SOLAR COLLECTORS and TYPES

 SOLAR COLLECTORS

A solar collector is a device that absorbs direct (and in some

cases, diffuse) radiant energy from the sun and delivers that energy to a heat transfer

fluid. While there are many different types of collectors, all have certain functional

components in common. The absorber surface is designed to convert radiant energy

from the sun to thermal energy. The fluid pathways allow the thermal energy from the

absorber surface to be transferred efficiently to the heat transfer fluid. Some form of

insulation is typically used to decrease thermal energy loss and allow as much of the

energy to reach the working fluid as possible. Finally, the entire collector package must

be designed to withstand ambient conditions ranging from sub-zero temperatures and

high winds to stagnation temperatures as high as 350 degrees F (177 degrees C).

 

COLLECTOR TYPES. The three major categories that have been used most

often are flat-plate glazed collectors, unglazed collectors, and evacuated tube

collectors. A general description of each collector type and its application is given

below.

FLAT-PLATE. Flat-plate solar collectors are the most common type used and

are best suited for low temperature heating applications, such as service water and

space heating. These collectors usually consist of four basic components: casing, back

insulation, absorber plate assembly, and a transparent cover. The absorber panel is a

flat surface that is coated with a material that readily absorbs solar radiation in the

thermal spectrum. Some coatings, known as "selective surfaces", have the further

advantage of radiating very little of the absorbed energy back to the environment.

Channels located along the surface or within the absorber plate allow the working fluid

to circulate. Energy absorbed by the panel is carried to the load or to storage by the

fluid. The absorber panel is encased in a box frame equipped with insulation on the

back and sides and one or two transparent covers (glazing) on the front side. The

glazing allows solar radiation into the collector while reducing convective energy losses

from the hot absorber plate to the environment. Similarly, back insulation is used to

reduce conductive energy loss from the absorber plate through the back of the collector.

UNGLAZED. Unglazed collectors are the least complex collector type and

consist of an absorber plate through which water circulates. This plate has no glazing or

back insulation. These collectors are often made of extruded plastic because they are

designed to operate at relatively low temperatures. Since they are not thermally

protected, these collectors should be operated only in warm environments where lower

thermal losses will occur. Swimming pool heating is the most common use of unglazed collectors.

EVACUATED TUBE. Evacuated tube collectors are best suited for higher

temperature applications, such as those required by space cooling equipment or for

higher temperature industrial process water heating. Convective losses to the

environment are decreased in this type of collector by encapsulating the absorber and fluid path within a glass tube that is kept at a vacuum. Tracking mechanisms and/or parabolic solar concentrating devices (simple or compound) are often used, resulting in somewhat higher equipment costs.