Bjorn’s Corner: The challenges of Hydrogen. Part 3. Application space.

By Bjorn Fehrm

August 7, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners we now look at the emission targets one is chasing and then discuss for what type of airliner does a hydrogen propulsion system make sense.

The CO2 emission target as expressed by Air Transport Target Group (ATAG) is shown in Figure 1. The graph is from the EU report on Hydrogen Powered Aviation.

Figure 1. The development of CO2 from aviation and the ATAG 2050 target. Source: EU Hydrogen Powered Aviation report.

The emission target for Air Transport

Figure 1 shows we were on route to increase the air transport CO2 emissions to about double the 2018 value by 2050 if the traffic had developed without the COVID pandemic. This assumes a 2% improvement of airliner efficiency per year (the present trend is about 1.5%/year).

The target as defined by ATAG is to halve the emissions level from 2005 by 2050. This is a sporty target as we only have 30 years to achieve it.

We need to prioritize the first moves to focus our innovation and development resources where they bring maximum effect.

The sweet spot for lower emissions

We shall focus the first airliner development for a segment that suits the constraints of a hydrogen powered airliner (the energy will take more space) at the same time as we achieve a maximum effect on the emission curve in Figure 1.

Figure 2 shows the hot spot of emissions to be in the segment between 81-165 seaters and the next segment 166-250 seaters.

Figure 2. CO2 emissions per aircraft type and range flown. Source: EU Hydrogen Powered Aviation report.

The reality is easier than that. The Boeing 737-800/8 and Airbus A320ceo/neo dominates this segment, and these have 150 to 180 seats dependent on when a cabin update was made. They represent 11,000 of the 25,000 airliners that fly in our skies (in normal times). The other members of these airliner families represent a much smaller part of the segment.

So we should focus a hydrogen-powered airliner at the segment of 165 seats plus any increase in seating that follows from now and to 2035 when it shall be introduced. In fact, we shall size it for its first ten years in service, not the year of entry into service.

We can also see the range sweet spot is between 500nm and 1500nm. This is important as LH2 has three times higher energy content per kg (which is good) but it takes four times the volume of jet fuel (which is problematic).

It will help us if we let the over 2,000nm range segment be flown with the jet-fuel powered aircraft in the segment. This range segment is not the center of emission and it will skew our aircraft to an inefficiently large LH2 storage tank if we try to cover these routes.

Conclusion, aircraft size and range

For the development of a hydrogen airliner that shall start bending the CO2 curve down by 2035, we shall focus the seating segment that represents the 165 seat aircraft of today. Would it be 180 seats (one seat up per year)? Perhaps, with the damping effect of the pandemic, we might land there.

We see from Figure 2, there is no point in going for the easier to achieve regional hydrogen airliner. We burn our powder without bending the emission curve appreciably.

As for range, we shall go for the sweet spot there as well. A practical 1,500nm range which equals a maximum trip length of three and a half hours, means we need a nominal range of around 2,000nm (for in-service deterioration and ATC/weather margins).

By avoiding going after the routes over 1,500nm we make the sizing of the hydrogen propulsion system easier and we wouldn’t gain much in emission suppression by sizing for longer range for the first project.

26 Comments on “Bjorn’s Corner: The challenges of Hydrogen. Part 3. Application space.

  1. Makes perfect sense, but how will the aircraft look like? Is an A321XLR with A320 cabin modified for H2 or LH2 sufficient or does it require a BWB like the TU Delft-KLM design?

    • We are closing in on this discussion. For everyone to innovate, I have seen no golden bullet design yet.

      • I guess EASA/FAA need to issue regulations on H2 and LH2 Aircrafts and its systems first before anyone can move from preliminary design to detail design with instrumented prototype testing.
        Not that easy but there are lots of experience in the liquid rocket industry and its regulations like what NASA imposed onto Space X. So probably will ESA and NASA influence EASA and FAA on what to write in the regulations and how to prove required safety levels. Certified rockets have seldom been more reliable than 1 crash/100 flights.

        • The Polaris is an intriguing design. Some aspects might be difficult to certify for safety.

          Having the LH2 tanks below the passengers has some risks, since spilled LH2 has hazards as a cryo-liquid ,and ignited LH2 burns in a rapid vertical jet.

          Also the use of open rotors surrounded by control surfaces (stabilators and rudders) would be problematic in a blade-out situation.

          It superconducting components are used in a turbo-electric design, with no other means of propulsion, they would need to be quench-proof, or at least quench-tolerant so they could continue operation in a non-superconducting state.

          So challenges exist, but perhaps they could be overcome.

          • I think hydogen tanks and pipes from it can be designed with double walled vented to ambient. In their design the cargo hold is consumed by LH2 tanks and the aircraft get very tail heavy. You need to fit all the power electronics with its cooling as well, Just doing the anti-ice treatment on it with inspection is not that easy. Hence a nice first try that can evolve into something better.
            Will be interesting to see RR ideas with the experience they have on shaft drive and counter rotating fans on the F-35B. Hence it might be one engine in the aft fueslage and shafts driving 2ea counter rotating composite fans.

