Bjorn’s Corner: The challenges of hydrogen. Part 27. Fuel cell APU gains

By Bjorn Fehrm

March 5, 2021, ©. Leeham News: We have discussed different auxiliary power generation principles for a hydrogen aircraft over the last weeks. We found a fuel cell auxiliary power system has many attractions, one being the possibility of making an elegant more-electric aircraft system architecture.

With or without such an architecture, the fuel cell alternative will save hydrogen consumption and cost compared to a hydrogen-converted APU alternative. What’s the value of the saving?

Figure 1. The Ballard/Audi FCgen-HPS fuel cell stack for cars and other mobility applications. Source: Ballard Power Systems Inc.

Hydrogen fuel cell APU gains

As we went through the different Auxiliary Power Unit (APU) alternatives for a hydrogen aircraft, we could see a fuel cell APU alternative had an efficiency about double that of a hydrogen-converted gas turbine APU.

We first calculate the gain from running a fuel cell APU instead of a hydrogen-converted APU during ground stops.

The APU is active during the landing (as a power backup), taxi in, the stop, and take-off. The total time per flight is 1 hour. The A320 APU fuel consumption is typically 120kg/hour during ground operation. We then assume 40kg hydrogen consumption for the gas turbine APU and 20kg for fuel cell APU, which give a daily consumption of 320kg versus 160kg with eight stops per day. Yearly consumption with 350 operations days results in 112t versus 56t hydrogen.

With a future hydrogen price of $3.00/kg (from EU report), we have a saving of $168,000 per year ($336,000 vs. $168,000) for the fuel cell alternative. For the 20 year lifetime of the aircraft, it’s a saving of $3.36m. For an OEM with an assumed production run of 2,000 aircraft (a reasonable assumption for a first-generation hydrogen airliner), his customer base cost gain is $6.7bn.

Gain with a more electric aircraft architecture

We now calculate the added gain, should the aircraft employ the more electric architecture we described here.

A typical yearly fuel burn for an A320neo class aircraft is 8,400t of Jet-A1. With a direct conversion of the A320neo engines to hydrogen, we assume an annual hydrogen consumption of 2,800t (in reality, the hydrogen aircraft has slightly different fuel burn due to changed weights, etc., but we ignore this here).

This gives us a yearly fuel cost of $8.4m, and a more electric architecture saves $84,000 for every percent gain in fuel consumption. We previously assumed a 2% lower fuel consumption with a more electric architecture, so we have a yearly cost saving of $168,000, or $3.36m over the aircraft’s lifetime.

For an OEM, the calculation is an additional customer cost gain of $6.7bn based on his 2,000 aircraft production run.

Conclusion

A fuel cell APU creates a cost-saving of $3.36m over an aircraft’s active life compared with one that is based on a hydrogen-fueled gas turbine. If we add a more electric system architecture, we can double this to $6.72m.

For an OEM with a 2,000 aircraft production run, the development of a fuel cell APU aircraft with a more electric system architecture could cost $10bn in additional development costs (which is way over what it should cost), and it would still be a good business case.

29 Comments on “Bjorn’s Corner: The challenges of hydrogen. Part 27. Fuel cell APU gains

  1. Bjorn, sorry to deviate from the subject of APUs.
    Against the backdrop of news this week regarding financial investments in the Boom supersonic plane project, might it be an idea to devote one of your articles to the potential use of LH2 to power a Boom-like aircraft?

    • Hi Bryce,

      I have written a lot about the Boom supersonic project and its major engine problem in previous Corners (search for Bjorn’s Corner and Supersonic transport revival, Part 14 is about the Boom engine). This problem is only exarbated if we also require it to be a hydrogen-fueled engine.

      The announcement doesn’t say how much AMEX ventures is investing. Boom needs north of $10bn to realize the project, let’s get excited when they’ve passed a billion in investments (to my knowledge it hasn’t passed $0.1bn) and have an engine partner that is prepared to develop an engine that passes noise regulations and can have a reasonable SFC at Mach 2.2 (which is close to mission impossible without a variable cycle engine, and it doesn’t exist outside DOD experimental projects at PW and GE).

