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.

ENERGY STORAGE AND AUXILIARY HEAT

Since effective sunshine occurs only about 5 to 6 hours per day (in temperate latitudes) and