Reading about using a water mass to keep greenhouses warm makes me think this is an easy way to heat up some water. I'm not sure on the management of the water and the dish and how long it could be left unattended, but this seems like a very simple and powerful use of the sun!
This one is excellent. It takes nothing more than greenhouse plastic braced on a frame with water in the middle to reflect enough heat to burn a board. The frame he the uses could easily be worked into a greenhouse structure.