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Summary

Irradiance at a given site is typically reported as three complementary components: Global Horizontal Irradiance (), Direct Normal Irradiance (), and Diffuse Horizontal Irradiance (). PlantPredict validates these components and pre-processes them before passing them to models.

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Inputs

NameSymbolUnitsDescription
Global Horizontal IrradianceGHIGHIW/m²Total irradiance on horizontal surface from weather data
Direct Normal IrradianceDNIDNIW/m²Beam irradiance perpendicular to sun’s rays from weather data
Diffuse Horizontal IrradianceDHIDHIW/m²Diffuse irradiance on horizontal surface from weather data
Solar Zenith Angleθz\theta_zdegreesAngle between sun and local vertical
Extraterrestrial IrradianceDNIextraDNI_{extra}W/m²Solar irradiance at top of atmosphere

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Outputs

NameSymbolUnitsDescription
Processed GHIGHIGHIW/m²Global horizontal irradiance (validated or calculated)
Processed DNIDNIDNIW/m²Direct normal irradiance (validated or calculated)
Processed DHIDHIDHIW/m²Diffuse horizontal irradiance (validated or calculated)

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Detailed Description

Irradiance components are:
  • Global Horizontal Irradiance (GHI): Total solar radiation on a horizontal surface from the entire sky hemisphere, including direct beam, diffuse sky radiation, and reflected radiation.
  • Direct Normal Irradiance (DNI): Solar radiation on a surface perpendicular to the sun’s rays, representing only the direct beam component.
  • Diffuse Horizontal Irradiance (DHI): Solar radiation from the sky dome after scattering by atmospheric constituents, excluding the direct beam.
The three components are related by the : GHI=DNIcos⁡(θz)+DHIGHI = DNI \cos(\theta_z) + DHI where θz\theta_z is the solar . PlantPredict processes irradiance data in the following order:
  1. Validate GHI: Negative GHI values are set to zero
  2. Resolve DHI:
    • If model selected (Erbs, Reindl, or DIRINT) → calculate DHI from GHI (overwrites any existing DHI)
    • If no decomposition model → use DHI from weather data (prediction will fail if missing)
  3. Resolve DNI:
    • If DNI is missing or the “Weather File DNI” option is set to OFF, calculate DNI from closure: DNI = (GHI − DHI) / cos(θz). If GHI ≤ DHI or θz > 87.9°, DNI = 0.
    • Else use DNI from weather data
  4. Pass to transposition: Validated components feed into models

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References

  • Duffie, J. A., & Beckman, W. A. (2013). Solar Engineering of Thermal Processes (4th ed.). John Wiley & Sons.
  • Iqbal, M. (1983). An Introduction to Solar Radiation. Academic Press.
  • Maxwell, E., Wilcox, S., & Rymes, M. (1993). User’s Manual for SERI QC Software: Assessing the Quality of Solar Radiation Data. NREL/TP-463-5608.