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Comment by baq | Hacker News Reader

If the tech takes off (pun intended) every major airport will need a SMR. Which is maybe good? But politically impractical today.

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Less of the "S". With current flight patterns you need multiple gigawatts. Calculations based on 737s leaving Gatwick:

Energy density of the fuel: 9.6kWh/L

900 flights per day = one flight every 96 seconds

26024.706L per flight

Total energy per flight: 9.6 x 26024.706kWh = 250MWh give or take = 900GJ

Total power supplied from Gatwick in the form of aviation fuel: 900GJ/96s = 9.375GW.

That's not only outside the range of SMRs, it's bigger than any single nuclear power station that's been built, by a comfortable margin.

To make electric flight work you can't think in terms of the way the current industry is structured because it's so distorted by the energy density of the current fuel.

That's assuming an overnight switch from what we have to all electric, for one of the busiest airports in the world.

Thinking in terms of disruption (from the innovator sense), their top 3 destinations [0] are Dublin, Barcelona and Malaga. Skipping barcelona becauese it's as busy, I don't think it's out of reach to consider that a 737 could do a return trip to dublin or Malaga without charging.

Another perspective is that taking off is significantly more energy intensive than cruising. According to [1], takeoff is equivalent to an hour of cruising. One way of looking at this is it only makes sense for mid haul travel instead. If we replaced transatlantic flights, or similar (us to Europe maybe) the savings would be immense and significantly more achievable

[0] https://www.gatwickairport.com/business-community/about-gatw...

[1] https://aviation.stackexchange.com/questions/47262/how-much-....

Yep. The thing is, even if you divide the power needed by two (by being smart about which planes you charge, or by how much) and then by two again (for a smaller airport) you still need a full new power plant to supply it. It's well out of SMR territory.

The way you'd have to do it is something like the Tesla approach: put small charging stations for luxury planes in as many airports as possible (because nobody, but nobody, will fly a plane into an airport they can't fly out of), and build out from there. That way you can do something financially interesting at SMR scale, and build momentum for the next step on something marketed as aspirational. Because the hardest SMR to build will be the first. Once you've got one, installing a second should be an easy sell. And two leads to four, and so on and so forth.

This is, of course, making the further assumption that something can be done about charging times. Getting 90GJ into a 737 currently takes about 23 minutes. That's 65MW, which is a nontrivial problem to solve all on its own; anything that slows down the recharge means longer queues to turn around, which, one way or another, means more land area or fewer flights for the airport, and worse economics for the operator.

Oof.

Jet engines are 35% efficient, I'd assume electric planes would be double that, does that change the calculation? Naively I'd say we 'only' need 4.5GW?

I feel like the back of the envelope calculation must have slipped a decimal point somewhere. 9GW is approximately 1/4 of the total electrical consumption in the whole of the UK. From memory aviation as a whole is only 2% of global emissions (although it might have an extra forcing effect due to being released directly into the upper atmosphere) where as electricity generation is 20-40% of emissions.

To paint a rough and ready picture, aviation emissions are very heavily weighted towards richer, less populous countries, whereas electricity generation (and particularly fossil fuel generation) is (to a lesser degree) tilted towards where the mass of population is: https://ourworldindata.org/carbon-footprint-flying#:~:text=W... vs https://ourworldindata.org/grapher/carbon-intensity-electric.... Note that the colour axis is a log scale. It's a compounding effect: more people => more energy; poorer => worse emissions and fewer flights per capita.

I thought I must have slipped a power of 10 too somewhere but if I did I can't spot it.

I found some reference to Gatwick using 2.6 billion litres of fuel a year. If I follow the logic above I get circa 8 billion litres. I think most of this is because a Boeing 737 has a 3000nm range fully fueled which they wouldn't be using normally. In fact I suspect it's impossible to take off fully fueled and with a full complement of passengers (it certainly is for lighter aircraft).

Between that and the efficiency difference mentioned elsewhere I think that explains about an order of magnitude. I'm totally willing to accept they'd need a 1GW power station to power Gatwick but 9GW seems high.

That weight constraint cuts both ways though, right? An electric plane charged for a 500 mile flight weighs the same as one charged for a 2000 mile flight, and the max landing weight of (e.g.) a 737 is substantially lower than the max takeoff weight. That means the maximum passenger load of the electric plane can never be as high as one fuelled by an energy source that leaves the plane over the course of the flight. So yes it's more efficient in terms of direct energy use, but it's less efficient in terms of the ratio of work done moving the passengers to work done moving the vehicle, first because you can't stuff as many on, and second because the mass of the vehicle itself doesn't drop over time.

EDIT: unless, of course, you have removable batteries that let you carry less weight for a shorter flight. That might be the only way to make this practical, and would have some other benefits: you could charge them off-site, for instance. It creates a hell of a logistics problem, but no bigger than liquid fuel.

Also, one factor to take into consideration is that the 9GW figure assumes that the refuelling is uniformly distributed throughout the 24 hours. That won't be true, I could believe peak usage being double the average. If that's true, the worst-case 9GW isn't what you need to work to, it's 18GW peak. If we go with the 2.6 billion vs 8 billion L ratio as telling us the true power requirement, that gets us back up to 2.925GW average, 5.85GW peak.

google and wolfram alpha tells me one fully tanked 737 stores 16 tons of kerosene, which translates to 261.1 GJ at 35% efficiency (72.5 MWh). doesn't sound too far off. assuming the same energy will be required for an electric airliner and you want to charge it to full in an hour... you probably need much more than 72.5MW power plant per aircraft because fast charging is nonlinear...? numbers which are hard to comprehend at scale in any case

With the energy efficiency attainable by traveling in the upper atmosphere, this might be the greenest possible long range transportation.

God such a tantalizing solar punk dream. I would love just to hear the inside of an electric commercial airliner at altitude.

Presumably they would use something like an electrically propelled ducted fan (basically the first stage of a high bypass engine). The noise I imagine would be reasonably similar.

I think a hybrid approach with a high bypass turbo fan powered by an electric motor. The fan could then switch over to a https://newatlas.com/automotive/inside-out-wankel/ when at cruising altitude. Using biofuels, or carbon air capture, we get long range and a closed carbon cycle.

I doubt it would be classical music and whale song playing over a beautifully calm scene..

More like kids watching movies without headphones, over loud conversations and screaming babies if other public transport is anything to go by.

But we can dream!

Experiences vary. I was on a Tokyo subway train (Chūō-Sōbu Line(Local)). For a couple of minutes after boarding it was so quiet that it was eerie. When I started hearing quiet noises I relaxed.

Anyway, mass transit does not have to be noisy. It varies by custom and culture.

What's SMR?

I would assume a Small Modular Reactor.

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