Bjorn’s Corner: Sustainable Air Transport. Part 17. Gas Turbine Propulsion

By Bjorn Fehrm

April 29, 2022, ©. Leeham News: Last week, we looked at the thrust generating device that aircraft propulsion systems use. We could conclude that independent of how we create the shaft power, we can choose different thrust technologies with desired characteristics. A propeller, open fan, or fan in nacelle covers different speed ranges and efficiency profiles.

Now we look at how we generate the shaft power for these devices. We start with the hydrogen-burning gas turbine alternative.

Figure 1. Airbus ZEROe hydrogen gas turbine turboprop concept. Source: Airbus.

The Gas Turbine alternative has a Fuel Cell

Figure 1 shows one of Airbus’ ZEROe airliner concepts. It relies on hydrogen-burning gas turbines to generate the shaft power for its propellers. Observe that it also has a fuel cell installed.

Once an aircraft has hydrogen as fuel, the first that happens is the gas turbine APU disappears. It’s designed to be simple, light, and small, as it’s only used to give the aircraft power during ground stops for cabin conditioning and supply of systems. It’s also used to start the main engines.

The simple design makes it noisy and inefficient. The efficiency is around 20% for APUs, which is very low. Thus it’s not used during flight other than as a power backup during takeoff and landing. As it’s noisy, it’s placed in the aircraft tail, far away from power consumers. It makes for long power conduits in the aircraft.

A hydrogen fuel cell is ideal for replacing the APU to deliver electric power and heat during the cold season. It’s quiet, reliable, and has twice the efficiency of the gas turbine APU. It makes a placement close to the consumers possible, in the middle of the aircraft.

It can be sized as an auxiliary power source like the APU but also scaled up to help the main gas turbines run more efficiently. How we describe in the next Corner.

Hydrogen burning Gas Turbines

We have learned that gas turbines can use hydrogen as fuel. It’s injected into the combustion chamber in gas form through modified injectors, burning at leaner mixture ratios than kerosene, Figure 2.

Figure 2. Possible mixture ratios for hydrogen and kerosene combustion. Source: Airbus Cryoplane study.

The lean mixture burning at lower temperatures leads to cleaner combustion with lower emissions, Figure 3.

Figure 3. Emissions from kerosene and hydrogen gas turbines. Source: Airbus Cryoplane study.

The above results in a cleaner gas turbine, emitting no CO2 or CO emissions, no soot, and about five times lower NOx emissions.

As previously discussed, the issue is the increased content of water vapor, where it’s not clear how large a problem this can cause. Because the exhaust contains few condensation nuclei, the formed ice crystals that build contrails will be larger and fewer.

We will need flight trials to know precisely how big a problem this is and the ways dangerous contrails can be avoided (by e.g., altitude changes) or mitigated (by engine changes).

As we discuss advanced ideas around hydrogen gas turbines in the next Corner, we will look at new ideas around water vapor emissions.

With a straight conversion, the hydrogen gas turbine has the same efficiency as today’s Jet-A1 or SAF burning versions. With liquid hydrogen onboard, there are several possibilities to do a more advanced engine,  both in environmental compatibility and efficiency. We look at these in the next Corner.

The advantages of the gas turbine shaft power alternative are:

  • It’s known technology. We have 80 years of gas turbine developments to build on. The change to H2 burn is straightforward; it has been done since the 1950s.
  • It keeps the gas turbine’s very high power to size and weight ratio, and we have extensive knowledge on how to package the turbines into our airliners.
  • The problematic technology for a gas turbine-engined hydrogen airliner is not the engines; it’s the tank and fuel system, something it shares with a fuel cell and electric motor alternative.

The drawback is an industry drawback. The 80 years of gas turbine experience for airliners is concentrated in a handful of OEMs. For these to convert existing engines to hydrogen is a modest job. But the development of an H2 gas turbine in a new power class (up or down) is the same billion-dollar project as for today’s engines. The industry is constrained to working with the existing OEMs for the gas turbine alternative.

A fuel cell plus electric motor alternative lowers the barrier for conceiving new propulsion alternatives, as we will see in coming Corners. It does this while presenting its unique problems, however.

