Solar Radiation at the Earth’s Surface
Solar radiations while passing through the earth’s atmosphere are subjected to the mechanism of atmospheric absorption and scattering. A fraction of the radiation reaching the earth’s surface is reflected back into the atmosphere and is subjected to these atmospheric phenomenon again, the remainder is absorbed by the earth’s surface. Absorption occurs due to the presence of water vapour and ozone in the atmosphere and other particulate matter. The sattered radiation redistributed in all directions, some going back into space and some reaching the earth surface.
Fig. 4.6 shows the position of terrestrial and extraterrestrial regions. The atmosphere absorption is due to Ozone (O3), Oxygen (O2), Nitrogen (N2), Carbon Disoxide (CO2), Carbon Monoxide(CO) and Water Vapor(H2O) and, scattering is due to Air molecules, Dust and Water Droplets. The X-rays and extreme Ultra-Violet radiations of the Sun are absorbed highly in the ionosphere by nitrogen, Oxygen and other atmospheric gases: ozone and water vapours largely absorb ultraviolet (l<0.4μm) and infrared radiations (l>2.3μm) respectively. There is almost complete absorption of short wave radiations (l<0.29μm) in the atmosphere. Hence, the energy in wavelength radiation below 0.29 μm and above 2.3 μm of the spectra of the solar radiation, incident on the earth’s surface is negligible.
Scattering by air molecule water vapour and dust particles in the attenuation of radiation. The range of wavelength radiation emitted from the sun, attenuation of its amplitude during propagation from the sun to atmosphere and further attenuation of radiation in the atmosphere and also the long wavelength radiation emitted from the earth as shown in ig.4.6.
The atmosphere attenuation is characterized by the term called Air Mass (AM or m). It is defined as the ratio of the optical thickness of the atmosphere through which beam radiation passes to the optical thickness if the Sun were at zenith. Large value of air mass indicates that solar radiation travel greater distance in atmosphere. Hence is prone to attenuation. An expression for air mass (Referring to Fig.4.7) is given by,
At Noon, ϕ=0, m=1, for ϕ=60o, m=2 and m=0 for outside earth atmosphere.
Thus, from the view of terrestrial applications of solar energy , only radiation of wavelength between 0.29 and 2.3 μm is significant.
The solar radiation, through atmosphere reaching the earth’s surface can be classified into two components, they are beam and diffuse radiation.
1. Beam Radiation (Ib) : It is solar radiation propagating along the line joining the receiving surface and Sun. It is also referred to as direct radiation.
2. Diffuse Radiation (Id): It is the solar radiation scattered by aerosols, dust and molecules. It does not have unique direction.
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