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Forum Index : Solar : cool idea
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| AllanS Regular Member  Joined: 05/06/2006 Location: Posts: 67 |
Imagine a deep insulated box (with water at the bottom) pointed at the night sky (a heat sink with a temperature of about -70C.) The water should radiate heat away, but because of the insulation, it would not be re-heated from radiation from the ground, trees etc. It should keep on getting colder. Come the morning, put on an insulated lid and try again the next clear night. Heat lost at night through the open end would be far greater than heat gained in the day, if the insulation is good enough. Doing a rough calculation, a 1m2 sky-fridge with R5 insulation would freeze a ton of water in a couple of months. That would be a handy amount of refrigeration. Any engineers out there? |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hi Allan Yeah I've heard of this before, an trainee engineer I knew wanted to try it out, but I never heard if it worked. It makes sense that it should work, but I wonder if we are missing something. Even though the night sky is cool, the air is still warm, and I wonder if the air will radiate its heat into the chamber. I think its one of those things where you need to build it to see if it works. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Joblow Regular Member Joined: 05/01/2010 Location: AustraliaPosts: 54 |
I also have wanted to try this but never got round to it, I believe that the theory is based on the latent heat actually radiating out into space, if this is true, and providing the insulation is good enough it should work. The real test would be to try it with a can of your favourite beverage, then at least it wouldn't have been a pointless excercise  The man who never made a mistake never made anything |
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| AllanS Regular Member Joined: 05/06/2006 Location: Posts: 67 |
 You sound like a man with a good grasp on reality.
A thin covering transparent to IR would stop drafts. Cold air sinks, so that should help too. Radiation coming back down from the atmosphere is already factored in by making the sky -70C rather than -270C. A couple of sensors could automatically close the lid if it blew a gale, or rained, or if the sky clouded over. So where's the catch? One possible problem. The cooling rate (an equation found in Wiki: black-body radiation) is 300W/m2 for water at 20C. I imagine the heat would be coming off the surface at all angles, but in the sky-fridge, only heat leaving pretty-much at right angles would exit the opening. This would reduce the cooling power considerably. Reflective walls and floor might help. Imagine the power saved if every house in the world turned off their fridge. |
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| VK4AYQ Guru  Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi All On the farm in the early days, prior to 1952 we had a cooler called a Koolguardi food safe, It was a galvanized metal frame with a a trough on the top to hold the water the outer walls where made of Hessian and the watter soaked slowly down the Hessian, and any slight breeze refrigerated the water, it cooled by the latent heat of vaporisation. It was good for its time but not socially acceptable in this day and age, other half wants shiny white fridge, in the future who knows. It kept things cool enough to keep butter firm and stopped vegetables going off in the heat, from memory it kept things at about the minimum night time air temperature. The biggest problem I can remember was it we prone to mold and grandma had to wash it out with vinegar every few days. My grandfather was an innovator for his time and tried all sorts of things to make a freezer, the only one I can remember that worked before he did an electric one was a salt pot freezer along the style used to make ice cream in the 1800's unfortunately I cant remember how it worked, he eventually made one with an old open fridge unit as used in shops at the time and drove it with 32 volts generated at the water wheel, it worked up till the mid 70's when grandma died and they rebuilt the house. I experimented with a compressed air fridge freezer in the early days, it ran off the VAWT and was a success but I got vetoed by wife as she wanted a chest type freezer, and in those power was so cheap. On your idea I think that the overnight minimum temperature would be the limit of cooling as when both the water and the air are at the same temperature vaporisation of the water would nearly stop so very little latent heat carried away. In a dam the water stratifies to a degree and the surface is always warmer than at depth and that surface is where the maximum vaporisation takes place. Admittedly there is no insulation and the deeper water is warmed / cooled by geo thermal energy. All the best Bob Foolin Around |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
