Evacuated Tube Collectors

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 Collectors

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 may

Figure 15-7.-Concentrating collectors for solar energy.

be evacuated. This type of collector must track the sun and can collect only direct radiation.

Figure 15-7, view (B), shows a linear-trough, fresnel lens collector. In this design, a curved lens is used to focus incoming rays onto a small absorber plate or tube through which the heat transfer liquid is circulated. This type of collector also requires a tracking mechanism and can collect only direct radiation.

Figure 15-7, view (C), shows a compound parabolic mirror collector. The design of the mirror allows the collector to collect and focus both direct and diffuse radiation without tracking the sun. Periodic changes in the tilt angle are the only adjustments necessary.

Direct radiation is intercepted by only a portion of the mirror at a time; thus this collector does not collect as much solar energy as a focusing collector that tracks the sun. It is, however, less expensive to install and maintain. The absorber tube is encased within an evacuated tube to reduce heat losses.

Many other types of concentrating collectors produce high temperatures at good efficiencies However, the high cost of installing and maintaining tracking collectors restricts their use to solar cooling and industrial applications where extremely high fluid temperatures are required. In addition, concentrating collectors must be used only in those locations where clear-sky direct radiation is abundant.

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