A while ago I had the idea to create a solar water heater. We need to have a tin box (the bigger the better). The box must be painted in black in the inside and isolated with expanded polystyrene or similar. On the inside of the isolated box we need to put a copper coil like the ones used on the back of refrigerators. The more the pipe turns, the more capacity it creates in liters. The water comes in from one side of the pipe through a hole in the box and out from the other side. In order to keep the coil with the water hot, we need to cover it with a glass and position it towards the sun. The glass allows the sun rays to get into the box, heating the water in the pipe and avoiding them to exit.

Solar water heater diagram