Bjorn’s Corner: The challenges of Hydrogen. Part 1. Background.

By Bjorn Fehrm

July 24, 2020, ©. Leeham News: What a difference three months make!

When I wrapped the 20 piece Corner series about e in ePlane not standing for electric, on the first of May, I was virtually alone in saying hydrogen is the best long term alternative to our airliners’ jet fuel.

Today it’s all about hydrogen, especially if you ask industry and authorities in Europe. What happened?

Figure 1. The Russian Tu-155 hydrogen fuel research aircraft that flew in 1988. Source: Tupolev.

Why hydrogen is now the focus for sustainable airliners

What happened? How can a World-wide conviction the next non-jet fuel airliner is electric turn to hydrogen in just three months? The actual turn has taken a bit longer but it happened fast. Let’s go through why.

It was all in our ePlane Corner series.  There we concluded:

  • We have a mounting environmental problem where global warming is the pressing one right now. The key action needed is to lower the CO2 emissions from our burning of fossil fuels for heating, industrial production, and transport.
  • Though air transport is 2% of the problem, it’s part is growing and it’s highly visible. Action is needed in all areas and this includes air transport.
  • To lower our CO2 emissions we must stop burning the stored energy in our fossil fuels. The dominant alternative energy store for vehicles is the battery.
  • But there are problems with planes that use the battery as an energy store. Its energy density (the amount of kg battery for each kilowatt-hour stored) is way to low for airplanes. It makes our cars more efficient only because our normal cars are such energy hogs, with an energy efficiency below 10%. When over 90% of the stored energy in the fuel is not used, it’s not difficult to find ways to improve. With such a low bar for improvement, battery-based solutions work.
  • Airliner propulsion system efficiency passes 50% as we speak and the goal is to achieve 60% or more. This raises the bar for any competing propulsion technology to a level where batteries can not be part of the solution.

Why didn’t the aeronautical community realize that batteries are not an option for airliners?

Other developments gave hope and the early movers didn’t have the experience to see the challenges the airliner application presented.

What happened over the last decade was the electric and hybrid car went from a curiosity to the mainstream. The classic car makers were cornered by Tesla (100% battery) and Toyota (battery-based hybrids). Entrepreneurs wanting to be the new Elon Musk predicted the same change would happen for aircraft. After all, aircraft are driven by combustion engines, as are cars.

Those who set about cornering the aircraft industry and create the aircraft industry’s Tesla or SpaceX didn’t do their homework before presenting their Powerpoint projects. Promises of new battery-driven airliners taking over from the Boeing 737 MAX and Airbus A320 before 2030 and being 30% more efficient occupied the industry magazine pages over the last years.

Gradually, experienced airplane designer teams from the OEMs made the sums (as did I) and the impossibility of achieving anything usable with present battery technology became clear. Then investigations into progress in battery technology showed it would be a struggle, even for a short-range airliner. There’s a huge difference between the performance of a research lab battery and what can be certified as an airliner propulsion battery system.

The focus then turned to hybrids. While it was possible to make it work, the same seasoned people saw it would not bring any real gains as long as batteries were the complementary energy store to jet fuel. A very little gain for substantial technical complexity and risk.

The key realization over the last year was the battery as an energy store for airliners didn’t cut it. Not today, not tomorrow, and probably not in the foreseeable future.

The alternative energy store that was continuously looked at was Hydrogen, H2. It was the fuel the world’s first jet engine used (H. von Ohain’s He-S-2 engine in 1937) and it was the fuel used in the Russian Tu-155 hydrogen research airliner in 1988, Figure 1.

It has some very attractive features like three times higher energy density than jet fuel (batteries have 70 times worse) but also challenges like four times worse volume density and a non-existent production ecosystem for air transport.

In the next Corners, we dig deeper into the challenges for H2 and the possible solutions to handle them.

PS. For anyone that wants to revisit the ePlane articles, go to our search box, top right, and enter “ePlane”. The series is then listed with all the parts. DS. 

