Page 57 - Design of Solar Thermal Power Plants

50 2. THE SOLAR RESOURCE AND METEOROLOGICAL PARAMETERS

with a wavelength falling in between. Regions vary by wavelength: less
than 0.4 mm is considered ultraviolet, over 0.76 mm is considered infrared,
and from 0.4 to 0.76 mm is considered visible light. Among solar radiation
that reaches the ground, the ultraviolet region accounts for about 8.03% of
global solar radiation, visible light accounts for 46.435%, and infrared
accounts for 45.54%.

2.3 ATMOSPHERIC INFLUENCES ON SOLAR
IRRADIATION

Because of the atmosphere, solar radiation energy that finally reaches
Earth’s surface has been influenced by various factors. Generally speaking,
major influencing factors include solar altitude, air mass number,
atmospheric transparency, geographic latitude, sunshine duration, and
elevation:

1. Solar altitude. Due to the spectral selectivity of atmospheric depth,
energy percentages for the various wavelengths of global solar
radiation are differentiated by their different solar altitudes. For
energy with a solar altitude of 90 degrees within the solar spectrum,
infrared accounts for 50%, visible light for 46%, and ultraviolet for
4%; with a solar altitude of 30 degrees, infrared accounts for 53%,
visible light for 44%, and ultraviolet for 3%; with a solar altitude of
5 degrees, infrared accounts for 72%, visible light for 28%, and
ultraviolet for approximately 0%.
2. Air mass number (AM). Earth’s atmosphere has an average depth of
100 km. When solar radiation travels through the atmosphere, it is
reflected, scattered, and absorbed, and the respective spectral
intensity distribution and corresponding total irradiance undergo
certain changes; the degree of these changes is determined by the
amounts of atmospheric substances that radiation has passed
through. The ratio of the distance traveled by solar radiation through
the atmosphere to the distance traveled by solar radiation through
the atmosphere while at the zenith is referred to as AM. For example,
AM0 refers to the solar radiation that reaches the atmospheric
surface of Earth before entering the atmosphere, namely that the air
mass that solar radiation travels through is equal to zero. Normally,
the distance that solar radiation travels through the atmosphere
when the sun is at the zenith, namely shining perpendicular to the
equatorial sea level (the spring equinox/autumn equinox) is called
one air mass. When the sun is at other positions, AM always exceeds
one. For example, at 8:00e9:00 a.m., AM is around two to three. As
AM increases, the distance that solar radiation travels through the