5 ms·

I’m missing the basic premise here, how does pointing an insulated box at the sky at night cool whatever’s in the box?

by jack_riminton 3y ago

mateo1 3y ago
I'm guessing he just froze water when the temperature dropped below zero at night, and everything else is nonsense.
- hifikuno 3y ago
  I wonder how you prove/disprove this. I imagine if you had a control box with water exposed to the air that wasn't "radiating heat," then that should show if the solar freezer was indeed working or not.
  - toast0 3y ago
    You could build a few test configurations. One as stated, insulated fridge, open on top. One, insulared fridge, open on top, under an open air shelter, no walls, just a roof, where the roof is a significant distance away from the top of the fridge, so airflow is not impacted, but there's no view or the sky. Possibly variations on this with the roof being ir transparent or not. Another test configuration could have the fridge closed, but with forced air ventilation.
  - tagawa 3y ago
    He did try a basic control with his first version: “i got ice consistently over several nights, even though there was no frost on the ground, and pots of water sitting next to the box cooker did not freeze.”
    - hifikuno 3y ago
      Ah you're right! I totally missed that line. Cheers.
- deleted 3y ago
  [deleted]
- satori99 3y ago
  Humans have been using this technique (radiating heat into the cold of space) to freeze water in hot places, for a very long time. https://www.realclearscience.com/blog/2018/07/09/how_people_created_ice_in_the_desert_2000_years_ago.html https://www.realclearscience.com/blog/2018/07/09/how_people_...
  - mateo1 3y ago
    That's an interesting article. I'm aware of this sort of radiative cooling, but usually convection and conduction, along with evaporative cooling will be much, much more important than radiative cooling. I'd like to see some kind of report or article showing the cooling potential in W/m^2. I remember someone made a metamaterial optimized for emitting heat in the wavelength of the atmospheric window, but the value was quite low.
    - satori99 3y ago
      You might be thinking of the work led by Professor Shanhui Fan @ Stanford some years back; https://news.stanford.edu/2017/09/04/sending-excess-heat-sky/ https://news.stanford.edu/2017/09/04/sending-excess-heat-sky... This is being commercialized, but I don't know how successful it has been to date. https://www.skycoolsystems.com/ https://www.skycoolsystems.com/
punnerud 3y ago
Almost all surfaces radiates heat. You can heat and cool using radiation. It’s the same way satellites is cooled, when you can’t cool with convection (interact with other atoms). Black absorb more radiation, but it also radiates more
vladms 3y ago
I guess that not having other warm objects pointing at the water, will not warm the water (example: a wall radiates infrared so it heats the water if the wall has "sight" over the water). Insulating the walls of the container with the water servers the same purpose - not have the water get heat from the surroundings.
JohnVideogames 3y ago
A black body will emit radiation (and receive radiation) until it's in equilibrium with its surroundings. In theory, the black bit of box radiates heat into space, and receives some of the CMB until both are at ~3K. It's how we cool satellites and spacecraft. But the effect is quite small, and I'm suspicious of it (as the nearest surroundings are the hot planet, which transfer heat more effectively!)
- bamboozled 3y ago
  Snow is a near perfect black body radiator
  - richardw 3y ago
    Therefore imagine the impact of reduced snow and ice due to global warming. It’s free cooling and we’re throwing it away.
    - thfuran 3y ago
      The albedo is the part that's bad to lose. I don't think snow emits especially strongly in the frequencies relevant for this. A highly imperfect blackbody emitter could cool more effectively.
  - alexpotato 3y ago
    This was a key plot point in the 1990s video game: Midwinter https://en.wikipedia.org/wiki/Midwinter_(video_game) https://en.wikipedia.org/wiki/Midwinter_(video_game)
- Scoundreller 3y ago
  If you grow crops, your crops can freeze when it's a clear sky at night with stagnant air even though the air temperature only goes close to freezing. In some agri areas, you'll see "wind machines" to blow the warmer air around the crops to keep them from radiating their heat and frosting. https://www.ontario.ca/page/wind-machines-minimizing-cold-injury-horticultural-crops https://www.ontario.ca/page/wind-machines-minimizing-cold-in...
deleted 3y ago
[deleted]
aaron695 3y ago
[dead]
richardw 3y ago
Great question! “PDRC surfaces are designed to be high in solar reflectance (to minimize heat gain) and strong in longwave infrared (LWIR) thermal radiation heat transfer through the atmosphere's infrared window (8–13 µm) to cool temperatures even during the daytime.” https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_cooling https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_co... I also posted this as a separate post for visibility. It’s an amazing technique that more should be aware of.

