Bjorn’s Corner: The challenges of Hydrogen. Part 7. The fuel system.

September 4, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners we look at the rest of the fuel system after we looked at the hydrogen tanks over the last weeks.

The cryogenic state of the liquid hydrogen (cryogenic=very low temperatures, -253°C) creates some new challenges when designing the fuel system.

Figure 1. The Tu-155 Hydrogen research aircraft with its aft fuselage tank and fuel system. Source: Tupolev.

How to design the fuel system of a liquid hydrogen airliner?

The low temperature of LH2 forces some special solutions around the fuel system of an LH2 based airliner.

Tanks layouts with engine collector tanks and feeder storage tanks like today’s airliners have been analyzed in Airbus’ Cryoplane studies. It works if special care is given to tank form and its insulation to avoid heat leaks and the material choices and insulation of pipes, valves, pumps, and sensors are done carefully.

Special hydrogen leak detectors with appropriate ventilation are required for all areas where the fuel system is present.

The tanks are isolated low-pressure tanks as in Figure 2, with both LH2 (B) and H2 (A) insulated fuel pipes and a dedicated insulated LH2 filling line (D) to each feeder tank. The tank feeds the collector tank via the tank pump at E.

Figure 2. LH2 feeder tank. Source: Leeham Co.

There is no specific problem having a central filling receptacle for the LH2, with LH2 flowing concurrently to each feeder tank.

There is long-time experience of cryogenic tanks, pipes, valves, pumps, and sensors from the Technical and Space industry. The Space experiences must be further developed to suit the longer storage times of the airliner application (typically a 12-hour case without excessive boil-off due to heat leakage).

The leakage gas at A, when needed to be supplemented with gas from the heat exchanger at C, can be routed to a hydrogen fuel cell providing the aircraft with electrical power, both when stationary and during flight, thus replacing the APU and the engine-driven electrical generators. With the appropriate development of the efficiency of such fuel cells, they can provide sufficient electrical power to feed a more electric aircraft architecture.

With electrically driven De-Icing, Environmental Control System (ECS=Air Condition System), hydraulic pumps, and engine starters, the aircraft can gain efficiency by not tapping this power from the Turbofan engines or an APU.

The Turbofans can be optimized for delivering propulsive power, where the LH2 is supplied to the combustor injectors at the appropriate temperature via a heat exchanger in the engine pylon.

In the next Corner, we will explain why we use LH2 based Turbofans as propulsors for an A320/737 class of aircraft, instead of fuel cell-driven electrical propulsors as outlined in the EU study.

22 Comments on “Bjorn’s Corner: The challenges of Hydrogen. Part 7. The fuel system.

  1. The idea of using the H2 losses for a fuel cell, to offload engine electrical generation for a more-electric aircraft, is intriguing. The fuel cell would have to be quite large (2 MW or more) and there would need to be two of them for redundancy, if engine generators are eliminated. If the generators are retained for backup, the requirements ae lessened.

    • It would probably have backup generators on the engines as well, but they need not be active. I think you would like to see fuel cells in the 2MW class working reliably in this context before betting more on them like having dual for redundancy purposes.

    • Most IDG’s (Integrated Drive Generators) on commercial aircrafts are between 100-250kVA so 2ea 200kW electrical output fuel cells running when needed should be sufficient. The modern starter-generators are similar except the 787 with 4x 250kVA + 2 x 225kVA from APU.

      • The more-electric aircraft is expected to have higher electrical power requirements, for the newly electric features Bjorn mentioned. The number tossed around in various studies is up to 2 MW, so I used that here as a reference.

        The 787 already has 1 MW of generator capacity, although it is not all used at the same time. But it gives an idea as to what Boeing felt was required in terms of redundancy.

        • Think if you go for electrical cabin screw compressors instead of engine bleed makes a big difference in electrical load. Normally should an engine compressor have much higher efficiency compared to cabin air screw compressors driven by electrical motors and you only use as much engine bleed as required nowdays. So the benefit of cabin compressors that feed its air into “normal” and pretty expensive air cycle machines are not that easy to derive. I can agree that the tubing can be reduced and its pretty low pressure makes for sizable tubing network. As the engines have starter/generators you don’t need bleed air for engine start and hence the engine air offtake will be limited for just cabin air and others for wing and intake anti-ice.

          • The 787 uses a combination of compressors and ram air. The electric compressors can be efficient because they are variable speed and are optimized for the load.

            Bleed air is withdrawn from the engines at higher temperatures and pressures than required for the application, which represents an overall loss.

            The more-electric design substitutes a different form of loss in terms of power electronics & conditioning, but these losses are being steadily reduced, so in the end more-electric offers the potential for better efficiency than bleed air.

  2. Using the cold LH2 to cool Engine parts to can gain efficiency. On a rocket Engines where its exhaust nozzle are cooled by high pressure LH2 and after heat pickup be routed up to the fuel injectors.
    Just cooling the vanes and housing of the compressor in addition to HPT vane cooling could give good thermodynamic and parts life benefits.
    RR are working on it with Reaction Engines coolers for fighter engines.

    • The chance of using the cold LH2 for compact intercooled gas generator designs in something I’m sure will be investigated in due course.