52 Comments on “Bjorn’s Corner: The challenges of Hydrogen. Part 1. Background.

  1. What happened ?
    Simple , the industry realized just how impractical such a technology was . In addition , awareness of the pollution caused by producing hydrogen increased .
    *Overall , there was no net gain .
    😎

    • What happened was the ‘industry’ was happy to play lip service to charlatans and child saints amount others. The PR won out over the engineers.
      Suddenly there is world changing event where expert opinion won out, even though the useful data was limited and contradictatory. Now was the chance for something where the data wasn’t contradictory to be heard and the PR spin pushed aside.
      Not the only area where there was a complete sea change , but we won’t go there.
      My view is 2020 will be known as the year everything changed

    • I don’t know how what you’re alluding to when you refer to the “pollution caused by producing hydrogen”.
      Currently, hydrogen is produced from alkane gases, because that’s a readily available production method that provides enough hydrogen to test concept vehicles. As hydrogen vehicles become more mainstream — which is starting to occur in (some countries in) Europe — a switch can and will be made to hydrogen production by dissociation/electrolysis of H20 (which has the added advantage of producing oxygen). This is an endoergic process that can be easily powered by solar, wind or nuclear power.
      When you look at the whole cycle, the hydrogen is merely acting as a storage medium for energy: energy is put in or order to produce it, and is released again when it is recombined with oxygen in a fuel cell of combuster. In that way, it’s just a far more efficient version of a battery.

      For road vehicles, the huge advantage of hydrogen is that it allows a range and fueling time comparable to present fossil-fuel vehicles, which makes it much more attractive than battery-powered vehicles. Moreover, it removes the problem of trying to recycle hundreds of kilos of spent batteries per vehicle when the batteries reach their EOL. Lithium for batteries is rare, and its production is very energy consuming.

      • It seems probable that the whole renewable power/battery cycle will become interlinked as vehicle batteries removed from scrapped vehicles gain a second life as charging/demand smoothing stores.

        If the cryogenic storage problem can be overcome hydrogen should be the preferred solution for road vehicles, but will the supply side be able to cope with demand? Many of the middle-eastern countries blessed with huge oil reserves are also fortunate to have very sunny climates well suited to solar energy and hence hydrogen production.

        • I agree that used batteries from EVs can have a “second life” in large-scale powerbanks/smoothers as you allude to. But, as with all batteries, they get more and more tired with every consecutive charge/discharge cycle, and they eventually have to be considered dead. At that stage, the huge headache of scrapping/recycling can no longer be circumvented. That’s already going to be a problem in developed economies…can you imagine how big the problem will be in developing economies, which are already far behind when it comes to recycling practices?

          It’s not just the middle east that has lots of sun, of course: there’s also lots of sun in central Australia, the US southwest, and the Sahara, Gobi and Atacama deserts. A nice opportunity for some emerging economies to generate extra GDP. Iceland could also generate H2 using geothermal energy, which would be a welcome diversification of its economy.

          • The used car Li-ion batterier are valuable in the recycling business and factories are starting to pop up..

      • “because that’s a readily available production method that provides enough hydrogen to test concept vehicles.”
        Electrolysis has been around longer and is just as easy to do. Using natural gas is now the primary method because of the cost is lowest and will remain so.
        Nothing to do with ‘concept vehicles’, as its a bit of smoke and mirrors just like batteries are.
        Increasing Renewable electricity isnt going to be cheap and the process of ‘transporting it’ via a high voltage AC grid requires frequency and voltage stability which isnt helped by increasing from a highly fluctuating wind and solar sources.

        • Exactly…
          And there is where the H2 fits in…
          You increase the stability and reduce transport problem producing H2 on site

    • Was any information published by the Russians (USSR in 1988) about the Tu-155 hydrogen powered aircraft? What performance, what problems they encountered with the fuel storage and the converters to bring stored fuel to a gas flow suitable for the engines? If the Russians published a cutaway picture of the plane then it wasn’t secret.

    • Using excess renewables capacity from wind and solar to power hydrogen production by electrolysis (rather than steam reforming of methane) is non-polluting and is also a cheaper and cleaner way than batteries to capture and store this energy.