I’m missing the basic premise here, how does pointing an insulated box at the sky at night cool whatever’s in the box?

mateo1 3y ago

I'm guessing he just froze water when the temperature dropped below zero at night, and everything else is nonsense.

hifikuno 3y ago
I wonder how you prove/disprove this. I imagine if you had a control box with water exposed to the air that wasn't "radiating heat," then that should show if the solar freezer was indeed working or not.
- toast0 3y ago
  You could build a few test configurations. One as stated, insulated fridge, open on top. One, insulared fridge, open on top, under an open air shelter, no walls, just a roof, where the roof is a significant distance away from the top of the fridge, so airflow is not impacted, but there's no view or the sky. Possibly variations on this with the roof being ir transparent or not. Another test configuration could have the fridge closed, but with forced air ventilation.
- tagawa 3y ago
  He did try a basic control with his first version: “i got ice consistently over several nights, even though there was no frost on the ground, and pots of water sitting next to the box cooker did not freeze.”
  - hifikuno 3y ago
    Ah you're right! I totally missed that line. Cheers.
deleted 3y ago
[deleted]
satori99 3y ago
Humans have been using this technique (radiating heat into the cold of space) to freeze water in hot places, for a very long time. https://www.realclearscience.com/blog/2018/07/09/how_people_created_ice_in_the_desert_2000_years_ago.html https://www.realclearscience.com/blog/2018/07/09/how_people_...
- mateo1 3y ago
  That's an interesting article. I'm aware of this sort of radiative cooling, but usually convection and conduction, along with evaporative cooling will be much, much more important than radiative cooling. I'd like to see some kind of report or article showing the cooling potential in W/m^2. I remember someone made a metamaterial optimized for emitting heat in the wavelength of the atmospheric window, but the value was quite low.
  - satori99 3y ago
    You might be thinking of the work led by Professor Shanhui Fan @ Stanford some years back; https://news.stanford.edu/2017/09/04/sending-excess-heat-sky/ https://news.stanford.edu/2017/09/04/sending-excess-heat-sky... This is being commercialized, but I don't know how successful it has been to date. https://www.skycoolsystems.com/ https://www.skycoolsystems.com/

punnerud 3y ago

Almost all surfaces radiates heat. You can heat and cool using radiation. It’s the same way satellites is cooled, when you can’t cool with convection (interact with other atoms). Black absorb more radiation, but it also radiates more

vladms 3y ago

I guess that not having other warm objects pointing at the water, will not warm the water (example: a wall radiates infrared so it heats the water if the wall has "sight" over the water). Insulating the walls of the container with the water servers the same purpose - not have the water get heat from the surroundings.

JohnVideogames 3y ago

A black body will emit radiation (and receive radiation) until it's in equilibrium with its surroundings. In theory, the black bit of box radiates heat into space, and receives some of the CMB until both are at ~3K. It's how we cool satellites and spacecraft. But the effect is quite small, and I'm suspicious of it (as the nearest surroundings are the hot planet, which transfer heat more effectively!)

bamboozled 3y ago
Snow is a near perfect black body radiator
- richardw 3y ago
  Therefore imagine the impact of reduced snow and ice due to global warming. It’s free cooling and we’re throwing it away.
  - thfuran 3y ago
    The albedo is the part that's bad to lose. I don't think snow emits especially strongly in the frequencies relevant for this. A highly imperfect blackbody emitter could cool more effectively.
- alexpotato 3y ago
  This was a key plot point in the 1990s video game: Midwinter https://en.wikipedia.org/wiki/Midwinter_(video_game) https://en.wikipedia.org/wiki/Midwinter_(video_game)
Scoundreller 3y ago
If you grow crops, your crops can freeze when it's a clear sky at night with stagnant air even though the air temperature only goes close to freezing. In some agri areas, you'll see "wind machines" to blow the warmer air around the crops to keep them from radiating their heat and frosting. https://www.ontario.ca/page/wind-machines-minimizing-cold-injury-horticultural-crops https://www.ontario.ca/page/wind-machines-minimizing-cold-in...

deleted 3y ago

[deleted]

aaron695 3y ago

[dead]

richardw 3y ago

Great question! “PDRC surfaces are designed to be high in solar reflectance (to minimize heat gain) and strong in longwave infrared (LWIR) thermal radiation heat transfer through the atmosphere's infrared window (8–13 µm) to cool temperatures even during the daytime.” https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_cooling https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_co... I also posted this as a separate post for visibility. It’s an amazing technique that more should be aware of.