Air Conditioning isn't free... but we're close.
Why Hacker News recommends it
HN points to it in threads on radiative cooling as a hands-on explanation of passive daytime cooling.
What it is
Tech Ingredients shows how to build a liquid panel with a specialty paint that, placed outside, cools below ambient air temperature. It demonstrates radiative sky cooling.
Recommended by 1 HN commenters across 1 different months, 2023 to 2025.
What Hacker News said
Recommends energ8 · 2025-06 · on “NASA's Voyager Found a 30k-50k Kelvin "Wall" at the Edge of Solar System”
It's a thing in from thousands of years ago [LINK] and today [LINK] for PDRC there are a couple good videos about it from NightHawkInLight [LINK] , [LINK] and Tech Ingredients [this video] [LINK]
Cites jcims · 2023-03 · on “Just How White Can White Paint Get?”
>Only active cooling can cool anything below the ambient temperature. Not exactly: [LINK] Couple of videos explaining the effect [LINK] [this video]
Cites simonjgreen · 2023-07 · on “Making Radiative Sky Cooling Paint (With Novel CaCO₃ Microsphere Synthesis) [video]”
There is a detailed example of this uploaded a few months ago here [this video] [In fact, I just noticed the author of the OP video posted on this one]
Cites p1mrx · 2023-11 · on “Passive radiative cooling ceramic with high solar reflectivity”
That problem can be solved by adding a mirrored ring around the cooling surface, so it can only "see" the sky: [this video]