      • Yes, todays engines compressor exit temperatures forces you to expensive powder metals in the rear end of the HPC like you have in the HPT and if I remember right does the compressor efficiency drop with raising temperatures. The HPT would like more efficient cooling but if might be to radical to input LH2 right into the blades & vanes and have it burn as it exits its cooling holes. More likely an heat exchanger of Reaction Engines model with LH2 cooling the cooling HPT blade and vane cooling air on its way to the burner, even the HPT Case cooling could be easier having access to cold LH2. Even the bearing chambers can be hydrogen cooled like it is on high power nuclear powerplant generators especially on SSBJ engines where bearing chambers cooling at sustained supersonic speeds can be a challange.

        • Isnt the burning temperature of H2 only 570C, well down on the temperatures in in the turbine. They would mix it with O2 from the air like JP to produce much higher temperatures

          • “570°C”
            Where did you get that number from?

            H2 in Air, open flame: 2100°C
            H2 in O2, open flame: 3050°C

            H2, O2 is one of the highest energy release applied to lowest mass reactions around.

          • Addendum:
            I see where you got that from.

            585°C is “Autoignition temperature” of H2 in Air.

          • The advantage of burning LH2 in air is its high flame speed hence the very hot flame can be really short and then mix in colder air to get an desired temp profile into the Turbine Nozzle guide vanes. A very short time at high temperatures reduces the NOX creation and hence the fuel nozzles can in theory be much simpler and cheaper.

  3. How would refueling time compare with jet fuel refueling? Just wondering if the same (relatively) rapid refueling with high flow rates can be achieved with LH2.

    When I’ve seen it done with Nitrogen, it seems that refilling liquified gas tanks is a considerably longer process than for regular liquids.

    • Studies of refueling LH2 transfer times have been made and are available online. The process is not as simple as pumping or gravity feed as for jet fuel, and the volume required is much larger, but with proper filling sensors & controls, the time can be comparable, or at least within practical limits.

      The fuel has to be added with less delay before takeoff, and offloaded at landing if there will be any extended or overnight delay. So those are some of the adaptations needed to the fuel handling process.

      A lot of learning required and certification safety standards to be established and met, before this could be implemented. Aviation is still not the best use of renewable resources, but it doesn’t hurt to consider it and do the research, to understand how it might be done.

      • Aviation is among the most high-tech industries around so putting new requirements onto aviation to solve, then old regular industry can copy the solutions for the big polluters when forced to: powerplants for both heat and electricity; large ship engines; concrete industry; trucking; steel industry and mining.
        So I think it is a combination of public and political pressure on the aviation industry with its large cash-flow and the image that aircraft industry suppliers are the most technologcal advanced there is and can solve it.

        • I hear what you’re saying, but I think the outcome is more likely to be the same as electric aircraft. In that case technology that is viable and cost-effective on the ground, was not viable or cost-effective in the air.

          So it makes sense to develop that technology on the ground first, as is happening in the automotive market. There is much low-hanging fruit on the ground to be had.

          Then if the technology begins to look viable for the air, that can be pursued. In the meantime the benefits on the ground are enormous, as compared to those in the air.

          All of this said, there is no harm in doing the research for aircraft utilization. It just shouldn’t be pursued in lieu of the easier availability & benefits on the ground. Ground first, is the best use of resources.

          • You are right that doing it for ground applications is easier, but with goverments and “The Rockefellers of the World” owning these industries and its distributions networks not much will happen until they easily and cheaply can copy the solutions from someone else (and charge for it) unless the problems get large enough (LA smog and car catalytic converters)

  4. Using hydrogen and possibly high-energy batteries might work for short and possibly medium ranges. But it seems to me the best way to preserve long-distance air travel is to use synfuels. This would allow us to use existing airplane technology, and the process would be totally carbon neutral.

    The downside is that until every oil refinery on the planet is either demolished or very carefully monitored, fossil fuels would creep into the supply chain and defeat the enterprise.

    https://www.nationalgeographic.com/news/2018/06/carbon-engineering-liquid-fuel-carbon-capture-neutral-science/

  5. Instead of using heat exchangers in the pylon (presumably using bleed air of some sort?), would it not make sense to use the cryogenic nature of LH2 as a precooler?

    There is some very interesting work being done with cryogenic-induced laminar flow, and combined with a Boundary Layer Ingestion engine you have the opportunity for significant efficiency improvements. Leaving the energy budget of a cryogenic fuel untapped seems like substantial waste.

    Another benefit is in the packaging. Like the diagram shown here, rear mounted engines in a BLI or ducted configuration along with a rear fuel tank gives you less overall plumbing (and it’s insulation, non-trivial for LH2).

    • Aircraft need the CoG to be in front of the CoL, for pitch stability. So if you put the major weights in the back, that presents a problem with traditional tube and wing. The TU-155 shown had the tank partially positioned over the wing.

      This problem is lessened somewhat if you go with a delta wing or blended wing-body design. But I’m sure there are tube designs that overcome this, they just might seem a bit odd.

      To generate the H2 stream for a 2MW fuel cell from LH2, will require a heat source. That could have many applications, engine intercooling as Claes suggests, or boundary later cooling as you suggest Also on-board refrigeration or air conditioning. The main thing would be to calibrate the heat sources to produce the needed stream rate of H2. The pylon heat exchanger could be a fallback or makeup if the other sources are not sufficient.

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