Eevacuated tube solar collectors convert energy from the sun into usable heat energy in a solar water heating system. This energy can be used for domestic and commercial water heating, pool heating, space heating or even air conditioning. Evacuated tube collectors are made up of a single or multiple rows of parallel, transparent glass tubes supported on a frame. Each tube consists of a thick glass outer tube and a thinner glass inner tube which is covered with a special coating that absorbs solar energy. The tubes are made of borosilicate which is resistant to high temperatures .Evacuated tube collectors do not heat the water directly within the tubes. Instead, air is removed or evacuated from the space between the two tubes, forming a vacuum . This vacuum acts as an insulator reducing any heat loss significantly to the surrounding atmosphere. With the assistance of this vacuum, evacuated tube collectors generally produce higher fluid temperatures .
ETC are cheaper than FPC( Flat Plate Collector ) based systems. They perform better in cold regions and avoid freezing problems during sub-zero temperature. But ETC systems are less effective in compare to FPC systems.