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B Radiant heating of air trapped in hollow glass spheres...

  1. Jun 17, 2016 #1
    I'd posted more of a real world version of this question, involving greenhouses, a few days ago at...
    https://www.physicsforums.com/threa...rior-much-cooler-if-reflective-inside.875575/
    ...but with no responses after 130 reads, I'm reducing it down to a barer and much better, IMO, hypothetical question here...

    Assume four hollow sealed clear glass spheres, let's say about 2 foot diameter each.
    Glass is typical thickness and transparency of single pane window, inside is just trapped air.

    Temperature of spheres and air inside and outside is the same, 70 degrees F, for all four spheres when first positioned outside in the morning before sun rise.

    Spheres then exposed to rising sun for hours, outside ambient air temperature climbs to over 100 degrees F.

    Question is; which sphere will have the highest internal air temperature and, most importantly, how significantly so compared to each other, if they vary in the following way...

    #1 sphere is as described above.

    #2 sphere is the same, except that it has a silver reflective coating on half of the inside of the glass and it stays positioned, relative to the sun, where that reflective coating is always on the bottom and sunshine is always streaming in on the clear top hemisphere.

    #3 sphere is the same as original in #1, no reflective coating, but it also has a 6" spherical lead mass painted black suspended by string in the center.

    #4 sphere is the same as #2 with the reflective coating and it also has the 6" lead spherical mass painted black suspended by string in the center.

    Air, inside and out and the spheres and lead mass all started out at the same temperature, 70 degrees F, before exposure to sun.

    I'd like to also assume, for this comparison to better isolate the radiant heat component, that there is no appreciative conductive heat transfer in or out through the glass itself for being in contact with the changing air temperature either outside or inside.

    What I'm trying to determine here is not just the ranking of, but the likely significance in the difference of, the comparative internal air temperature between all four spheres, while the sun is still shining on them all.

    For instance, likely expected difference between #1 and #2, is of greatest interest to me. Not looking for any number crunching and all, just a feel for how significant a difference might be expected between the two.

    Secondly, I'm curious how significantly the black mass might affect the internal air temperature, considering it is both intercepting and absorbing radiant heat that might have otherwise passed through the sphere, but is also still a heat sink for as long as it is still cooler than the rising internal air temperature via conduction. Again, solely the impact, and how significant, the black mass has on the internal air temperature is what I'm most curious about.

    Appreciate any thoughts.
     
  2. jcsd
  3. Jun 18, 2016 #2
    Hi Shane:

    I am guessing that you are describing a thought experiment, and that you have not actually performed any experiment in which sunlight caused the raising of temperature of air within a glass sphere. Is this correct?

    My understanding of the behavior of radiant heat with respect to directly raising the temperature of air is that it mostly does not happen that way. The way air is heated from radiation is that the radiation warms something solid or liquid, and the air is then warmed by conduction and convection. In normal conditions whenever a molecule of air absorbs a photon, and thereby increases it's internal energy, the molecule will almost always spontaneously emit a similar energy photon before the molecule can interact with another molecule. This is because in a gas the molecules are sufficiently far apart that the spontaneous emission of a photon by an exited molecule is much faster than the time between molecular interactions. It is the interaction of an exited molecule with another molecule that converts the exited energy into a rise in temperature.

    Regards,
    Buzz
     
  4. Jun 19, 2016 #3
    Buzz,
    Thank you for the response.

    Yes, just thinking out loud, nothing real-world tested..

    I was thinking something like what you described might be the case for empty sphere 1 &2, especially when compared to the other end of the spectrum with spheres 3 & 4, that had a black mass to absorb radiant heat and then, via conduction and convection, went to heating up the air.

    Do you think a sealed up vehicle in the hot summer sun, if all the interior, except for the windows, was covered in radiant barrier everywhere, would be appreciatively cooler compared to if it was not? Assume for the question that it was very well insulated, that the hot ambient outside temperatures were not conducting well through the skin of the vehicle or the windows on into the interior.

    Let me ask you something else, too, sort of related, that I'll probably be posting a new thread here to ask about...

    I'm interested also in nighttime radiant cooling. I've seen where some have even made ice in their solar cookers at night when the ambient temperature was still above freezing. The reflective sides apparently added to the radiant heat sink footprint, while also shielding the water down in the bottom from 'hotter' terrestrial sources that might be in line of site of the water.

    In making ice like above, the ambient air temperature can't be too high above freezing or it would be conducting with the water, adding heat to it, countering the ice making. Makes sense that you'd have a better shot when air temp was in mid 30's, than in the mid 90's.

    Anyways, the question I have is, not to make ice but just very cold water when ambient temperatures are high, wouldn't running the water through a double walled glass vacuum tube, like what's used in solar collectors, negate the outside higher ambient temperatures and make for some very chilled water? This is assuming it'd be positioned like the solar cooker above, though bigger, but still evacuated tube and reflective panels would only 'see' the sky, nothing terrestrial and warmer.

    Am I missing anything there?

    - Shane
     
    Last edited: Jun 19, 2016
  5. Jun 19, 2016 #4
    Scratch that about the "double walled glass vacuum tube" above, it won't be emitting any long waves from warmer water.

    - Shane
     
  6. Jun 19, 2016 #5
    Hi @shane2:

    I am not sure what you mean by a radiant barrier. If you covered the all interior surfaces with a completely reflective material, I would guess that this would (substantially) eliminate the greenhouse effect, and the car's interior air temperature would be (almost) unchanged. This seems like an experiment that could be reasonable approximated with available reflective material

    I don't have time now to respond to your other questions, but I plan to do so later.

