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Comment by barrkel | Hacker News Reader

Indeed. I expect we'll synthesize jet fuel or a close equivalent (something that burns and reduces airframe weight) from electric or bio sources long after most other engines have moved to electric. Perhaps we'll generate electricity on board, but it will be with a dense, wasting fuel.

And we certainly aren't going to use solar to power commercial human air transport any time this millenium; it will always be better, for people, to compress energy before a flight and travel faster, than try to make the flight itself self-sustaining and travel slower. The only exception could be recreational flyers.

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We've been synthesizing one liquid hydrocarbon fuel for thousands of years; ethanol is pretty similar to jet fuel in energy density and combustion. I think the possibility that in the future we'll have cheap solar/wind grid power and use that to produce ethanol, rather than charge batteries, is more likely than people think. Ethanol production isn't terribly efficient, but if your input power is sustainable it doesn't really matter.

Ethanol is about 70% of the energy density of Jet A-1: https://en.m.wikipedia.org/wiki/Energy_density

You'd want to produce methanol. Ethanol does not have sufficient energy density.

http://phys.org/news/2014-11-artificial-photosynthesis-fuel-...

It's generally accepted that a "hydrogen economy" [1] doesn't make sense, and isn't going to work. I think we can say with reasonable certainty that that's true for cars where it looks like batteries are going to reign supreme.

I'm not at all sure that that's true for large jet aircraft though. Storing liquid hydrogen is a non-trivial exercise. On the other hand the larger the tank you're storing it in, the easier the problem becomes -- and large passenger jets would by necessity be using very large tanks. These tanks would be so large that they would impact jet performance in terms of increased drag. However, even with the larger tanks, liquid hydrogen will still provide a much better energy-to-weight ratio than you can get with jet propellent. This means you need less lift to fly the airplane and you have lower energy losses to lift-induced drag. I don't know that this would get you to break-even, but it seems likely that a hydrogen powered jet can get comparable performance to a jet using conventional jet fuel.

Of course just handling liquid hydrogen is no easy task. But a large airport should benefit from considerable economies of scale. It wouldn't be like trying to provide liquid hydrogen to cars at your local gas station.

There has been at least one experimental liquid hydrogen fueled aircraft, the Tupolev Tu-155 [2].

[1] https://en.wikipedia.org/wiki/Hydrogen_economy

[2] https://en.wikipedia.org/wiki/Tupolev_Tu-155

The problem with hydrogen is that it has abysmally low density, being a gas (8mg / L at STP, 40 g / L at 700 bar). Even liquid hydrogen has a density of only 70 g / L. Compare jet fuel, which ranges around 800 g / L. Hydrogen is a more efficient fuel per weight, but it really loses out when you compare volume. This inefficient use of volume is probably a more severe limiting factor than the weight.

> This inefficient use of volume is probably a more severe limiting factor than the weight.

I don't necessarily disagree with you. However, weight is a really, really big deal in aircraft. And the volume becomes less of an issue as aircraft get bigger, thanks to the square-cube law. Are modern day jet aircraft big enough to largely offset the costs of fueling them with liquid hydrogen? I don't know, but they're pretty damn big.

I looked up the stats for a Boeing 787 Dreamliner. The fuel capacity is about 126,000 L. Judging from Wikipedia, you'd need ~5× the amount of space to store pressured hydrogen for the same amount of energy. Wikipedia also says that the cargo capacity of the same 787 is... 172,000 L. I don't have hard numbers on the size of passenger space, but guesstimating that as the upper half of a cylindrical tube gets you in the region of 1,800,000 L. So you're looking at taking out 10-20% of the revenue space of an aircraft to fit in hydrogen fuel tanks.

Now the 787 isn't the largest craft out there, but spitballing the 747 gave similar numbers. I also don't have numbers on how much less fuel you'd need for the lighter weight (you'd reduce the weight of the aircraft by ~⅓ with equivalent amounts of fuel). If you assume it's more or less linear, you're talking about using ⅔ the fuel, which would be cutting out "only" 5-10% of revenue space or cutting your range by ⅔ and keeping revenue space the same.

Given the amount that airlines are trying to squeeze out of the existing revenue space already, it doesn't look like a particularly appealing trade-off.

I'm not claiming it would be cost competitive with current jet fuel prices. However, if fossil fuel prices skyrocket (either because of a substantial carbon tax or simple resource exhaustion), then the economics change in hydrogen's favor.

I'm also not claiming that existing airframes could be easily retrofitted to fly on hydrogen. Not only do you need a much larger fuel volume, you can't practically store it in wing tanks. On the other hand if you took an existing design and stretched the fuselage to accommodate larger tanks, or attached large external tanks to the wings, you could get something that's economical. That's because you have an aircraft that's larger (more expensive, all other things being equal) but also lighter (cheaper, all other things being equal).

I agree hydrogen will probably never be used as a general purpose fuel, but if you have energy you can synthesize hydrocarbon fuels.

I wouldn't be so sure this would not happen this millennium as you say. Maybe not this decade, but I certainly imagine sooner or later we will have I.e. a big satellite beaming power directly to a plane (laser or microwave, you choose). Once launch costs make this plausible and it's deemed safe and secure, why not?

Bonus, you now have an orbital death ray if you decide to go Bond villain.

That would be about as much of a "bonus" as the Chernobyl disaster was a "bonus" to nuclear power. In other words, a threat.

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