This should probably be its own thread, but when I discovered that they've developed white paint that passively cools the surface over 15 degrees below the ambient temperature even in direct sunlight. It reflects 98.1% of sunlight. They've tuned it to the band of infrared heat that escapes directly into space. That blows my mind. If you covered your entire roof with this it would passively cool about the same energy as a home AC system. They're currently testing panels with circulating water for AC and refrigeration units.