Many potential applications of solar heat require larger temperatures than
those achievable by even the best flat plate collectors. In particular a
working fluid at 500 C can drive a conventional heat engine to produce
mechanical work and thence (if required) electricity. Even larger temperatures
∼2000C are useful in the production and purification of refractory
materials.
A concentrating collector comprises a receiver, where the radiation is
absorbed and converted to some other energy form, and a concentrator,
which is the optical system that directs beam radiation onto the receiver,
(e.g. Figure 6.11). Therefore it is usually necessary to continually orientate
the concentrator so that it faces the solar beam. (Section 6.8.4 considers a
non-tracking case.)
The aperture of the system Aa is the projected area of the concentrator
facing the beam. We define the concentration ratio X to be the ratio of the
area of aperture to the area of the receiver:
X = Aa/Ar (6.10)
For an ideal collector, X would be the ratio of the flux density at the receiver
to that at the concentrator, but in practice the flux density varies greatly
across the receiver. The temperature of the receiver cannot be increased
indefinitely by simply increasing X, since by Kirchhoff’s laws (Section 3.5.4)
the receiver temperature Tr cannot exceed the equivalent temperature Ts of
the Sun. Moreover the Sun (radius Rs, distance L) subtends a finite angle
at the Earth which limits the achievable concentration ratio to
X < L/Rs2 = 45 000 (6.11