Free Cooling
If you design your roof right you might never have to do any cooling at all
(The following post is slightly speculative engineering. It at least makes sense on paper. It hasn’t been vetted by an expert and may have some fatal flaw I don’t know about. But it’s at least plausible and is what I came up with after spending some time thinking about the problem.)
During the summer in warmer climates you need to expend energy for air conditioning to keep a house a livable temperature. But there’s a loophole. The sky isn’t all hot, only the sun part of it is hot. The rest of it is very cold. Polyethylene has the magical property that it’s a thermal insulator while letting through radiant heat, allowing you to cool off by touching the sky.
Let’s say that you want to build a house in Los Angeles, which has constant need for cooling and where the sun is always due South (It roams from East to West, but always at least a little South). Surprisingly the best way to build a roof under those conditions is to make a portion of the roof completely uninsulated, utilizing a technology you may have heard of called ‘shade’. The ideal arrangement of a roof looks something like this:
This design is a complicated set of tradeoffs between the cost of materials, the power from the solar panels, the amount of cooling, the pitch of the roof, and how evenly cooling happens. You can make cooling greater by allowing the sun to peek in a bit at high noon on the longest day of the year, but that few hours gets uninhabitable fast. The design’s effectiveness falls off fast if the house doesn’t have a wall facing directly South. For insulation ideally the East, West, and South walls should have no windows at all but short of that the windows should be well insulated and have outside shutters which are on motors to make them automatically shut when the sun is shining in and open at other times.
A very different geometry would be necessary directly over the equator where the sun is close to directly overhead but never all that far North or south. In places which are both too hot in the summer and too cold in the winter you’d ideally like to be able to completely reconfigure the roof between seasons but that’s probably impractical. It would be easier to have something which looks like the above but the whole roof is insulated and there’s a water recirculation system which routes through the cold western part of the roof in the summer and through the hot eastern part of the roof in the winter.
As a New Englander, my knee jerk reaction is "That will perform poorly with snow."
Great idea!! Simple and beautiful.
But: During the day, is the sky actually colder than ambient air? ChatGPT4 says it's not: " However, even when considering a part of the sky that is not directly exposed to sunlight, the sky temperature during the day is generally still not lower than the ambient air temperature. This is because the Earth's atmosphere absorbs and scatters solar radiation, which warms the air and contributes to a higher sky temperature even in areas far away from the sun"
Is it hallucinating and you found other data?
Also, even if it is a few degrees colder, the radiative cooling effect would be minimal, since heat transfer is proportional to DeltaT**4.
I read somewhere that it might work for other wavelengths... and indeed:
"The Earth's atmosphere consists of several gases, such as water vapor, carbon dioxide, and ozone, which absorb and emit radiation in specific wavelength bands. These absorption bands are known as "atmospheric windows," and they allow certain wavelengths of IR radiation to pass through the atmosphere with minimal absorption. For example, the 8-13 micrometer wavelength range is one of the most prominent atmospheric windows in the IR range.
If a surface can selectively emit thermal radiation in these atmospheric window regions, it can achieve radiative cooling even during the day because the emitted radiation can escape into space without being significantly absorbed by the atmosphere. In this case, the sky temperature, as seen by the surface at these specific wavelengths, can be lower than the ambient air temperature, allowing the surface to cool down.
Developing materials and coatings that have high emissivity in these specific wavelength bands and high reflectivity in the visible and near-infrared range (to avoid solar heating) is an area of active research in the field of radiative cooling. These materials can potentially enable passive cooling of surfaces during the day, even in the presence of sunlight."
(ChatGPT4)