33 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 17. Gas Turbine Propulsion

  1. If LH2 water emissions condensate to big droplets its mass might be enough for rain droplets falling to ground. The A380 trail should be done with engine #2 shut off not to create soot particles close to the LH2 burning Passport engine for steam emissions to condensate onto. It will condensate onto the remaining ones but further downstream

    • -Most of the waste heat in a fuel cell that must be rejected is in the electrolyte. Relatively little is in the exhaust stream. That suggests the exhaust stream will be relatively easy to cool and condense In fact the exhaust of the Toyota Mirai needs a water tank to collect the condensation much of which forms simply in the exhaust pipe. It can be released at the kerb by the push of a button. I assume water flying out of the exhaust its annoying to other motorists.
      -With ambient air at -20C at 20,000ft it should be possible to collect all the water and dump it in batches that reach the ground or to freeze it into small hail stones (say 3 mm diameter) that reach the lower atmosphere.

      • For fuel cells I also think water waste is managable, but for LH2 burning gas turbines at 30 000′ the problem is maybe harder to solve.

        • “new problem water waste”

          Hydrogen:
          2 H2 + O2 -> 2H2O

          Kerosene:
          C12 H26 + 25 O2 -> 12 CO2 + 13 H2O

          There is quite enough water vapor in fossil Kerosine combustion.

          • H2:
            1MJ thermal needs 7g H2 produces 63g H2O
            Kerosene:
            1MJ thermal needs 22g Kerosene prooduces 26g H2O + 66g CO2

            napkin math.:-)

          • I think @claes and Bjorn may have been alluding to:

            50 H2 + 25 O2 -> 50H2O (using the same amount of O2 as in your kerosene reaction).
            50H2O is an unpleasant surprise when you were used to 13 H2O .

  2. I know from previous Bjorn’s Corners (and it’s repeated in the present article) that electric motors have shortcomings relative to gas turbines — but it would still be nice if motors could be used to replace turbines in even a small segment of the commercial aviation fleet. The high costs and long waits associated with new turbine development have become a major headache for aircraft OEMs — as well as representing a huge risk for the engine manufacturers.

    • Its more than just ‘shortcomings’. They are practically unusable in the
      next 10 years in the commercial sense unless we want to return to the early 1920s when small planes flying small distances WERE commercial aviation
      Feb 1919 Farman F.60 Goliath converted to carry 12 passengers from Paris to London

        • The point is even with latest tech it’s only workable for say 12 seater on shorter distances
          The Goliath plane was powered by a low octane radial water cooled petrol engines not electric

          • My comment above specifically referred to “even a small segment of the commercial aviation fleet.”

            Regarding new tech:

            “One of the major challenges of scaling up electric propulsion to larger aircraft is the power-to-weight ratio. In other words, today’s electrical systems simply do not meet the necessary power requirements without adding excess weight to the aircraft. But high-temperature superconducting technologies are emerging as a promising solution to this technical conundrum, notably by increasing power density in the propulsion chain while significantly lowering the mass of the distribution system.

            “This is where ASCEND comes in. The three-year demonstrator project aims to show that an electric- or hybrid-electric propulsion system complemented by cryogenic and superconducting technologies can be more than 2 to 3 times lighter than a conventional system—through a reduction in cable weight and a limit of 30kW/kg in power electronics—without compromising a 97% powertrain efficiency.”

            https://www.aero-mag.com/electric-hydrogen-propulsion-aircraft-16042021/

          • Its not an issue about ‘more efficiency’ from the electric motor turning energy into propulsive effort. They are already extremely high efficiency.
            Its about the energy storage on the airframe to provide for the flight distance and payload.
            Thats why we are back to say 12 seaters and short distances. Unless you solve the energy storage problem then even 50 passengers on a turbo prop for 300km is out of reach.
            Your system for ‘saving weight in cables’ doesnt address the large weight to store energy for flight and payload using foreseeable battery tech
            Sounds like they can get an extra 25 km range , who knows as they dont quantify the real world advantage as range-payload other than ‘lighter than existing cables’

          • @ DoU
            Who’s talking about “battery tech” (except you)?
            The current series is about hydrogen.
            It’s possible to use electric motors in conjunction with fuel cells and hydrogen.

          • Fuel cells for main engine power in airliners has already been dismissed by Bjorn
            This story mentions it’s minor use as APU only.
            Time for some revision of previous ‘classes’ on these topics by Bjorn

          • Fuel cells haven’t been “dismissed” by Bjorn at all: he merely pointed out that they are less attractive than gas turbines from a power point of view. On the other hand, they have the advantage of producing zero NOx emissions, and allowing capture of produced water.

