Bjorn’s Corner: The challenges of Hydrogen. Part 8. Fuel cell electric or Turbofan propulsion?

September 11, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners we look at whether we use an LH2 burning Turbofan as propulsion or as the EU study proposed, a Parallel Hybrid feed by a fuel cell, Figure 1.

Figure 1. The Parallel Hybrid 165 seat aircraft from the EU Hydrogen study. Source: EU.

Fuel cell electric hybrid or Turbofan propulsion?

The EU study presented a Fuel cell parallel electric hybrid aircraft at 165 seats for introduction in 2035 as the alternative to today’s Airbus A320 and Boeing 737 aircraft. Let’s call this the LH2 Hybrid. Key data and assumptions are shown in Figure 1. Let’s examine these a bit closer.

The size and configuration is a modern variant of an A320/737. This is the size class we dissect in our series. The configuration of the propulsion should be the bottom alternative in Figure 2.

Figure 2. The different hydrogen propulsion concepts we study. Turbofan, Serial fuel cell, and Parallel Hybrid fuel cell. Source: Leeham Co.

The study assumes the cruise power needed to propel the aircraft is 10MW, taken from an 11MW fuel cell, through the inverter, and driving a motor that drives the fan on each side (1MW disappears in chain losses). During takeoff and climb an LH2 gas turbine core is assisting.

The picture assumes 11MW fuel cell output is enough to propel the cruise for the aircraft at 0.72M cruise speed. After losses through the Inverter, the Distribution network, and Motor we are at 10MW fan power for the LH2 Hybrid case.

Takeoff power to the fans for today’s A320 is 40MW, with cruise power of 14MW at the average weight on the average sector. Cruise speed is then 0.76M.

Let’s first calculate the weight of the components for the LH2 Hybrid and then discuss how realistic this configuration is for a 2035 realization.

The fuel cell weighs 5.5t with the assumption of 2kW/kg. The inverters weigh 0.6t and the motors 0.9t (kg/MW values from the Electric Corner series).

To understand the weight of the parallel helper core we need to assume an additional power need on top of 10MW. With today’s A320neo it would be 30MW.

The EU LH2 Hybrid assumes to be more efficient than today’s A320neo, so it should need less takeoff power. It’s easy to think it should be (let’s disregard the battery in the concept, in order to give the LH2 Hybrid it’s best shot. We know by now batteries kill any configuration).

But we know the empty weight and wetted are of any LH2 aircraft will increase because of the tanks as a carbon fueled aircraft gets the tanks for free (in the wingboxes) and the isolated LH2 tanks weighs a lot and take considerable space in the fuselage. So before we look at the LH2 Hybrid propulsion system we have a heavier aircraft with a larger wetted area.

The fuel weight fraction will be lower (LH2 has three times the energy per kg of Jet-A1) but on the short-range flights we look at it can’t fully compensate. We fly a larger and on average heavier aircraft on the sector we fly. This is consistent with the conclusions of the Airbus Cryoplane study (we talk of around 10% higher drag = energy consumption for an LH2 aircraft compared to carbon fueled variant according to the Cryoplane study, everything else being equal).

The fix for this is a wing with a higher aspect ratio, to reduce the wetted area of the wing and offer a higher effective span, by it, reducing induced drag. But we are span limited to 36m, otherwise, the aircraft doesn’t fit the airports. In reality, we are lucky if we can improve the power need from today’s A320neo.

Let’s give the LH2 Hybrid its best shot

To not be to negative, let’s assume we can by some magical means keep takeoff power at 30MW and cruise power to the fans at 10MW, a 25% or more improvement from today.

We then need a core of 10MW to complete the hybrid. Today’s PW1100/LEAP engines weigh 3t. Let’s assume 1.5t is the fan and the structure that we need in both cases. The 20MW turbofan cores are then 1.5t each (we need 2*20=40MW to get an A320/737 in the air). Our helper cores then weigh 0.75t each (we assume a linear scaling).

To summarize our Carbon fueled propulsive power to the fans weighs 2*1.5t=3t and the LH2 Hybrid’s power 5.5+0.6+0.9+2*0.75=8,5t (we don’t count the tanks or additional structure in either case, we just analyze the power section right now).

So we have LH2 Hybrid power that weighs 8.5t with a 60%*97%*97%=56% cruise energy efficiency compared with a PW1100/LEAP core of 3t that has a 53% cruise efficiency as of today (I don’t know where they got the 45% efficiency from, it’s not a present core).

