Saturday, October 29, 2016

Hydrogen powered planes: can they save the airlines?





Not exactly the same thing as the current generation of planes! To run on hydrogen, the airlines would require a completely new generation of planes. (source)


Years ago, a Ukrainian colleague told me about a plan that the Soviet Union had for their military presence in the Mediterranean Sea. Because of the long supply lines from the home bases, they were thinking of using their nuclear-powered battle cruisers to produce hydrogen in order to fuel their warplanes.

I have no way to verify whether this story is true or not; I couldn't find any trace of it on the Web. But it is not unreasonable that the idea of hydrogen fueled warplanes was seriously taken into consideration in the 1980s, when the Soviet Union still had dreams of being a superpower. In any case, nothing came out of it and there are good reasons for that: a hydrogen-powered plane is an engineering nightmare for several reasons that are well described in a post by S.H. Salter that Sam Carana published on his blog a few months ago. The full post is reproduced below.

If running a warplane on hydrogen is a nightmare, doing that with the civilian airlines is much worse. Salter makes it clear how complex and difficult the task is. Hydrogen was a good fuel for the Space Shuttle, but the shuttle was not a passenger plane and it carried a gigantic external tank full of liquid hydrogen. This is because hydrogen is a good fuel in terms of weight, but it is bulky. In a passenger plane, the fuel is carried mainly in the wings, but there is just no way to do that with compressed or liquid hydrogen without completely redesigning the whole plane. And that implies replacing the whole fleet of the civilian airlines.

In a little more than a century, we went from the flimsy planes of the Wright brothers to the current generation of wide-body aircraft. The lifetime of the present planes is supposed to be around 30 years or more and it took seven years to deliver the first Airbus A380 (in 2007) from when the decision was taken to design and produce it. And the A380 makes use of proven technologies - it is just one of a long line of aircraft that have been developed and tested over more than 50 years. How long would it take to rebuild the whole airline fleet? Can we afford to do it? Will we have to ground the airlines before it is too late to avoid the worst disasters of climate change?

So, it is easy to write books about the upcoming "hydrogen based economy," assuming that all technical problems can be solved by throwing a little money at them. It is not so easy. Then, of course, there are other renewable fuels that could be used instead of hydrogen, but I will discuss that in another post, but let me tell you that things are not much better. Making a "sustainable plane" is a technological nightmare, at least if we pretend from it the performance we pretend from the current generation of planes.

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From Sam Carana's blog




Can we Design Hydrogen-Fuelled Aircraft?



S H Salter, Engineering and Electronics, University of Edinburgh.EH9 3JL.

The collection of temperature measurements by David Travis following the 3-day grounding of all US civilian flights after 9/11 showed the astonishing effect of jet exhaust on the environment. If burning hydrocarbon fuel in the stratosphere ever becomes a criminal offence, the aviation industry will have an interesting problem. A possible solution is the use of hydrogen as a fuel. Is this technically possible?

The Airbus 380 carries 250 tonnes of fuel with a total calorific value of about 1013 joules. Fuel is stowed in wing tanks but this would be a volume of about one eighth of the fuselage. The calorific value per unit mass of hydrogen is about 3.5 times that of jet fuel and so the weight of hydrogen for the same range would be only about 70 tonnes. Unfortunately the ratio of density of jet fuel to un-pressurized hydrogen is about 9000, so the design problem is how to reduce the volume ratio by about 2500. If we compress hydrogen to reduce its volume by a factor of, say, 100 we still have a fuel volume of 25 times the liquid fuel one or 3.2 times the fuselage volume. The cube root of 3.2 is 1.47 so by increasing all three fuselage dimensions by this factor we could have an aircraft with enough volume for all fuel in the fuselage but no passenger space. An increase by a factor of about 1.6 in both diameter and fuselage length would give enough volume for passengers provided they did not feel unhappy about being close to so much hydrogen.

The immediate reaction against the proposal will be triggered by embedded folk memories of the Hindenburg. Any use of hydrogen will need careful public relations. The Hindenburg survival rate was 64%, much better than crashes of modern conventional aircraft. Deaths were caused by jumping not burning. People who stayed aboard until the wreck reached the ground were unharmed. It is likely that the fire started in the fabric dope rather than the hydrogen. Because spilt hydrogen moves rapidly upwards there is much less risk than from a liquid fuel or heavier-than-air gases like butane or propane which regularly cause devastating explosions in boats and buildings. Furthermore the heat radiated by the invisible hydrogen flame is much lower than that from carbon particles in hydrocarbon flames. We can argue that hydrogen is actually safer than jet fuel, petrol and hydrocarbon gases.