            They haven’t been “dismissed” by Airbus, either.

            https://simpleflying.com/airbus-build-hydrogen-engines-in-house/

          • ‘As for the battery-based electric or hybrid airliners, we find the promotors of fuel cell propulsion are far off the reality, even when we use their most optimistic data and leave all spiral consequences of the inefficiencies we find by the side. Why is this?’
            https://leehamnews.com/2021/02/12/bjorns-corner-the-challenges-of-hydrogen-part-24-propulsion-choice

            ‘Far off reality’ is what Bjorn establishes.
            APU is what Airbus is looking at ‘as a supplement’ to the GT for the planes electrical power. Not the main propulsive effort

          • “In a strategic partnership with automotive systems supplier ElringKlinger, Airbus is investing to mature fuel cell propulsion systems for the aviation market.”

            “While hydrogen fuel cell technology to power alternative-propulsion systems is still new to aviation, cross-industry collaboration, like the strategic partnership between Airbus and ElringKlinger, will be essential to maturing the technology’s potential in the years to come.”

            https://www.airbus.com/en/newsroom/news/2020-10-hydrogen-fuel-cells-explained

          • Covered that . The fuel cell is the APU replacement .
            They even say so
            ‘In fact, Airbus’ ZEROe concept aircraft is expected to use hydrogen fuel cells to create electrical power that complements the modified gas-turbine engines,”

            “Still far off reality “. seems that applies to some comments as well.
            This spells it out
            ‘The values for the fuel cell airliner are almost three times those for the gas turbine variant. It could perhaps make sense if the efficiency is considerably higher. It isn’t.

            The GTF core efficiency at cruise is 55% (GasTurb simulation data), and burning hydrogen can only improve that. The above fuel cell alternative has an efficiency of 55% with the data given in the report. This is without counting losses in the distribution system.”

          • @DoU
            You evidently missed the phrase “propulsion systems” in my post — it’s used TWICE.
            An APU is not a propulsion system.
            Bjorn’s past comment relates to *current* fuel cell technology; the Airbus link that I posted related to *future* fuel cell technology.

          • Gas turbine APU efficiency is abysmal. Especially for loads below full power.

            IMU a battery backed fuel cell APU makes a lot of sense as efficiency is retained even with lowish loads.

          • Doesnt matter about ‘future propulsion’ systems either

            Bjorn establishes they are far off reality.
            Even the airbus project for a hydrogen powered GT with a fuel cell APU is over 10 years away. And in the way of the aviation world mostly likely even longer.

            The fuel cell propulsion is theme is essentially green washing as they in theory still need LH2 for takeoff/climb phase

            ‘The EU aircraft is a parallel hybrid with 5.5MW electric cruise motors and 14.5MW gas turbines in parallel for takeoff and climb”- Bjorn

            Its an absurdity as the complexity doesnt add up even as a paper project. Small improvements in fuel cell tech are like those of batteries, incremental and no where near the orders of magnitude required

          • @Dukeofurl, the majority of conversions of turbo props to fuel cell powered electroprops using gaseous hydrogen are using ATR72, Q300 and D328. Passenger count drops by about 10% but is still around 56 passengers.
            Electroprops need Half the power of electro fan jets and this alleviates the need for high density fuel cells, electric motors and cabling.

  3. “For these to convert existing engines to hydrogen is a modest job. But the development of an H2 gas turbine in a new power class (up or down) is the same billion-dollar project as for today’s engines.”

    If I understand the above comment correctly, switching an existing engine such as GTF or LEAP to run on liquid hydrogen is relatively simple/inexpensive but it becomes a billion dollar project to build a clean sheet “next generation” hydrogen turbine, just as it would be a billion dollar project to build a next generation kerosene turbine.

    • A billion, peanuts.
      As long as I have been alive (a very long time) people have been proclaiming how simple it is to produce hydrogen from water. If it was really that simple to produce and distribute, it would have been done by now.

      • It is simple…but it’s endoergic. And, in today’s world, energy expenditure increasingly needs to be green.

        If you have a solar panel on your roof, you can start making green hydrogen tomorrow.

      • It is simple, it just costs more than using hydrocarbons. We have optimized the production and distribution of hydrocarbons.

        We can search for crude 1000’s of feet underground or below an ocean. Drill down, pump it up, store and transport it halfway around the planet, refine it and then distribute it to consumers selling it for about the price of a quart of water and still make huge profits.