As we can see it’s break-neck developments for a Hybrid power that 2035 is not more efficient than today’s PW1100/LEAP cores. And this assumes we can develop all the components for the Hybrid at the assumed weights and efficiencies. We are today at 1MW motors/inverters and aircraft fuel cells of below 1MW.

In the next Corner, we examine LH2 fueled Turbofans as alternative propulsion.

51 Comments on “Bjorn’s Corner: The challenges of Hydrogen. Part 8. Fuel cell electric or Turbofan propulsion?

  1. Yes you need to reduce TO-Power, one way is have folding wing tips for extreme wing span and a L/D close to sailplanes and you still have the operational cost of the whole system. Just having the fuel cell replace the APU and drive a fan to accelerate the boundary layer should be doable letting the turbofans/UDF’s drive the Aircraft similar to NASA-Boeing Trussed Brazed Aircraft.

    • OK, but folding wingtips increases weight further and we are already some 10-15t heavier on an aircraft type that has carbon fuel OEW of 40t. Wide wings on an aircraft doing 500-1000nm sectors are doubtful, weight is an important factor in such designs.

      • You need to do trade-off’s with less Engine Power available you need to lift-off at lower speeds even though you are heavy, poiting to a Boeing SUGAR type of slender wing and big UDF’s giving high thrust at low Aircraft speeds.
        They migh be less optimal once airbore and as you pick up speed at altitude but you need to get there.. so with a great L/D number thanks to the wings, low SFC from the UDF’s and burning all LH2 you filled up just as you reach top of decent trusting the batteries for powering the fan for any TOGA you are getting closer.

        • Bjorn: If I am tracking, 1 MW loss in the Inverter, but the motor has at least 10% losses (assumes an extremely efficient motor) . Gears 7%?

          Does this all account for hotel power (air condition, lights, galley? )

          • Bjorn was assuming 10% loss in the electrical to mechanical conversion path, so motor included. Real-world losses likely would be higher, but he is putting the best possible case forward for the hybrid configuration, as a check. If it won’t work for the best case, no need to look further.

            Hotel loads are not included, the fuel cell(s) and inverters would have to be sized 10% or more larger for those loads.

        • More lift for takeoff AND less drag during cruise….there was a turbine powered plane with the ‘ideal’ configuration for this , indeed the first jet passenger plane DH Comet.
          The wings were mostly straight rather than highly swept, which allowed it to take off for nonstop flight to London from Idlewilds old short runways. (The early 707 had to do a fuel stop both ways until the extra concrete allowed a full weight TO from New York). The buried jet engines reduced drag and gave a cleaner wing.

          • Maybe it’d make sense for an E-fan type of configuration where the engine and generator are enclosed in the wing, like the Comet, and then multiple propulsion motors/fans are located outside the wing. The engine core doesn’t need that much airflow and they’re much more reliable now so access isn’t as important as it used to be. Fuel burn is now less, so one doesn’t need as big a fuel tank in the wing. Total wetted area is probably less,so less high speed drag. Rotor burst still an issue though, we wouldn’t want such an event to take the wing off ….

          • “”Maybe it’d make sense for an E-fan type of configuration where the engine and generator are enclosed in the wing, like the Comet, and then multiple propulsion motors/fans are located outside the wing.””

            Yes, that’s a great idea, keeping much weight in the wings.

          • @Dave, yes having one engine mounted in a slimline nacelle driving Fans/UDF’s by shafts is a possibility, RR-Bristol got F-35 Liftfan shaft experience, but you wake up memories of the Bristol Brabazon with 2 engines each having a shaft together drivning one counter rotating prop as a bad omen.

        • Reconsider how?
          It was a great idea, but those engines 70 years ago were small.

          Wouldn’t the air intake instead of a nose be a good place?

          • Yes , more smaller engines would be needed but there is the efficiency advantage from LH2. The final Comet derivative the military Nimrod MR4 version had modern BR710 engines in the wings.
            The cruise power seems achievable , its the takeoff power thats a challenge, so look at methods that reduce that.

          • So maybe 3 small engines in each wing and in cruise switching 4 engines off? Maint might not be bad because 4 engines run only short time.
            But engines off might have lots of drag. Like the Hudson river landing, the engines fell down; engines in the wings and the wings would brake.

