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Could hybrid-electric aircraft rule the skies?

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Companies are proposing a hybrid airliner with electric motors, a smaller battery, and a liquid fuel powered generator.

One advantage of this design is you can get the redundancy that safe flight needs a different way. Today all commercial airlines have 2 or more liquid fuel engines. They can still fly if they lose one.

A hybrid would have two or more electric motors, and the battery pack would be split as well, but it might need only one generator. If the generator fails, the small battery would still have enough range to get you to an abort airport.

This would not be suitable for flying over the ocean more than a few miles, so it would be limited to short-haul craft. The one in the article is for only 700 mile range. As long as it stays within 100 miles of an airfield, it would be able to survive loss of its generator.

Electric batteries today just weigh too much. Liquid fuel has many times the energy density of batteries, so they don't work very well for aircraft. An additional downside is that batteries weigh the same full or empty. Today's jets weigh a great deal less when they land than they did when they take off. On the other hand, electric power is cleaner, more versatile and more reliable. As the energy density of electric storage increases, the trade-off line will be crossed.

The hybrid design, however, offers the energy density of liquid fuel with many of the advantages of electric power. The need for only one generator can save a lot of weight, but for now the reserve batteries could cancel that saving. But consider some of the advantages:

  • The high torque and efficiency of electric motors is in general great for take-off, which is the only time that aircraft engines use their full power.
  • The scariest situation in any aircraft is a loss of power on takeoff. Electric motors are very reliable, and loss of the generator would cause no loss of thrust on takeoff. The aircraft would be able to climb and return to the runway.
  • It can make sense to have many smaller electric motors and props without much efficiency cost. This offers superb redundancy and reliability. Failure of lots of motors can be close to zero probability.
  • Electric torque can be so high that VTOL can be possible in tilt-wing designs. More radical options become open.
  • While the generator will still make noise, overall there should be much less. Landing could be done on batteries with the generator at idle, and be very quiet. This could also be possible in certain take-off zones.
  • In fact, since electric motors can start instantly with high reliability, it could be possible to land much of the time as a glider, in close to silence.

Some things a hybrid design won't do, though:

  • A pure electric aircraft can go to extremely high altitudes (above 70,000') where drag is vastly less and travel is efficient. They don't need to breathe air to burn the fuel.
  • As noted, you can't get too far from an emergency landing airport in case your single generator fails. That can alter flight plans and means no travel over the ocean. A two generator design would escape this but would add a lot of weight.

Another potential breakthrough could come in hydrogen storage for a fuel-cell based generator. Hydrogen has a great energy density by weight. It is high volume, however, and needs high pressure tanks. Today, people are not ready to store it in the wings. New ways to store hydrogen could change the game. Alternately, methane is easier to store, but requires a much hotter fuel cell. One prototype plane has flown.

Another possible power source for busy routes would be microwave beamed power. This could be reasonably practical over busy air routes like San Francisco to Los Angeles. A small number of transmitting stations on the hilltops of that route could provide megawatts to clean, silent, low cost aircraft. The passenger bodies would of course be Faraday cages.

Comments

Well, not exactly... The footprint at which the cone of your glide-path plus battery power intersects the earth needs to contain an emergency landing strip. However, the area of the footprint increases _vastly_ as altitude increases. So that's another argument in favor of higher altitudes.

But that just changes the meaning of "too far." On loss of the generator you would need to know that you could glide to near a suitable airport, and have enough battery to complete the landing including a go-around if need be. Plus what range the rest of the battery can give you. And presumably governed by live weather. Non trivial but doable.

It might also be possible to consider a 2nd generator that is smaller than the main. It would not be able to provide enough power to climb to altitude but it might be able to give you enough power to fly for a few hours once at altitude. I am not sure how much smaller it could be to do that, since the main generator also doesn't have to be sized for much more since the battery provides extra power on takeoff and possibly on climb. The two-generator design might still be a lower cost approach even to transoceanic, given that the aircraft does not go far over the ocean until it has climbed to high altitude and also recharged any battery used in takeoff/climbing. It now has two forms of stored energy -- battery and altitude.

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