Bjorn’s Corner: Electric aircraft, Part 13

By Bjorn Fehrm

September 22, 2017, ©. Leeham Co: After 12 articles about electric aircraft, it’s time to wrap up. We will go through what we have learned and discuss future developments.

Our designs were aimed for the next decade and the result was sobering. Electric aircraft have important challenges to traverse. As had electric cars, and they have turned the corner.

Figure 1. Idea for future aircraft that could use electric propulsors. Source: NASA.

What we learned
The battery problem

We learned that batteries as energy stores leave a lot to be desired. Here a summary:

  • The battery stores 40 times less energy per kilo than Jet fuel.
  • While jet fuel gets consumed during flight, the battery weighs the same at take-off and landing.
  • A battery needs 20 times more space than jet fuel for the same energy content.

The inefficiencies make the battery virtually impossible as an energy store for longer range aircraft. In addition, the battery has four times higher maintenance costs than gas turbines; it needs replacement after 1,500 charge cycles.

But batteries will improve. The car industry is turning electric with force, and it needs efficient and lower cost batteries. This will result in batteries with improved characteristics.

The problem is that we are 40 times behind and batteries might improve two to three times in specific energy over the next decade.  We need at least 10 times to make longer range battery driven aircraft practical.

Fuel cells

Fuel cells hold the promise to solve the problem. But the promise has been there for the last 30 years and we are not closer to a breakthrough. Former US Energy Secretary Dr. Steven Chu summarized the problem: “We need four miracles to happen (for fuel cells to become practical for transportation) and Saints only need three:”

  • Production of hydrogen would be based on reforming natural gas, in the process losing energy content. Not ideal.
  • After production, there is no efficient way of storing the hydrogen.
  • As a consequence, there is no distribution system for hydrogen.
  • And the compact fuel cells for transportation use are not there

The problem with fuel cells is that they need a new eco system. Aviation cannot be the driving force to solve all these problems and the automotive world isn’t close to a fuel cell driven car.

Gas turbine hybrids

Hybrids in cars are successful because the electric motor driving the wheels can be reversed into an energy-recovering generator when the car decelerates for the stoplight. The wasted energy in a car’s journey is recovered. Electric cars can therefore compete on efficiency, if not range.

The airliner flight cycle doesn’t have such waste parts in its trajectory. The descent is done with the engines at idle, transforming the aircraft’s height energy to forward motion. The stored height energy is compensating for aircraft drag on the way down.

Comparing a gas turbine-driven airliner with a hybrid, where the gas turbine drives a generator which in turn drives a motor-fan combination, we found the hybrid is not competitive.

This is no wonder. We add complexity and weight, and unlike the car, we have no energy gains.

The peak power take-off phase could be where the battery complements the gas turbine, so it could be sized for climb/cruise. But we find the battery can’t be the power surge back-up, it’s just too inefficient.

Instead, we go for a small, light (and rather inefficient) gas turbine, an APU, to complement at power surges and for redundancy. The inefficiency is no problem, it’s only active a fraction of the mission time.

So now we have a solution, but it’s not a good one. The main power chain of gas turbine-generator-inverter-motor-fan is more complex, heavier and more inefficient than today’s gas turbine-fan combination.

If the battery was an efficient energy store, we would be better-off. We could charge it from the power grid during our ground stops, then use it for take-off. The APU could then fill it up again when in the air. But the batteries are too inefficient, weighing in itself more than the whole hybrid chain.

Future developments

Electric aircraft (and quad-copters) will make sense as urban commuters at first. They can be made quieter and more neighbor-friendly than gas turbine or piston aircraft (no noise, no fuel or exhaust odors).

Gradually, their practical range will increase. Battery swap systems will be developed to shorten turn times. It will also allow charging the batteries in an optimal way.

Once batteries can take part as an efficient energy source, hybrids can start to make sense for longer range flights.

A gas turbine that can spin at an ideal constant rpm can be made simpler (and by it, lighter/cheaper) and more efficient than one that has to modulate its power during the flight cycle.

The battery would cover power peaks and at the lower fan power need after climb, the turbine-generator can recharge the battery.

But it will take time until we are there. It might be that other technologies like fuel cells mature during this time and changes the picture.

Aircraft architecture gains

There is potential for more optimal aircraft architectures when the motor-fan unit can be made smaller/simpler. Propulsors can be placed more freely. But the gains are counted in percent. Perhaps one can ultimately achieve a 20-30% efficiency gain.

The problem we create with going electric is a 4,000% efficiency loss (battery versus jet fuel specific energy). So, while one happily accepts the architecture gains, they don’t solve the problem. Only battery development will (or a miracle or two on the fuel cell side).

Summary

There’s much hype around the electric aircraft. It can be instructive to look at the well-known hype versus progress cycle, Figure 2. It has proven it selves in many industrial developments.

