Computing radiation under overcast conditions

To compute the overcast irradiance from clear-sky raster maps, the thermal solver uses a correction factor that accounts for cloud cover attenuation.

This method uses the clearness index Kt, which is the ratio of radiation on a horizontal surface to the extraterrestrial radiation to correct the direct radiation [27].

First, the direct beam atmospheric transmittance under clear sky conditions, Knc, is calculated. Then, correction coefficients a, b, and c are determined based on the value of Kt.

If Kt ≤ 0.60:

If Kt > 0.60:

Using these coefficients, the correction to the clear-sky transmittance ΔKn, the corrected transmittance Kn, and the direct normal irradiance In are computed as:

where:

  • m is the air mass.
  • I0 is the extraterrestrial irradiance.

The corrected direct beam transmittance Kn also refines the calculation of diffuse radiation under overcast conditions.