[icegreentea]

Don't know what happens if there's condensation. As for orientation, here's the paper.

http://www.nature.com/nature/journal/v515/n7528/full/nature1...

You're looking for Extended Data Figure 1b). It shows relatively constant emissivitiy from 5 through 60 degrees of incident light.

Speculating on water condensation. This would probably lower its efficiency. That said, they only claim cooling of ~40Wm^-2, corresponding to ~5 degrees below ambient. Realistically, this stuff is most useful when ambient is above ~20-25 degrees (this is when most people want cooling), where the drop in temperature by ~5 degrees shouldn't cause too much of a condensation problem. Of course, that's just one application.