        • The drilling heads often cost $600,000 each and are made of same alloys gas turbines in jet engines are made of. They have tungsten and diamond cutting heads. Within them are inertial navigation systems with accelerometers, gyroscopes and tiltmeters that can measure the position of the head which can be drilled sideway for maybe 10 kilometres.
          If fracking is used shaped charges blow holes into the earth about ever 1.4m and 1.5m deep and water is pumped in. You can see the pressure go up to hundreds of bar on dial gauges and then relax as the earth fractures.
          production of electro fuels and hydrogen will become cheaper. Main thing is to get started so that we get experience.

      • It is hard to do cost effective with optimal catalysts. Lots of research on new catalysts to reduce electrical power required. Still H2 from natural gas “Blue hydrogen” cheapest.

        • 10 %of payments to UK windfarms are “constraint payments”, paying them not to produce electricity when there is too much. Effectively this electricity is cheaper than free electricity. It’s still not viable to store in the form of hydrogen, or someone would have done it.

          • Wind turbines have brakes. Ive seen wind farms with say half turning and the other half not. Thats solves the generation matches the demand problem.
            if you are going to bring a a new demand for power they will pay like any one else as those braked turbines can be released.
            it seems that no one wants to pay the true price of green hydrogen from only renewable sources , even when taking it off peak

          • The bulk of electricity production in the UK is probably Combined Cycle Gas turbine and peak load gas turbine both of which can despatch power fairly rapidly when wind power cuts out or to meet peak demand. These turbines in theory could be either modified to run of hydrogen or replaced by ones that ran of hydrogen.
            The problem, as I see it, is that there simply isn’t enough hydrogen around to make this viable to quite justify the effort and the reason for that is that there aren’t enough renewables around yet.
            I suspect it might be possible to blend in hydrogen up to say 20% in to the natural gas going in to the power station thereby at least ensuring this excess wind power is utilised. Hopefully this can be done with few modifications to the turbines apart from control settings.
            The rough figures I read on the Norsk Hydro site was that an electrolysers capital and running costs add 30% to electricity costs. Not every precise but best I’ve got. For reference the additional transmission line costs of renewable add 80% to the cost of power compared to conventional.
            I think we are at the cusp of hydrogen becoming a viable option to handle excess green energy. If combined with blue and even some brown hydrogen it will start to work.
            Obviously batteries are much more desirable (nearly 50% efficient) compared to hydrogen (at most 50%) but its better than nothing,

      • There is a long history of electrolysis of water to produce hydrogen.
        1 Norway: Norsk Hydro in the 1930s, was used to produce hydrogen for production of ammonia.
        2 Canadian Hydro in the Same time period also for production of hydrogen for production of ammonia by the haber-bosch process.
        3 Aswan High Dam in Egypt in the 1950s/60s also produced hydrogen.
        -These used alkaline electrolysers that could produced hydrogen at an efficiency of 60% to 70% (depending on load) at 15 Bar.
        -Norsk Hydro continued to sell electrolysers of a modest size to this day for companies that need a hydrogen supply (margarine makes, welders, certain industrial processes) but don’t want the inconvenience of bottled delivery.
        -Modern PEM electrolysers can produce hydrogen at 700 bar at 80% efficiency.
        -Most hydrogen is produced by reforming of CH4 natural gas into hydrogen. This is of course cheaper.
        -The costs are mainly in the electricity not the electrolyser.
        -The way forward in the short term for Europe and the Free world while avoiding, OPEC, Russian Oil and unsavoury producers is the production of Blue hydrogen through gasification of coal and the sequestering of the CO2 by-product underground. (Plenty of places such as deep aquifers and old oil wells).
        -Synthetic Natural Gas can also be made from coal with the CO2 similarly sequestered in the same plant if desired.
        -Thus blue hydrogen and renewable green hydrogen can be fed into the same pipeline. No CO2 is emitted to atmosphere.

        When synthetic fuel made from coal was a viable option in the 1980s before greenhouse warming concerns the oil price typically fluctuated between $25/barrel to $70/barrel. The cost to make coal into fuel with a profit was about $40-$50 barrel. The problem was that anyone who invested a billion dollars in a large plant with a 15 year life near a large coal mine would simply be undercut by the oil producers. Wouldn’t it be nice to however cap their top prices?

    • A billion dollars was the price for the PW2000 engine project for the 757 back then. Now it cost more.

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