eventually works out, I don’t know. Maybe hydrogen will be the primary solution, maybe not, maybe there will be a blend of hydrogen and other solutions. But electric has been investigated, and now hydrogen. That work is valuable even if not ultimately adopted.

      • It sometimes seems practical thinking is missing. Where do we get the most carbon reduction for dollar spent. I doubt it is in aviation. Probably it is in de-carboning the grid. That enables low carbon cars, industry, air-condition etc.

        Providing energy to disconnected entities, aircraft or ships seems a high investment low return proposition.

        • “Where do we get the most carbon reduction for dollar spent.”
          That sums it up perfectly…though you could also add “per unit of time”.

    • The LH2 powered Rocket business are much more marginal as you go for superlight structures. Airliner structures are much lighter loaded and have much bigger design margins. Rocket systems are equally marginal and many rocket designs have problems to keep accidents below 1 per 100 flights. If you look at jet engines only the fuel system with shut off valve, fuel/oil cooler and HMU/Fuel Control with fuel distribution valve with tubing and injectors will see LH2 and those are LRU’s (Line Replaceable Units) that can be LLP’s (life limited parts) for replacement and overhaul with NDT (non destructive testing) during the shop visits. The rest of the LH2 system inside the aircraft is bigger and harder to design as the designs as well need to comply with LH2 system regulations that are not issued yet. Tubes, tanks, pumps, valves including check valves and safety valves can be based on rocket designs but made heavier with increased life requirements

      • Modern commercial aircraft are almost (_almost_) automobile like in their daily maintenance requirements.

        check the tires, check the oil, check the fuel level, check for warning lights, do a FOD and Function walk, fly.

        the almost part comes more in the response to anything being off nominal – no fly, fix immediately, and in documentation. where in a car, you probably drive it for a couple weeks before bothering to get it looked at, and throw out the receipt.

        the idea that you would have to routinely change parts other than tires and brakes more often than a C check is anathema. the idea of having to open up dozens of access panels daily, weekly or even monthly to hand check every single connector, valve and line in a fuel system is unacceptable.

        sure, “flight weight” in an aircraft is different from “flight weight” in a rocket (and it varies in both directions depending on component), but it still needs to be as light as possible and very few rockets have 250 lives depending on them 7 times a day, 7 days a week, 365 days a year.

  13. Is the acceleration profile of a jet engine powered by LH any different than one powered by JetA? Are auto-throttle changes required? Does the eight second time to full power in a go-around still apply?

  14. Hi there,
    Yes. The early fuel tanks for the most volume in aircraft were bag types. That was because integral tanks were leak prone from flight and thermal stresses. There was no need for insulation. That challenge was solved by special coatings so that in essence the flexible material that was previously a bag surface now became integral with structure that was surrounding it. The use of carbon fibre as continuous shells and skins shows promise for lightweight containers for LH2

  15. The cool light of morning shows the need to re-phrase the question.

    Does the energy extracted from burning hydrogen deploy at the same rate and manner as JetA? Is it sufficient to meet the 8 second spool-up to go-around power requirement? Does that or anything else require auto-throttle modifications to meet certification requirements?

    • The flame speed of hydrogen is greater than most carbon fuels, so combustion increases should not be a problem. However a gas turbine uses a continuous flame burner, so there may not be any meaningful difference in time response to throttle. It’s just a matter of adding more fuel, and the response is dependent more on the mechanics and angular momentum of the spools.

  16. Has anyone compared Hydrogen to Sustainable Aviation Fuel (SAF) in terms of cost and emissions? Green (as opposed to Brown) Hydrogen has to be produced using renewable energy – and SAF can be manufactured by reversing the combustion process by applying (renewable) energy to H2O and CO2. Burning both H2 and SAF in aircraft turbines are net carbon neutral. The big difference is that you don’t need a whole new aircraft fleet or new infrastructure at airports to be able to use SAF. The (green as opposed to brown) Hydrogen industry may take off for sectors other than aviation, but leaving that aside, what is the case for aviation to invest in Hydrogen rather than SAF? It seems to me that to make Hydrogen the better choice (that is to pay for all the additional investment in aircraft and infrastructure) it needs to be a whole lot cheaper to produce H2 than to produce SAF. Given that the cost of renewable energy is falling so rapidly, unless producing H2 is a lot less energy intensive it’s hard to see where H2 could become sufficiently lower cost to produce?

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