Comment by vilhelm_s | Hacker News Reader

Comment by vilhelm_s | Hacker News Reader

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I guess a more low-tech approach would be to fuel jets with ethanol. Googling a bit now, the U.S. uses about 18 billion gallons/year of jet fuel[1], and produces 17 billion gallons/year of fuel ethanol [2]. The price per gallon is about $1.5 versus $1.94 [3,4]; because jet fuel has 46% higher energy density that means that the cost per Joule would increase by a factor of 1.9.

Obviously this is only a back-of-the-envelope thing, but it seems that even if we switched to renewable fuels using the technology that are available today, ticket prices should about double. Historically, that corresponds to going back to circa-1980 prices[6], which I guess is bad, but not exactly apocalyptic.

[1] https://www.eia.gov/todayinenergy/detail.php?id=31512 [2] https://www.eia.gov/todayinenergy/detail.php?id=32152 [3] https://markets.businessinsider.com/commodities/ethanol-pric... [4] https://www.indexmundi.com/commodities/?commodity=jet-fuel [5] https://en.wikipedia.org/wiki/Energy_density [6] https://www.theatlantic.com/business/archive/2013/02/how-air...

The problem with ethanol is it absorbs water, which at greater altitudes (lower temperature and pressure) freezes and blocks fuel lines. For that reason aviation fuels have limits on ethanol content.

Could we redesign the fuel system to (safely) remove the water somehow? Or prevent the fuel from absorbing moisture in the first place?

Alternatively we could use ethanol in fuel cells, and use electric motor, unfortunately it requires either finding new catalists or a cheap source of platinum https://en.m.wikipedia.org/wiki/Direct-ethanol_fuel_cell

It would have to be hermetically sealed from creation to combustion at 37,000 feet. I’m sure it’s theoretically possible but so difficult as to be untenable.

Maybe you could chill it to precipitate the ice just before loading it on the plane?

Ground vehicles can run fine on ethanol but the increased fire risk (relative to kerosene) probably makes it impractical for commercial passenger flights.

You answered your own question.

" jet fuel has 46% higher energy density"

That's why. Few things matter more when lifting things into the air than weight.

I guess the current long routes would not work, but apparently airliner fuel efficiency improved by almost exactly this much since 1980 (https://www.greenaironline.com/news.php?viewStory=684), so it still seems that's the era we'd go back to...

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