A flat-plate collector has a transparent cover made of specially toughened glass, a coated metal absorber plate (coating is usually matt black paint), and a well-insulated weatherproof casing. Flat plate collectors are robust, economical, and versatile.
Vacuum-tube collectors consist of a series of glass tubes connected together. The absorber strip in an evacuated tube collector is placed inside the toughened glass tube, which can withstand considerable pressure.
The heat transfer fluid flows through the absorber directly in a U-tube or sometimes in a tube-in-tube system. Several tubes are connected to one another or to a manifold that makes up the solar collector. A heat pipe collector contains a special fluid that vaporises at low temperature.
The hot vapour rises in the heat pipes and warms the heat transfer medium in the main pipe before condensing and re-circulating.
The pipes must be at a certain angle to facilitate the process. Too flat or too steep an angle and the system will not work.
Vacuum tubes function well in low radiation conditions as well. They can often produce higher temperatures in applications such as hot water heating, steam production and air conditioning.
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