      As renewable capacity grows, this will become increasingly viable. It’s a question of scaling up this method of production.

    • Is this a typo, and the reference to pollution should refer to battery manufacture?

    • It help to be older, you have perspective .

      Last big thing was open rotor. That is so yesterday.

      Band Wagon, mass hysteria, whatever, then electric.

      Of course we still have not cycled through liquefied natural gas (pesky density issue)

      Same old issues and with Hydrogen, pressure vessels need round shapes, they don’t fit in wings well.

  2. Not Airlines but small Airport to city helipads works with batteries. Expensive sport planes for Tesla/Porsche owners like Lilium can have a market especially once they don’t require a pilot on-board and fly automatic connected to ATC.
    For Airlines it is H2 and jet Engines or fuel cells that are growing in Power like Toshiba stacking “Toyota Mirai-type” fuel cells to a MW class unit
    https://asia.nikkei.com/Business/Companies/Toshiba-s-megawatt-fuel-cells-have-enough-juice-to-power-a-factory
    Seems like lots of industries are working on fuel cells and gas turbines pretty easy can burn H2 but most likely need new burner designs to get NOX within desired limits.
    But the main driver might be Goverment tax revenues opening up for H2 production/distribution as a fuel instead of JET-A1 in the EU, especially if Aircrafts use supercooled liquid LH2 that need to be empied between flights and reloaded just before pushback.
    The main competition might come from direct capture CO2 and bio-fuel with the argument that they just “borrow” the CO2 already on the Earth Surface/atmosphere, see Aw Week Aerion CEO talk.

  3. Bjorn, thank you for this planned series of articles.
    One aspect that you might wish to dwell upon is an objection that is already been voiced by Greenies (surprise!), i.e. that (very) high-temperature combustion of hydrogen in turbines can catalyze the formation of NOx/NH3 from atmospheric nitrogen. Of course, farts from wildlife produce far greater cumulative quantities of nitrogenous compounds, as does all that organic material decomposing at the bottom of lakes and ponds, but there has to be SOME aspect for the Greta fans to knock…right?

  4. My take on what happened is different.

    First, for decades hydrogen has been seen as ideal and designers haven’t suddenly woken up to its /kg density advantage or the inherent weakness in current battery technology /kg density. Nor will they have suddenly decided the ongoing research into alternative chemistries, structures etc won’t work. Also, the underlying tech pipeline has not changed substantially during Covid-19 and will not change substantially in an hurry. Suitably qualified researchers take years to develop. Allocating research budgets, organising research departments, dealing with noses knocked out of joint when professionals discover the battery tech they’ve based their careers on under government guidance is no longer what governments want have to be dealt with. Etc etc.

    Really, the only thing that has changed in airliner terms is that the French government has recognised the enormous economic dislocation of Covid-19 as the optimum time to dislocate in its favour, with hydrogen as the means. They recognise that China dominates battery raw resources, manufacturing, and probably (my guess) research efforts, and that in the West it is the USA and Germany (largely through Musk’s factories) that dominate. Hydrogen is though, as yet, all to play for. So they are, all in. They see that not only is there an opportunity in aviation, there is also an opportunity in rail (battery is ahead but hydrogen close), power generation and, maybe automobile. Time will tell whether they manage to turn Airbus to hydrogen, and how competition with other countries (eg UK, which has a key tech provider in ITM, and is a few tentative years into starting a switch of domestic energy from natural gas to hydrogen) over broader hydrogen eco-systems play out.

    None of this though has much bearing on the air taxi market that has been buzzing these past few years. That will continue, prett much as before. It is not France’s hydrogen push that will do for many startups. Simply, and just as before, it is the same poor business plans, inadequate management, IP weakness or bad luck that it always is. Unless there is a tax grab on them the vast funds that the very wealthy have sloshing around looking for a high return will, just as before, continue to slosh into the (seen as potentially lucrative and with first mover advantage possibilities) nascent air taxi market. And, just as before, these startups will base their designs on the already existing, already proven, cost effective, well supported (by autombile industry) battery ecosystem.

