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Horizontal Irradiance Processing models prepare input irradiance data (measured or modeled) for use in energy production calculations. These models handle component separation, clear-sky estimation, and—when is the input—reverse decomposition to derive horizontal components.

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Models in This Section

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Horizontal Irradiance Components Pre-Processing

Validates input irradiance data using the (GHI = DHI + DNI × cos θz) and applies quality control checks to ensure physical consistency.

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GHI Diffuse-Direct

Estimates DHI from GHI, then calculates DNI using the . This is the standard decomposition path, applied when DHI or DNI measurements are unavailable or when the user chooses to override measured values. Three model options:
  • Erbs: Simplest model; uses only GHI and extraterrestrial irradiance via a correlation
  • Reindl: Adds air temperature and optionally relative humidity for improved accuracy under varying atmospheric conditions
  • DIRINT: Uses adjacent timesteps to detect rapid irradiance changes (e.g., cloud transients), providing better accuracy for variable sky conditions

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POAI Diffuse-Direct

Reverse-decomposes measured (POAI) into GHI, DNI, and DHI using the GTI-DIRINT algorithm. This path is activated by the Frontside POAI toggle in Simulation Settings and bypasses the standard GHI decomposition. The derived horizontal components feed into the normal transposition pipeline for each DC field.

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Bird Clear Sky Model

Calculates theoretical clear-sky irradiance components using atmospheric transmittance factors. Used for spectral correction calculations.

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Processing Workflow

PlantPredict supports two entry points for irradiance data, selected via the Frontside POAI toggle:
  1. GHI-based (standard path) — GHI is validated, then DHI and DNI are resolved based on user settings (see Horizontal Irradiance Components for the full processing sequence):
    • DHI: calculated from GHI when a decomposition model is selected, or read from the weather file when decomposition is set to None
    • DNI: read from the weather file when the Weather File DNI toggle is on, or derived from the closure equation otherwise
  2. POAI-based (alternative path) — Measured POAI is reverse-decomposed into GHI, DNI, and DHI using GTI-DIRINT, bypassing standard GHI processing.
Both paths produce validated horizontal components (GHI, DNI, DHI) that feed into the transposition models for each DC field (Hay-Davies or Perez). In either case, the Bird clear-sky model runs independently to provide reference irradiance for spectral corrections.