[Medium] How much do irradiance transposition models differ?

[kandersolar]

The irradiance on a tilted PV module (the plane-of-array, POA, irradiance) can be estimated using an irradiance transposition model. POA irradiance comprises three parts:

• direct irradiance, which is almost always calculated as poa_direct = dni * cos(AOI) where AOI is the angle of incidence.
• ground-reflected irradiance, which is almost always calculated as poa_ground = ghi * (1 - cos(tilt))/2 where tilt is the angle formed from horizontal by the tilted surface.
• sky diffuse irradiance. Here, an empirical expression is often used. A transposition model is often referred to by the name of its sky diffuse model.

pvlib-python includes several transposition models developed by the solar PV research community, including common ones like Perez and Hay-Davies, and less common ones as well. Here is a listing of the transposition functions available in pvlib: https://pvlib-python.readthedocs.io/en/stable/reference/irradiance/transposition.html

Different transposition models produce different estimates of POA irradiance. How much variation is there between model predictions for the same weather inputs?

You can use a weather file of your choosing or calculate a year of clear-sky irradiance as described here.

Helpful links:

• https://pvlib-python.readthedocs.io/en/stable/reference/generated/pvlib.irradiance.get_total_irradiance.html
• https://pvlib-python.readthedocs.io/en/stable/gallery/irradiance-transposition/plot_transposition_gain.html
• https://pvlib-python.readthedocs.io/en/stable/gallery/irradiance-transposition/plot_ghi_transposition.html