Page 152 - Design of Solar Thermal Power Plants

3.2 HELIOSTAT FIELD EFFICIENCY ANALYSIS 137

and particles with the size of 0.1e0.4 mm have the largest
contributions to the total extinction coefficient; peak value of
extinction appears when the particle size approximates 0.2 mm,
b(440) ¼ 0.36/km. Along with the increase of particle size, extinction
coefficient decreases rapidly; when the particle size reaches up to
1.0 mm, extinction coefficient has already approached 0. Then, based
on the variation of extinction coefficient over wavelength, aerosol
particle has the best extinction ability when the wavelength is
440 nm; along with the increase of wavelength, extinction ability of
the particle gradually decreases; comparing with other wavelengths,
the extinction coefficient decreases with a higher speed when
wavelength is 440 nm. Yet at the same time, it is also discovered that
the relationship between extinction coefficient and wavelength is
not onefold, which can be concluded from 0.4 to 1.0 mm section of
the particle size curve. In this section of the particle size curve,
curves of various wavelengths intersect with each other; there is no
obvious law to follow. Many studies have indicated that variation of
extinction coefficient over wavelength can normally be manifested
as follows:
A
b ¼ (3.7)
r g
in which A is a constant; index number g may vary from 4 (suitable
for Rayleigh scattering and extremely small-sized particles) to
0 (suitable for the scattering effect of mist on visible light or near-
infrared light).
However, only for extremely large and extremely small particles, can
such dependency relationship be manifested by a simple function within
a comparatively broad wavelength range. Sand dust particles for a power
plant fall in between these two extreme values. Based on the entire
wavelength and particle size ranges, variation of extinction coefficient still
follows a certain rule.
To sum up, extinction coefficient b in Eq. (3.4) is 0.02. For a 10 MW
power plant, the last row of heliostats is located about 0.8 km away from
the receiver tower. By substituting extinction coefficient and light trans-
mission length L ¼ 0.8 km into Eq. (3.3), atmospheric transmittance of
heliostat from the last row can be obtained:
T ¼ expð bLÞ¼ expð 0:02 0:8Þ¼ 0:984 (3.8)

The mean value is
T ¼ expð bLÞ¼ expð 0:02 0:4Þ¼ 0:99