The flat-plate collectors are much simpler than the concentrating collectors. They do not need to face directly at the sun; they can absorb diffused light and almost anyone can make one. We know that dark surfaces absorb radiation, and lighter surfaces reflect it. A flat-plate collector is a black sheet of metal with fluid channels or conduits running over, under, or even through it.
A flat-plate collector works much like a greenhouse. Rays come through the glass, reflect off the walls and the floor of the greenhouse, but cannot escape back into the atmosphere. When rays of short wavelength hit the absorber plate, some of their energy is reradiated back toward the source, but their intensity is weakened - thus increasing the length of the waves. Because they cannot pass back through the glazing, they hit the absorber repeatedly, giving the plate several chances to absorb them.
Figure 15-6 shows an evacuated tube collector. The vacuum tube collector has a vacuum between the absorber and the glass outer tube. This reduces convection and conduction heat losses.
Evacuated tube collectors operate essentially the same as flat-plate collectors. Solar radiation passes through the outer glass tube and the coated absorber receives the heat. The heat energy is transferred to the fluid flowing through the absorber. Most evacuated tube designs collect both direct and diffused radiation efficiently, but certain types are designed for more efficient collection of direct radiation. Although evacuated tube collectors are considerably more expensive than typical flat-plate collectors, they are much more efficient when high collection temperatures are needed for operating absorption chillers of for industrial processing.
Figure 15-6. - Evacuated tube solar heat collector.
They may not be as efficient as flat-plate collectors at low temperatures, such as domestic water heating and space heating.
Concentrating, or focusing, collectors intercept direct radiation over a large area and focus it onto a small absorber area. These collectors can provide high temperatures more efficiently than flat-plate collectors, since the absorbtion surface area is much smaller. However, diffused sky radiation cannot be focused onto the absorber. Most concentrating collectors require mechanical equipment that constantly orients the collectors toward the sun and keeps the absorber at the point of focus.
There are many types of concentrating collectors. The most popular types are the parabolic trough, the linear-trough fresnel lens, and the compound parabolic mirror. Figure 15-7, view (A), shows a linear concentrating or parabolic-trough collector. It collects energy by reflecting direct solar radiation off a large curved mirror and onto a small absorber tube that contains a flowing heat transfer liquid. The absorber tube is encased in a glass or metal tube that mayContinue Reading