    • My take is even harder.

      French government was eager to inject cash to protect it’s aeronautical industry and needed a descent argument to present it as “green”.
      Non feasibility of full or hybrid electrical commercial airplane finally made its way through stakeholders minds. So they used hydrogen as a spotlight to green their action…
      (Btw doubtful arguments / counterpart requireemts were also used to justify Air France help).

      It is not a matter or being or not in favour of hydrogen A/C (also I am doubtful that technology would be mature enough to certify an airliner before two decades…), it is just that, so far, there is no invest to develop the infrastructures needed to provide and store enough hydrogen for an airline application.

      To be short, IMHO, any hydrogen airliner would have no future if it can’t be refueled wherever it might fly (including diversion airports).
      Just look at how airport equipments limited the use of A380…

  5. Road vehicles will continue to build on electric technologies. Hydrogen might become a part of that if fuel cell technology advances sufficiently, but isn’t really needed as electricity distribution systems already exist. Renewable sources of electricity can be substituted for the grid source as their cost drops and storage methods become more economical.

    Aviation remains one of the best and most appropriate uses of fossil fuels. It’s ideally matched to the application. Switching entirely to hydrogen tomorrow would barely make a dent in the climate issue. There are far more important terrestrial reductions to make, that can more readily be accomplished and have a much larger impact on the world.

    I suspect that this is being driven by the perceived goal to eliminate all fossil fuel use, which also drove the electric aviation endeavors. But that goal is neither necessary nor practical.

    We would need to reduce terrestrial consumption by over 50% before aviation use would become a significant contributor, and over 90% before it would become dominant. At those levels, we wouldn’t have a climate issue. Fossil fuels are one element of our energy toolbox. The goal is to stop abusing them as the easiest and most readily available source, as we have thus far.

    So as Bjorn mentioned in his earlier series, the best aviation approach at present, is to increase the efficiency of flight, and focus our efforts on terrestrial consumption, which will have the greatest impact. I think the pursuit of hydrogen flight will be another iteration of electric flight. A large investment but not a practical solution forthcoming.

    Although the focus has switched in a brief time from electric to hydrogen, the technology and real-world challenges have not changed. In the hype curve that Bjorn discussed, we are just resetting back to the start, but the cycle is likely to repeat again, unless our eyes are clearly open.

    And I say that as an optimist, having worked with hydrogen as a propellant in my career. Hydrogen may have many applications, but aviation would be among those that make the least sense.

  6. Hydrogen will be popular until people are willing to admit that 90% of the world’s hydrogen comes from fossil fuels. Steam Methane Reforming is a primary production process, which not only liberates as much CO2 as when equivalent kerosene is burned in a jet engine but also requires heat. The additional heat comes from fossil fuels. This two step burn is dirtier than a single step kerosene burn in a jet engine.

    Until all hydrogen can be generated by electrolysis, any additional demand for hydrogen by aircraft will satisfied with a dirtier burn path using fossil fuels.

    • “Until all hydrogen can be generated by electrolysis…”

      What’s the problem with producing all hydrogen by electrolysis?

      Being done right now, all over the planet. With rapidly decreasing cost per kilogram of H2.

      Stop rowing backward, help row forward instead.

      • The problem is that less than 5% of hydrogen worldwide is produced by electrolysis. Until things change the substitution of hydrogen for other fuels in aircraft and other equipment results in a net increase in carbon. I don’t know why you think this is not a problem.

        We can all hold hands and wish with all our might, but these are the facts until someone makes it different.

        • All those projects start saying that it only works with “green hydrogen” and some “dare” to day “blue hydrogen”

          The scaling of electrolysis is happening as we speak

  7. Too often e-plane or H2-plane entrepreneurs fail to recognize that for these concepts to work, super-efficient airframes are needed. If during the next 10~15 years we can indeed reduce the energy needs of such platforms by 50% through technological developments (higher wing aspect ratio, lighter structure, bwb etc.) and operational compromises (lower cruise speed, longer take-off distance etc.), then it may still be easier to keep using conventional fuel, or mixed it with bio-fuel.

