Structure of the Evacuated Tube Solar Collectors

Evacuated Tube Solar Collectors, just like ordinary collectors, capture solar radiation by means of a flat metallic absorber plate. However in vacuum collectors, the plate is split into parallel strips.

Each strip is attached to a metallic tube in good thermal contact with it. It can be a direct flow tube or a so-called heat pipe. We will focus on the latter option.

Plating tubes are manufactured with a selective coating to reduce infrared emission losses as much as possible. In addition, each plated pair is surrounded by a glass tube which is then evacuated. Hence, the name of the collector. The vacuum makes conduction and convection losses negligible.

Finally the upper parts of the heat pipes are embedded inside the horizontal duct.

The water to be heated is circulated through this duct.

Operation of The Heat Pipe

The interior of the heat pipe is also evacuated and it only contains a small amount of a volatile fluid such as an alcohol. In the absence of solar radiation, the plate and tube temperatures are low and the fluid lies at the bottom of the tube in liquid state.

But if the sun shines, the plate transfers heat to the tube. Most of the heat is used to vaporize the fluid, that is, it is the latent heat.

The vapor rises by natural convection and reaches the top of the tube where a bulge acts as a condenser.

Water at relatively low temperature circulates outside the bulge. Therefore, the fluid transfers its latent heat to the water and condenses.

The bulge is intended to increase the heat transfer surface so that thermal exchange is more efficient. Upon condensation, the drops of fluid slide down the tube.