Page 21 - Applied Photovoltaics

It is common to consider separately the ‘direct’ (or ‘beam’) radiation from the solar
disk and the ‘diffuse’ radiation from elsewhere in the sky, with their sum known as
‘global’ radiation. A table of AM1.5 global (AM1.5G) irradiance versus wavelength
for an equator-facing, 37° tilted surface on earth is given in Appendix A. Since
different types of photovoltaic cells respond differently to different wavelengths of
light, the tables can be used to assess the likely output of different cells.

For the spectrum of Appendix A, the total energy density, i.e. the integral of the
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power density over the entire wavelength band, is close to 970 W/m . This spectrum,
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or the corresponding ‘normalised’ spectrum of 1000 W/m , is the present standard
used for rating photovoltaic products. The latter is close to the maximum power
received at the earth’s surface. The power and photon flux density components
corresponding to the ‘normalised’ spectrum can be obtained by multiplying the
Appendix A values by 1000/970.

To assess the likely performance of a photovoltaic cell or module in a real system, the
standard spectra discussed above must be related to the actual solar insolation levels
for the site at which the system is to be installed. (Fig. 1.12 illustrates the global and
seasonal variation in daily insolation levels.)

1.5 DIRECT AND DIFFUSE RADIATION
Sunlight passing through the earth’s atmosphere is attenuated, or reduced, by about
30% by the time it reaches the earth’s surface due to such effects as (Gast, 1960;
Iqbal, 1983):
1. Rayleigh scattering by molecules in the atmosphere, particularly at short
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wavelengths (~Ȝ dependence)
2. Scattering by aerosols and dust particles.
3. Absorption by atmospheric gases such as oxygen, ozone, water vapour and
carbon dioxide (CO 2 ).

The latter produces the absorption bands apparent in Fig. 1.3. Wavelengths below
0.3 ȝm are strongly absorbed by ozone. Depletion of ozone from the atmosphere
allows more of this short wavelength light to reach the earth, with consequent harmful
effects on biological systems. The absorption bands around 1 ȝm are produced by
water vapour absorption, complemented by CO 2 absorption at longer wavelengths.
Changing the CO 2 content of the atmosphere also has consequences for the earth’s
climatic and biological systems.

Fig. 1.7 shows how atmospheric scattering results in a diffuse component of sunlight
coming from all directions in the sky. Diffuse radiation is predominantly at the blue
end of the spectrum because of more effective scattering at small wavelengths.
Hence, the sky appears blue.
AM1 radiation (radiation when the sun is directly overhead), has a diffuse component
of about 10% when skies are clear. The percentage increases with increasing air mass
or when skies are not clear.
Cloud cover is, of course, a significant cause of radiation attenuation and scattering.
Cumulus or bulky, low altitude clouds, are very effective in blocking sunlight.

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