Thought I would share this with everyone. On my way thru Tennessee this year I stopped at a rest area and saw this outside the bathroom area. A older gentleman came up to me and said "This is how they get the hot water in the rest room".
I used to build stuff like this before the wreck. That's a fairly simple system, we used to have solar water heaters on my uncles old house. It had solar heat too, just at night it would turn into a cold draft.
In Florida, after we get hit with a hurricane and lose power, we lay the garden hose out in the sun. Provided we still have water pressure, you can get about 1 minute of hot water to take a quick shower.