Hi MattM,
The Pex looks fine to me so far. The black paint over the PEX "ends" is still sticking good so It will probably be fine for years.
Of course, the majority of PEX is covered with the aluminum absorber strips. And if one wanted to, you could always cover the "U-turn" PEX ends with a 10" wide piece of aluminum flashing to completely block UV from ALL the PEX.
Thats another reason I wish I had made a bigger 4x8' collector out of those Polycarb panels instead of tempered glass. The Polycarb panels have 100% UV protection they claim, and less heat loss than glass. (single pane)
For my other projects I got the Polycarb panels from Lowes for just $20 ea.
They are called Tuftex.
http://tuftexpanel.com/
They certainly are tuff because I had them laying in my living room floor for weeks, and kept accidently stepping on them in the dark numerous times! Left no mark at all.
-Stores stock very few of these I found out. So I'm going to get some more panels NOW for future projects, before the world ends in a few weeks...
http://online.wsj.com/article/SB10001424052748703574604574500580285679074.html
-Thanks.