Solar radiation that earth receives varies inversely with the square of the distance between the sun and earth. Due to continuous motion of the earth in the orbit, radiation coming on earth also varies.
Extra-Terrestrial Solar Radiation
It is solar radiation just outside the earth’s atmosphere. The figure shown below represents extra-terrestrial solar irradiation.
Two terms are commonly used while studying solar spectrum namely “Irradiance” and “Insolation”
Irradiance: It refers to the solar power received by the collector per unit area and its unit will be kW/m2/nm.
Insolation: It refers to the energy received by the collector per unit area over a given period of time and its unit will be kWhr/m2/nm.
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Figure: Reduction in radiation at the earth's surface because of absorbption and scattering of radiation by the ozone layer, CO2, water vapour and dust particles present in the atmosphere. (Source: https://rredc.nrel.gov/solar//spectra/am1.5/)
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Solar Spectrum atEarth’s Surface
After entering into the atmosphere about 6% of the irradiation is reflected and around 16% irradiation is absorbed and scattered by the ozone layer, CO2, water vapour and dust particles present in the atmosphere. Radiations that gets scattered in the atmosphere are called Diffused Radiation and radiation that gets reflected from the ground is called Albedo Radiation.
Airmass
Airmass gives an idea of the path length through which spectral irradiation passes through the atmosphere before reaching the earth’s surface. This length varies continuously due to the varying position of the sun with respect to earth. To avoid confusions with varying path lengths, standard Airmass coefficients (AM0, AM1, AM1.5, AM2) are made to compare different solar panels and same is used worldwide.
Airmass Coefficient
It is defined as the direct optical path length of the insolation through the atmosphere. It is expressed relative to the zenith path length which is normal to the horizon plane at sea level.
Air mass approximation is given by
AM = 1/Cos(z)
Where z is zenith angle in degree.
AM0 represents radiation spectrum just outside the earth’s atmosphere.
When sun is exactly at overhead position, spectrum is referred to as AM1. In this case, sun radiations travel the shortest distance and is considered unity(1).
When sun is at 48.5 degree from zenith, spectrum is referred to as AM1.5. In this case obviously, the radiations will have to travel a longer distance in the atmosphere.
Example
For air mass =1.5 (AM1.5), calculate zenith angle (z).
AM = 1/Cos(z)
1.5 = 1/Cos(z)
z = 48.5.
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