I did watch a real good set of videos by Tech Ingredients, the chap’s gotta be either a lecturer (and) or his special interest is DIY-ing everything from pulse detonation rocket engines (that turned out better than the ones ol muskrat’s using) to using an underground heatsink made of plastic tubing and carbon saturated cement outside his workshop for an in-window AC unit, turning an 800W drain down to a 300W drain for the same cooling.

He was using some kind of fine particle paint to create a large Infrared radiating surface that shed heat so well it got below ambient air temp because it was shedding IR directly up out into space

The only unfortunate limit is shielding it from catching any outside radiation and making sure all the IR actually leaves the system, he had to build a shade to protect it from trees and buildings which would have been effectively shining IR back into the system.

This video;

https://youtu.be/dNs_kNilSjk