          • In the piston (and turbo prop) era I dont think engines in wings was a real problem for water landings, and they were more common.
            US Airways 1549 was a unique situation , the plane wing is so strong anyway especially if the engines are near the fuselage. If one wing dips and digs into the water then it will certainly tear off. The situation is still very similar to a gear up landing, the water is almost as inflexible as the concrete when you are at high speeds

  2. The BLI fan has only a small gain as well, for the weight that is added. Bjorn estimated it at a few percent in his Corner on BLI. So I would think the more-electric aircraft use of the fuel cell as APU is more justifiable right now, recognizing as Bjorn mentioned last week that the generator weight would still be needed.

    Although the engine load would be less in flight, the overall system would still be heavier, and so the fuel cell efficiency would need to be higher than the core to justify the weight. There would be some advantage as the APU is divorced from engine operation, it could be used for starting with a starter/generator, also potentially for performance improvement and rotor bow remediation.

    • Most likely will a fuel cell APU run the whole flight just to generate electrical Power for the Aircraft more efficient than the Engine+generator and powering the boundary layer acceleration fan-stage in the tail.

      • As I am reading the charts out on this, 30% less maleficent so in affect its a tax on fuel (for a goal, not necessarily a negative)

        But will the US, China and or Russia (if they ever get the4ir act together) go Kero and EU is wafting in the wind? (pun intended)

        Pretty much my take has been (pr below) maximize what you can on aircraft using conventional and make up for it on the ground.

        Just getting rid of all coal plants would more than offset aviation (even if you go natural gas).

        • YES!!! For reducing carbon dioxide emissions one should always look to the most cost effective ($ per ton avoided) and not to the coolest. Worldwide, which is the only metric that matters) coal is the biggest problem. China alone uses about 4 billion tons per year. That is twice the carbon mass of all the coal, oil and natural gas burned in the US.

          Replace coal with a mix of nuclear, gas, wind and solar (plus some storage) and you are about half way to a low carbon world. Hydrogen and electric aircraft ar a waste of time, effort and money.

  3. A320 and 737 have 36m wing span.
    Why less speed? Then propeller could be possible too instead of fans.

    I can’t follow the calculation completely but it doesn’t matter. No need to choose a hybrid.

    • Less speed reduces in-flight power. And the calculations are interesting. As are the assumptions. For example there is no change to the wings, and as others have posted, changes to the wings can reduce takeoff power needs, or cruise power needs, or both.

      • But if you have less speed you need more lift or you fly with more AOA.

        The A321neo burns 2.44t/h fuel at Mach 0.80 and 2,2t/h at Mach 0,76, but these examples are for low weight and a hybrid is heavy.
        So big wings would be needed.

  4. Great story. Could we collaborate? Several US groups have just won contracts to study SOFCs as range extenders for 737-sized electric airplanes. I agree that first application will probably be a fuel cell APU like we (Boeing) and Tech. U. of Munich studied several years ago.
    Cheers, Dave Daggett
    Fellow, U. Of Louisiana

  5. “The fuel cell weighs 5.5t with the assumption of 2kg/MW. ” Can you confirm the fuel cell assumption?

  6. Although I’m a fan of hydrogen propulsion for ground-based transport, it really seems to be a headache for aircraft propulsion. It might be better for the airline industry to just keep things as they are (kerosine-based), and finance atmospheric CO2 extraction facilities as a way of negating the associated CO2 footprint. There’s already such a facility in Switzerland: it supplies its CO2 to a soda manufacturer. We need more such facilities, in combination with storage in empty gas/oil fields.

    • Trees does the same CO2 thing with rain and sunshine. Hence massive tree plantations that are well managed (also with interrupting fire streets) of a mix of spieces can easily compensate for all aviation CO2 emissions. The US Midwest from Lake Michigan to The Gulf of Mexico were forests before the immigrants kind of took it and coverted it to farmland.

      • I love trees, but trees are actually horribly inefficient and slow when it comes to CO2 absorption. Also, they have the nasty habit of returning the absorbed carbon to the atmosphere when they burn or decompose. Remember that vast tracts of the Northern Boreal Forest are in areas of Canada, Siberia and Scandanavia where nobody lives, so there’s nobody around to put out fires. Huge areas of Siberia burned this summer.

        Romanticizing about ancient forest cover isn’t going to do much good, because the farmland that has replaced it is needed to feed people. And as long as people continue to indulge themselves in procreation, that need won’t be going away any time soon.

        • As an aside, despite farmland use, I have seen reports the US has not more overall forests.

          I too agree with Bryce, it seems a beyond able to do the Hydro thing for aircraft. Better clean fuel and more efficient engines.