      • very elegant way to say that it is a challenge!
        the MAC KINSEY report gives two orientations:
        1/ There should be two tanks, in front and rear
        2/ Length of SA plane will be increased by 10m
        due to the pressure of LH2 (around 350 bars is mentioned) a sphérical shape is very advisable.
        diameter of each tank will therefore be around 5m !
        same as cabin diameter…
        definitely no golden bullet.

  2. This is another good series, well done.

    What I am worried is that the industry & journalist are following a non-scientific approach.
    There is simple no viable fuels system to the current hydrocarbon fuels.
    If the industry wants to be close to carbon neutral now they can shift to biojet fuels. These are now available and qualified as drop in replacement.
    Fuel efficiency will continue to be improve, as it is always been the case, particularly if all of the funding wasted on alternative stuff is dedicated to that.

    It is time to speak the truth to the general public and educate them. Even a 100% reduction in aviation emission will not significantly impact the CO2 emission. For that we need to go after air transport and electricity generation.

  3. Regarding the range figures mentioned: in Fig. 2 these are quoted in [km], in the text in [nm]. The proposed 1500 nm practical range equals 2775 km, i.e. a bit more than just the sweet spot.
    That is indeed off-optimum for airlines only serving the sweet spot class of route lengths, but probably much more attractive to everybody else. Historically, aircraft with range design limit equal to the sweet spot range have never been commercially successful.
    In this case, because it is a tank volume limit, even a MTOW increase later in the aircraft’s life would not help.
    Obviously the trade-off should be discussed with the potential launching customers.

    • Hi Jaap,

      if you have flown airliners you know that range is not an exact science and the 2,000km in the report is the 2k km bracket. That’s why I don’t bother with 1080nm for the conversion of 2,000km, it conveys the wrong impression on what the report tells you.

      Re tank volume. If you have a tank that scales with for instance fuselage length it will increase the available energy with a stretched fuselage, higher Gross Weight variant. Stuff you do once you know your airplane from flight test and use.

      You design to give you a tight design but with room for stretch once you have achieved your original targets. The history of aircraft is full of failed design that started off with a too ambitious spec, only to fail as you are not efficient enough at your design center.

  4. The target area ignores the real market.

    In the US we would call it Trans Con, but that is around 3000 NM. Throw in diversion, fuel reserves and you are more 3200 (not exact).

    The A220 claim to fame is its got Tran Con range vs the Embrare regional ranges that inhibits it (mixed up in scope clause and weights).

    So while the segments are often less, airlines have gone with the longer ranges to add flexibility to a single aisle that in turn displaced wide aisle at lower costs. they may only use it 20% of the routes but they do buy them for that reason.

    Any hyro model has to replace like for like.

    • Yes, the Red eye flight LAX-JFK. The US with its lage oil companies and political power might not be at the forefront of converting to LH2 powered aircrafts and might team up with Russia thinking the same, but there will be a fairly big movement from US sun- and wind-power to produce LH2. The EU and Japan might be the drivs as they are big oil/gas importers (UK and Norway as non EU). China might team up with Europe for wind/sun/nuclear powered LH2 factories.

  5. “As for range, we shall go for the sweet spot there as well. A practical 1,500nm range which equals a maximum trip length of three and a half hours, means we need a nominal range of around 2,000nm (for in-service deterioration and ATC/weather margins).
    By avoiding going after the routes over 1,500nm we make the sizing of the hydrogen propulsion system easier and we wouldn’t gain much in emission suppression by sizing for longer range for the first project.”
    I won’t quibble over whether that is a sweet spot from a bang-for-the-buck/effect-for-the-euro engineering basis, but doesn’t that leave-out a somewhat non-trivial slice of the US market at least? 1,500 nm gets you only ~2/3 of the way across the country. Half if one is going to/from Boston to the West Coast.

    “The reality is easier than that. The Boeing 737-800/8 and Airbus A320ceo/neo dominates this segment, and these have 150 to 180 seats dependent on when a cabin update was made.” They also have something like 3,550 and 3,500 nm range respectively (MAX 8/ A320neo). I have to assume that is also a non-trivial part of their dominance even if they aren’t as often flown those distances.

    • Probably part of the equation is they don’t have to fuel at each of the short hops.

      And never discount c the flexibility to mix and match as needed.

      So you can have a short range hydro power high cost bird or you can try to tax into compliance.

      Or you can make improvements as much as possible with Jet A type, and put your money into where you can make it work (solar to me, I don’t like wind as you wind up with a cluttered up Windmill laden world)

      Wind also has a huge impact on bird kills.

