The article suggests compressed hydrogen. Although that has obvious problems (at some point there is probably a tradeoff between increased density and the weight of the storage tank required to safely hold that density, which directly related to energy density) and probably more subtle problems, it would be clean-burning.

Edit: Random wikipedia page[1] gives energy density of "jet fuel" as 35 MJ/L and liquid (i.e., about as compressed as you're going to get) hydrogen as ... 8.5 MJ/L. Yeah, that's not even close, nevermind. LNG gets you to 22 MJ/L, which is still 2/3 of the density and requires storage at -160°C, which costs more energy, etc, and isn't clean-burning (but maybe slightly better than kerosene).

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

But a liter of compressed hydrogen weighs 38 grams, while a liter of kerosene weighs 810 grams. So the energy density by gram, rather than liter, is in favor of compressed hydrogen by a factor of more than 5x.

I don't know what the tradeoff is between volume and weight in aviation, but it's worth noting that we can measure density by both volume and weight, and by weight compressed hydrogen defeats kerosene.

If you are going to compare hydrogen and kerosene by weight you need to include the weight of the containment vessel. For kerosene that is a thin piece of aluminum. For hydrogen it is...not.

In aviation weight is more important. Volume is important, but weight (and distribution) is more important.

So, for sleek jets, heavy but compact jetfuel will rule. But for zeppelins, the weight-to energy ratio is much more important, as they have plenty of volume to spare..

That's a good point I hadn't considered, thanks.

They are not suggesting hydrogen. The article is specifically about _electrofuel_, which is extracted from a mix of hydrogen and carbon dioxide in a very power-hungry process; it’s a transparent liquid similar to diesel, and energy density is not too far off.

As jemfinch points out the value that is important is the Wh per kg, or MJ per 1000 kg, not the liter volume. Though certainly the carbon fiber composites tanks required to store hydrogen at very high pressures are not light in weight.

One of the reasons why electric aircraft are currently very short range is that the best li ion batteries top out at 250Wh/kg.

This was largely the point I was making

Liquid ammonia contains 15.6 MJ/L (half of diesel). Ethanol - 24MJ/L. Methanol - 15.6NJ/L. Mix of ethanol or methanol with ammonia can replace gasoline[1][2]. Both ethanol and ammonia can be generated from air using solar energy.

[1]: https://nh3fuelassociation.org/wp-content/uploads/2012/05/le... [2]: https://asmedigitalcollection.asme.org/IMECE/proceedings-abs...

Hydrogen is much lighter though. You wind up with big ugly planes that are lighter than the planes we have now. Apparently they’ll be able to use shorter runways as well.

You're forgetting the tank. There are a few hydrogen powered cars on the road, their tank weights about 125kg and stores a whopping 5kg of hydrogen.

The article mentions hydrogen at the start but then says the hydrogen would be used to produce synthetic jet fuel with the properties of regular fuel.

So the idea being: if you pull the carbon out of the air to make your kerosene, burning it again is carbon neutral? That makes sense.

"Synthetic fuels" means jet fuel. There are various ways to make synthetic kerosene with atmospheric CO2.

https://en.wikipedia.org/wiki/Jet_fuel#Synthetic_jet_fuel

I believe the idea is that you can use hydrogen to synthesize kerosene, which assuming you use clean energy inputs, is carbon neutral. There was a similar sort of startup that showed up on HN a few months ago.

It is already taking over the light plane market. Short haul flights will all be electric in less ten years.

I think you're being wildly optimistic here about the state and future of the technology