Page 146 - Design of Solar Thermal Power Plants

3.2 HELIOSTAT FIELD EFFICIENCY ANALYSIS 131

above parts. Thus in the power tower plant with concentrators
located far away from the receiver, influences of atmospheric
humidity on radiation transmission shall be considered. It is
suggested not to build up large-scale power tower plants in areas
with high atmospheric humidity.
2. Molecule scattering. There is always the statistical deviation of
partial density from the mean density in the atmosphere, which is
known as the density fluctuation. It ruins the optical uniformity of
the atmosphere; partial solar radiation light will transmit toward
other directions, which leads to the scattering of the solar radiation
in various directions.
In visible light and near-infrared wave regions, radiation
wavelength is always much greater than molecular dimensions.
Under such conditions, scattering is referred to as the Rayleigh
scattering. The intensity of Rayleigh scattering light is inversely
proportional to the biquadratic value of wavelength l. The empirical
equation of Rayleigh scattering coefficient is:

s m ¼ 0:827 NA 3 l 4 (3.6)

in which N refers to the quantity of molecules within a unit volume,
3
2
cm ; A refers to the molecular scattering section, cm ; l refers to the
length of the optical wave, cm. The bigger of wavelength l, the
weaker of the scattering effect; the smaller of l, the stronger of the
scattering effect.
When encountering atmospheric molecules or aerosols particles,
the optical wave will interact with them and once again emit the
light (called wavelet) of the same frequency and less intensity with
the incident light in all directions, which is referred to as the optical
scattering. The wavelet is called the scattering radiation;
atmospheric molecules and aerosol particles that receive the original
incident light and emit wavelets are called scattering particle. When
scales of scattering particles are much smaller than the wavelength
of incident light (for example, scattering effect of atmospheric
molecules on visible light), it is called molecular scattering or
Rayleigh scattering, in which scattering lights have been distributed
on an even and symmetric basis.
As molecular scattering is inversely proportional to the
biquadratic value of wavelength, the bigger of the l, the weaker of
the scattering effect; and the smaller of the l, the stronger of the
scattering effect, scattering effect of visible light is stronger than that
of infrared light, and scattering effect of blue light is stronger than
that of red light. In a clear sky, other particles are insignificant in
content; thus Rayleigh scattering has been playing a dominant role.