Never seen this before.

[WetSaw]

Water filter on the end of some solar panels heating a pool. On further inspection I found this

I'm not a plumber but I'm quite confident that non return valves should have something in them!

 

[Chalker]

No need for a filter on solar, unless it's direct fed to hot water. Either way it should be out of the sun.

 

[WetSaw]

Exactly, not much point filtering water once it's actually in the pool and they don't like being in the sun. Just surprised that it was fitted so close to the solar panels and the non return valve having no insides!