Bifacial

Bifacial modules can be used that collect light from both front and back surfaces. The same Decomposition, Transposition, and View Factors are used for the backside to determine the backside irradiance.

Note
  • When running locally only the 2D calculation can be used for bifacial modules. It is not supported for the 3D calculation.
  • When running in the cloud bifacial modules are supported in both 2D and 3D calculations.
  • See Calculation Feature Comparison to see some of the differences in feature support between running calculations locally and in the cloud.

After completing the near shading model we have a backside component in addition to the frontside component, denoted \(G_{poa,near,back}\).

The soiling loss effect and spectral effect are not modelled for the backside irradiance. The incidence angle modifier effect is only modelled for the backside irradiance when running the calculation in the cloud (depending on the model used) and not in the local calculation. The backside irradiance is a small part of the total irradiance, so omitting these small effects from what is already a small effect should not introduce a significant error.

Two additional irradiance effects are modelled for bifacial modules:

  • structural shade factor - accounts for the fact that the rear of modules can be shaded by items such as junction boxes
  • transmission factor - accounts for the effect that some light passes through the racks/trackers and reaches the ground, thereby increasing the ground reflected light that illuminates the back-side of the modules

These are both simple loss factor models where the user must specify the loss parameter (specified for each layout region in the Design Layout task in SolarFarmer).

$$G_{\text{back,shade}} = G_{poa,near,back} \bullet \left( 1 + K_{\text{shade}} \right)$$ $$G_{\text{back,transmission}} = G_{back,shade} \bullet \left( 1 + K_{\text{transmission}} \right)$$

This backside irradiance is then combined with the front-side irradiance.
$$G_\text{eff,back} = G_\text{spectral} + G_{poa,near,back}$$ $$G_\text{eff,back,shade} = G_\text{spectral} + G_{back,shade}$$ $$G_\text{eff,back,transmission} = G_\text{spectral} + G_{back,transmission}$$