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  • dp
    replied
    Originally posted by J Tiers
    yes, a surface would..........Unless it kept off the condensing dew that would warm it back up and mess up the cooling................. Then some efficiency loss would be preferable to a complete failure to work.

    That cover could allow the same cooling system to work OK in a wetter climate, which otherwise would mess it up.
    I think you're missing a fundamental point - even dew is chilled if it is attracted to the water film that flows over the radiating surface. It too is collected in the storage tank along with the water sprinkled over the surface. It isn't important that the surface is warmed - the water is still cooled. It is the temperature of the stored water, not the surface temperature of the radiator that is important.

    A wetter climate, as in rain, would end the process. There is no need to protect it from rain because it would be turned off until the sky clears again. Dew just becomes more cold water when it's running under a clear sky.

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  • J Tiers
    replied
    Originally posted by dp
    There's no cover over the night sky radiator as there is in a solar collector. There is nothing between the radiating surface and the cold uncaring perpetual dark of space except a bit of atmosphere. Any intervening surface would make the process less efficient.
    yes, a surface would..........Unless it kept off the condensing dew that would warm it back up and mess up the cooling................. Then some efficiency loss would be preferable to a complete failure to work.

    That cover could allow the same cooling system to work OK in a wetter climate, which otherwise would mess it up.

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  • dp
    replied
    There's no cover over the night sky radiator as there is in a solar collector. There is nothing between the radiating surface and the cold uncaring perpetual dark of space except a bit of atmosphere. Any intervening surface would make the process less efficient.

    The only parts you have are a metal surface and a thin film of water moving over it. The heat in the water is absorbed into the metal and then radiated out into space, never to be seen again. Assuming a water temperature initially of 54؛ F, and a night sky temperature of -70؛ C (-94؛ F), that represents quite a bit of entropic opportunity. Might be needed to add some Prestone to the water to keep it from freezing during some parts of the year.

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  • J Tiers
    replied
    I'm still thinking that you are going to have a hard dew point limit...... as dp agrees.

    Dunno about other places, but we routinely have overnight dewpoints that are quite high in the summer, I want to say they get into the 70s at times.

    IR transparent coverings might help, but many are not cheap, or are impractical for other reasons. I think the cover would have to be VERY transparent, very low loss, to work at all well. Are there extremely IR-transparent materials with decent rigidity and low cost?

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  • Evan
    replied
    I did a few tests this summer to see how much cooling I could get from my solar collector. I turned it around to point north and turned on the circulating pump at night instead of day. I haven't crunched any numbers yet but it did cool the storage tank below ambient. The idea was to keep the house just a little cooler during the ridiculous hot spell we had.

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  • dp
    replied
    Originally posted by J Tiers
    Believe it or not, I DID manage to catch that point it was tough to finally get it through my head, but ............. otherwise what I said would have no point, would it?

    Now, you DO realize that this is what cools the entire earth, right?
    Yes - I said so in the second paragraph of #1 in this thread

    And, of course, the greenhouse effect is what prevents the cooling from taking place as effectively. So, all this "greenhouse gas" business is exactly what cuts the effect of radiation from the earth (and your cooler). The GG is only a small percentage effect, but....
    Of course. I said so in #7. The temperature of the present day night sky is -70؛. Without all the intervening atmosphere components it would be much colder. What this means is the night sky is warmer than -70؛ in some locations, and likely colder in others. I think the -70؛ represents an average.

    The dryer the environment, the better it ought to work. In fact I very much doubt it would work nearly as well in a high humidity environment. If there is anything at all to the greenhouse gas biz, water vapor is just about the best common greenhouse gas (far better than CO2 and it is very common), so you are down a bit right there.
    Mentioned in #5. Have you been skipping over these posts ?

    Also, in a humid environment, as the item is cooled, water vapor will condense on it and rapidly warm it up towards ambient.

    It's for the desert.
    Eyep - see #5. I recommended Arizona. The energy transfer mechanism is water on the radiant surface, so any atmospheric water will be collected and stored. Imagine you have a corrogated roof on a building such as a Quonset hut, and install sprinklers on it. On clear evenings you turn on the sprinklers and the combination of evaporative cooling and radiative cooling of the water on the Quonset hut chills the water as it runs down the sides. Having been chilled it is now stored in a large tank.

    In the morning as the sun rises the reverse happens - the water is heated by the sun and that is collected to a different tank so that you finally have a large tank of hot water and a large tank of cold water.

    What to do with all this water? Circulate it to a Stirling motor that runs pumps that feed the sprinklers and the circulation lines and you have a solar powered sprinkler that requires only water to run. Not particularly useful except as a concept machine. Energy produced in excess of what the Stirling engine requires is available to do useful work.