We can spend the 180 tonne fuel weight-saving on gas storage bottles in the form of a low-permeability skin surrounded by wound carbon fibres. A helical winding of aluminium sheet with a low diffusion coefficient for hydrogen looks good. It can be made with the linear equivalent of spot welding. The axial stress in a thin-wall tube under pressure is only half the hoop stress, so we can use the gas tubes as fuselage strength-members. Once the fuselage bending moments are known, we can choose the wrap angle of the windings to give the right balance of directional strength. One structure might be a bundle of nine tubes in a hexagonal array with six full of hydrogen and three containing passengers. A cross section is sketched in the figure. Other configurations are being studied.

The smooth stress paths of the gas bottles would be badly disrupted by the conventional design of landing gear. Can we get rid of it? The requirements for processing the variable energy flows from renewable-energy sources have led to the development of new high-pressure oil machines using digital rather than analogue control of machine displacement. These machines have very high conversion efficiencies and very easy interfaces to computers (see http://www.artemisip.com/ ) . The extremely accurate control of very large energy flows allows many new applications. One of these involves replacing the landing gear of large passenger aircraft with a ground vehicle. Please suspend disbelief until you have considered the following facts:

  1. The landing gear of the A380 weighs 20 tonnes, say, 200 passengers. This weight is carried round the world for many hours and then used for only a few minutes on each flight.
  2. The landing gear occupies a substantial volume of the internal space. The volume restriction limits the travel of the landing gear and so increases acceleration forces.
  3. The requirement for openings compromises the structural integrity of the fuselage and adds weight, even more passengers.
  4. Landing gear must perform with very high reliability despite the weight penalty and extreme temperature cycling.
  5. The full weight of the aircraft must be passed to the ground through highly stressed points.
  6. Gas turbines are very inefficient for moving aircraft on the ground at slow speeds.
  7. On the A380 the shape of the landing gear doors and opening spoils the aerodynamic fairness. 
  8. There is a severe design conflict between tyre weight, tyre life and braking performance.
An alternative might be to provide the function of the landing gear by a special-purpose ground vehicle. It would of course have to have VERY reliable links to the aircraft ground approach electronics so as to be in exactly the right place and moving with the right velocity underneath an aircraft on final approach. However there would be no weight, volume or temperature compromises.



The contact between the landing vehicle and the aircraft would be provided by a nest of large air-filled tubes like very large, very soft V-block, running the full length of the fuselage. This would spread the weight evenly into the aircraft skin. The tube surfaces could have vacuum suckers, like an octopus, which could apply shear forces evenly to the aircraft skin. The bags could be on a frame which could have hydraulic actuators to give a much longer travel than the legs of the landing gear. Tilting this frame would remove the need for the angling of the rear underside of the fuselage required to prevent ground contact at V-Rotate. This would further reduce drag during flight. The absence of fuselage penetrations could allow safe water landings for emergency. Runways can have parallel lakes presenting a much lower fire hazard if fuel is spilt. The impact loading on the runway would be much reduced and it might even be possible to revert to grass runways with several parallel operations from any wind direction.

The ground vehicles could use Diesel engines with much higher efficiency at taxi speed than gas turbines. They could have higher acceleration during take off and higher deceleration during landing. The hydraulic transmission would also allow regenerative braking, so the kinetic energy from one landing could be used for the next take-off. All-wheel steering and the option of direct side movement would allow much better use of ground space. The ground vehicle could have many more tyres, which need have no weight or volume compromise to achieve high braking. It could have an air-knife to dry runway surfaces and remove snow. There would be plenty of time to inspect and exchange landing vehicles and they would be in use for a much higher fraction of the time. The landing vehicles could gently lower aircraft on to passive supports at each loading pier and be used for other movements while aircraft were being boarded or serviced.


Images by S H Salter, University of Edinburgh.
The volume of most aircraft wings is much below that of the fuselage and so there is not a strong reason to use gas tubes as structural wing members. However they would offer a way to offset the extra drag of the larger frontal cross-section. From the original work by Prandtl, it has long been known that sucking air from the upper surface of an aerofoil section will reduce the drag by an amount which far offsets the power needed for a suction pump. Schlichting in figure 14.9 of Boundary Layer Theory gives a graph showing a factor of more than two. An objection to suction on wings, where the outer skin is a structural member, is that perforations and slits cause stress concentrations. This should not apply to wing spars made as gas tubes supporting an unstressed skin.

