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3 Solar Radiation
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
 (6)
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:
(7)
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:
 (8)
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:
 (9)
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.
 (10)
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