[fix] Diffuse irradiance is not fully anisotropic

Summary

Diffuse irradiance is not fully anisotropic.

Current behavior

Diffuse is computed as the sky view factor multiplied by DHI, not taking into account the cos of the angle between the incident angle and the receiving surface. Results in overestimation of low altitude contribution.

Desired behavior

Implement cosine correction : contributions coming in on the horizontal ground surface with a zenith angle of 0° are not affected (multiplied by cos(0) = 1), and for increasing zenith angles the contributions are decreasing.

Sources

Solar irradiance - projection effect

Linked features or branches

n/a