DISC Model

This routine implements the DISC correlation function, which is not included by itself as an option in PlantPredict; however, it is used by both the DIRINT decomposition and Hay transposition models.





1.) Compute the clear-sky normal transmittance for all solar zenith angles.

DISC Algo23
2.) Compute coefficients a, b and c for the beam transmittance equation.

    1. If Kt ≤ 0.60

DISC Algo24

    1. If Kt > 0.60

DISC Algo25
3.) Compute the direct normal transmittance.

DISC Algo26
4.) Compute the DIRINT direct normal irradiance.

DISC Algo27
5.) Set Gb,n,DISC to 0 if θZ > 90°, GT < 1, or Gb,n,DISC < 0

Please note that in Version 9 and earlier step 5 above used θZ > 87°.  This caused some unrealistic spikes when using Weather File POAI with sub-hourly data.  Beginning with Version 10 we have adjusted this condition to be θZ > 90°, which eliminated these spikes.


Maxwell, E. L., A Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation.

Technical Report No. SERI/TR-215-3087, Golden, CO: Solar Energy Research Institute, 1987.