      • Bjorn – I read those articles with great interest and one of the key takeaways was that for high supersonic applications turbojets are inherently more efficient than medium and high bypass turbofans.

        the downside of course is takeoff noise and subsonic fuel efficiency.

        with all the work done for the F-35 developing a flight rated clutch system for the lift fan, I would think that you could have the best of both worlds (modulo the weight/complexity penalties) by putting a clutch on the fan and using variable intake geometry. the aerodynamic penalties of the large diameter engine housing could be mitigated by integrating the engine and intakes into the rear of the fuselage rather than on the wings….

  2. I think the LH2 boil off needs to be added. The LH2 narrowbody aircraft is most likely defueled after a days work but need some LH2 to remain for the next morning APU start and running when towed to the gate with APU on for electrical power heating/AC. Hence every night there will be LH2 boil off at outdoor parking or in the hangar.
    If one assume the LH2 tanks can take some internal pressure, the fuel cell APU could light up and run a compressor to fill a H2 tank “Like a Toyota Mirai composite overwrapped tank” to lower the LH2 tank pressure down to desired pressure level and avoid the defueling cost.

    • You should consider the boil-off in a comparison of carbon fuel to hydrogen fuel APUs.

      Here the comparison is hydrogen fueled gas turbine versus fuel cell APU. Boil-off is a tank design-related issue, it will not depend on the hydrogen APU architecture.

    • Boil of is of course not an issue during flight but I suspect that at an airport the aircraft will be connected to a system to capture boil off. If no such facility is available the boil off will be channeled to the aircraft Fuel Cell and the excess electricity sold to the airport. The BMW 7 series hydrogen car would begin boil of after tank pressure reached about 6 bar. This was about 18 hours (worst case) upon which the hydrogen would be burned in a catalytic burner. I think the parameters on an aircraft will be similar. On board reliquification shouldn’t be ruled out. Aircraft LH2 tanks may handle quite a high pressure with composites so maybe boil of won’t happen so quick.

      • A car gas tank contains much less fuel than a gas tank of an airplane and with volume to the third, area squared so boil off in an airplane should much less of a problem.

      • William, that is one of the main questions of how high pressure the LH2 tanks will be certified to and its safety valves set to. The simplest solution is double wall thermos Al-Li friction stirr welded structures, a bit like Space X and NASA Orion. Using the gas bottle, composite overwrapped pressure vessel, like the Toyota Mirai and rockets helium pressurisation tanks you can pretty easy make them take 70-100 bar overpressure (depending on diameter) to avoid boil off and not needing high pressure fuel pumps that might not like a mix of LH2 and H2, but every bar adds mass. So the carbon fiber wrapping will be on the inner pressure vessel and the vacuum chamber outer wall be one aircraft fuselage section maybe in ceramic due compressive loads and low heat transfer. We will see what solutions they allow to be certified.

        • You can’t compress a liquid so high pressure is useless for the liquid phase and above a certain pressure you don’t get a clear transition between liquid and gas phase. I have always assumed that there are technical advantages to keep the LH2 below that pressure. Also the gas part of the tank is minor and the density is much lower so it will only contain a small part of the fuel. Only advantage is boil off but that is not a problem during operating hours. The tank is large enough and the fuel is cold enough to be able to not have boil off for the first few hours after tanking

          • The gas phase in the tanks is compressed by letting the boil off from the LH2 not being vented under a certian pressure. That also forces the fuel truck deliver the LH2 above tank pressure. The defueling of LH2 is pretty quick with the help of tank pressure. Not letting the tank pressure cycle too much help with tank life.

