It started with [KB9ENS] looking into paints or coatings for passive or radiative cooling, and in the process he decided to DIY his own. Not only is it perfectly accessible to a home experimenter, his initial results look like they have some promise, as well.
[KB9ENS] read about a type of ultra-white paint formulation that not only reflects heat, but is able to radiate it into space, cooling the painted surface to below ambient temperature. This is intriguing because while commercial paints can insulate and reflect heat, they cannot make a surface cooler than its surroundings.
What really got [KB9ENS] thinking was that at its core, the passively-cooling paint in the research is essentially a whole lot of different particle sizes of barium sulfate (BaSO₄) mixed into an acrylic binder. These two ingredients are remarkably accessible. A half-pound of BaSO₄ from a pottery supply shop was only a few dollars, and a plain acrylic base is easily obtained from almost any paint or art supplier.
[KB9ENS] decided to mix up a crude batch of BaSO₄ paint, apply it to some things, and see how well it compared to other paints and coatings. He wetted the BaSO₄ with some isopropyl alcohol to help it mix into the base, and made a few different concentrations. A 60% concentration by volume seemed to give the best overall results.
There’s no indication of whether any lower-than-ambient cooling is happening, but according to a non-contact thermometer even this homemade mixture does a better job of keeping sunlight from heating things up compared to similarly-applied commercial paints (although it fared only slightly better than titanium dioxide-based white paint in the initial test.)
[KB9ENS] also painted the battery section of a solar recharger with his homemade paint and noted that while under normal circumstances — that is to say, in full sunlight — that section becomes too hot to touch, with the paint coating it was merely warm.
Actual passive cooling can do more than just keep something less warm than it would be otherwise. We’ve seen it recently used to passively and continuously generate power thanks to its ability to create a constant temperature differential, day and night.