It is important that using fuel does not move the centre of gravity of the aircraft. This happens automatically with fuel stowed in wing tanks. If large quantities of fuel are to be stored in the fuselage it will be necessary to have the centre of pressure of the wings close to the centre of gravity of the fuselage-engine combination. The choice of a ground-based landing vehicle suggests high wings and engine placement above the wing. In theory at least, this will give some advantage from higher air-velocity over the upper wing surface and lower noise transmission to ground level. It is much easier to service and inspect equipment if you do not have to reach above your head. Cranes lifting an engine upwards are much more convenient than forklift trucks working from below. While some change in the architecture of maintenance hangers would be required, high engines accessed from above would by no means be unwelcome to ground crew.

Gas tubes may not be ideal for connections to a low-chord wing and so the longer attachment line of a delta wing, such as used in the Vulcan and Concord and many fighter designs, should be investigated. A flat underside will relax the requirement for precision in yaw during landing. Suction may be able to offset some of the disadvantages of the delta wing as applied to civilian aircraft provided always that they can land safely after a failure of the suction system. A delta wing with a deep thickness and a leading edge made from very strong but transparent material, perhaps poly carbonate, might even allow passengers to sit in the wing enjoying a splendid view if their vertigo allows.

The range of the A 380 is 15,000 kilometres. While this may have been chosen for passenger convenience with the properties of present fuels, it is larger than necessary for trans-Atlantic flights and could allow a further volume reduction. The San Francisco to Sydney distance is only 12000 km and stops in mid Pacific could be very attractive.

Before we waste time on radical new aircraft designs and ground-based landing systems, it is necessary to confirm that burning hydrogen in gas turbines at high altitudes will be a chemically appropriate solution. If we burn hydrogen in ambient air there will be no release of carbon dioxide but there will still be the formation of nitrogen–oxygen compounds collectively known as NOXes. If these are cooled very rapidly, as in the adiabatic expansion of an internal combustion engine, they can be ‘frozen’ at the high-temperature equilibrium state with lots of very nasty acids. The lower combustion pressure and slightly slower cooling of a jet exhaust should be less severe but we want to quantify the severity of the problem. There may even be problems from ice crystals formed from the exhaust. I have asked colleagues at the National Centre for Atmospheric Research at Boulder Colorado for an opinion.

There is one engine design in which the combustion products cool slowly enough for almost all the NOX production to revert to ambient values. This is the Stirling engine originating from 1815 but abandoned because of the absence of materials with good thermal conductivity and high hot strength. Much better materials are now available. By an extraordinary coincidence, the digital hydraulic systems needed for the speed and accuracy of the ground-based landing gear can also radically change the design of Stirling engines by using hydraulics to replace the crank and connecting rods of the conventional Stirling engine. A Stirling-engined aircraft would probably have to use a ducted fan or propeller propulsion but these could still allow civilian aviation to continue in a NOX-sensitive world.

The best way to do experiments on high-altitude engine-chemistry might be from a balloon. Do we know anyone with an interest in this area?

16 comments:

  1. In Salter's article: "The Airbus 380 carries 250 tonnes of fuel with a total calorific value of about 1013 joules". No. A joule is a very small unit. One joule in everyday life represents approximately the energy required to lift a medium-size tomato (100 g) 1 meter. The idea is daft anyway.

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    1. I think it is a misprint. Should read 10^13 J

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  2. Why not, but why ? Do we need this ? Is this not an answer to a question that nobody asks ? If I think at Jeff Rubin " Why your world is about to get a whole lot smaller", I wonder if this is a priority. What is the ROI of a transatlantic flight ? What is the CO2 difference if I would take a ship ? For trips inside one continent, we had hotel trains, why not office trains in the future ?

    Best regards,

    Etienne

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  3. Air travel alone, as we now practise it, is sufficient to blow our remaining C budget. The need to rethink the extravagance of flight at will may be a major,conscious or unconscious, sticking point for the wealthy to accept AGW.The likelihood of finding any truly sustainable alternative is close to zero.