        • BMW were talking about boil off occurring after as much as 8-10 days and certainly 2-4 days with a 15 minute drive to reduce pressure was claimed achievable.. One technology was using magnets to suspend part of the inner vacuum tank. They also did work on irregular shaped tanks suitable for smaller hatchbacks. One technology to collect boil of would be the kind of lithium titanium metal hydride used in the German Navy’s Type 214 submarines. It’s a very safe material that stores hydrogen at 2 x higher volumetric density than liquid and at low pressure. Unattractive for automotive due to cost of materials and measures needed to prevent oxygen contamination these issues are not a factor in aviation. The metal hydride might even be next to the fuel cell and allow ultra long term storage of hydrogen for the APU for an defueled aircraft.

          Tanks capable of high pressures to prevent boil off are also strong crashworthy tanks. I would assume zero or low pressure on takeoff as one wouldn’t want a rupture spreading LH2. On the other hand pressure stabilizes the structure.

          • A chemistry professor I had about 30 years ago said he knew a guy at Dow or Dupont who worked on metal hydrides, which would react with water to liberate hydrogen gas. The colleague claimed they could mass-produce calcium hydride for 2 dollars a brick, and the brick could power a car for 200 miles.

            There was also a guy on Usenet who promoted his “PowerBall” concept of plastic-covered sodium hydride balls that would be cut open and submerged in water whenever hydrogen was needed. So I’ve been wondering for awhile whether solid metal hydrides could be viable for land and air transportation.

            http://web.archive.org/web/20040603191630/http://powerball.net/process/index.html

          • Aviation has to deal with crashes. Making a container that survives a crash so the crash site isn’t light up by the lithium titanium metal hydrids is hard. but it could be a solution for the real emergency fuel supply.

            But high pressure tanks go hard when they go and boil off is not a real issue.

  3. It’s quite remarkable that there are such low hanging fruits available for reduced emissions. Another low hanging fruit is electric taxiing, which Safran believes can save 4% fuel consumption over the life of an aircraft; not to mention less engine FOD damage, less noise and fume emissions, and much smoother ground operations. Another benefit is a more pleasant working environment for ground service crew who will have to enduce less noise and smelly fumes.

    Engines are least efficient when the aircraft is moving slowly.

    • There are electric push back tractors that are becoming popular. With electrical motors on some wheels you add mass unless someone comes up with electrical brakes/motors. In theory todays aircraft brakes made up or rotors and stators that are mechanically forced together instead of electromagnetically coupled. If made as electrical rotors/stators it can convert the brake energy to electrical power and use it to make LH2 from some of the water produced by the fuel cell?

      • The electric ground taxiiing traction motors are optimized for a speed of about 20kmh at maximum motor RPM. Having them handle a landing at 250kmh with probably 50 times the power level seems unlikely in a light weight structure. There are PEM fuel cells that can work as electrolyser in reverse.

      • “If made as electrical rotors/stators it can convert the brake energy to electrical power and use it to make LH2 from some of the water produced by the fuel cell?”
        The British have a term for that engineering approach- Heath Robinson, the US equivalent is Rube Goldberg

    • Yes, it advertises the type of power density that I have used in the calculations, 2kW/kg.

  4. Bjorn,

    Your analysis ignores the time value of money. Nobody would invest $10 billion today in order to generate $10 billion in savings over a 30-50 year timeframe. They might be willing to invest $1-2 billion depending on the minimum internal rate of return they expect.

    • Sure, you need to do an NPV or similar analysis.

      The development cost of such a system would probably be in the region of $1bn (fuel cell with systems and packaging and the system components for a more electric aircraft) but you can use the technology for more projects than the first hydrogen aircraft, so worth it.

      • And getting a turbine APU rated for H2 is also not free. A fuel cell APU is also so low hanging, obvious stuff for a H2 airplane that using a H2 turbine APU would be seen as an obvious only green-washing, not serious project which makes creating buy-in hard.

        This also assumes that a bleed air system is possible for a H2 airplane. If it has a too high a risk of explosions because of high H2 concentration in the bleed air during engine problems than the cost for a more electric aircraft don’t hit the fuel cell development as it is money you have to spend regardless of APU choice.

  5. A vehicle fuel tank contains considerably less fuel than a fuel tank of a plane and with volume to the third, region squared so bubble off in a plane should significantly less of an issue.

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