Solar Power » Radiation

Effects in Atmosphere

In various layers of the atmosphere, the sun light gets reflected back outside, absorbed and scattered. As a result, the radiation that hits a horizontal surface on the earth is

  • attenuated,
  • split into diffuse and direct light, and
  • has a different spectrum to the one outside the atmosphere.


Attenuation


To express the amount of intensity that is lost through absorption, the clarity index is defined as the ratio between the observed (global) hourly irradiance on earth, Hg, and the hourly radiation H0 just outside the atmosphere:

Clearness Index           

The actual values for K have to be measured. Typical values are:

Clear sky at sea level

0.6 – 0.8

Cloudy

0.1 – 0.3

Around 18% of the extraterrestrial radiation is absorbed or reflected back. Higher latitudes experience lower values, as the path through the atmosphere under a larger zenith angle is much longer.

The clarity index is usually either daily or hourly to average out short-term fluctuations. It is assumed that clouds are uniformly distributed over the sky. Drifting clouds are not considered in this technique.

The diagram below shows ranges for the monthly average clearness index in three locations: Zurich, Grude (Bosnia-Herzegovina) and Dakar (Senegal).

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Diffusion


Direct beam radiation is one which strikes the surface from one angle only, directly from the sun. Conversely, diffuse light, as a result of absorption and scattering, approaches the horizontal surface from almost any angle. It can therefore not be focused or concentrated.

The global hourly irradiance on a surface can be expressed as the sum of direct (beam) and diffuse radiation as:

Global Radiation

Similar to the clearness index, the diffusion index is defined as

Diffusion Index

The beam fraction is 1 – KD.

Diffusion Index

We have plotted the typical relationship between the beam fraction and the clearness index for latitudes around 50°Ν. As expected, clear skies cause less diffusion. However, where there are clouds, the ratio of diffuse light can be in excess of 75%. Any devices that concentrate light onto a single point rely on a high proportion of direct beam and are therefore not suitable in locations with high diffusion index.
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