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  4. I would venture that H₂-fueled aviation can be made practical: Prof Salter’s numbers are basically correct. Also, the design configuration he offers is only one of many. The obstacles are surmountable… but not in the time frame available.

    Back in the 1970’s, I worked in GM’s “Emissions Impossible” crash program and the subsequent high-fuel-economy crash program. I worked on one Emissions Impossible scheme which we thought was wonderful: improved fuel efficiency and nigh-zilch NOᵪ and CᵪHᵪ₂ emissions, not to mention a basically simpler & cheaper engine. Problem: really squirrelly controls, and worst of all, requirements for technologies that did not exist anywhere. No surprise altho to our immense disappointment, GM management turned it down: they needed a product that would be in production in only a few years and manufacturable with current capital infrastructure. So the world got Exhaust Gas Recirculation first, followed by the Catalytic Converter (we engineers named the latter the “Catholic Converter” — we were young back then).

    Those two were both klutzy kludges. Oh, if properly funded R&D had started back when Arie Hagen-Smit proved how automotive combustion resulted in smog (late 1950’s), by the 1970’s we could have had the technology. Indeed, the technology of which I speak (stratified air-fuel-water cylinder charging) was proposed. But unfolding History, in the hands of scoundrelly Captains of Industry and their “democratically elected” lackeys, had something else in mind.

    Indeed, at GM I witnessed (but did not participate in) programs that turned out poorly, to put it mildly. Like one in which a simple substitution of what was thought to be an improved sound-deadening material resulted in removing a promising car being taken off the market.

    For any solvable problem, there are at most a very few practical solutions and a semi-infinite number of ways to do it wrong. And H₂-fueled airliners are a far longer leap than achieving a quieter ride in an inexpensive car.

    In 1925 my father traveled to France to study art. He returned in 1927. He traveled by steamship both ways. The lack of cheap airplane flights did not deter him. It need not bring civilization to a screeching halt.

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  5. Hello,

    To come back to the first question : "can the airlines be saved ?", maybe, but probably not with a propulsion engine. I heard that propeller would have a much higher efficiency, and furthermore it works with an electrical motor, maybe a fuel cell. In a Seneca cliff type of scenario, it is easier to change the concept with existing technology than to change the technology in order to keep the existing concept. The time of the Concorde airplane is over anyway.

    Best regards,

    Etienne

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  6. I don't think we can save the passenger aeroplane, but we can surely save aviation. And we already know the fuel: Blau Gas, which in 1929 flew the Graf Zeppelin around the world.

    Buoyant flight is sustainable; heavier-than-air flight is not.

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  7. If you are going to try to maintain air travel with Hydrogen, why use planes at all? Just use airships like the Hindenberg, or create some kind of hybrid. You're going to have the potential for explosion no matter what, and in least in theory engineers today could do better than they did with the Hindenberg.

    Given the fact you could also use prevailing winds to drive the travel, it would be far more energy efficient.

    RE

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  8. travel must have a purpose

    travel of itself does not provide that purpose.
    This applies to cars, trucks, trains, planes---anything we use to move us or our goods.

    travel does not generate prosperity, no matter how prosperous our method of travel, so unless there is something at our destination that creates wealth in real terms, travel has no ultimate purpose in the context of human lifestyle
    We face finite resources, so consuming those resources to build ever more complex transport systems that serve only to consume yet more resources would appear to be fatuous

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    1. No human endeavor creates wealth Norman. It just consumes wealth, as all animals do. The only "wealth creation" that comes on Planet Earth is from the plants harvesting energy from the Sun, but that isn't wealth creation either, because the Sun is spinning downward in total energy output. The only "wealth creation" came with the Big Bang, everything since is harvesting that wealth in one way or the other.

      Far as planet earth is concerned, travel and shipment of goods can move wealth around the planet and make some places more hospitable for human life, for a while. For instance, Saudi Arabia would be a shitty place to live, but if you can export oil and import food, and then use oil to desalinate water, it becomes a somewhat more hospitable place to live.

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  9. The relative efficiencies of aeroplanes versus airships can be calculated from the known data. As a passenger carrier, the Hindenberg was about 5 times as efficient (measured as energy per passenger kilometer) as today's A380, and the passengers travelled in almost unimaginable luxury.

    But the real w9inner is the cargo airship, which would be about 20 times as efficient - as good as a lorry, and needing no infrastructure to speak of.

