Page 58 - Design of Solar Thermal Power Plants

2.3 ATMOSPHERIC INFLUENCES ON SOLAR IRRADIATION 51

atmosphere increases as well, and as a result it undertakes more
attenuation and less energy reaches the ground.
Assuming Earth is a perfect sphere with a mean atmospheric depth
of 100 km and a mean radius of 6400 km, it can be calculated that at a
position with a latitude around 48 degrees, the solar radiation
spectrum will be AM1.5, whereas at the pole (a ¼ 90 degrees) it will
be AM11.4. The distance that solar radiation travels to pass through
the atmosphere increases with increments in latitude, as does the
atmospheric influence. Thus areas with higher latitudes normally
tend to have smaller irradiances.
3. Atmospheric transparency. Atmospheric transparency is a parameter
used to describe the transmittance of solar radiation. During clear
weather, atmospheric transparency is high, so more solar radiation
reaches the ground. During overcast and stormy skies, atmospheric
transparency is quite low, so less solar radiation reaches the ground.
Currently, atmospheric transparency in China can be categorized into
six levels; level 1 means that the area’s atmospheric transparency has
reached its maximum, namely that solar irradiance is at its highest
level, while levels 2 through 6 decrease in sequence.
4. Geographic latitude. When atmospheric transparency is unchanged,
the atmospheric distance gradually increases from low latitude to
high latitude, and solar radiation energy weakens correspondingly
from low latitude to high latitude.
5. Sunshine duration. Sunshine duration is one of the most commonly
used physical parameters for describing the solar resource.
Presently, all operating meteorological stations can carry out a
sunshine duration observation, which observes the sunshine
duration of a specific area (the cumulative time for the ground
observation site under solar direct irradiance that is equivalent to
2
and above 120 W/m ). The unit is an hour, which can be as precise
as 0.1 h. The longer the sunshine duration, the greater the global
radiation received by the ground.
6. Elevation. Generally speaking, the greater of the elevation, the better
the atmospheric transparency and the greater the solar direct
radiation.
SuneEarth distance, topography, terrain, and the like also influence
solar radiation. For example, the mean temperature when Earth is at

perihelion is 40 C higher than it is when Earth is at aphelion. Another
example is that for the same latitude, basin area has a higher temperature
than surface area, and sunny slope is hotter than shaded slope.
To sum up, many factors can influence ground solar radiation, yet the
amount of solar radiation in a specific area is determined by the foregoing
factors in a comprehensive manner.