11 ms·

Just goes to show there's nothing remotely like fossil fuels for energy density.

nine_k 9y ago
Nuclear is! The problem is that the minimal feasible size of the whole system is too big for an airplane. Nuclear airplanes were actively designed in 1950s, but proved to be excessively heavy and unsafe.
- Sephr 9y ago
  > excessively heavy and unsafe Quite the understatement. I'd describe them as apocalyptically unsafe, seeing as many designs literally exhausted nuclear fallout.[1] [1]: https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion#Direct_Air_Cycle https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion#Di...
  - stingrae 9y ago
    I stand corrected. That is an interesting concept.
    - crooked-v 9y ago
      > Direct cycle nuclear engines would resemble a conventional jet engine, except that there would be no combustion chambers. The air gained from the compressor section would be sent to a plenum that directs the air into the nuclear reactor core. An exchange takes place where the reactor is cooled, but it then heats up the same air and sends it to another plenum. The second plenum directs the air into a turbine, which sends it out the exhaust. The end result is that instead of using jet fuel, an aircraft could rely on nuclear reactions for power. A direct cycle design (which is the only one that was ever thought practical to use) passes outside air through the core for cooling purposes, then sends it through the exhaust. > Indirect cycling involves thermal exchange outside of the core. The compressor air would be sent to a heat exchanger. The nuclear reactor core would heat up pressurized water or liquid metal and send it to the heat exchanger as well. That hot liquid would be cooled by the air; the air would be heated by the liquid and sent to the turbine. The turbine would send the air out the exhaust, providing thrust. An indirect cycle design doesn't expose outside air to the core, but "never came anywhere near producing flight-ready hardware", presumably because the amount of water needed for cooling would have required an absurdly large plane and even more power, leading to recursive issues.
- civilitty 9y ago
  Almost any time nuclear is mentioned, I find this link to be relevant: https://en.wikipedia.org/wiki/Nuclear_lightbulb https://en.wikipedia.org/wiki/Nuclear_lightbulb The thing is, even if you got the system small enough (lightbulb reactor or otherwise), you'd have one hell of a time with cooling. It works with rockets because they need to dump massive amounts of heat into propellant and generate a lot more thrust with it but airplanes fly slower and generate force through lift. If the money were there though, using something like the lightbulb for synthetic jet fuel could massively change their environmental impact.
  - chrisseaton 9y ago
    Why do you need to cool the reactor? I thought the whole point was the reactor was hot - that was how you got useful energy from it. Doesn't using the heat energy dissipate it?
    - corndoge 9y ago
      You have to keep it cool enough to sustain a chain reaction in order to prevent runaway, what the media refers to as a meltdown
      - deleted 9y ago
        [deleted]
      - deleted 9y ago
        [deleted]
    - nine_k 9y ago
      Also, if you care about mechanical stability (an engine, not a bomb), you either cool the reactor down to temperatures of its containment material (all attainable solid materials are limited to a few thousand K), or use something like magnetic containment, which is not always an option (you need everything inside electrically charged, and a low neutron flux).
  - ridgeguy 9y ago