    Norman Pagett, travel can indeed create wealth - by moving goods from places where they are less valuable to places where they are more valuable, as Adam Smith described. And if we can reduce the transport cost of perishable cargo by 95% ...?

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  10. The amount of fossil fuels to keep airplanes in the air is astonishing. When the ressource crisis hits air travel and air transport will be one of the first sectors of mobility to crash. The Lufthansa even admits that and plans to run on biofuels in the future (see: http://www.zeit.de/zeit-wissen/2012/02/Gruenes-Fliegen-LT/komplettansicht )

    But the reality is that hydrogen or "aviation biofuels" production will not even come close to provide enough sustainable power to keep the planes flying, even if we already had the technology and infrastructure today.

    I roughly estimate that we would need 500 million tons of energy crops to keep the planes in the air with biofuels, which would be the eqivalent of the food supply for 5 billion people. Aviation can never be sustainable.

    The industry reacts to that reality with total denial. The London Heathrow and Munich Airports, two of the largest airports in europe, are to be expanded, because the industry argues that air travel grows by 3,6% per year in the next 30 years! (Source: Lufthansa)

    Capitalism, as always, is utterly unable to react to long term trends and solve the problems it creates. In all sectors of our current fossile fuel powered industry we are expanding the infrastructures for production and operation as if we were on speed.

    It is quite probable that even in the lifetime of the infrastructure and machinery we have today, the cars, planes, trucks etc. we will run out of fuel to operate all or most of them. In spite of this we produce more cars than ever, more planes than ever and use more trucks than ever.

    This capitalist system is sick and crazy and no "hydrogen fueled jet" or other technological marvel will save it from self destruction. We should really get rid of it.

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  11. By the way, to fuel airplanes with hydrogen would bring lots of steam into the higher regions of our atmosphere (condensation trails) and would therefore contribute to the greenhouse effekt.

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  12. The newest Article by George Montbiot (Guardian):
    http://www.monbiot.com/2016/10/19/the-flight-of-reason/

    "The inexorable logic that should rule out new sources of oil, gas and coal also applies to the expansion of airports. In a world seeking to prevent climate breakdown, there is no remaining scope for extending infrastructure that depends on fossil fuels. The prime minister cannot uphold the Paris agreement on climate change, that comes into force next month, and permit the runway to be built.

    While most sectors can replace fossil fuels with other sources, this is not the case for aviation. The airline companies seek to divert us with a series of mumbo-jumbo jets: mythical technologies never destined for life beyond the press release. Solar passenger planes, blended wing bodies, hydrogen jets, algal oils, other biofuels: all are either technically impossible, commercially infeasible, worse than fossil fuels or capable of making scarcely a dent in emissions."

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  13. Claudio Della Volpe and Antonio Zecca

    Comments given until now do not mention the two most effective ways to reduce the negative impact of air traffic on the planet.
    First: reducing the speed of airplanes dramatically reduces the fuel consumption - no matter if oil or hydrogen. Speed is presently set around 900 km/hour as in the last 40 years. This speed has been established on a technical basis (speed of sound is not much larger) and on the evaluation of the maximum stress the airline can impose to travelers. The second criterion is ready for deep revisions. Just to give an example, reducing the speed to 450 km/hour, reduces the fuel consumption by a factor of the order of four. Depending on the total duration of the trip, on the distance, on the moment of the day of the travel, many travelers would accept a 450 km/hour speed.
    Second: a substantial part of present flights can be attributed to the so-called "parcel-post" tourists. Parcel post refers to a bunch of persons who are "parceled" in a bus to the airport; then parceled into an airplane and delivered to another bus in a very distant airport; then moved to a nearby hotel where they "enjoy" bating on a beach non-distinguishable from several beaches in the neighbourhood of their homes; eventually they are parceled to a museum where they can look for a few minutes to a few art works. At the end of a week these people are parceled back home. It is clear that any small shaking of the costs of travel will make a big change in the number of parcel tourists; as there is no doubt that those changes would not make happy both airlines and tourist operators. The difficulty is set today by the quoted categories will try to extend the BAU trend as much as possible.
    [should anyone think that the above changes are of very difficult implementation: yes, you reached the conclusion that the present "neoliberal" economy is not fit to deal with the present world.]

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Who

Ugo Bardi is a member of the Club of Rome and the author of "Extracted: how the quest for mineral resources is plundering the Planet" (Chelsea Green 2014)