Page 155 - Design of Solar Thermal Power Plants

140 3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

P AP refers to the radiation flux through the receiver aperture after
solar radiation is reflected by the concentration field.
P
P REFCAV ¼ r COV AP
0 1
(3.12)
a w
¼ @ 1 A P AP
A 1
1 ð1 a w Þ 1
A 2
P AP can be obtained through some heliostat field simulation
software, and corrected through experimental measurements.
In Eq. (3.12), a w is a constant that changes along with (A 1 /A 2 ). We
assume a w ¼ 0.85, and substitute it into Eq. (3.11)
0:85
r COV ¼ 1 (3.13)
A 1
1 0:15 1
A 2
Fig. 3.12 displays the variation rule of reflectance, according
to which, equivalent reflectance of cavity receiver is normally
around 0.05.
In case that the area ratio (A 1 /A 2 ) is a constant and a w changes
accordingly, we assume A 1 /A 2 ¼ 0.2, and substitute it into Eq. (3.11)

a w
r COV ¼ 1
1 ð1 a w Þð1 0:2Þ
(3.14)
a w
¼ 1
1 0:8ð1 a w Þ
According to Fig. 3.13,along with theincreaseofabsorptance
of absorber, equivalent reflectance decreases rapidly. Thus it
is useful to increase the thermal efficiency of the receiver by

Equivalent reflectance

A / A 2
1
FIGURE 3.12 Equivalent reflectance of cavity receiver (A w ¼ 0.85).