The physics of solar radiation
The sun radiates energy as a black body having a surface temperature of about 6000°C over a spectrum of wavelengths from 300 to 470 nm. Nine per cent of the energy is in the ultra-violet region but 91 per cent of the energy is in the visible part of the spectrum (380780 nm) and in the infra-red. Figure 7.1 shows a typical spectral distribution of the energy reaching the surface of the earth. The peak intensity of the solar energy reaching the upper limits of the atmosphere of the earth is about 2200 W irf2 at 480 nm but the average total,
Wavelength in nanometres 1 nm = 10 9 m
Fig. 7.1 A typical solar spectrum at sea level. The maximum irradiance (beyond the atmosphere) is 2130 W m-2 at 451 nm (in the green visible spectrum).
Termed the solar constant, is 1367 W m“2, according to Iqbal (1983). The orbit of the earth about the sun is an ellipse and the earth is slightly closer to the sun in January than it is in July. Consequently the solar irradiation has a maximum value of 1413 W m-2 in January and a minimum of 1332 W trf2 in July.
A total of only about 1025 W rrf2 reaches the surface of the earth when the sun is vertically overhead in a cloudless sky. Of this figure, about 945 W m-2 is by radiation received directly from the sun, the remainder being solar radiation received indirectly from the sky.
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