It's not so far-fetched. When I was in the middle-east (Jordan) 30 years ago, the locals (bedouin) would put out milk in a shallow pan, on some towels. We were seeing 46+ degrees C every day out there in the desert, yet at night the sky temperature was low enough for them to make icecream using this technique. There were several factors to this. The desert is very dry, so there is little moisture in the atmosphere and IR transmission (both ways) is quite good. The desert rarely had clouds in summer. (I think I was there for 4 months before I even SAW a cloud, much less any rain). At night, the air was generally quite still. Using a shallow tray was an important part of this, you want to get rid of as much heat as possible, compared to the total volume. |
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| AllanS Regular Member Joined: 05/06/2006 Location: Posts: 67 |
Hi Bob, My hypothetical fridge is sealed at the top to stop drafts. The air inside would be 100% humid and so no cooling would be caused by evaporation. It would cool purely because the night sky has a lower temperature than the water in the fridge. The heat has only one way to flow, and that is up and out. (The insulation stops heat coming in from the sides.) So long as the outflow at night is greater than the inflow during the day, the water should get colder. |
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| AllanS Regular Member Joined: 05/06/2006 Location: Posts: 67 |
Very interesting... Presumably most of that cooling would have been caused by evaporation more than by radiation leakage into the sky. Another thought. We get a hard frost a few times a year where I live. Suppose I have a straw-bale shed containing hundreds of coke bottles full of water. When a frost hits, a fan blows cold air past the bottles, freezing them. How much ice would you need for domestic refrigeration, I wonder, if you piped in cold air from the ice-shed to cool an upright fridge. At R-50, straw bales would keep the bottles cold for quite a while. |
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MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Crew I was reading old threads just for fun and ran across this one, which interests me. It seems to me, if what you're after is cooling, there are two main factors driving its success. First off, you'd need a source of delta T (temperature difference) which could be, as stated above, the night sky. Second, you need a plaece to store the working fluid (air or water). What I'm envisioning here is a heat exchanger coupled to a buried vault. An underground "receiver" insulated by several feet of dirt contacting its surface would likely not change temperature over the course of a year much more than only a very few degrees. There's likely research on this somewhere on the Web where a fellow could look it up and use it. Using a pump, a working fluid of water could be cycled from the receiver to a bare copper heat exchanger open to the night sky. When a sensor on the heat exchanger detects a heat difference between the exchanger and the receiver of a preset amount, the thing would run the working fluid around in a loop until the temperature gradient lessened, approaching zero or some pre-set cut-off point. If the outside temperature chilled further, the pump would again circulate the working fluid. Water is a perfect vehicle for this as it has a high specific heat. It takes 540 thousand calories to elevate one gram of water one degree Centegrade. A vat of water buried undergraound, the contents of which cycling through a heat exchanger on the surface would be a great way to capture heat (or its absense) for use elswhere, like in a cooler. Air could be blown through another exchanger (called an evaporator) to manufacture cooled air. Actual surface evaporation and latent heat would not figure into this arrangement, unless it rained, getting the heat exchanger wet, followed by a dry breeze, which would cause the water to subsequently evaporate and lose its latenet heat of vaporization, which would serve to cool the surface of the exchanger. If there were sensors on both the receiver as well as the heat exchanger element, rain and subsequent evaporation would enhance things even more by creating temporary cooling. This difference would be stored in the receiver, thus taking advantage of the situation. In the scorching heat of the day, as long as the pre-sets of the pump controls were set up correctly, the water in the surface-mounted exchanger would get hot, but not circulate to the buried (cooler) receiver and due to the physical arrangement, it would not thermo-siphon either. Point of fact: if you run water out of a hot line onto your hand, the temperature of the water "Just Before" the hot volume emerges, actuall decreases in temperature! I don't know WHY this works, I just know it DOES. So, the fact that the heat exchanger gets hot in the day MAY (or may not  ) cause the initial cooling due to exposure to the night sky to remove even more heat, I dunno.
If someone out there has lots of open space (Bob?) this might be something to try. If nothing else, it'd be a good place to store some emergency potable water, right? 
. . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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