Estimate diffuse and direct component from global irradiance

Question

I am looking to separate the diffuse and direct component of global irradiance and found the Erbs model to do this in pvlib (see pvlib.irradiance.erbs) however, I am getting ver

Answer 1

DNI > GHI is common at low solar elevation. GHI decreases much faster than DNI as solar elevation decreases. For example, think of a clear day with the sun right near the horizon. DNI will large because it's measured on a plane normal to the sun vector, but GHI will be near zero.

The values of DNI that are much greater than 1000 W/m2 are likely at very high zenith, since the Erbs model basically divides by cos(zenith). In practice, I limit using decomposition models like Erbs to zenith<85 degrees, to avoid the non-physical results.