    But to expand on this as a practical solution, recall that the estimates of energy are between 30 and 500 watts per meter squared. The RTG's on the Pioneer space craft product 40 We of energy (the RTG itself generates much more energy, but efficiency limits electrical output to 40 watts), and look what that 40 watts has done for the last 30 years. And in a world of global warming mania, all this energy is made available by radiating energy into space which is a good thing. It is of greater potential than reflecting heat into space by painting roofs and parking lots white as it is creating useful work.

    Anyway - that's all science. I'm still interested in anyone's experience with actual installations. There are commercial products that use these principles.

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  • J Tiers
    replied
    Originally posted by dp
    You radiate it out into space. In fact you emphasize the natural tendency of energy to radiate out into space. Heat moves from where it is to where it is not and there's not much in space.
    Believe it or not, I DID manage to catch that point it was tough to finally get it through my head, but ............. otherwise what I said would have no point, would it?

    Now, you DO realize that this is what cools the entire earth, right?

    And, of course, the greenhouse effect is what prevents the cooling from taking place as effectively. So, all this "greenhouse gas" business is exactly what cuts the effect of radiation from the earth (and your cooler). The GG is only a small percentage effect, but....

    The dryer the environment, the better it ought to work. In fact I very much doubt it would work nearly as well in a high humidity environment. If there is anything at all to the greenhouse gas biz, water vapor is just about the best common greenhouse gas (far better than CO2 and it is very common), so you are down a bit right there.

    Also, in a humid environment, as the item is cooled, water vapor will condense on it and rapidly warm it up towards ambient.

    It's for the desert.

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  • A.K. Boomer
    replied
    If I was wealthy there is one huge change id do in the form of home energy consumption ---- I would have two huge insulated tanks buried in the ground under or next to my house...

    I would then have a series of collector panels - the panels would track the summer months (and a little in the winter during mid day) and expose maximum surface area and super heat one of the tanks --- in the winter they then get re-directed and feed the other tank at night, - The biggest real challenge would be coming up with a cost practical coolant/boil over fluid that's non corrosive.

    Both tanks would be thermostatic solenoid controlled, I tiny little heat pump would do the circulation when needed.
    At least where I live it would take care of all heating and cooling energy needs and depending on the amount of panels and storage even electricity if you ran a highly efficient stirling heat pump between the variances (most stirlings use variances in air temp on their heads - these would use fluid which would make them more powerful for their displacement if the same variances are used)

    heat or cool would be suckled off each tank when needed to keep the house livable - the house would be designed super efficient - and also for solar gain in the winter months.


    This system would cost a fair amount of money but once built would cost zero to run except for occasional maintenance.

    To me it makes sense that If your going to have panels for energy collection don't just use them for one end of the spectrum (either just hot or in the OP just cool)
    Make the panels do double duty and Go for the opposing variance as that's not only the most direct measure for what we need during the seasons, its also a very efficient on demand storage devise for energies that can be utilized elsewhere.

    The thing that keeps us from doing this is these energies of course happen at opposing times of the year and of course when we don't want any part of them ---- but the immense capabilities (and thermal mass) of two well insulated storage tanks make it possible to use the seasons against each other.

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  • Evan
    replied
    Making Ice by night sky radiative cooling was known by the ancient Arabs. They would build a small culvert behind a thick wall that kept the culvert and that side of the wall in shadow during the day for a good part of the year. At night a thin layer of water would freeze producing ice for consumption the next morning. The method could also be used to purify brackish water.

    The method relies on the low effective temperature of the night sky as Dennis has already related. However, the object being cooled must have a view of only the night sky. If any other object is in the field of view of the cooled surface it will radiate IR that will warm the cooled surface. That includes other structure and vegetation such as trees. Think in terms of illumination. If a tree is in the field of view it will be illuminated by IR emitted by the ground, other objects and the cooled surface itself which will then be reflected from the tree. If the cooled surface can "see" it then it will be warmed by the reflected IR.

    This problem can be lessened by shielding the cooled surface with baffles that prevent it from "seeing" any warmer object. The baffles should be made of an insulating medium and should be reflective on both sides.

    In order to analyze such a system one may simply turn the situation upside down and imagine the cooled surface as a cold collector rather than a radiator. One then plans the orientation and sheilding so that it can collect the maximal amount of cold.

    I have a designed and a partly constructed prototype for a refrigeration unit that operates on this principle. It would serve as a portable cooler the size of a large ice chest and could be used to provide refrigeration for vaccines. I think I should finish the development of this device as it can be built for minimal cost and requires zero maintenance or tech training to operate.

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  • dp
    replied
    Originally posted by ckelloug
    It is my understanding that radiating heat to the night sky was actually used by the ancient romans to make ice. Don't know where I saw the reference though.

    --Cameron
    It is a part of what causes black ice on bridges and frost on your windshield over night, even if the air temperature doesn't drop below freezing. Bridge decks are large radiating surfaces without a lot of thermal mass, and much of what heat they have is carried off by convection as well as radiation. The radiation is what can cool the deck below ambient temperature.