    The nuclear lightbulb is new to me, thanks for the link. Has anybody thought through the engineering of those silica reaction vessels (I haven't searched)? I don't think it can work. If I understand the concept shown in the Wikipedia page, it depends on uranium hexafluoride operating at 25,000°C in fused silica containers which transmit hard UV that heats hydrogen external to the silica containers and is exhausted through nozzles, propelling the vehicle. Silica has very poor thermal conductivity, which means it can't be cooled readily by conduction. This means that the inner container wall will melt even if the outer wall coolant is liquid hydrogen because the wall material can't transport the heat flux. One might be able to keep a 10µm silica shell from melting, but nothing thicker. Too thin to be mechanically sound. Also, the uranium hex will be completely dissociated and ionized at 25K°C. This means the inner silica wall will be exposed to atomic fluorine, and will corrode away immediately. Uranium at 25k°C probably reacts with SiO2, but I don't know that. Uranium silicides and oxides are known compounds, so it's likely. It's a horrific corrosion environment. At temperatures like this, everything reacts with everything else. In short, this nuclear lightbulb can't be built with any material we currently know of. We'll have to wait for Scotty's transparent aluminum. Perhaps the MIT drone could use a Stirling engine with a Pu238 RTG heat source and air as the working fluid. If so, it could stay up forever. I'd bet on that before the nuclear light bulb.
walrus01 9y ago
For comparison, from wikipedia: Gasoline: 12,889 Wh/kilogram Diesel: 13,333 Wh/kilogram Lithium Ion battery: Ranges from 100 to 243 Wh/kilogram. Even when you consider that something like 50% of the energy in gasoline is lost to waste heat in a typical internal combustion engine, it's still a huge difference.
- imaginenore 9y ago
  Hydrogen: 33,300 Wh/kg (compressed: 39,443 Wh/kg)
  - jandrese 9y ago
    The problem is that you need a big heavy tank to keep all of the Hydrogen molecules from escaping, especially if you're keeping it under pressure. Hydrogen is much more problematic to work with.
  - scrooched_moose 9y ago
    But look at energy per volume [1]: Diesel: 35.8 MJ/L Hydrogen: 9.17 MJ/L at 700 bar (10,000 psi). It will require a tank of 4x the volume for the same energy which comes with many additional challenges. Plus, a tank that can withstand 700 bar will have to be extremely strong and heavy which hurts your energy/mass ratio. 1. https://en.wikipedia.org/wiki/Energy_density#Energy_densities_of_common_energy_storage_materials https://en.wikipedia.org/wiki/Energy_density#Energy_densitie...
  - deleted 9y ago
    [deleted]
schiffern 9y ago
> Just goes to show there's nothing remotely like organic molecules for energy density. GTFY (generalized that for you). ;) It's all about the energy in that carbon-hydrogen bond. Lipids have a similar energy density, which is why migrating birds have a similar nonstop range as fossil fueled airplanes (11,000 km vs 14,000 km for the 747 long range variant). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2864269/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2864269/
- folli 9y ago
  Very interesting, according to Wiki: Fat has an energy density of 37 kJ/g Diesel: 48 kJ/g Lithium Ion: 0.8 kJ/g
- traviscj 9y ago
  Reminds me of Fight Club's premise, but with a different output. Not sure a tiny biodiesel internal combustion engine is a very suitable power source for laptops, though. :-)
  - honestoHeminway 9y ago
    https://www.toshiba.co.jp/about/press/2009_10/pr2201.htm https://www.toshiba.co.jp/about/press/2009_10/pr2201.htm Exists. Stop disguising your nostalgia as future entusiasm :D
    - traviscj 9y ago
      Really cool, thank you for sharing! Would totally be interested in buying one, if you happen to have a stockpile :-)

