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