Do you know how a heat pipe works for solar collector? Here we'd like to make a brief introudction for you as follows:
Sunlight passing through the glass tubes is absorbed by the black coating. The fins on the central copper pipe become hot.
There are actually two glass tubes, one inside the other. The air space between the inner and outer glass tube is evacuated to greatly reduce heat loss due to convection.
The liquid sealed in the central copper tube has a low boiling point and turns to vapor. Because heat rises, the vapor flows to the top of the pipe.
The top of the tube fits inside a copper "nipple" in the stainless steel tank. The nipple transfers the heat to the water surrounding it inside the tank.
This cools the tip of the copper collector tube and the vapor condenses back to liquid. It then rolls down the back side of the tube (which is cooler because it isn't being heated by direct sunlight) and returns to the liquid pool at the bottom of the tube. Whereupon it gets heated, starting the process over again.
The beauty of a thermosiphon system is that it has no moving parts. It works by efficiently transfering heat from the absorber to the water without your water ever coming in contact with any dangerous or toxic fluids. Even if the glass tubes break, no toxic fluids are released to the environment or contaminate your water.
Because each tube acts independently, if one breaks, the others keep working until you can replace the broken Heat Pipe.