Glass material primary and secondary transmission

1

Hi all,

Is it possible to create a window build-up where I specify each glass layer’s inner/outer emissivity (longwave) and inner/outer absorptivity (shortwave), as well as solar transmission, in order to account for re-radiated absorbed solar energy? I want to account for the solar absorption in order to measure the surface temperature of my windows.
image

If this isn’t possible, can I create a complex fenestration system in e.g. LBNL WINDOW and import this as my window constructions?

Thanks!
Joe

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2

@jroberts Hi! Here is some relevant information:
In EnergyPlus, the absorptivity of a window glazing layer is not directly input as a single parameter in the WindowMaterial:Glazing object. Instead, it is derived from the solar and visible transmittance/reflectance values that you specify.

Here’s how it works:

Relevant fields in WindowMaterial:Glazing

  • Optical Data Type (SpectralAverage, Spectral, SpectralAndAngle)
  • Solar Transmittance at Normal Incidence
  • Front Side Solar Reflectance at Normal Incidence
  • Back Side Solar Reflectance at Normal Incidence
  • (similar fields for visible and IR ranges)

Relationship:

For solar radiation, absorptance is computed as:

α=1−τ−ρ\alpha = 1 - \tau - \rhoα=1−τ−ρ

where:

  • τ\tauτ = Solar Transmittance (at normal incidence)
  • ρ\rhoρ = Solar Reflectance (front or back, depending on direction)

image

This is wavelength‑averaged if you use SpectralAverage, or calculated across wavelengths if you supply Spectral Data.

Example (SpectralAverage glazing):

idf

WindowMaterial:Glazing,
  Clear 3mm,              !- Name
  SpectralAverage,        !- Optical Data Type
  ,                       !- Window Glass Spectral Data Set Name
  0.837,                  !- Solar Transmittance at Normal Incidence
  0.075,                  !- Front Side Solar Reflectance at Normal Incidence
  0.075,                  !- Back Side Solar Reflectance at Normal Incidence
  0.898,                  !- Visible Transmittance at Normal Incidence
  0.081,                  !- Front Side Visible Reflectance at Normal Incidence
  0.081,                  !- Back Side Visible Reflectance at Normal Incidence
  ...

For this glass:

  • Front absorptance = 1−0.837−0.075=0.0881 - 0.837 - 0.075 = 0.0881−0.837−0.075=0.088
  • Back absorptance = 1−0.837−0.075=0.0881 - 0.837 - 0.075 = 0.0881−0.837−0.075=0.088

and Output Variables applicable:

Per-Surface Window Absorbed Solar Radiation

  • Surface Window Transmitted Solar Radiation Rate [W]
  • Surface Window Absorbed Solar Radiation Rate [W]
  • Surface Window Absorbed Solar Radiation Rate per Area [W/m2]
  • Surface Window Absorbed Shortwave Radiation Rate [W]
  • Surface Window Absorbed Shortwave Radiation Rate per Area [W/m2]
  • Surface Window Absorbed Beam Solar Radiation Rate [W]
  • Surface Window Absorbed Diffuse Solar Radiation Rate [W]

(these give total absorbed energy, split by beam/diffuse if desired)

Per-Layer (Glazing Layer) Absorptance

For multilayer glazing (e.g., double-pane, triple-pane), EnergyPlus can report absorbed solar by each layer:

  • Surface Window Absorbed Solar Radiation Rate Layer {i} [W]
  • Surface Window Absorbed Solar Radiation Rate Layer {i} per Area [W/m2]

where {i} = glazing layer index (1 = outside pane, 2 = inside pane, etc.)

Optical Properties / Absorptance Coefficients

If you want the actual absorptivity values, not just absorbed energy:

  • Surface Window Transmitted Solar Radiation Fraction
  • Surface Window Reflected Solar Radiation Fraction
  • Surface Window Absorbed Solar Radiation Fraction

These fractions are basically:

image

Example Output:Variable block

idf

Output:Variable,
  *,  !- Key Value (all surfaces)
  Surface Window Absorbed Solar Radiation Rate per Area,  !- Variable Name
  Hourly;  !- Reporting Frequency

Output:Variable,
  *,  
  Surface Window Absorbed Solar Radiation Fraction,  
  Hourly;

Output:Variable,
  *,  
  Surface Window Absorbed Solar Radiation Rate Layer 1,  
  Hourly;

Hopefully some of that is helpful!
best
-trevor

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Thanks Trevor,

That’s really helpful! It looks like in honeybee-energy glass construction the shortwave absorptivity is calculated like you say as \tau_sol - \rho_sol, whereas the longwave emissivity is input directly. I guess we don’t really care about the back solar reflectance.

  • shortwave transmittance _transmittance_
  • shortwave reflectance _reflectance_ (front only)
  • longwave transmittance _t_infrared_
  • longwave emissivity _emiss_front_
  • longwave emissivity _emiss_back_

Thanks for helping me get my head around this!

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