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.
- If Kt ≤ 0.60
- If Kt > 0.60
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.