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Using a solar oven as a radiant refrigerator at night

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Re: Using a solar oven as a radiant refrigerator at night

#81

Post by Enginerrrd »

mamediz wrote: post #78
K0balt wrote: post #69

Earlier quoted context omitted.

Fascinating! That does seem like the most likely explanation. This reminds me of the ancient ice ponds that made ice thousands of years ago in Persia. I read somewhere that they were able to make ice through a combination of radiative and evaporative cooling at night temperatures around the same as you experienced, about 5C.

Yes, I like this explanation, when that phenomenon happened in our house I thought one of our neighbours was dumb because he just put a blanket over the collector on the roof, as he said, "to keep it warm". But now I think he was right. Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve…

>Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating.

This doesn't exist and can't even in principle because it would be a fundamental violation of thermodynamics. Basically, it would be a Maxwell's demon for radiation that would allow you to arbitrarily reduce entropy.

Re: Using a solar oven as a radiant refrigerator at night

#82

Post by KMag »

Enginerrrd wrote: post #81
mamediz wrote: post #78

Earlier quoted context omitted.

Yes, I like this explanation, when that phenomenon happened in our house I thought one of our neighbours was dumb because he just put a blanket over the collector on the roof, as he said, "to keep it warm". But now I think he was right. Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve…

>Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating. This doesn't exist and can't even in principle because it would be a fundamental violation of thermodynamics. Basically, it would be a Maxwell's demon for radiation that…

But, it might be possible to have a material that has different characteristics at different temperatures, as long as it's symmetric. If it's nearly opaque to IR in its cold state, hopefully sunlight at dawn would warm it rapidly enough that it would automatically "shut off" on cold nights, and still "turn on" shortly after dawn even on cold days.

Re: Using a solar oven as a radiant refrigerator at night

#83

Post by smallnamespace »

mamediz wrote: post #53
contemporary343 wrote: post #25

If you insulate it in vacuum with an IR transparent window (unfortunately $$$) you can get substantially cooler than ambient: https://www.nature.com/articles/ncomms13729

Here in Brazil we use a system with copper pipes in an insulated "box" to heat water during the day using this energy from the sun. A couple of years ago, in a cold winter day with a minimum of 5 degrees (celsius), just after the sunrise the water froze, broke the pipes and the glass above it. I couldn't understand what happened because the ambient temperature was above freezing point, maybe it was something like thi…

The handwavey explanation in my head is that “ambient” for this emitter is not just the immediate physical environment, but also deep space, which is very very cold. This wouldn’t work if the temperature of space and the air were the same.

An emitter is also an absorber so if space were as hot as the emitter then it would not shed heat.

Re: Using a solar oven as a radiant refrigerator at night

#84

Post by sgc »

KMag wrote: post #82
Enginerrrd wrote: post #81

Earlier quoted context omitted.

>Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating. This doesn't exist and can't even in principle because it would be a fundamental violation of thermodynamics. Basically, it would be a Maxwell's demon for radiation that…

But, it might be possible to have a material that has different characteristics at different temperatures, as long as it's symmetric. If it's nearly opaque to IR in its cold state, hopefully sunlight at dawn would warm it rapidly enough that it would automatically "shut off" on cold nights, and still "turn on" shortly after dawn even on cold days.

Light meter controlled / tracking louvers would be the quick fix.

Re: Using a solar oven as a radiant refrigerator at night

#86

Post by jack_riminton »

zettabomb wrote: post #17
JohnVideogames wrote: post #8

Earlier quoted context omitted.

A black body will emit radiation (and receive radiation) until it's in equilibrium with its surroundings. In theory, the black bit of box radiates heat into space, and receives some of the CMB until both are at ~3K. It's how we cool satellites and spacecraft. But the effect is quite small, and I'm suspicious of it (as the nearest surroundings are the hot planet, which transfer heat more effectively!)

It's possible to make ice in the desert with this effect. Not too hard to believe.

It’s also commonly freezing in the desert at night

Re: Using a solar oven as a radiant refrigerator at night

#87

Post by tim333 »

mamediz wrote: post #53
contemporary343 wrote: post #25

If you insulate it in vacuum with an IR transparent window (unfortunately $$$) you can get substantially cooler than ambient: https://www.nature.com/articles/ncomms13729

Here in Brazil we use a system with copper pipes in an insulated "box" to heat water during the day using this energy from the sun. A couple of years ago, in a cold winter day with a minimum of 5 degrees (celsius), just after the sunrise the water froze, broke the pipes and the glass above it. I couldn't understand what happened because the ambient temperature was above freezing point, maybe it was something like thi…

Yeah, on a clear night the pipes radiate heat to space but nothing much radiates the other way as space is cold and dark. It's also why the tops of cars get frosty on clear nights.

Re: Using a solar oven as a radiant refrigerator at night

#88

Post by tim333 »

tintor wrote: post #85
contemporary343 wrote: post #25

If you insulate it in vacuum with an IR transparent window (unfortunately $$$) you can get substantially cooler than ambient: https://www.nature.com/articles/ncomms13729

Will it work equally well on nights with cloud cover?

No. It doesn't work the same as clouds are at ~0C+ whereas space is like -270C (~3K).

Re: Using a solar oven as a radiant refrigerator at night

#89

Post by Enginerrrd »

KMag wrote: post #82
Enginerrrd wrote: post #81

Earlier quoted context omitted.

>Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating. This doesn't exist and can't even in principle because it would be a fundamental violation of thermodynamics. Basically, it would be a Maxwell's demon for radiation that…

But, it might be possible to have a material that has different characteristics at different temperatures, as long as it's symmetric. If it's nearly opaque to IR in its cold state, hopefully sunlight at dawn would warm it rapidly enough that it would automatically "shut off" on cold nights, and still "turn on" shortly after dawn even on cold days.

Yes I think that'd be ok. You'd basically just be taking advantage of the thermal gradient between the radiating body and the object being heated. It'd be analogous to adding more insulation as it heats up and removing insulation as it cools down.

You'd be slowing the flow of heat into your reservoir just as much as you'd be slowing down the loss of that heat later though.

You might still get some benefit in preventing freezing at the expense of needed a larger area to get the same amount of heat flow in to your water system though.

Re: Using a solar oven as a radiant refrigerator at night

#90

Post by kragen »

mianos wrote: post #59
kragen wrote: post #36

Earlier quoted context omitted.

my air conditioner is rejecting 3000 watts to the outdoors right now, even though my bedroom is the same temperature as the nighttime outdoors. doing that with radiative cooling would require 8 square meters of emitter area at 400 watts per square meter and a clear, low-humidity sky (not necessarily night, but in the daytime, you additionally have to keep the sun off the panels). my air conditioner's condenser is sig…

I am disappointed you could not come up with a more concrete term for an improvement over a 'motherfucking miracle'. :) But, yes, heat pumps are magic shite.

magic shite!
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