Energy FAQ

Question:

I’m planning to get some coolth from exposing a collector to the cold black night sky.  But can anybody tell me if it will help to mount reflector wings on the side, as it helps when you’re doing flat-plate solar heat collecting during the day?  - Bill Fisher p.s. my guess is that they will help, at least when you have them blocking the colletor’s view of the relatively hot terrestial environment, and converting that into a view of the sky.

Response:

I guess your main heat losses will be conduction/ air convection and NOT radiation (unless you have a particularly cold night), which means that the reflector will make little difference. The principle is correct, however. adrenalin

> I’m planning to get some coolth from exposing a collector to the > cold black night sky.  But can anybody tell me if it will help to > mount reflector wings on the side, as it helps when you’re doing > flat-plate solar heat collecting during the day? >  - Bill Fisher > p.s. my guess is that they will help, at least when you have them > blocking the colletor’s view of the relatively hot terrestial environment, > and converting that into a view of the sky.

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Response:

> I’m planning to get some coolth from exposing a collector to the > cold black night sky.  But can anybody tell me if it will help to > mount reflector wings on the side, as it helps when you’re doing > flat-plate solar heat collecting during the day? >  - Bill Fisher > p.s. my guess is that they will help, at least when you have them > blocking the colletor’s view of the relatively hot terrestial environment, > and converting that into a view of the sky.

Well, *assuming* no conduction/convection is involved, no I don’t think they would help.  If a given amount of energy is radiated from your ‘collector’ at night, the path the radiation takes after leaving it isn’t too important (unless the reflector actually reflects it back to the collector surface). To maximize radiant cooling, I should think you want to fold the reflectors down flat away from the collector.  This would maximize the angle that the collector can radiate thru to the night sky. daestrom

Response:

– Hide quoted text — Show quoted text -> I’m planning to get some coolth from exposing a collector to the > cold black night sky.  But can anybody tell me if it will help to > mount reflector wings on the side, as it helps when you’re doing > flat-plate solar heat collecting during the day? >  - Bill Fisher > p.s. my guess is that they will help, at least when you have them > blocking the colletor’s view of the relatively hot terrestial environment, > and converting that into a view of the sky. > Well, *assuming* no conduction/convection is involved, no I don’t think they > would help.  If a given amount of energy is radiated from your ‘collector’ > at night, the path the radiation takes after leaving it isn’t too important > (unless the reflector actually reflects it back to the collector surface). > To maximize radiant cooling, I should think you want to fold the reflectors > down flat away from the collector.  This would maximize the angle that the > collector can radiate thru to the night sky. > daestrom

My feeling is that they would help, but to see much benefit you’d need a clear sky. Your radiator will radiate energy depending upon its temperature and the surface material. It will also absorb radiated energy striking it. Whether the body cools or heats up (for radiated energy) will be dictated by which energy flow is higher. By placing the body in the reflector, you are receiving minimal energy (from the direction of the reflector surface), but the radiated energy flow is maintained. The best part is that the reflectors would not need to track any part of the sky. In fact the reflector surface could be extended beyond the radiator to decrease the radiation from the surroundings. Whether it would be worth it is another matter. (You still have ambient temperature and conduction to think about of course). What do you think? Simon.

Response:

Simon says: >Whether it would be worth it is another matter. (You still have ambient >temperature and conduction to think about of course). >What do you think?