    Regards,
    Buzz
     
  7. Jun 19, 2016 #6
    Hi @shane2:
    I am not familiar with this phenomenon. I also have difficulty visualizing the setup. Can you post a link to a reference that discusses this in more detail?

    Regards,
    Buzz
     
  8. Jun 19, 2016 #7
    Buzz,
    Looks to be a pretty neat trick, I've got a narrow 1,000ml vacuum flask I'm going to be trying it with,
    though with night temps in the low 70's here I'll just see how much cooler than ambient by morning.

    According to https://en.wikipedia.org/wiki/Radiative_cooling night sky ice making has been around awhile...

    In India before the invention of artificial refrigeration technology, ice making by nocturnal cooling was common. The apparatus consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with a clear exposure to the night sky. The bottom and sides were insulated with a thick layer of hay. On a clear night the water would lose heat by radiation upwards. Provided the air was calm and not too far above freezing, heat gain from the surrounding air by convection would be low enough to allow the water to freeze by dawn.[1]

    Towards the bottom of http://solarcooking.org/plans/funnel.htm you'll see there...

    How to Use the Solar Funnel as a Refrigerator/Cooler

    A university student (Jamie Winterton) and I were the first to demonstrate that the BYU Solar Funnel Cooker can be used - at night - as a refrigerator. Here is how this is done.

    The Solar Funnel Cooker is set-up just as you would during sun-light hours, with two exceptions:

    1. The funnel is directed at the dark night sky. It should not "see" any buildings or even trees. (The thermal radiation from walls, trees, or even clouds will diminish the cooling effect.).

    2. It helps to place 2 (two) bags around the jar instead of just one, with air spaces between the bags and between the inner bag and the jar. HDPE and ordinary polyethylene bags work well, since polyethylene is nearly transparent to infrared radiation, allowing it to escape into the "heat sink" of the dark sky.

    During the day, the sun's rays are reflected onto the cooking vessel which becomes hot quickly. At night, heat from the vessel is radiated outward, towards empty space, which is very cold indeed (a "heat sink").

    As a result, the cooking vessel now becomes a small refrigerator. We routinely achieve cooling of about 20º F (10º C) below ambient air temperature using this remarkably simple scheme.

    In September 1999, we placed two funnels out in the evening, with double-bagged jars inside. One jar was on a block of wood and the other was suspended in the funnel using fishing line. The temperature that evening (in Provo, Utah) was 78º F. Using a Radio Shack indoor/outdoor thermometer, a BYU student (Colter Paulson) measured the temperature inside the funnel and outside in the open air. He found that the temperature of the air inside the funnel dropped quickly by about 15 degrees, as its heat was radiated upwards in the clear sky. That night, the minimum outdoor air temperature measured was 47.5 degrees - but the water in both jars had ICE. I invite others to try this, and please let me know if you get ice at 55 or even 60 degrees outside air temperature (minimum at night). A black PVC container may work even better than a black-painted jar, since PVC is a good infrared radiator - these matters are still being studied.

    I would like to see the "Funnel Refrigerator" tried in desert climates, especially where freezing temperatures are rarely reached. It should be possible in this way to cheaply make ice for Hutus in Rwanda and for aborigines in Australia, without using any electricity or other modern "tricks." We are in effect bringing some of the cold of space to a little corner on earth. Please let me know how this works for you.
     
  9. Jun 19, 2016 #8
    Hi @shane2:

    Thanks for the references.

    Regards,
    Buzz
     
  10. Jun 20, 2016 #9

    sophiecentaur

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    There was a tradition of high technology in ancient Islam. Keeping cool was a big thing, of course and there was no Air Con. Apparently, there is evidence that, centuries ago, inhabitants of the Sahara used to procure ice for their drinks by radiation to outer space from water in shallow ice-ponds, via clear skies at night and store it in insulated vessels for use in the day. This requires special conditions of low humidity and totally clear skies of course - plus a load of low paid workers to operate the system whilst the rich guys were living in luxury. ('twas ever thus.) I would have thought that central Australia could have similar conditions over night but I don't know what the climate is like in Rwanda.
     
  11. Jun 20, 2016 #10
    I read about that in Persia, pretty big production, some structures still survive.

    Also, heard about some water wells with ice in bottom from night sky, while ambient temp still above freezing.

    Internet produced stories of Australians with their camping coolers purposefully positioned and rigged for making ice overnight when ambient air temps above freezing.

    Water container insulated on bottom and sides and skyward positioned to where terrestrial radiant sources are not line-of-site with it, adding heat to system, insulating the production from convective/conductive hotter air seems like the only major limiting factor. Camp coolers and solar ovens will use thin IR transparent plastic film to try and minimize hotter ambient air stealing it's "coolth".

    If coil of copper tubes flowing water could be under Long Wave IR transparent dome that would hold a partial vacuum, that could largely eliminate that limitation.

    Any clues what might can be found to re-purpose, for an IR transparent dome, to test and experiment with?
     
  12. Jun 20, 2016 #11

    sophiecentaur

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    I love the word "coolth", which I first heard many years ago. I also heard "dryth" which is in the air and takes water away from washing on a line. :smile:
     
  13. Jun 20, 2016 #12

    JBA

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    Those following this topic might be interested the following article from "Science Daily" some months past.
     

    Attached Files:

  14. Jun 20, 2016 #13
    JBA, thanks, great read!

    - Shane
     
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