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.