  6. I’m not convinced using narrowbody hydrogen aircraft for short range and traditional aircraft for longer range can be competitive. It has often been pointed out that economics of scale for a fleet type will only work up to a certain fleet size. But even pre-corona large airlines were moving to standartize their fleet on fewer aircraft types justifying it with reducing complexity in operations and saving cost in return.
    People are still booking by best-price. Knowing you have done the right thing for the environment is cold comfort when beeing driven out of business by competitors who don’t give a damn. Unless its enforced by regulations it’s not going to happen.

    • Think the H2 aircrafts will be mainly used by countries that have plenty of electricity and a big gas industry “Air Liquid”.
      Countries with plenty of oil/gas will not move as fast, so it will not be as popular in Kuwait as in Paris.
      Qatar with lots of natural gas to make cheap hydrogen might do it to be able to sell LH2 in the EU. They are big on Liquid Natural Gas and can make a deal with Airbus to become an aggressive launch customer again…

      • Or you can turn all that natural gas into totally clean Jet A (Qatar has a huge clean diesel plant that derives from Natural gas stock)

  7. Thanks for the series. It makes sense to go after the aircraft segment that uses the most fuel, where it has the greatest benefits. Hopefully the series will consider other storage approaches, like solid storage, chemical approaches including synthetic liquid fuels.

    It will be interesting to also consider sources of hydrogen, the cost of creating the H2, the energy required to liquefy / compress plus the weight penalty of a cryo or pressure tank.

    Hydrogen reforming from natural gas is apparently the cheapest source, but it is not carbon neutral unless sequestration is included. Electrical approaches are improving, but we don’t have enough carbon neutral generation currently.

    There is research to develop nuclear with high burn up rates, reducing waste by an order of magnitude. Nuclear has almost insurmountable challenges in the realm of public opinion which may prevent this.

    Off shore wind turbines in deep water are being mooted with onsite production of liquid fuels using the energy. This eliminates the need for long distance electrical transmission, but still requires collection of the fuel by ship or pipeline.

    • Nuclear is also encountering cost pressure from natural gas and renewables. In my area, nuclear plants are being decommissioned as they lose money for the utilities.

      Fast reactors that can consume spent fuel are difficult to operate with water as coolant, since water is a moderator. So that means molten salt, molten metal or inert gas cooling, which raises costs further.

      New generation reactors will need to reduce costs substantially in order to make the business case, Then as you mentioned, they still face some public opposition.

  8. It’s going to be impractical to wean the world off fossil fuels entirely, but really we should be making the best possible use of what we “have” to use. So, we should be looking at converting plastic that cannot be recycled, and which now go into landfill, back into lower grade products such as hydrocarbon fuels as well as working at increasing production of biofuels.

    Aviation is probably a unique case where the use of hydrocarbon fuels will remain the only practical option for most purposes. It is unrealistic to assume that there will be a single solution to the provision of that fuel but provided sufficient reductions can be made in other areas of fossil fuel consumption it should not be an issue. The biggest problem we face is probably generating sufficient electricity to eliminate our current heavy dependence on oil and gas for domestic heating.

  9. Compared to aircraft, much bigger sources of CO2 are 1. Land transport (cars and trucks), 2. Generation of electricity using coal, oil and gas, and 3. Winter-time heating of buildings, using oil and natural gas. While it is a good academic exercise to look at aircraft powered by hydrogen, IMHO it is a wild goose chase. Because H2 has to be produced somehow, it may not make much difference to the carbon foot print, especially since only a segment of air transport (as Bjorn points out) should be made hydrogen-powered. While the principal advantage is lack of pollution at the point of emission (including greenhouse gas CO2), I am afraid flying public will not readily accept riding on hydrogen-powered aircraft. One fatal hull loss with photos of Hindenburg-style flames distributed around the world by social media will be a death knell. Hydrogen-powered cargo aircraft might me more feasible, but that segment makes very little difference.
    That said, as an academic, I am interested in what hydrogen-powered aircraft might look like, the design trade-offs etc. This Bjorn’s series will shed some light on that and so it is very welcome.

  10. The average Southwest or Ryanair sector is 750 miles. How much would it improve emissions if we used planes that were designed to closer match that requirement?

  11. On the CO2 emmision graph, efficiency gains (sfc) have not been 2% per year. Rather upto 1%.

    An entirely new generation engine offers 15-20% better efficiency every 20 yrs. E g. Genx, Trent1000, Leap & PW1000. Meanwhile aviation has been growing ~4%

    Ambitions & projections by ICAO and IATA tend to have a window dressing component in them, because real solutions didn’t pop up despite billions in R&D in the last 30 years. Ambitious outlook to cover up worrying realities today.

    IMO massive clean production of electricity is the only way forward. E g. to produce hydrogen, which requires a dissapointing amount of electricity..

    We have massive electricity generation already, but it is far from clean. Which we usually prefer to leave out if the equation.

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