Figure 2. Gartner hype cycle. Source: Wikipedia.

The hype is often a decade before the reality slump, then comes the time of real progress. We are somewhere at the first peak right now.

So, electrical aircraft will come, just not as fast as many thinks.

22 Comments on “Bjorn’s Corner: Electric aircraft, Part 13

  1. The future of fuel cells will be based on Methanol.
    First commercial applications are already available (see e.g. efoy.com).
    Methanol can be produced by biochemical processes from plants.

  2. I have to disagree that four miracles are required in order to substitute fuel cells for batteries;

    Production of hydrogen would be based on reforming natural gas, in the process losing energy content. Not ideal.
    Fuel cells can work with dry ammonia which can be split into nitrogen waste and hydrogen fuel by the application of heat and a catalyst. If the installation is in a hybrid configuration, the gas turbine can provide heat (that might otherwise be wasted – e.g. use it as an intercooler and/or recuperator) to split the ammonia molecules into hydrogen and nitrogen.
    After production, there is no efficient way of storing the hydrogen.
    Yes there is. It’s called dry ammonia! A fixed mass or volume of this stuff contains more hydrogen than compressed pure hydrogen and at far less pressure. Its far less flammable compared to kerosene though it can be used in a combustion engine, and would also power the gas turbine part of the hybrid configuration as well as the fuel cell.
    As a consequence, there is no distribution system for hydrogen.
    Yes there is. Dry ammonia is produced by the mega-tonne and is distributed widely around the world, and is transported in trains and trucks today principally for use as fertilizer, but also for many other industrial processes.
    And the compact fuel cells for transportation use are not there
    That’s true, but that only leaves one miracle rather than four. But its one I’m optimistic about, you’d think its much more do-able than a wafer thin plasma TV that ordinary people can afford and you know, you can get those at your local supermarket.

    Using ammonia as fuel would mean that the weight of the aircraft would decrease during the flight, unlike battery powered aircraft.

    And did I mention that if the dry ammonia is produced from renewable energy sources (nuclear, wind, solar, wave, geothermal) then it has almost zero carbon footprint!

    • This is all very true. Still there is one aspect that should not be overlooked: The energy density is only 1/3 of Kerosene, which renders long range flight impossible and significantly reduced productivity of short range flights.
      I expect that a new technology will have a break through only when all important factors/features/characteristics are equal or better than the old tech.

      • The energy density is way better than batteries.

        I would expect breakthroughs in membrane technology and perhaps catalysts to improve the rate of reaction in the fuel cell to be necessary, but I’m no expert in that field.

        Getting the fuel cell small enough, light enough and yet powerful and responsive enough would be problems more acute for aircraft applications than others. Perhaps the technology will be driven more at first as a standalone powerplant for domestic use, then ships & trains and even trucks & cars before its good for aircraft, which will likely be the last application. And I agree, short range commuters before long range aircraft

        I understand that fuel cell development is also driven by the requirements of submarines since they produce power almost silently.

        But keep your eyes on the prize, for the prize is huge – air travel without its carbon footprint! Much quieter aircraft! Local production of fuel from renewable resources, no reliance on unstable oil producers. Stable prices for fuel! What’s not to like?

      • Ammonia fuel density is apprx. 18 MJ/Kg vs Kerosene 43MJ/Kg, according to articles on the web.

    • The ancient method of producing methanol is pyrolysis of wood. This was primary method until 1900. Later more and more replaced by methods via syngas.

      There is also ongoing research to store surplus energy of solar and wind power as methanol.

      At normal temperatures ammonia is a very toxic gas. You should avoid drinking methanol.

      A hybrid aircraft could be powered by methanol fueled turbines during take off assisting the fuel cells. Cruise mode is fuel cell powered only.

  3. Today’s Aw Week has a RR chief technology officer Paul Stein analysis on electrical propulsion for short range small regional aircrafts that indicates RR belives it is getting feasible.

  4. In these 13 very useful articles, there was no description and analysis for parallel hybrid/electric propulsion configurations on aircraft. In the meantime parallel (automotive term) systems could possibly compete with TF propulsion by downsizing TF core by 50% or more on twins. Another winning factors are: 1. Low noise on take-off/climb and landing; 2. Low emission CO and NOx on take-off/climb and landing.

    • No, it’s a boundary layer ingestion concept. What drives the fans is not given, but it’s one of the developments discussed to increase electric aircraft’s viability.

      • Yes, it is a BLI concept. However, it was optimized with gas turbine technology.

  5. I believe the next all-new regional 50-seater will be a hybrid-electric with distributed propulsion and reduced size empennage. Range limited to one hour missions at first, with technology allowing for one pilot operation.