    It is a low cost energy differential that can be exploited - Stirling engines come to mind, that run on the imbalance of collected daytime solar energy and night time energy loss to do radiation.

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  • ckelloug
    replied
    It is my understanding that radiating heat to the night sky was actually used by the ancient romans to make ice. Don't know where I saw the reference though.

    --Cameron

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  • dp
    replied
    Originally posted by J Tiers
    How do you exceed the normally available cooling? You need a surface with a better emission.
    You radiate it out into space. In fact you emphasize the natural tendency of energy to radiate out into space. Heat moves from where it is to where it is not and there's not much in space. As Evan says, the night sky as measured from the surface is -70؛ C. Were there no atmosphere to radiate heat back at us, the sky would be closer to 3؛ Kelvin.

    The trick in radiating heat is to keep the radiator hot, so you bring heat to it and carry away the chilled component that coveys the heat. In many cases this is water. Warm water is spread in a thin layer on a sky facing surface which is chilled below ambient by the radiation of heat into space, chilling the water at the same time. That chilled water is collected and used later. A large parabolic dome with a sprinkler at the top center would do the trick.

    You also need cloudless nights, as clouds hold in heat.
    I mentioned the need for clear nights. The requirement is for the sky to be cooler than the radiating object. The greater this difference the greater the watts radiated.

    Basically, anything that obstructs heat emission, like CO2 (to a small extent) or water vapor (a huge contributor), will keep your cooler from working as well. Absent clouds the effect won't be so much.
    The night sky in Arizona is very cold. Not talking about the air - it's the night sky that enables the heat transfer.

    Strikes me that solar-powered cooling is about as green as nighttime heat loss.... the heat input was already there, although you may be reflecting less of it.

    Nothing stops you from doing both. One offsets the other, and the combination should be much more effective than the radiative cooling alone.
    Absolutely. Nothing prevents someone from using both, but not so many people are aware of the night sky radiator effect.

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  • J Tiers
    replied
    How do you exceed the normally available cooling? You need a surface with a better emission.

    You also need cloudless nights, as clouds hold in heat.

    Basically, anything that obstructs heat emission, like CO2 (to a small extent) or water vapor (a huge contributor), will keep your cooler from working as well. Absent clouds the effect won't be so much.

    Strikes me that solar-powered cooling is about as green as nighttime heat loss.... the heat input was already there, although you may be reflecting less of it.

    Nothing stops you from doing both. One offsets the other, and the combination should be much more effective than the radiative cooling alone.


    BTW, the rich folks in north Africa and the middle East did something like that, using air as an intermediary.

    Radiation cooled the ground, and thus the air. In the daytime, a fairly massive chimney-like device heated up, and drew out hot air. That air came in through a brick "checkerwork" which was cooled the previous night by the cooled air.

    The hot chimney continued to pull air in at night, exhausting hot air, and pulling cool air into the house through the checkerwork, cooling it for the next day's heat.

    The house presumably kept at a pretty stable temperature.

    I live in a brick house, and we do a version of that.

    95% of our cooling is via a 1/6 HP window fan, since we don't have the heat chimney. We also have just one small AC unit, that runs for a week or so in summer when night temps stay over 80 F. And, we live in a place with hot and sticky summers. This while our neighbors have their A/C roaring to keep their houses colder in summer than they would tolerate in winter........ go figure.
    Last edited by J Tiers; 09-20-2009, 12:00 AM.

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  • dp
    replied
    Originally posted by J Tiers
    A somewhat more direct way of cooling is to use heat to cool. Gas refrigerators are fairly common, depending on where you are.

    Once you realize that you want the cooling most when the hottest sun is available, and that "coolness" can be stored, you are on your way. Absorption refrigeration. It is already being used in some Caribbean resorts.

    And my own workplace is currently cooled by a gas refrigeration system, which we will, when the time is available, see about converting to alternately use the "solar furnace".
    The technology is used to chill water overnight to provide cooling during the day. I'd done some additional reading and one site suggested radiative heat transfers of 500 w/meter ^2 which is a bit more than 1/3 of the maximum incoming radiation energy. That seems a bit optimistic, but may stand up to verification.

    As this is energy lost to space it represents a net cooling and so is very global warming friendly. It works best in areas with clear nights and dry air - that would be places like Arizona.

    There may be enough efficiency to keep a shop comfortable for a whole lot less than AC costs.

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  • J Tiers
    replied
    A somewhat more direct way of cooling is to use heat to cool. Gas refrigerators are fairly common, depending on where you are.

    Once you realize that you want the cooling most when the hottest sun is available, and that "coolness" can be stored, you are on your way. Absorption refrigeration. It is already being used in some Caribbean resorts.

    And my own workplace is currently cooled by a gas refrigeration system, which we will, when the time is available, see about converting to alternately use the "solar furnace".

    Leave a comment:

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