 Well, it depends on how cheaply you can make the cooler and how dear electricity is.  I hope I can get some free time in a couple of weeks to do some actual experimenting. Even if it’s not economical, I may try to do it anyway: it’d really be a kick to be able to freeze a little ice in August this way.  I found a paper at this site: http://wire0.ises.org/wire/doclibs/SWC1999.nsf/0/e678b47954fc0b69c125… 03d61a4?OpenDocument  that claims to get 40 watts/sq meter of cooling power with their cooling radiator design. A small room air conditioner might take 6000 btu/hr  =~ 1758 watts, so you could get the equivalent with a collector of 44 sq m, or about 400 sq ft, about 20′ x 20′, unless I’ve mis-calculated.   And I’m trying to figure out a way to pull & hold a vacuum around the collector, which should minimize conductance losses. And if it’s a flat plate, covering the back & sides with shiny aluminum foil should minimize radiative losses to the earth.  I haven’t found a real good source for cheap flat copper or aluminum for flat plates, so my first real experimental system may be an evacuated-tube design, using copper tubes sold for plumbing, a transparent surrounding tube made from a flourescent-light protector that Lowe’s sells, evacuated with a small hand pump sold by Edmund Scientifics, and a large reflector made from cardboard covered with aluminum foil.  I plan to do some preliminary experiments using two wide-mouth thermos bottles side by side, each lidded with a sandwich of two sheets of plastic or glass with a vacuum between, one with a large funnel reflector and one without, each with an electronic thermometer inside. This should tell me what cooling power I can typically get, and whether or not a reflector really helps.  And I’m going to sneak some flat wires out of the thermos bottles running to my computer, so I don’t have to sit up all night recording data.  - Bill F.

Response:

>   And I’m trying to figure out a way to pull & hold a vacuum > around the collector, which should minimize conductance losses.

It would also bounce the radiation (long wavelength) from the radiator back, preventing the cooling effect. One possible way of looking at the idea of using reflectors to cool is this. A flat plate insulated and silvered on the bottom is only radiating energy from half of its’ surface area. By using reflectors, you can double it, along with the radiative cooling effect. Perhaps as an experiment you could fill some copper tubes with warm water and make use of reflectors with one of them, and record how quickly their temperatures drop, and how low their temperatures go. Should be fairly quick and easy to do. Simon. Here’s a though. Since the radiation is long wavelength, the reflectors need not be shiny or opaque. Glass could do the trick!

Response:

– Hide quoted text — Show quoted text -> > I’m planning to get some coolth from exposing a collector to the > > cold black night sky.  But can anybody tell me if it will help to > > mount reflector wings on the side, as it helps when you’re doing > > flat-plate solar heat collecting during the day? > >  - Bill Fisher > > p.s. my guess is that they will help, at least when you have them > > blocking the colletor’s view of the relatively hot terrestial > environment, > > and converting that into a view of the sky. > Well, *assuming* no conduction/convection is involved, no I don’t think > they > would help.  If a given amount of energy is radiated from your ‘collector’ > at night, the path the radiation takes after leaving it isn’t too > important > (unless the reflector actually reflects it back to the collector surface). > To maximize radiant cooling, I should think you want to fold the > reflectors > down flat away from the collector.  This would maximize the angle that the > collector can radiate thru to the night sky. > daestrom > My feeling is that they would help, but to see much benefit you’d need a > clear sky. Your radiator will radiate energy depending upon its temperature > and the surface material. It will also absorb radiated energy striking it. > Whether the body cools or heats up (for radiated energy) will be dictated by > which energy flow is higher. By placing the body in the reflector, you are > receiving minimal energy (from the direction of the reflector surface), but > the radiated energy flow is maintained. The best part is that the reflectors > would not need to track any part of the sky. In fact the reflector surface > could be extended beyond the radiator to decrease the radiation from the > surroundings.

What you’re saying is true, *if* the reflector blocks the line-of-sight between the collector and some warm body here on Earth.  But I was thinking of folding any such reflector down so that there is a wider arc of *sky* exposed to the collector.  If the reflector is just between the collector and some portion of the sky, then the radiant flux from the reflector *must* be higher than the portion of the sky it obscures.  And in that case, no reflector is better. But yes, if instead it blocks radiant energy from some earth bound objects, then it would improve the cooling. daestrom

Response:

> > > I’m planning to get some coolth from exposing a collector to the > > cold black night sky.  But can anybody tell me if it will help to > > mount reflector wings on the side, as it helps when you’re doing > > flat-plate solar heat collecting during the day?

Lets see… objects on earth have temp determined by conduction and radiation to and from other objects around them, including air. If we vacuum insulate something, there is very little conduction, and temp is determined by the equilibrium of radiated heat to and from the object. Now if we add sky pointing reflectors all around our insulated object, the object still radiates but the sky radiates very little back. Dose this mean that oevr time this object would fall well below earth air temp? Regards, NT

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