20
A. Funde and A. Shah
in comparison with the solar spectrum at midday: consequently, the sun itself looks
reddish at sunrise and sunset.
(d) Scattering by aerosols and dust particles
This type of scattering is also termed as “Mie Scattering” and originates from
scattering particles in the atmosphere that are larger than the wavelength of sunlight.
Mie scattering is caused by one or many of the pollen grains, dust, smoke, water
droplets, and other particles in the lower portion of the atmosphere. The extent of
Mie scattering at a given location on the Earth depends on the local environmental
conditions. Thus, the attenuation of sunlight in the atmosphere is stronger in industrial
regions and in densely populated areas.
2.1.2 Nature of Solar Irradiance Received on Earth’s Surface
There are a variety of factors affecting the total amount of solar irradiance received
(W m
−2 ) and the intensity of solar irradiance at each wavelength (W m
−2 nm
−1 ),
which is the spectral irradiance (or simply the spectrum), at a given location on earth.
These factors include latitude, season, time of day, cloud cover and altitude. The
nature of solar spectrum received outside the atmosphere is different from the solar
spectrum received at sea level on the Earth’s surface, at over-head sun position. This
is because of the atmospheric effects (mentioned above in Sect. 2.1.1) encountered
by the solar radiation when travelling from outside the atmosphere to a given location
on Earth—absorption, reflection or scattering in the atmosphere. Figure 2.2 depicts
the different atmospheric phenomena encountered by solar radiation.
The Air-Mass Ratio (AM)
Here, “air-mass” literally means the thickness of atmosphere crossed by the solar
radiation before it reaches a given location on earth. Thus, outside the atmosphere,
the air mass traversed by solar radiation is zero. The “air-mass ratio” is, as shown in
Fig. 2.3 is the ratio of the atmospheric path traversed, in a given case, by the solar
radiation to the atmospheric path traversed by the solar radiation if it had come in
perpendicular to the Earth’s surface (in overhead sun position). Thus, the air-mass
ratio outside the atmosphere is referred to as air-mass ratio = 0 or simply AM0. The
total length of atmosphere at sea level in overhead sun position is normalized to be
1. Thus, the air mass ratio at this situation is air-mass ratio = 1 or simply AM1.
At any other zenith angle θ z , the air-mass ratio can be estimated as:
Air-mass ratio =
Y
X
, where Y is the actual path travelled by the solar radiation and
X is the unit length of air mass when the sun’s position is overhead at that location.
With simple trigonometry this quantity turns out to be sec θ z = 1/cos(θ z ).
Solar PV modules are characterized by their performance specifications IEC
61215 [3] under standard test conditions (STC), which are:
(1) Solar irradiance of 1000 W/m
2
A. Funde and A. Shah
in comparison with the solar spectrum at midday: consequently, the sun itself looks
reddish at sunrise and sunset.
(d) Scattering by aerosols and dust particles
This type of scattering is also termed as “Mie Scattering” and originates from
scattering particles in the atmosphere that are larger than the wavelength of sunlight.
Mie scattering is caused by one or many of the pollen grains, dust, smoke, water
droplets, and other particles in the lower portion of the atmosphere. The extent of
Mie scattering at a given location on the Earth depends on the local environmental
conditions. Thus, the attenuation of sunlight in the atmosphere is stronger in industrial
regions and in densely populated areas.
2.1.2 Nature of Solar Irradiance Received on Earth’s Surface
There are a variety of factors affecting the total amount of solar irradiance received
(W m
−2 ) and the intensity of solar irradiance at each wavelength (W m
−2 nm
−1 ),
which is the spectral irradiance (or simply the spectrum), at a given location on earth.
These factors include latitude, season, time of day, cloud cover and altitude. The
nature of solar spectrum received outside the atmosphere is different from the solar
spectrum received at sea level on the Earth’s surface, at over-head sun position. This
is because of the atmospheric effects (mentioned above in Sect. 2.1.1) encountered
by the solar radiation when travelling from outside the atmosphere to a given location
on Earth—absorption, reflection or scattering in the atmosphere. Figure 2.2 depicts
the different atmospheric phenomena encountered by solar radiation.
The Air-Mass Ratio (AM)
Here, “air-mass” literally means the thickness of atmosphere crossed by the solar
radiation before it reaches a given location on earth. Thus, outside the atmosphere,
the air mass traversed by solar radiation is zero. The “air-mass ratio” is, as shown in
Fig. 2.3 is the ratio of the atmospheric path traversed, in a given case, by the solar
radiation to the atmospheric path traversed by the solar radiation if it had come in
perpendicular to the Earth’s surface (in overhead sun position). Thus, the air-mass
ratio outside the atmosphere is referred to as air-mass ratio = 0 or simply AM0. The
total length of atmosphere at sea level in overhead sun position is normalized to be
1. Thus, the air mass ratio at this situation is air-mass ratio = 1 or simply AM1.
At any other zenith angle θ z , the air-mass ratio can be estimated as:
Air-mass ratio =
Y
X
, where Y is the actual path travelled by the solar radiation and
X is the unit length of air mass when the sun’s position is overhead at that location.
With simple trigonometry this quantity turns out to be sec θ z = 1/cos(θ z ).
Solar PV modules are characterized by their performance specifications IEC
61215 [3] under standard test conditions (STC), which are:
(1) Solar irradiance of 1000 W/m
2