In heat pipe evacuated tube collectors, a sealed heat pipe, typically made of copper to increase the collector's efficiency in cold temperatures, is attached to a heat-absorbing reflector plate inside the vacuum-sealed tube. The hollow copper heat pipe within the tube is evacuated of air yet contains a small quantity of a low-pressure alcohol/water liquid and some extra additives to prevent corrosion or oxidation.
This vacuum permits the liquid to vapourize at terribly lower temperatures than it would ordinarily at atmospheric pressure. Once sunlight in the type of solar radiation hits the surface of the absorber plate within the tube, the liquid in the heat pipe quickly turns into a hot vapor type gas because of the presence of the vacuum. As this gas vapor is now lighter, it rises to the top portion of the pipe heating it up to a high temperature.
The top part of the heat pipe, and so the evacuated tube is connected to a copper heat exchanger known as the “manifold”. Once the hot vapors still inside the sealed heat tube enter the manifold, the heat energy of the vapor is transferred to the water or glycol fluid flowing through the connecting manifold. because the hot vapor loses energy and cools, it condenses back from a gas to a liquid flowing back down the heat pipe to be reheated.
The heat pipe and so the evacuated tube collectors should be mounted in such a way as to have a minimum tilt angle (around 30o) for the internal liquid of the heat pipe to come back down to the hot absorber plate at the bottom of the tube. This process of turning a liquid into a gas and a liquid again continues inside the sealed heat pipe as long because the sun shines.
The main benefit of heat Pipe evacuated Tube Collectors is that there is a “dry” connection between the absorber plate and the manifold making installation easier than with direct flow collectors. Moreover, in the event, an exhausted tube cracking or breaking and the vacuum turning into lost the individual tube can be exchanged without emptying or dismantling the whole system.
This flexibility makes heat pipe evacuated tube solar hot water collectors ideal for closed-loop solar designs because the modular assembly permits for simple installation and ability to simply expand by adding as several tubes as you wish.