Evacuated tube collectors are exactly just what the name implies. They are made up of glass tubes that the air has been evacuated (withdrawn) out. Actually each tube is made up of a glass tube within a glass tube. Each tube is made of two layers of strong borosilicate glass (glass with very low iron content).
The reason there is no air in the tubes is to prevent heat loss through convection and radiation. The vacuum helps the tube to retain the heat that has been collected allowing a much greater heat gain. In addition to the vacuum, a special absorbing film is placed between the two tubes. This special film allows the tube to absorb as much as 90-92% of the sun’s ultraviolet rays.
One of the evacuated tube solar collectors has inside the inner tube of the evacuated tube collector, a metal heat pipe made up of a flat or curved aluminum or copper fin that is attached to a metal heat pipe transfers the collected heat to a copper header at the top of the collector. Inside the heat pipe is a small amount of acetone or alcohol that turns into steam when heated and rises to the top of the pipe.
When sunlight strikes the collector the liquid acetone or alcohol boils
and turns to steam. After if turns to steam it rises up the heat pipe
to the copper header at the top of the collector where it gives up its
heat to the water in the copper header. The acetone or alcohol steam
now condenses and falls back down the heat tube where the process starts
all over again. This type is also know as an evacuated tube heat pipe collector.
Since there is a "dry" connection between the evacuated tubes and the
copper header this solar heating system can still operate with a broken
tube and the tubes can be easily replaced without turning off the
system. And the tubes start working very quickly even at low
temperatures due to the low boiling point of the acetone or alcohol.
Another type of evacuated tube solar collectors is known as a U-Tube collector. The construction is similar to the evacuated tube solar collector with a heat tube but instead has a long copper tube in a U shape inside the inner tube that a glycol/water mixture flows. The U type tubes are also connected to a copper header and the glycol/water mixture flows from the header through the U tube and then back to the header.
The U-Tube collector has the advantage of being much cheaper than the "dry" type evacuated tube collector. But unlike the "dry" type collector the tubes cannot be changed out while the system is working and if a tube is broken it will affect the entire system.