Solar Radiation
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3 Solar Radiation

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The spectrum of solar radiation extends from 200 to 3000 nm wavelengths. It is almost identical with the 6000 K black-body radiation spectrum as shown in Figure 5. The radiation is distinguished as:

(a) ultra-violet radiation, 200 to 380 nm, producing photochemical effects, bleaching, sunburn, etc.

(b) visible light, 380 (violet) to 700 nm (red),

(c) infra-red radiation, 700 to 3000 nm, radiant heat, with some photochemical effects.


When the sun's rays passing through the earth's atmosphere,

(a) a portion of solar radiation is scattered when striking on molecules of air, water vapour and dust particles. That portion scattering downward from the atmosphere arrives at the earth surface in the form of diffuse radiation.

(b) another portion of solar radiation is absorbed, and

(c) the remaining portion of solar radiation transverses through the atmosphere and reaches the earth surface in the form of direct radiation.


3.1 Direct Solar Radiation


where A = apparent solar irradiation just outside the atmosphere, W/m2

B = atmosphere extinction coefficient

The intensity of direct radiation (ID) incident upon a surface at sea level is:


where Fc = air clearness factor


3.2 Diffuse Solar Radiation

Diffuse solar radiation is that portion of solar radiation that is scattered downwards by the molecules in the atmosphere. During clear days, the magnitude of diffuse radiation is about 10 to 14% of the total solar radiation received at the earth's surface. Only diffuse radiation may reach the earth's surface during extremely cloudy days.

The solar intensity of diffuse radiation from a clear sky falling on any surface is approximately given by:


where     C = diffuse radiation factor

              S = tilted angle of the surface (see Figure 4)


3.3 Reflected Radiation

When the solar radiation irradiates upon a surface which is opaque, a portion of radiation is absorbed and the remaining portion is reflected in diffuse or specular nature depending on the roughness of the surface. The reflected radiation falling on an adjacent surface, Iref, is calculated as:


where r ref = reflectance of reflected surface

Fr-s = shape factor between reflected surface and the receiving surface

ID = direct solar radiation

Id = diffuse solar radiation


3.4 Total Intensity of Solar Radiation

Total intensity of solar radiation falling on a surface, It, is the sum of the direct radiation ID, diffuse radiation Id and reflected radiation Iref.