Evacuated Tube Collectors
Among the three types of solar collectors, evacuated tube collectors reach the highest temperatures at 170 to 350 degrees Fahrenheit. They are also about twice the cost of flat plate collectors. Flat plate collectors, in comparison, do not heat water or air beyond 180 degrees Fahrenheit. The third type, the integral storage collector, uses passive water heating techniques in locations where outdoor temperatures never reach freezing.
A direct flow evacuated tube consists of flat, metal absorbers housed within parallel glass tubes. The metal absorbers, also called fins, are attached to a header pipe. As the fins absorb heat, a heat transfer fluid flows through the tubes to collect that heat. There are three different configurations for a direct flow evacuated tube system. The most common consists of dual piping for an inlet/outlet flow system. Air is removed from the vacuum between the two glass tubes to prevent heat loss. The other types consist of two glass tubes fused together at one end and a concentric inlet and outlet fluid.
A heat pipe evacuated tube collector consists of a copper absorber plate placed on a sealed solar tube. The plate is located in a hollow metal heat pipe, where liquid comes to a boil and rises up to the top of the pipe in the form of steam. There, water or a heat transfer fluid, such as glycol, collects the heat, and the condensed water returns to the bottom of the pipe. The absorber strip can also be located in an evacuated glass tube, and the heat transfer fluid flows through the tube. The heat pipe evacuated tube must be placed at an angle to allow for vaporization and condensation.
For more information on installing evacuated tube collectors, give our certified solar contractors a try.
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