  8. Two big issues for Hydrogen
    1/ Electrolysis is awfully simple in principle, but awfully difficult in volume production. Water needs to be demineralized, and électrodes have to be plated by sophisticated coatings. so far, nobody has made it in a cost effective way….
    2/ Energy Density looks fine as long as you forget about tanks. Usually a very high pressure is used (700 bars…) and even with rocket technologies, this requires heavy and bulky tanks. Please Bjorn, show us a sketch of these tanks on a commercial plane!
    two smaller issues:
    1/ with such high pressures, there is some leakage 24X7…
    2/Hydrogen refueling in airports would require mobile tanks which would be ideal targets for cheap and simple RPG (aka bazookas)

    • The hydrogen storage issue will be difficult from a safety & certification standpoint as well. In aa accident, the passengers are exposed to cryogenic fluid, which will readily flash into vapor and expand in a huge volume ratio, while combusting in am invisible flame as well, once in vapor form.

      So there are numerous possible causes of death. It seems not very survivable, unless fuel can be stored in wings only, but then the wing volumes have to expand by a factor of 4 or 5. If the wings could be designed as propulsive units with fuel and engine, that separate in a crash, that might save passengers but still pose risks for those on the ground.

      Other safety designs may be possible, and I’m sure Bjorn will address this in his series. It would be one of the problems to solve.

  9. Hydrogen is not a fuel. We would make hydrogen [at some cost], and store it for later use. Its appeal for aircraft is its energy density and convenience in storage. In practice, storing hydrogen is notoriously tricky.

  10. I think 8 or 10 years ago, BMW had a Hydrogen optional care (that was when it was the latest thing)

    No luggage because the tank took up the whole trunk.

    Range was 70 miles (I can get 600+ out out of a 16 gallon tank on the Passat)

    Hint.

    ps: We can haul 4 people (5 in a serious pinch) and all their travel luggage as well. No bag left behind.

    • 10 years ago, everybody had slower internet.
      10 years ago, almost nobody had touchscreen phones.
      10 years ago, planes were noisier and less fuel efficient.
      10 years ago, hormonal treatments for cancer were basically non-existant.

      Things evolve!

      • No disagreement.

        But, evolve vs a revolution?

        So, 100 some years into it and liquid fuel at ambient temps still rules.

        Propane? Mostly in forklift in warehouses. For a good reason. Don’t ask me how many bottles and how many times they ran out over the years.

        Airframes? When a 737 airframe from the 60s is still not totally gone against the most modern airframe (A220) then it tells you some areas of technology y simply are almost immune from significant progress.

        You can whiz bang a tank, you can whiz bang pressure (which is what you need to get in a viable amount) but you can whiz bang tank shapes. And you can only shave so much weight from those tanks.

        The only aspect I have seen in tech was Space X making a return first stage work.

        But they still shuck the first stage for a reason. Its a big bulky and heavy tank.

        And the high value of a few tons to orbit vs the tonnage of an aircraft than can carry pax and luggage and freight?

        Two, three, four – five years from now we are back to open rotor.

        In the meantime, gasoline and diesel still work.

        • The airframes comparison is meaningless. The engine development is the key , both have engines from the last decade or so. Bjorn has previous series on the small gains in changing fuselage materials especially for short haul. The accomplishment was many decades ago in having aluminium monocoque pressurised fuselage thats remarkably thin , light and fatigue resistant.

          • Actually its not. Its a classic example of tech that simply is not easy to improve upon.

            Ergo, no Leap (pun intended) in airframe, then the only improvement to be had is in engines.

            No one has changed the basic car in 100 years. The form perfect for the function. Tricycle cars were not. Cycles are not (as much fun as they can be an transportation in some areas)

            You can talk about electronics all you want, data moving around the speed of light opens up all sorts of possibility.

            Storage can move from moving disks to solid state memory chips.

            Sky is the limit for that area. Data movement allows leaps in those areas.

            But it does not mean that you can improve on a basic rock. It does a perfect job being a rock and you arn’t going to change that.

