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Clean energy seems wildly impractical for airplanes. There is no battery tech approaching kerosene for power density, and unlike fuel, the battery doesn't get l

by Aloha 7y ago

Clean energy seems wildly impractical for airplanes. There is no battery tech approaching kerosene for power density, and unlike fuel, the battery doesn't get lighter as it's used up.

sitkack 7y ago
It is already taking over the light plane market. Short haul flights will all be electric in less ten years.
- Aloha 7y ago
  I think you're being wildly optimistic here about the state and future of the technology
loeg 7y ago
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 https://en.wikipedia.org/wiki/Energy_density
- joe_the_user 7y ago
  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.
  - loeg 7y ago
    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.
- viburnum 7y ago
  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.
  - 08-15 7y ago
    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.
- jemfinch 7y ago
  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.
  - godelski 7y ago
    In aviation weight is more important. Volume is important, but weight (and distribution) is more important.
  - loeg 7y ago
    That's a good point I hadn't considered, thanks.
  - evgen 7y ago
    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.
  - dr_dshiv 7y ago
    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..
- walrus01 7y ago
  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.
  - Aloha 7y ago
    This was largely the point I was making
- jiofih 7y ago
  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.
- corrund 7y ago
  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/leeladhar_nh3.pdf https://nh3fuelassociation.org/wp-content/uploads/2012/05/le... [2]: https://asmedigitalcollection.asme.org/IMECE/proceedings-abstract/IMECE2014/46514/V06AT07A071/257569 https://asmedigitalcollection.asme.org/IMECE/proceedings-abs...
wffurr 7y ago
"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 https://en.wikipedia.org/wiki/Jet_fuel#Synthetic_jet_fuel
war1025 7y ago
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.

by Aloha 7y ago

Clean energy seems wildly impractical for airplanes. There is no battery tech approaching kerosene for power density, and unlike fuel, the battery doesn't get lighter as it's used up.

sitkack 7y ago

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

Aloha 7y ago
I think you're being wildly optimistic here about the state and future of the technology

loeg 7y ago

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 https://en.wikipedia.org/wiki/Energy_density

joe_the_user 7y ago
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.
- loeg 7y ago
  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.
viburnum 7y ago
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.
- 08-15 7y ago
  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.
jemfinch 7y ago
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.
- godelski 7y ago
  In aviation weight is more important. Volume is important, but weight (and distribution) is more important.
- loeg 7y ago
  That's a good point I hadn't considered, thanks.
- evgen 7y ago
  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.
- dr_dshiv 7y ago
  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..
walrus01 7y ago
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.
- Aloha 7y ago
  This was largely the point I was making
jiofih 7y ago
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.
corrund 7y ago
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/leeladhar_nh3.pdf https://nh3fuelassociation.org/wp-content/uploads/2012/05/le... [2]: https://asmedigitalcollection.asme.org/IMECE/proceedings-abstract/IMECE2014/46514/V06AT07A071/257569 https://asmedigitalcollection.asme.org/IMECE/proceedings-abs...

wffurr 7y ago

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

war1025 7y ago

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.