Concentrating collectors use mirrored surfaces to concentrate the sun's
energy on an absorber called a receiver. Concentrating collectors also
achieve high temperatures, but unlike evacuated-tube collectors, they
can do so only when direct sunlight is available. The mirrored surface
focuses sunlight collected over a large area onto a smaller absorber area
to achieve high temperatures. Some designs concentrate solar energy onto
a focal point, while others concentrate the sun's rays along a thin line
called the focal line. The receiver is located at the focal point or along
the focal line. A heat-transfer fluid flows through the receiver and absorbs
heat. These collectors reach much higher temperatures than flat-plate
collectors. However, concentrators can only focus direct solar radiation,
with the result being that their performance is poor on hazy or cloudy
days. Concentrators are most practical in areas of high insolation (exposure
to the sun's rays), such as those close to the equator and in the desert
southwest United States.
Concentrators perform best when pointed directly at the sun. To do this,
these systems use tracking mechanisms to move the collectors during the
day to keep them focused on the sun. Single-axis trackers move east to
west; dual-axis trackers move east and west and north and south (to follow
the sun throughout the year). In addition to these mechanical trackers,
there are passive trackers that use freon to supply the movement. While
not widely used, they do provide a low-maintenance alternative to mechanical
Concentrators are used mostly in commercial applications because they
are expensive and because the trackers need frequent maintenance. Some
residential solar energy systems use parabolic-trough concentrating systems.
These installations can provide hot water, space heating, and water purification.
Most residential systems use single-axis trackers, which are less expensive
and simpler than dual-axis trackers.
There are four basic types of concentrating collectors:
Stationary concentrating collectors
Special Subject:Solar Cooking