            Armaments are the same. What a 1000 years and its still a bullet or a shell. Well developed, still easily recognizable from what they started with in guns and cannon.

            Precision bombs the same, the for is identical back to WWI, finally just stick a guidance kit (electronics) on it and it does the job and its cheaper than any rocket.

    • All three comercialy available hydrogen cars right now have the same autonomy that a petrol car

  11. Large scale hydrogen for power application is coming soon, contracts are issued and concepts defined: https://www.power-eng.com/2020/07/21/uniper-partners-with-ge-to-convert-euro-gas-fired-fleet-with-carbon-free-hydrogen/ There are still challenges in the combustion of H2 in gas turbines.
    I am curios, what Bjoern will present about lightweight H2 storage for aircraft. Unless there is a major breakthrough incatlysts for storing without cryogenics, I am not optimistic that H2 as fuel will become popular. I expect more Ethanol production from renewable sources (e.g., algae) as fuel for the aviation industry.

  12. I was always told aircraft would probably the vehicles to take the last available gas reserves, because almost every other vehicle is easier to switch over to other fuels, so is more productive for the environment to spend your R&D there.

    • Yep. Prius works because you have the right balance and you arn’t going 550 mph.

      But as I told them at work, yep, you get an efficiency gain and it cost you a lot of money to get it and it has to be setup exactly right or you loose all you got.

      And then only a super trained tech that knows the mfg software can tune it.

      Not to mention, when it quits working, what it costs to fix it.

      So when your Prius has an electrical zark, money flows out the door and the battery has to be replaced at a cost of 5 grand.

    • Keesje, under normal circumstances your comment would be 100% correct…and it will probably remain correct for a long time for military and freight aviation.
      However, passenger-carrying airlines are starting to see the effects of Greta’s rantings: it may not yet be prevalent on other continents, but “flight shame” is becoming an issue in Europe, and it’s hurting load factors. Moreover, increasing lobbying by Greenie parties is leading to discussion of banning relatively short national flights altogether. It’s unfortunate for the aviation sector, because it’s one of the most innovative in terms of the continuing drive for greater efficiency…a natural effect of the huge fuel bill associated with aviation. But it’s currently fashionable to be anti-aviation, and the mob will follow that fashion in order to “fit in” with convention. Airlines and IATA can see this happening, and know that something needs to be done about it.

      KLM are currently accelerating efforts to run 100% on fuel made from used cooking oil. But the easily-manipulated mob will be told by Greta that combustion of such fuel still produces CO2, so it’s questionable whether this step will be enough. In that context, it’s not hard to see why hydrogen proposals are being floated again.

      It’s worked for road vehicles. Despite TW’s comments above, mainstream car manufacturers are now introducing models that use hydrogen fuel cells, and that have the same range and performance as gasoline cars…and also have normal trunk space. There’s a hydrogen fueling station in the city where I live (just one for now…more will come). There are hydrogen buses running in a province next to mine, and hydrogen trains running in the country next to mine. In Europe at least, the hydrogen revolution is starting.

  13. It costs a tonne of coal to create the electricity to produce hydrogen. Sun / wind don’t produce any sort of quantity even close enough to produce hydrogen. And we have to warm our houses somehow.

    Still everybody like the idea. So politicians put aside billions. It took 20 years and tens of billions to realize biofuels are non-sense. The opportunism & lack of understanding of decision makers is close to criminal in my opinion.

    Meanwhile the environment goes down the drain. Massive nucliar energy isn’t without disadvantages, but the only way to produce the huge amounts of electricity while halting our enormous polution I’m afraid.

    Windmills at sea cost more energy to produce/ remove than they’ll produce & we smartly fly in cheap solarcells from China without anyone asking how much energy/ pollution it costed to get them on our roofs. We just make a new start.

    And a lot of people make their incomes like that and pay tax.. uncomfortable realities.