Nuclear is! The problem is that the minimal feasible size of the whole system is too big for an airplane. Nuclear airplanes were actively designed in 1950s, but proved to be excessively heavy and unsafe.

Sephr 9y ago
> excessively heavy and unsafe Quite the understatement. I'd describe them as apocalyptically unsafe, seeing as many designs literally exhausted nuclear fallout.[1] [1]: https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion#Direct_Air_Cycle https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion#Di...
- stingrae 9y ago
  I stand corrected. That is an interesting concept.
  - crooked-v 9y ago
    > Direct cycle nuclear engines would resemble a conventional jet engine, except that there would be no combustion chambers. The air gained from the compressor section would be sent to a plenum that directs the air into the nuclear reactor core. An exchange takes place where the reactor is cooled, but it then heats up the same air and sends it to another plenum. The second plenum directs the air into a turbine, which sends it out the exhaust. The end result is that instead of using jet fuel, an aircraft could rely on nuclear reactions for power. A direct cycle design (which is the only one that was ever thought practical to use) passes outside air through the core for cooling purposes, then sends it through the exhaust. > Indirect cycling involves thermal exchange outside of the core. The compressor air would be sent to a heat exchanger. The nuclear reactor core would heat up pressurized water or liquid metal and send it to the heat exchanger as well. That hot liquid would be cooled by the air; the air would be heated by the liquid and sent to the turbine. The turbine would send the air out the exhaust, providing thrust. An indirect cycle design doesn't expose outside air to the core, but "never came anywhere near producing flight-ready hardware", presumably because the amount of water needed for cooling would have required an absurdly large plane and even more power, leading to recursive issues.
civilitty 9y ago
Almost any time nuclear is mentioned, I find this link to be relevant: https://en.wikipedia.org/wiki/Nuclear_lightbulb https://en.wikipedia.org/wiki/Nuclear_lightbulb The thing is, even if you got the system small enough (lightbulb reactor or otherwise), you'd have one hell of a time with cooling. It works with rockets because they need to dump massive amounts of heat into propellant and generate a lot more thrust with it but airplanes fly slower and generate force through lift. If the money were there though, using something like the lightbulb for synthetic jet fuel could massively change their environmental impact.
- chrisseaton 9y ago
  Why do you need to cool the reactor? I thought the whole point was the reactor was hot - that was how you got useful energy from it. Doesn't using the heat energy dissipate it?
  - corndoge 9y ago
    You have to keep it cool enough to sustain a chain reaction in order to prevent runaway, what the media refers to as a meltdown
    - deleted 9y ago
      [deleted]
    - deleted 9y ago
      [deleted]
  - nine_k 9y ago
    Also, if you care about mechanical stability (an engine, not a bomb), you either cool the reactor down to temperatures of its containment material (all attainable solid materials are limited to a few thousand K), or use something like magnetic containment, which is not always an option (you need everything inside electrically charged, and a low neutron flux).
- ridgeguy 9y ago
  The nuclear lightbulb is new to me, thanks for the link. Has anybody thought through the engineering of those silica reaction vessels (I haven't searched)? I don't think it can work. If I understand the concept shown in the Wikipedia page, it depends on uranium hexafluoride operating at 25,000°C in fused silica containers which transmit hard UV that heats hydrogen external to the silica containers and is exhausted through nozzles, propelling the vehicle. Silica has very poor thermal conductivity, which means it can't be cooled readily by conduction. This means that the inner container wall will melt even if the outer wall coolant is liquid hydrogen because the wall material can't transport the heat flux. One might be able to keep a 10µm silica shell from melting, but nothing thicker. Too thin to be mechanically sound. Also, the uranium hex will be completely dissociated and ionized at 25K°C. This means the inner silica wall will be exposed to atomic fluorine, and will corrode away immediately. Uranium at 25k°C probably reacts with SiO2, but I don't know that. Uranium silicides and oxides are known compounds, so it's likely. It's a horrific corrosion environment. At temperatures like this, everything reacts with everything else. In short, this nuclear lightbulb can't be built with any material we currently know of. We'll have to wait for Scotty's transparent aluminum. Perhaps the MIT drone could use a Stirling engine with a Pu238 RTG heat source and air as the working fluid. If so, it could stay up forever. I'd bet on that before the nuclear light bulb.

walrus01 9y ago

For comparison, from wikipedia: Gasoline: 12,889 Wh/kilogram Diesel: 13,333 Wh/kilogram Lithium Ion battery: Ranges from 100 to 243 Wh/kilogram. Even when you consider that something like 50% of the energy in gasoline is lost to waste heat in a typical internal combustion engine, it's still a huge difference.

imaginenore 9y ago
Hydrogen: 33,300 Wh/kg (compressed: 39,443 Wh/kg)
- jandrese 9y ago
  The problem is that you need a big heavy tank to keep all of the Hydrogen molecules from escaping, especially if you're keeping it under pressure. Hydrogen is much more problematic to work with.
- scrooched_moose 9y ago
  But look at energy per volume [1]: Diesel: 35.8 MJ/L Hydrogen: 9.17 MJ/L at 700 bar (10,000 psi). It will require a tank of 4x the volume for the same energy which comes with many additional challenges. Plus, a tank that can withstand 700 bar will have to be extremely strong and heavy which hurts your energy/mass ratio. 1. https://en.wikipedia.org/wiki/Energy_density#Energy_densities_of_common_energy_storage_materials https://en.wikipedia.org/wiki/Energy_density#Energy_densitie...
- deleted 9y ago
  [deleted]

schiffern 9y ago

> Just goes to show there's nothing remotely like organic molecules for energy density. GTFY (generalized that for you). ;) It's all about the energy in that carbon-hydrogen bond. Lipids have a similar energy density, which is why migrating birds have a similar nonstop range as fossil fueled airplanes (11,000 km vs 14,000 km for the 747 long range variant). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2864269/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2864269/

folli 9y ago
Very interesting, according to Wiki: Fat has an energy density of 37 kJ/g Diesel: 48 kJ/g Lithium Ion: 0.8 kJ/g
traviscj 9y ago
Reminds me of Fight Club's premise, but with a different output. Not sure a tiny biodiesel internal combustion engine is a very suitable power source for laptops, though. :-)
- honestoHeminway 9y ago
  https://www.toshiba.co.jp/about/press/2009_10/pr2201.htm https://www.toshiba.co.jp/about/press/2009_10/pr2201.htm Exists. Stop disguising your nostalgia as future entusiasm :D
  - traviscj 9y ago
    Really cool, thank you for sharing! Would totally be interested in buying one, if you happen to have a stockpile :-)