It doesn't need to go back into space. If you don't want the scattered sunlight to heat your house, you can always add the same ceramic to your house...
I'm pretty sure that doesn't work. The surface works because it's emitting a band of radiation that the atmosphere is mostly transparent to. If it sees radiation in that same band it's going to absorb it, not emit. Point enough of these at each other and you'll approach zero.
The material works by having very low emissivity / absorptivity at sunlight wavelength so that sunlight is scattered back out, and by having high emissivity at typical earth temperature blackbody peak wavelength, so it radiates out heat effectively.
Sunlight that is scattered off the roof can scatter back off other roofs before heading into space. Likewise radiation that is emitted by the roof can be absorbed and then re-emitted by other roofs before heading off into space. Neither of these preclude the roof from doing its job.
At the end of the day, the roof will effectively reflect back sunlight from the high temperature sun, while radiating infrared radiation into the low temperature sky, and have a neutral energy balance with the surrounding earth / houses / trees / objects which are at a similar temperature to it.
> Sunlight that is scattered off the roof can scatter back off other roofs before heading into space.
… but that's not what they're asking about?
Imagine you're standing in a second story window, looking out. Out the window, you're seeing your neighbor's roof, coated with this stuff. A lot of sun is hitting that roof — How much heat is coming in through that window, off that roof?
There have been stories in the past of reflective surfaces cooking nearby apartments, so I get the concern. I think in this case it boils down to it not being reflective, mostly, so it's fine.
You’re missing the point here. Almost willfully so.
Two pieces of this material in an overlapping field of reflection are putting out exactly the wavelength the other will absorb. I can’t defend my building from being baked by yours by coating it with the same stuff. I am simply going to return the favor in the process.
Not to mention the big problem of creating a new case of the haves versus the have-nots.
Less than 70% on non-commercial structures. And on commercial structures the roof can be a negligible percentage of the total building envelope.
This is cool stuff, but it’s just stuff. It’s not a silver bullet. We here are all old enough, many of us by half, to no longer get sucked in by magic thinking.
