Solar Energy Perspectives - IEA

Chapter 2: The solar resource and its possible uses
Figure 2.5 shows that there are days where the polar regions receive higher irradiance than
all other places on earth, with about 13.5 kWh/m 2 /day at the December solstice for the South
Pole and 12.6 kWh/m 2 /day at the June solstice for the North Pole, versus about 10 kWh/m 2 /day
at the equator. This gap is probably accentuated, not diminished, by differences in the
transparency of the atmosphere in both places. Poles are the sunniest places on earth, but
only for a few days per year because summer days within the polar circles last 24 hours,
against only 12 hours at the equator. 3
However, this basic model is complicated by the atmosphere, and its content in water vapour
and particles, which also vary over time and place. Clouds bar almost all direct beam radiation.
The composition of the atmosphere has two main implications for availability of the solar energy.
First, the cosine effect is compounded by the greater distance the sun’s rays must travel through
the earth’s atmosphere to reach the earth’s surface when the apparent sun is lower in the sky –
twice as much when the sun’s direction forms a 60° angle with the direction of the zenith. This
is termed an “air mass” value of 2, versus a value of 1 when the sun is exactly overhead.
Second, the atmosphere scatters and absorbs some of the solar energy – particularly infrared
radiation absorbed by the water vapour and CO 2
trace gases of the atmosphere, and
ultraviolet radiations absorbed by ozone. Visible light, near and short-wave infrareds, being
the more energetic types of solar radiation (the shorter the wavelength, the higher the energy),
comprise more than 95% of the solar radiation at sea level (Figure 2.6).
Figure 2.6 Solar radiation spectrum at the top of the atmosphere and at sea level
2.5
Spectral irradiance (W/m 2 /nm)
UV Visible Infrared
2
Sunlight at top of the atmosphere
1.5
5250 °C Blackbody spectrum
1
H 2
O
Radiation at sea level
0.5
O 2
H 2
O
Absorption bands
0
O 3
H 2
O
H 2
O CO 2
250 500 750 1 000 1 250 1 500 1 750 2 000 2 250
Wavelength (nm)
H 2
O
2 500
Note: The solar radiation at the top of the atmosphere is in yellow, Figure the 2.6 radiation reaching the sea level is in red. The bulk of the energy
is made of visible light and near infrared.
Key point
Visible light and near infra-red constitute the bulk of incident solar radiation.
3. The South Pole is sunnier than the North Pole because the earth’s orbit is an ellipse and the sun closer to us during the winter of the
northern hemisphere. Thus, the so-called solar “constant” is in reality only about 1 320 W/m 2 on 2 July versus 1 415 W/m 2 on 2 January.
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© OECD/IEA, 2011