Irradiation models
The effective incident irradiation is defined as the solar energy actually reaching the PV cells. It is obtained according to the steps described in this page. If only monthly meteorological data are available: hourly synthetic data is first generated (global horizontal irradiance and ambient temperature).
PVsyst Nomenclature
This page uses standard names for irradiance componants. The corresponding variable names in PVsyst are:
- Global Horizontal Irradiance (GHI) :
GlobHor - Diffuse Horizontal Irradiance (DHI):
DiffHor - Direct Normal Irradiance (DNI):
BeamNorm - Beam horizontal irradiance (BHI):
BeamHor
1. Irradiance Decomposition
In order to compute shadings, the global irradiance must be decomposed into diffuse and direct components. The relation between the global horizontal irradiance (GHI), the diffuse horizontal irradiance (DHI), and the beam horizontal irradiance (BHI) is given by: $$ \text{GHI} = \text{DHI} + \text{BHI}. $$
Where BHI is related to Direct Normal Irradiance (DNI) by: $$ \text{BHI} = \text{DNI} * sin(\text{SunHeight}) $$
Depending on what input data is available, different steps might be required to obtain DHI and BHI. The formulas are listed in the following table:
| Input Data | DHI | BHI |
|---|---|---|
| GHI only | Diffuse irradiance models | GHI − DHI |
| GHI, DHI | Input | GHI − DHI |
| GHI, DNI | GHI − BHI | DNI × sin(SunHeight) |
| DHI, DNI | Input | DNI × sin(SunHeight) |
| POA only | Reverse transposition (Hay Model) | Reverse transposition (Hay Model) |
| DNI only | Concentrating systems only, Global approximated from clearsky model | DNI × sin(SunHeight) |
Note that when importing POA irradiance, you must use the Hay Model as transposition model to get consistant results. POA can only be imported alone.
2. Far shading
If a horizon (far shading) is defined: the effective beam component is computed.
3. Transposition
Now that the horizontal irradiances are known, the irradiance on the PV tilted plane can be computed. This is done using a Transposition model.
At this stage, the plane irradiance is composed of global, diffuse, beam and albedo components, with the relation: Gp = Dp + Bp + Ap.
4. Shadings and IAM
Finally, additional irradiance losses have to be taken into account:
- Applying the near shading calculations (shading factor on beam, diffuse and albedo components), either linear or according to electrical array connections,
- Applying the IAM (Incidence Angle Modifier factor),
This finally results in the Geff irradiance, the flux effectively useable for PV conversion. Heff will be the corresponding irradiation over a given time period.
Note
We usually use G for designing irradiances (flux expressed in [W/m²]) and H for irradiations (energies in [kWh/m²].