You may want to check with the manufacturer to see if this is recommended or will void the warranty. It seems to me that most enclosed systems I've seen are in permanent roof-mount systems for household hot water--the "greenhouse" may well prevent freezing. But that's just a guess.

Usually, since it's for summer swimming, it's not an issue. Even if my panels can keep my water warm when it's 50 degrees and windy out, I'm not going in! (unless I have to repair a leak--been there, done that...brrrrr! THAT will convince you that a wetsuit is great to have around!)