          Ground based stuff has far less an issue as demonstrated by Hybrids.

        • If you manage the forests well you harvest timber for construction and just the residues are used for papermills or burned. Many countries does not enforce replantation after preparations hence losing years of growth and CO2 conversion. Of cause does trees grow pretty slow approx 8 to 20 m^3/acre per year but huge first generate good results and the residue is O2 from the forests. Of cause you need fire streets for harvest and stopping wild fires, also used for WRC rally car races.

    • These are ideas for 2050, at earliest.

      You cannot do experiments with CAT passengers and so AFTER (and if) the technology will be mature you will need some years of experience before replacing airliners engines.

      In the meanwhile what we have for 2030 or 2040?

      Natural gas?

      100% biofuel?

  7. Bjorn, I think you should write about CLH2, and why it works or why it doesn’t work in airplanes. CLH2 for Compressed Liquified Hydrogen. Not to be confused with Compressed Gaseous Hydrogen.

    • He has explained why compressed is out .
      “Hydrogen is the most common chemical substance in the universe. It melts from solid material to liquid at 14K (-259°C) and boils to gas at 20K (-253°C). It means, unless we want to store it under extreme pressure (which is not weight efficient for an aircraft), we need to store the LH2 between 14K and 20K in our tanks.”

    • The main issue is it requires cryo-compressed tanks, which basically need to have cryogenic thermal isolation, as well as pressure ratings going to 250 atm or so, about a third to half of compressed hydrogen gas. So the tank becomes more complex and heavier than those requiring either cryogenic (vented LH2) or pressure (gas) properties alone.

      The main benefit is longer storage time, as you can allow pressure to build up in the tank as opposed to venting. But there is a limit before you still have to vent, and you can get into supercritical states of hydrogen, which may add extra processing for filling or discharge. Hydrogen has a small liquid phase envelope.

      It’s not impossible, so maybe Bjorn can illuminate it further for us.

      • Since the LH2 isnt created in the liquid form- unlike jet fuel- it needs to have the temperature reduced below -253 C which consumes energy as well. Wont that have to be considered as part of the energy used to create the gaseous H2 .

        For a comparison of the compressed but gaseous version to LH2.
        At 700 bar ( 70Mpa), 5kg of H2 is stored in 125 litre tank
        At -253C and 1 bar, 5kg is in a 75 litre tank
        I dont know the weight of the 700 bar pressurised tank is but for rocket engines the tank weight is close to 20% of the weight of the LH2 inside ( Ariane 28 tonnes of LH2)

        • A certain rate of heat leakage and boil-off occurs in cryogenic LH2 vessels. This means there is a conversion of heat energy into gas volume. The expansion ratio is about 850 to 1.

          For a cryo-compressed tank, the energy goes into gas pressure instead, until the pressure limit of the tank is reached. Then the tank vents and the energy goes into volume again. With well-insulated tanks, this method can work for overnight storage, which can mitigate loss or the need for off-loading, but not for longer periods.

          Last week we discussed that the energy invested in LH2 liquefaction can be harvested as cooling, if applications can be found on-board, that match gas production to engine/fuel cell consumption. These could be superconduction, climate control, refrigeration, engine intercooling, boundary layer control, etc.

          Bjorn represented this schematically as a heat exchanger, and mentioned it could be in the pylon as is done now for bleed air. That might be needed for makeup or emergency use, even with other applications.

          The other alternative is LH2 injection into the combustion chamber, where engine heat provides the vaporization. That could have benefits as well, lower flame temperature and lower NOX formed by dissociation.

        • 95% tank weight for compressed storage.

          Numbers for compressed (700Bar) storage are in the range of 5% _useable hydrogen_ in relation to tank mass.
          5kg H2 -> 100+kg tank mass.

      • Thanks for the info. Do you know the weight penalty and volume advantage for Cryo-Compressed H2 vs LH2? How much extra fuel could one fit on an airplane, and might that be worth the trade-off?

        • There is a substantial increase in the amount of H2 that can be stored with cryo-compression (about 50% to 60%) over compressed gas storage. However LH2 is still 30% to 50% better than cryo-compression. So an LH2 tank still has the greatest storage mass per unit volume.

          The weight of the H2 stored in cryo-compression could range up to 10% of the total tank weight, as opposed to 5% for compressed gas, and 30% or more for cryo-only vented tank. Bottom line is:

          — Compressed H2 has least storage but least loss, with heaviest tank weight (at 350 to 700 bar), and can handle room temperatures.