  14. Hydrogen is an energy carrier and not a readily mined energy source as fossil fuels are. It has to be created somehow, stored and transported and this requires energy in some form. Renewable solar and wind energy and electrolysis appear attractive but that is not what is being done at present. The one positive thing about hydrogen is that it is pollution-free at the point of use and so it won’t be a distributed source of pollution like fossil fuels are. Its production may lead to large point sources of pollution and how hydrogen is going to be produced needs attention before getting carried away by the hype.

    • What does hydrogen come from? And how is it produced? I thought I read once, that the easiest way to get hydrogen is from fossil fuels. TIA

      • The main choices of hydrogen production are currently from reformation of methane, as a byproduct of other chemical production, or from electrolysis of water.

        Right now, the methane method is the dominant source, which is dependent on fossil fuels and still has a carbon problem. The byproduct method cannot produce hydrogen at sufficient quantities.

        The electrolysis method is really energy conversion, from electricity to hydrogen at 70% to 80% efficiency at commercial scale. So we need an abundant supply of electricity that does not rely on fossil fuels or carbon emissions for generation. That means either nuclear or renewable.

        Once the electricity is converted to hydrogen, storage is the next issue. The options for storage include compressed gas up to 700 atmospheres, which gives a density of 40 kg per cubic meter (reducing to 1 kg at 1 atmosphere).

        Storage as cryogenic liquid can be done at 100 atmospheres at temperature of 33 K, reducing to 1 atmosphere at 23 K. That increases storage density to 70 kg per cubic meter. At liquid temperatures, even minimal heat leakage will cause boil-off. This loss can be accepted as several percent, or a small liquefaction plant can be used for recovery.

        For either form of storage, there is an additional electrical cost of compression and/or liquefaction. This reduces overall conversion efficiency to generally less than 70%. Also there are significant safety issues as liquid hydrogen temperatures can liquify or solidify the components of surrounding air. Also the uncontrolled release of either compression or liquefaction energies are immense, not to mention the combustion energy stored in the hydrogen itself.

        Storage can also be accomplished by absorption or chemical binding in receiver materials. This has the potential to increase storage density, at the cost of the weight of the receiver, and the energy/temperature requirements for release. Many options have been pursued in the laboratory but none have yet become commercially viable at large scale. Research on this continues but there is no simple or easy solution on the horizon.

      • To give an idea of the end-to-end conversion efficiency of the electrolysis to hydrogen to storage to gas turbine power output process: Siemens estimated in a study that 175 MW of renewable energy capacity would need to be installed for a 50 MW turbine power output. So a little less than 30% efficiency overall.

        • That 30% efficiency is better than what is achieved in gasoline engines in cars, which have a Carnot limit of about 35%, but which rarely achieve more than about 20-25% in practice.

          State-of-the art gas turbines achieve about 45% efficiency in simple-cycle operation, and about 60% in combined-cycle operation.

          • The study above assumes a combined cycle turbine efficiency of 50%.

            The main point was that for each NB aircraft with twin 20 MW engines, you’d need 12o MW of installed renewable or nuclear capacity to operate the aircraft with hydrogen from electrolysis.

            If the hydrogen is obtained from fossil fuel reformation, the conversion efficiency is slightly better but the cost is reduced because the energy source is low-availability heat rather than high-availability electricity. With on-site carbon capture, this advantage is diminished due to the energy penalty of separation/capture.

            So there likely will be competition between carbon capture and renewable technologies development, in terms of lowering cost for each. In the short-term, carbon capture can buy time to build out a more robust renewable infrastructure for the long-term..

            This is especially true if we push out hydrogen in the role of heat source first. Existing natural gas distribution systems are being evaluated for compatibility with hydrogen, and preliminary results are that concentrations of 5% to 25% are feasible, without significant modifications. We used gas mixtures for years before quality refinement became available.

            At 20%, separation at the destination becomes economically feasible (although at higher cost), for those applications that must have pure methane or pure hydrogen. For most combustion applications, the mixture will work well without separation. However separation still would be required for liquid storage.

            As time goes on and distribution/appliances are upgraded, the percentage of hydrogen can be increased to above 50%, and methane can be slowly phased out. Also the source of hydrogen can be shifted over the same time period to favor renewables over methane reformation with capture.

