Dear Aika,
why doesn't SAM use all radidation components (beam, diffuse and total) when calculating teh tilted surface radiation?
One reason is that for the radiation incident on a surface, each of the radiation components can be calculated from the other two. If SAM were to use values for all three from the weather file, because of measurement artifacts or other data issues, the radiation values in the weather file for each of the components may not be consistent. To ensure that the total, beam, and diffuse values are consistent, SAM always either calculates the total component of the incident radiation from the beam and diffuse values in the weather file (and information about the array inclination and solar angles), or it calculates the diffuse component from the total and beam values in the file.
The other reason for not using all three radiation components from the weather file is that in some cases data for only two of the components is available. This makes SAM flexible enough to run with these files.
I always choose Perez model and "Beam and Diffuse" on the Array page.(Flat PV panel analysis)
And help page says that SAM ignores the GHI and calculates it from BNI and DHI.
Why dose SAM ignore GHI??
We recommend using the Perez tilted surface radiation model with the Beam and Diffuse option unless you have a reason to choose different options. With this option, SAM calculates the total incident radiation from the beam and diffuse components as I explained above, and it ignores the GHI data from the weather file.
And also I would like to know why "beam and diffure" is better than "Total and Beam".
Calculating the diffuse component from the beam normal and global horizontal components introduces uncertainty in the resulting incident total radiation value that depends on the model used to calculate the sky diffuse radiation (Perez, HDKR, or Isotropic). These models are widely discussed in various articles and textbooks. For example, see Duffie and Beckman's textbook Solar Engineering of Solar Processes (Section 2.14 in the 2013 edition).
Best regards,
Paul.
