CPC are non-imaging concentrators. These have the
capability of reflecting to the absorber all of the incident
radiation within wide limits. Their potential as collectors of
solar energy was pointed out by Winston [44]. The necessity
of moving the concentrator to accommodate the changing
solar orientation can be reduced by using a trough with two
sections of a parabola facing each other, as shown in Fig. 4.
Compound parabolic concentrators can accept incoming
radiation over a relatively wide range of angles. By using
multiple internal reflections, any radiation that is entering
the aperture, within the collector acceptance angle, finds its
way to the absorber surface located at the bottom of the collector. The absorber can take a variety of configurations. It can be cylindrical as shown in Fig. 4 or flat. In the
CPC shown in Fig. 4 the lower portion of the reflector (AB
and AC) is circular, while the upper portions (BD and CE)
are parabolic. As the upper part of a CPC contribute little to
the radiation reaching the absorber, they are usually
truncated thus forming a shorter version of the CPC,
which is also cheaper. CPCs are usually covered with glass
to avoid dust and other materials from entering the collector
and thus reducing the reflectivity of its walls.