            This is a path forward which is available quickly, and with higher conversion efficiencies since the end product is heat rather than electricity or mechanical work. It also has the potential for significant climate impact as together, building & industrial process heating/cooling represent about 35% of carbon emissions.

  15. Hello Bjorn,

    please discuss also other exhaust gases than CO2 in this series!
    AFAIK the influence of NOx and water vapor on our athmosphere is not fully understood yet, and research is ongoing.
    But what seems clear, is that an H2 fuel cell based airplane is the only viable technical solution for an airliner, which doesn’t exhaust any gases into our atmosphere – just liquid water.

    Jörg

  16. Liquid water released at cruising altitude will quickly solidify and sublimate in the cold dry air. So it all ends up as water vapor, regardless of source. Unless you store it on board, which introduces a weight problem.

    The impact of aviation water vapor release is estimated to be no more then the impact of CO2 release, a few percent of the warming problem at most.

    Turbine manufacturers have reductions in development for the NOX emissions problems as well, whether burning hydrogen or fossil fuels. The problem is the same in either case since the nitrogen and oxygen come from the atmosphere, and are formed due to high combustion temperatures, which can be better controlled.

    As Bjorn showed in his earlier series, the energy density and efficiency of a hydrogen fuel call (plus fuel) are not yet high enough for practical commercial aviation use.

    As with all other aspects of aviation, the byproducts of combustion are only a small fraction of terrestrial sources.

  17. Best way to store hydrogen is as Green Ammonia, better than liquid hydrogen and a much lower pressure. The North American X-15 used it (albeit with liquid oxygen).

    Still nasty stuff in an accident, but I can’t say it’s any worse than liquid hydrogen or indeed Jet A1 if the stuff is on fire.

    Hydrogen/ammonia can be used as part of a fuel-cell/gas-turbine hybrid and used as an intercooling medium and then again as a recuperating medium.

    Good riddance to batteries.

    • Ammonia has a 1.5 to 1 advantage in both volumetric density & volumetric energy density, but a 5 to 1 disadvantage in gravimetric density, and a 7 to 1 disadvantage in gravimetric energy density, over liquid hydrogen. It’s far below jet fuel in both categories.

      Plus the additional nitrogen release results in higher NOX emissions if the ammonia is burned directly, although those can be addressed with exhaust treatments for terrestrial turbines. Might be more difficult for an aviation turbine.

      The combustion issues can be addressed if the hydrogen is separated first, but that requires additional equipment and weight, as would a fuel cell. Neither batteries nor fuel cells are a yet at a point that is suitable for aviation.

  18. It’s still the way to go – the stats you quote don’t include the weight of the metal required to hold the fuel in and there’s already an infrastructure in place to produce , handle and transport the stuff. You’re right, the technology is not yet mature enough but it will be, that relationship I mentioned with gas turbines, if they need cooling and catalytic conversion of ammonia needs heat . . .
    Nitrates aren’t the big problem either, it shouldn’t stand in the way of addressing the big problems.

    • The energy density of ammonia is well below jet fuel, so there would be a large weight penalty, even with the same tank technology. A 7 to 1 advantage for liquid hydrogen provides some headroom for extra tank weight.

      There is no ideal hydrogen storage solution, as opposed to jet fuel. You just have to choose which penalty you’re willing to accept. There will be a trade-off no matter what solution is selected.

      This points again to the suitability of fossil fuels to aviation, as opposed to other applications that are much more readily converted to alternative energy.

  19. To be taken seriously you need to stop saying that battery powered vehicles are disadvantaged by their long charging times. The reality is that ICE vehicles must go to service stations whereas more than 99% of my Tesla charging happens at home. Less than 10 seconds of my time is used to charge my car at home each week. Only three times in the last year have I had to use charging stations And in each incident I needed to take a rest after four hours of driving. ICE vehicles are disadvantaged that they MUST go to service stations But since this is our paradigm for fuelling vehicles we in correctly apply that to battery powered vehicles but your argument is false.

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