The evacuated tube collector consists of some rows of parallel clear glass tubes connected to a header pipe and that is employed in place of the blackened heat absorbing plate.
These glass tubes are cylindrical in form. Therefore, the angle of the sunlight is always perpendicular to the heat absorbing tubes that allows these collectors to perform well even once sunlight is low like when it is early in the morning or late in the afternoon, or once shaded by clouds. Evacuated tube collectors are particularly helpful in areas with cold, cloudy wintry weather.
So how do solar evacuated tube collectors work?
Evacuated tube collectors are created by one or multiple rows of parallel, clear glass tubes supported on a frame. Every tube varies in diameter from between 1" (25mm) to 3" (75mm) and between 5′ (1500mm) to 8′ (2400mm) in length relying upon the manufacturer.
Every tube consists of a thick glass outer tube and a thinner glass inner tube, (called a “twin-glass tube”) or a “thermos-flask tube” which is covered with a special coating that absorbs solar power, however, inhibits heat loss. The tubes are made from borosilicate or soda-lime glass, which is strong, resistant to high temperatures and includes a high transmittance for solar irradiation.
Unlike flat panel collectors, evacuated tube collectors don't heat the water directly inside the tubes. Instead, the air is removed or evacuated from the space between the 2 tubes, forming a vacuum (hence the name exhausted tubes). This vacuum acts as an insulator reducing any heat loss considerably to the surrounding atmosphere either through convection or radiation making the collector much more efficient than the internal insulating that flat plate collectors need to provide. With the help of this vacuum, evacuated tube collectors usually make higher fluid temperatures than they’re flat plate counterparts so it may become extremely hot in summer.