I think the title says it all. I see these used hoops offered for free regularly. The base is generally a black plastic water tank. They are flattish and quite robust. They get hot in the sun. One of these mounted on a south facing surface, could heat water and only a little plumbing is required. A covering of clear glass or plastic would improve efficiency.
That's about it. Dale strikes again.
I brought this back from the farm where they grow the tiny ads: