Your task, should you accept it, is to make ice in a typical British summer. Your system must be 100% passive, so the following aren't permitted: electricity, fuel, motive power, compressed gas or vacuum. Evaporating a chemical to produce cold is also not allowed. Placing your water in contact with something else pre-cooled isn't permitted. Importing supercooled water is not permitted.
The obvious question is: is it even possible? If it is, how?
NT
Yes that was excluded. What I had in mind has been partially mentioned.
Outer space is well below freezing, and we can use that on earth. Silver tape line a large parabolic dish or similar. Well insulate the outside. Cut plastic discs of assorted diameters & sit them in the bowl to create glazing layers. Point the dish upward & place water container in dish under the glazing. Nights with heavy cloud cover will not freeze.
This came up as part of answering the question how could one take completely barren desert & make it livable & workable.
NT
I was hoping you would provide more in view of the many points raised in this thread which you do not address there. It leaves me with many questions. But I'll confine myself to just 3:
a. just what do you mean by "silver tape"? I ask as I have used various tapes which have a silver colour but none came with specification as regard their ability to reflect in the "atmospheric window" band;
b. where can we see pictures of this device and reports of its performance - stuff like date(s) and place(s) operated, quantity of ice produced?
What happened to c?
Virtually all glazing apart from germanium and diamond are strong attenuators/absorbers of infrared. So I have no idea how anything below "plastic discs of assorted diameters", are going to see anything other than ambient temperatures and lose absolutely no heat.
Hence my suggestion about using cling film, which is thin enough to provide very little attenuation in the IR so the water below could see the cold space.
Certainly a parabolic reflector below would assist in providing a near
360degs of view to a cold sky. Except a near flat reflector would do the same. Aluminium foil would be an idea reflector material.Anything just "silver" in colour would normally be useless where pigments are generally ineffective in the IR wavebands where all that would be seen is the paint base material. This would be very IR absorbing unless of course it is germanium or diamond, unlikely and of course very expensive.
I don't think TB has really thought this one through. Perhaps he can demonstrate his idea and put it on Youtube?
On 10/09/2020 21:13, Tim Streater wrote: <snip>
A person from Porlock.
Thanks for those comments - though I was rather hoping to have NT's views first.
AIUI a simple, all-round, horizon-to-horizon (2 x Pi steradian) field of view is rarely optimal for radiative cooling because the radiation temperature increases from the zenith to the horizon. There is a threshold angle at which it is better to stop the target "seeing" the sky. That can be done simply by walls. But a parabolic adds the benefit of the target "seeing" only the sky near the zenith where the sky is coldest.
aluminium tape.
I've not kept notes, it's not something I plan to do in future. If you're out in the desert with no power, options are very limited, this has at least been done.
NT
film is cheaper too
Certainly you don't need parabolic, you just need to exclude anything on earth from the water's view. A flat reflector won't do that.
silver coloured tape is normally ali foil.
Others have done it. I saw this stuff back in the 90s, I have no notes kept. I think the idea dates back at least to the 1960s, not certain on that.
NT
And might transit some of the IR radiation, unlike your several "plastic discs of assorted diameters" which might just as well be discs of wood or any IR absorbing material.
No it's not:
Not with several "plastic discs of assorted diameters".
I also see lots of things that are not credible.
I would normally be surprised if an engineer specified silver tape when they mean aluminium given the wide number of silver coloured cloth tapes sold.
I don't think much of Britain counts as a desert, so we don't have empirical evidence or detailed modelling to show it works "in a typical British summer".
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