The evacuated tube is made up of 2 layers ofglass. 

The solar energy passes though the outer glass and is absorbed by theselective coating on the inner glass, thus the inner tube becomes hot. 

The heat is transferred to the heat pipe by the aluminum fins, making the heat-transportliquid inside evaporated very fast. 

The evaporation steam goes up to the condenseron the top and the heat is transferred to the heat transfer fluid inside thepipe, and therefore the heat is kept inside the water tank.