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2.7.1 Solar Collectors. Because of the effects of the earth's atmosphere,
three ways exist for solar energy to arrive at the solar collectors. The first way is
straight through the atmosphere.  On the average, 27% of the amount that was
available above the atmosphere arrives by this method. Because it travels directly
to the surface, this energy is called "direct" or "beam" radiation.  (Figure 2-16)
The second type of solar energy at the earth's surface is called "diffuse" or
"scattered" radiation. This energy travels in a series of collisions with dust, water
vapor and other particles in the atmosphere before arriving at the surface. This
represents about 16% of the amount available above the atmosphere. Finally, solar
energy reflected from the surface of the earth is sometimes available, averaging 5%
of what was available. The rest is absorbed or "reflected" by the atmosphere, and is
unavailable for collection.
Direct, Diffuse and
Reflected Solar
Flat-plate and evacuated tube solar collectors can collect all components of the
sun's energy. Of the two, flat-plate types are the most-used type of collectors.
The typical "liquid-cooled" flat-plate collector consists of a black absorber plate with
tubes running through or attached to it to take the collected heat away. Above this
surface is a glazing, usually glass, to help trap the heat. A frame holds the entire
package together, and usually includes some provision for mounting. Insulation
surrounds the absorber to retard the loss of heat from the collector. (Figure 2-17)


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