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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

#51

Post by voisin »

madaxe_again wrote: post #47
dave333 wrote: post #22

This can also be used to heat and cool a building. Harold Hay experimented with a "Skytherm roof" back in the 1970s and built a house at Atascadero, CA. https://www.sciencedirect.com/science/article/abs/pii/S09601... Harold Hay LA Times article: https://www.latimes.com/archives/la-xpm-2007-nov-10-fi-harol...

This is how buildings heat and cool, mostly - every part of a structure that can see the sky allows radiative heating AND cooling . People forget about the second part. And this is why we built our current house on a north facing slope, in a forest - against all common wisdom about siting. Place stays cool in the summer, warm in the winter - the other side of the valley literally caught fire this summer, and gets int…

> And this is why we built our current house on a north facing slope, in a forest - against all common wisdom about siting.

Can you comment or give more detail about the level of tree coverage required to have a noticeable effect of radiative cooling?

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

#52

Post by stewpy »

madaxe_again wrote: post #47
dave333 wrote: post #22

This can also be used to heat and cool a building. Harold Hay experimented with a "Skytherm roof" back in the 1970s and built a house at Atascadero, CA. https://www.sciencedirect.com/science/article/abs/pii/S09601... Harold Hay LA Times article: https://www.latimes.com/archives/la-xpm-2007-nov-10-fi-harol...

This is how buildings heat and cool, mostly - every part of a structure that can see the sky allows radiative heating AND cooling . People forget about the second part. And this is why we built our current house on a north facing slope, in a forest - against all common wisdom about siting. Place stays cool in the summer, warm in the winter - the other side of the valley literally caught fire this summer, and gets int…

Which hemisphere are you on? If you're in the southern hemisphere do you want to have a south-facing slope(aka away from the sun)?

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

#53

Post by mamediz »

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 this effect?

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

#54

Post by s0rce »

ShakataGaNai wrote: post #42
simonblack wrote: post #34

Many years ago, I had an indoor swimming pool with solar heating. Some days in the middle of summer that water got too warm. So I'd run the water through the black plastic solar collectors at night-time to throw away that excess heat.

I installed a new pool solar (water) heating system a few years ago. It's got electronic controls for when to send the water up based on temperature etc. It has an integrated cooling mode, which does exactly what you described. Not useful for my location but I've learned in more warm climates it can be quiet a problem.

Better than this https://www.youtube.com/watch?v=lcnr6YiT-g8

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

#55

Post by s0rce »

JohnVideogames wrote: post #8
jack_riminton wrote: post #2

I’m missing the basic premise here, how does pointing an insulated box at the sky at night cool whatever’s in the box?

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!)

The black body temperature of the night sky on earth is much warmer than the temperature of the CMB in the universe. You can test this by aiming an IR temperature gun on a clear night.

Relevant stack exchange: https://physics.stackexchange.com/questions/153839/what-is-t...

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

#56

Post by lukan »

madaxe_again wrote: post #47
dave333 wrote: post #22

This can also be used to heat and cool a building. Harold Hay experimented with a "Skytherm roof" back in the 1970s and built a house at Atascadero, CA. https://www.sciencedirect.com/science/article/abs/pii/S09601... Harold Hay LA Times article: https://www.latimes.com/archives/la-xpm-2007-nov-10-fi-harol...

This is how buildings heat and cool, mostly - every part of a structure that can see the sky allows radiative heating AND cooling . People forget about the second part. And this is why we built our current house on a north facing slope, in a forest - against all common wisdom about siting. Place stays cool in the summer, warm in the winter - the other side of the valley literally caught fire this summer, and gets int…

"~4h of sunlight and 20h of radiation - our more sheltered side gets 0h of sunlight, but the slope and tree cover drastically reduce radiative cooling at night"

I still would always take that 4h of sunlight over 0h and just invest in better insulation. Don't you miss the sun in winter?

But if you don't, well, then it makes sense. North facing sites are usually quite cheaper as well.

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

#58

Post by toast0 »

londons_explore wrote: post #28
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

Why isn't this replacing aircon over the whole world? Unless that IR-transparent window is made of gold dust, it has to work out cheaper than the energy cost of running an air conditioner.

Adding to everyone else, but local climate makes a big difference. This isn't very useful unless you have dry still air, and it works best at night, but could maybe work during the day if you have a good view of the sky but have blocked the sun.

Most places with dry still air don't need specific cooling at night, because "everything" already cools due to this effect, and it doesn't stay hot for long after the sun goes down. Areas that need a lot of cooling overnight tend to be humid, which disrupts radiative cooling. Also, it doesn't take much airflow for convection to transfer more heat than radiation, and most places have variable wind... so you can't really count on it.

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

#59

Post by mianos »

kragen wrote: post #36
londons_explore wrote: post #28

Earlier quoted context omitted.

Why isn't this replacing aircon over the whole world? Unless that IR-transparent window is made of gold dust, it has to work out cheaper than the energy cost of running an air conditioner.

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.

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

#60

Post by madaxe_again »

lukan wrote: post #56
madaxe_again wrote: post #47

Earlier quoted context omitted.

This is how buildings heat and cool, mostly - every part of a structure that can see the sky allows radiative heating AND cooling . People forget about the second part. And this is why we built our current house on a north facing slope, in a forest - against all common wisdom about siting. Place stays cool in the summer, warm in the winter - the other side of the valley literally caught fire this summer, and gets int…

"~4h of sunlight and 20h of radiation - our more sheltered side gets 0h of sunlight, but the slope and tree cover drastically reduce radiative cooling at night" I still would always take that 4h of sunlight over 0h and just invest in better insulation. Don't you miss the sun in winter? But if you don't, well, then it makes sense. North facing sites are usually quite cheaper as well.

We own both sides so the siting was by choice - the place is well insulated but having a more moderate ambient temperature outside, and having the windows shaded from the cold of space make a big difference.

As to the sunlight - while we don’t get any directly into the house for 2.5 months in the winter, the far side of the valley, which is practically barren, ends up brightly lit, bringing a lot of indirect light in.

Finally, for us, the summer is a killer - it can be over 40 for weeks on end, and the nights on the shadier side are much cooler, as we don’t have superheated rocks re-emitting IR, instead the canopy reflects and absorbs most of the solar irradiance. The daytime air will still be sweltering, but it doesn’t transfer heat anything like direct insolation. So the effect is more about the environment around the structure than the structure itself - although regarding the house the roof is exposed to the sky, as it’s heavily insulated and doesn’t matter, but the walls, which have a decent thermal mass being solid lumber, are always shaded, as are the windows, mostly through self-shading due to the orientation. Again, punishingly bright in there in the summer even without direct sunlight - most folks near us just eschew windows, or cover them with heavy shutters and so have no light and no view in the summer.

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