          — Cryo-compressed H2 has greater storage and relatively low loss over short periods, with heavier tank weight (at 250 to 500 bar), and can handle a range of temperatures up to 120K, Above that, loss is inevitable. Tanks are designed to delay this temperature ramp-up for 8 to 12 hours, for a partial remaining fill after daily use.

          — LH2 with vented tank has greatest storage but highest loss, with lowest tank weight (at 1 bar), but must be kept at 20K, otherwise loss is inevitable.

          Thus cryo-compression may be the best choice for vehicles, with daily use and overnight storage capability prioritized over weight and volume. For aircraft where weight and volume are prioritized over loss (unused LH2 needs to be offloaded), LH2 may be best.

  8. Bjorn, I wanted to thank you for your contribution to the Dominic Gates article at Seattle Times, on 737 MAX safety. It’s good to have a clear and factual assessment from someone with experience and knowledge. That goes a long way toward helping people understand the issues and risks surrounding the MAX return to service. Leeham has always provided this, with both you and Scott contributing.

    • Is this in some way related to LH2 aircraft?
      Is the MAX suddenly going to be flying on hydrogen when it graces our skies again?

  9. Bjorn, I wanted to thank you for your contribution to the Dominic Gates article at Seattle Times, on 737 MAX safety. It’s good to have a clear and factual assessment from someone with experience and knowledge. In particular, thank you for labeling ‘the design flaws that caused the two 737 MAX crashes “absolutely unforgivable” ‘.

    • Bryce, thank you for also affirming all of Bjorn’s comments and conclusions in the article. The more people that can acquire that level of understanding, the better.

    • I don’t think the MAX follows regulations, therefore it can’t have safety standards, especially when it wasn’t normally certified before. We know that 96% were self-certified and certifying engineers were threatened to get the results. The FAA didn’t even check all Boeing self certifications till now.

      FAA also didn’t pick up JATR concerns too. For example the “novel” use of the stabilizer. Self-certified by Boeing and FAA keeps it under the rug.

      Bjorn didn’t check all Boeing self-certifications too, so how can he know. Like many others he was fooled before too but is not checking deeper. Bjorn is just a part of the industry and the MAX must fly.

      With Bjorn’s point of view future planes don’t need to follow all regulations too. So why even have regulations, especially when Jedi mind tricked self-certifications which were signed under mangement pressure are no problem.

      Certifying planes for public use is not about believe or considerations, it’s about prove.

      • Leon, I agree.
        Although the MCAS software MAY now be sufficiently safe (I still have my doubts, purely from a control-theory point of view), god knows what other “delights” are waiting under the MAX hood, as yet unknown to the flying public.
        We know that the MAX also has a FOD problem, for example (a Boeing whistleblower revealed that months ago). Has anyone checked the further manufacturing quality? Or will that only be done in 10 years time, as part of a “routine audit”? I hope the regulators are very hawkish on this: they should be, in view of ongoing shoddiness in the Dreadliner program.

        And, of course, the Chinese will be tempted to make a meal of recertification, seeing as it gives them enormous leverage in the ongoing trade dispute…and in view of their own, domestic NB program.

        Interesting weeks ahead!

  10. If a two engine a/c requires 40MW take-off power, then a similar three engined a/c will only need 30MW (any two of the 10MW engines giving 20MW) and a four engined aircraft likewise reduced (any three of the four providing 6.67MW each).

    Anyway, the point isn’t to be as efficient as today’s aircraft, but to not emit CO2.

    • Efficiency is always important in the airline industry, in view of the huge capital outlay and tiny margins.

      • Eyes on the prize. It’s unreasonable to write off new eco-tech on the grounds that it isn’t quite up to par with the status quo post 60 years development. Subsides, tax breaks, preferential routing & landing slots, penalties for polluters and much more has to be both considered and reckoned with. Bjorne can’t do this here, which is ok its not in his remit, but big picture you have to reckon, if you are a potential investor, with those things and the estimate of future political will. Example – aircraft noise; lots of politics and legislation led to noisy but financially viable old coal burners being retired before their time. The newer “more efficient” aircraft were less profitable (initially) but were forced on the airlines.

  11. Part 7 Part 2? This is the second post in this series that has been titled “The challenges of Hydrogen. Part 7”, the previous one was posted on 9-4-20. I have found this series interesting and informative, as Bjorn’s series always are; however, the nitpicker within me cannot refrain from commenting on the post numbering anomaly.

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