Simulation process: Irradiances and PV-array

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Simulation process: Irradiances and PV-array

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The simulation involves about fifty variables, which are all accumulated in monthly values.

When starting, the early parameter definition parts in the program have already verified the consistency of all input parameters.

If  Near shadings are defined for the simulation, the "Shading factor tables" are computed (if not already done).

The diffuse attenuation factor should be calculated, by integrating simultaneously the shading factor due to horizon, the near shadings factor according to the table, and the IAM attenuation factors over the visible part of the sky hemisphere.

The same thing holds for the albedo attenuation factor.  

As these factors (integrals) don't depend on the sun's position, they are constant over the year.


Then the hourly simulation performs the following steps, for each hour:

Incident "effective" energy calculation

-Reading one hour data on the Meteo file  (Horizontal global irradiance, temperature, eventually diffuse irradiance and wind velocity).
-If the diffuse irradiance is not available, it is computed with the Erbs (near to Liu-Jordan) correlation model,
=> defined variables at this stage:   GlobHor, DiffHor, BeamHor, T Amb, WindVel.
-Performs the transposition (global, diffuse, albedo irradiances) in the collector plane, using either Hay or Perez model according to your user's preference.
This is done using solar angles at the middle of the time interval, taken from  the meteo file  (with possible time shift if defined in your meteo data).
=> defined variables at this stage:   GlobInc, BeamInc, DiffInc, DiffSInc, AlbInc,
-If  horizon is defined, applies the horizon correction on the beam component (ON/OFF),
-If near shadings defined,  applies the shading factor on the beam component  (from the Shading factor table, or recalculated) => evaluation of the "Linear" shading loss (loss due to the irradiance deficit).
-Applies the IAM factor on the beam component.
-Applies the diffuse and albedo attenuation factors (previously computed)  on Diffuse and Albedo parts.
-If soiling defined, applied the soiling factor to all components (global, diffuse, Albedo).
=> defined variables at this stage:   GlobEff, BeamEff, DiffEff, Alb_Eff,

This leads to the so-called  "Effective incident energy", i.e. the irradiance effectively reaching the PV cell surface after optical corrections.

Other secondary variables (essentially ratios of the above energy quantities) are available for displays:

=> Bm/Gl, Diff/Gl, DifS/Gl, Alb/Gl, Ftransp, FIAMBm, FIAMGl, FShdBm, FShdGl, FIAMShd.

Array MPP "virtual" energy  EArrMPP  and  effectively used energy EArray

For each sub-array  (i.e. each orientation independently), the simulation calculates:

-The array temperature  TArray  (energy balance between absorbed and heat loss energy),

It applies the one-diode model for the module, and evaluates:

-The MPP operating point of the array, calculated by the one-diode model if the system was running at STC efficiency  (1000 W/m² and 25°C).
-The irradiance loss, i.e. the loss due to the low-light performances of the module.
-The temperature loss due to the cell's temperature TArray.
-The spectral loss if defined   (amorphous modules or Sandia model).
-The electrical mismatch loss due to shadings  
-The module quality loss.
-Eventually the LID loss if defined.
-The Mismatch loss.
- The Array wiring loss.

This results in the  MPP virtually available energy EArrMPP.

This is not necessarily the true Array output energy:

-The operating point may be displaced by the rest of the system  (inverter in overpower or other limit conditions, direct coupling on the battery, etc), resulting in a MPPLoss.
-The energy may be unuseable if the battery or water tank is full: this will lead to EUnused loss.

The energy really used by the system is called EArray.  

For sub-array with mixed orientations, the whole meteo calculation is repeated for the second field orientation, output meteo variables are accumulated as averages between the two orientations, weighted by the field area ratio.

Then both array characteristics are electrically combined (on a same inverter input), in order to search the real maximum power point. The loss with respect to a common orientation is accounted as "MixLoss". It is usually negligible or very close to 0, as the mismatch when combining two different sub-arrays in voltage is very low.    

System energy

The next simulation stages are system dependent :

-        Grid connected system,

-        Stand-alone system,

-        Pumping system,

-        DC-grid system.


NB.All energies are calculated here as average power during one hour. They are expressed in [kWh/h] or [MJ/h], that is in a power equivalent unit. Therefore with hourly steps Power and Energy hold the same numerical values. Although most calculations are indeed related on power quantities, we will express them as energies for simplification.