Comment by ryndbfsrw | Hacker News Reader

Comment by ryndbfsrw | Hacker News Reader

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> I would bet on EV planes replacing regional jets

Energy density of batteries are around 1/40th of that of kerosene (0.9 mj/kg vs 44mj/kg) so we are talking 44x the energy density we need to make up for to get perfect parity. From what I can find I’ve seen 35% thermal efficiency for modern airliners and I believe 95% for electric motors. Ok so this means we can reduce the density gap required for parity to just under ~ 44/2.5 so ~18/19 mj/kg? (I’m doing this in my head so it’s rough). Rough calls show we need batteries with 20x the capacity to replace what we have right now with electric alternatives. While the gains in battery densities have been impressive we need to keep these gains in mind that we need to be making much bigger gains for this to become a reality. Personally I don’t see it being feasible for a very long time given the progress and I believe we’re better off taking fewer unnecessary flights than hoping on some miracle battery breakthrough

(Edited for clarity)

Your calculations are correct, but you've baked a few other assumptions into them.

- Fuel has no structural strength. Batteries are rigid and can double as parts of the airframe, saving weight.

- Combustion engines are complex and expensive. This precludes mounting a dozen tiny engines on a plane and integrating them at just the right spots. Electric fans are cheaper and easier to integrate into the airframe, reducing drag. This also helps with engine-out capability. If you have two engines and one fails, you've lost half your thrust. If you have 36 engines and one fails, you've lost 3% of your thrust.

- Air becomes less dense with altitude. This reduces drag, but it also reduces the power of combustion engines. Electric aircraft don't need oxygen, so their power remains the same at altitude. There are losses due to the fan having to rotate more to move the same mass of air, but in general, electric aircraft become more efficient at altitude.

- Electric fans respond much faster to throttle inputs and are easier to gimbal than combustion propellors or jets. This allows designs with reduced or even eliminated control surfaces. The lack of rudder and elevator reduces drag, allowing for greater range.

That last trick might sound insane for human-rated aircraft, but there are already flying prototypes that use this approach.[1][2]

1. https://www.youtube.com/watch?v=5ukmS9ZJm40

2. https://www.youtube.com/watch?v=ZH7DSFRCqDQ

I wonder if you could significantly charge the batteries from the energy of a vibrating wind. Or (and this is out there) from atmospheric electricity

You can definitely significantly charge batteries from sun - see https://en.wikipedia.org/wiki/NASA_Pathfinder - but it's at the moment far from a convenient commercial solution.

Did we forget that electric vehicles were some of the first sold in 1900's? Now imagine what battery tech we would have if a hundred years of development went into battery technology?

Be patient, we'll figure something awesome out.

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