SOLAR CONSTANT
Considering
the Sun’s temperature of 5760K, the total power emitted by thye Sun is about
3.8x1030 W (or) 6.25x1011W/m2. Because of
large distance between the Sun and the earth, the amount of solar radiation
that reaches just outside the earths atmosphere is quite low.
And
also orientation of the earth’s around the Sun is such that the Sun-earth
distances varzies only by 1.7 percent and since the solar radiation outside the
earth’s atmosphere is nearly of fixed intensities, the radiant energy flux
received per second by a surface of unit area held normal to the direction of
sun’s rays at the mean earth-Sun distance, outside the atmosphere, is
practically constant throughout the year. This is termed as the Solar Constant
Isc and its value is now adopted to be 1367 W/m2. It is
measured at the surface perpendicular to the sunrays at the average sun-earth
distance on the top of the atmosphere. Due to the variation in the Sun-earth
distance over the years the actual radiation reaching the earth’s atmosphere
varies through out the year. Variation in the solar radiation is also caused by
the difference in the emission intensity from the sun itself. The
extra-teriestrial solar radiation increase to about 1367 W/m2. The
solar constant used for calculation is only the average values of extra-terrestrial
solar radiation over a year. However, this extraterrestrial radiation suffers
variation due to the fact that the earth revolves around the Sun not in a
circular orbit but follows an elliptic path, with Sun at one of the foci. The
indensity of extraterrestrial radiation Iext measured on a plane
normal to the radiation on the nth day of the year is given in terms
of solar constant (Isc) as follows,
The
variation of extraterrestrial radiation with nth day of the year is
shown in ig.4.5.
Since,
the cosine function varies from +1 to -1, the extraterrestrial radiation flux
varies by +-3.3 percent over a year.
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