  6. Very interesting set of articles. I am more optimistic of an electric hybrid commuter/utility plane that competes with the Pilatus PC12 and its new jet sibling. A 9-12 seater plane has more empty weight/passenger carried headroom than a 200 seater commercial plane. Also electric planes have safety benefits that are important in that market. The issue is scaling up. It’s an order of magnitude more challenging to produce a competitor for a regional plane like the ATR or Embraer ERJs.

    • Yes, there was much hope for fuel cell cars but (https://en.wikipedia.org/wiki/Fuel_cell_vehicle):

      “In 2017 Daimler phased out of its FCEV development, citing declining battery costs and increasing range of EVs, and most of the automobile companies developing hydrogen cars had switched their focus to battery electric vehicles.”

      So battery technology, which is 40 times worse than jet fuel, is more promising than fuel cell technology.

        • Well here is another car manufacturer’s thinking, who has studied the problem at dept:
          Volkswagen’s Rudolf Krebs said in 2013 that “no matter how excellent you make the cars themselves, the laws of physics hinder their overall efficiency. The most efficient way to convert energy to mobility is electricity.” He elaborated: “Hydrogen mobility only makes sense if you use green energy”, but … you need to convert it first into hydrogen “with low efficiencies” where “you lose about 40 percent of the initial energy”. You then must compress the hydrogen and store it under high pressure in tanks, which uses more energy. “And then you have to convert the hydrogen back to electricity in a fuel cell with another efficiency loss”. Krebs continued: “in the end, from your original 100 percent of electric energy, you end up with 30 to 40 percent.”

          • At the end of the day what it matters is the price not the efficiency, for example if you generate the hydrogen from the excess of win/solar energy the efficiency is 40% vs 0%…

            But anyway, for the problem at hand, only matters the second part; if the energy density is better than the batteries, it will allow more range, less weight and maintenance (battery replacement) …

  7. Good set of articles, and I can understand fully why you’re being conservative here. I suggest everyone read this excellent paper by Mark Moore and Bill Friedricks at NASA Langley:
    https://arc.aiaa.org/doi/pdfplus/10.2514/6.2014-0535
    They tackle four misconceptions that people have about electric aircraft/ propulsion and do a pretty good job, imo.
    Among the things not discussed in Bjorn’s series of articles that I think are worth thinking about:
    – Electric propulsion as an enabler for novel technologies and the architectures: The D8 in the picture above uses boundary layer ingestion (BLI), which reduces fuel burn by up to 13% compared to a 737NG. Electric propulsion also enables distributed propulsion, allowing for more, smaller motors and reducing the overall size of the propulsion system for a given thrust requirement. (Of course, it comes with its own problems.) So, electric propulsion should not IMO be looked at as just replacing the two engines on an aircraft by two hybrid electric motor-fan systems, but rather as expanding the design space to allow for more possibilities.

    – Gas turbine technology is relatively mature, compared to (hybrid)-electric technology. With time and research and experience, electric propulsion systems will also mature. Bjorn’s last graph illustrates this point very well.

    – Superconductivity could be a game changer. NASA right now is working on a self-cooling superconducting motor for aircraft applications rated at 1.4MW with a specific power of 16kW/kg – this specific power is about 8x higher than the motors found in electric cars today. See this paper by Jansen et al:
    https://arc.aiaa.org/doi/pdfplus/10.2514/6.2017-4701

    So in summary, yes there are issues, but I see a brighter future than discussed here 🙂

  8. From somebody with a Honda Clarity Fuel Cell on order and two Hydrogen fueling stations close by, that Fuel Cell ‘Miracle’ looks a lot closer to reality than this article makes it out to be. Most notably the size of that fuel cell is now down to ICE size (only for Honda Clarity, not yet for Toyota Mirai). And Honda claims they have now figured out how to produce those fuel cells at affordable prices (when produced at quantity…). There are many other challenges left, of course.

  9. Thanks Bjorn for the series. Well done. Confining ourselves to electric and electric-gas turbine hybrid propulsion, you are right in that the battery technology must advance significantly and the battery specific mass must come down substantially before these kinds of propulsion can hope to compete with turbofans. However, as you pointed out, small commuter type aircraft for small distance travel could very well be electric for obvious reasons of minimal pollution and noise, in spite of their efficiency disadvantages. Thanks again.

  10. Great series Bjorn! If short range hybrid/electric commercial is >10 years out, what about a GA 4-seater, IFR certified, 4 hr endurance, 2700 gross weight, 800 pound payload, $19/hr operating cost, 55-120KT for $349K, see below link? I’m a 56 year private pilot that wants to buy <$200K airplane and fly for <$20 /hr, 4+ hr endurance, at 120KT when I retire in 2021. What is the % likelihood guys??
    https://www.aopa.org/news-and-media/all-news/2017/july/24/four-seat-sun-flyer-in-the-works

Leave a Reply

Your email address will not be published. Required fields are marked *