Page 105 - Design of Solar Thermal Power Plants

2.7 DISTRIBUTION OF SOLAR DIRECT NORMAL RADIATION 93

scattering transparency; and s Cl refers to the cloud absorption or scat-
tering transparency.
Based on NREL data, China’s annual mean daily DNI distribution can
be obtained. The data have not been calibrated through ground
measurement.
2
2
Daily DNIs of “2 kWh/m ” and “4.4 kWh/m ” occur most frequently
on the ground in China. The maximum value appears in the Tibet
2
Autonomous Region at 9.365 kWh/m while the minimum value appears
2
in the Sichuan Basin at 0.785 kWh/m . The annual mean daily DNI in
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China is 4.033 kWh/m .
2.7.2 Influencing Factors for Spatial and Temporal
Distributions of Solar Direct Irradiation
Solar direct radiation decreases along with reduced elevation, in that
when solar radiation enters the atmosphere from above, the further it
travels to the lower level, the greater the content of water vapor, aerosol,
and mixed gas it contains and the greater and faster its attenuation is. In
the upper part of the troposphere (with a height from the top of 7e17 km)
and above, the attenuation of solar direct radiation by the atmosphere is
insignificant.
In different places and at different time points, the distribution,
constitution, and concentration of water vapor and aerosol in the atmo-
sphere vary from each other, which leads to different absorption and
scattering of solar radiation.

2.7.3 Basic Characteristics of China’s Solar Resource
Characteristics of China’s solar resource are mainly determined by
geographic latitude, topography, and atmospheric circulation conditions.
Normally, the bigger the solar altitude (lower the latitude), the shorter the
path that solar radiation follows when passing through the atmosphere
(higher the elevation) and is less weakened by the atmosphere with more
solar radiation reaching the ground.
1. Characteristics of latitude and season influences. The influence of
latitude on global radiation distribution is mainly manifested in

East China (to the east of 105 E), including the zonal tendency of the
annual total irradiation isoline. It serves as the general background
for global radiation distribution, which has manifested the
dominant role of the astronomic factor (astronomic radiation) on
global radiation. Along with seasonal changes, the influence level of
latitude may change according to variation of the astronomic factor
and atmospheric circulation factor effect contrast.