Chapter: Electric Energy Generation and Utilisation and Conservation - Solar Radiation and Solar Energy Collectors

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Solar Constant

Considering the Sun’s temperature of 5760K, the total power emitted by thye Sun is about 3.8x10pow(30) W.

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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