Earlier quoted context omitted.
As in when it’s 36 outside?
Units, dammit!
Clear F. I don’t see you passively cooling anything to below freezing when it’s 36C outside.
Using a solar oven as a radiant refrigerator at night
31–40 of 94 posts
Re: Using a solar oven as a radiant refrigerator at night
#32I’m missing the basic premise here, how does pointing an insulated box at the sky at night cool whatever’s in the box?
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Re: Using a solar oven as a radiant refrigerator at night
#33Earlier 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!)
Snow is a near perfect black body radiator
Therefore imagine the impact of reduced snow and ice due to global warming. It’s free cooling and we’re throwing it away.
Re: Using a solar oven as a radiant refrigerator at night
#34Many 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.
Re: Using a solar oven as a radiant refrigerator at night
#35If you insulate it in vacuum with an IR transparent window (unfortunately $$$) you can get substantially cooler than ambient: https://www.nature.com/articles/ncomms13729
this is a great paper, thanks
a thing to keep in mind with research papers is that in many cases they're concerned with finding how to get the largest effect, or demonstrate the effect in a way that's most clearly due to the reasons they claim and not some experimental error, or measure the effect most precisely, and only in rare cases are they concerned with how to get the effect most cheaply, a consideration which conflicts with the others
there are a lot of cheap materials that are transparent in the thermal infrared, like ldpe, potassium chloride, sodium chloride, silicon, and rock crystal. they're mostly a pain in one way or another (not that i have any experience with this)
Re: Using a solar oven as a radiant refrigerator at night
#36If you insulate it in vacuum with an IR transparent window (unfortunately $$$) 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.
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 significantly less than 1 square meter as seen from the sky and also works when it's cloudy or humid outside
basically forced-air convection heat transfer is a motherfucking miracle, and vapor-compression refrigeration even more so
Re: Using a solar oven as a radiant refrigerator at night
#37If you insulate it in vacuum with an IR transparent window (unfortunately $$$) you can get substantially cooler than ambient: https://www.nature.com/articles/ncomms13729
Checking my understanding here: in a very simplified sense, is this saying that we’re taking energy, be it from ambient temperature during the evening or that plus solar radiation during the day, and essentially bottlenecking/‘forcing’ it to radiate in the spectrum that’s atmospherically transparent and can thus escape to space? Seems the use of insulating it in a vacuum would be to minimise heat loss until it’s cooled to the respective temperature that radiates in that specific spectrum.
Incredibly cool (hah) paper to read, thank you.
Re: Using a solar oven as a radiant refrigerator at night
#38If you insulate it in vacuum with an IR transparent window (unfortunately $$$) 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.
If my napkin math is right, solar panels give around 2x higher electricity production per square meter than these devices give cooling power. Then that goes into a COP of ~3 for your aircon, and the solar panels have have a factor of 6x better cooling performance per area covered.
Then factor in that electricity can be used for lots of other useful things than cooling, and that solar power variation during the day is perfectly matched with cooling demand, it's a no brainer.
Re: Using a solar oven as a radiant refrigerator at night
#39I’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!)
> as the nearest surroundings are the hot planet, which transfer heat more effectively!
The box is insulated. In one case it's an unused refrigerator that opens on the top with more insulation apparently surrounding it. The planet may be hot but on a cold night the ground does nothing to warm me. Soil is not a good conductor of that heat at the surface nor is the air between it and the container. The planet stays "hot" because it is "insulated" as well.