Calculation and Model

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Calculation and Model

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Shading calculations

In order to evaluate the shading losses during the simulation, we have to treat each of the three irradiance components of the input meteo data in the appropriate manner:

-For the Beam component, we will define a Shading factor which depends on the sun position.
-For the Diffuse component, we will define a factor as an integral over all sky directions. This will result in a Shading factor  for diffuse, which is independent on the sun position.
-For the Albedo component, we will perform an integral according to the near obstacles on the ground. This will result in a Shading factor  for albedo, independent on the sun position.

Shading losses

When applying these shading calculations in the hourly simulation, we observe two kinds of losses:

Linear shading losses

The Linear shading losses are computed from the Shading factors described above, and represent the irradiance deficit on the PV array.

At each time step, the simulation will evaluate the shading loss on the Beam, Diffuse and Albedo contributions of the Meteo input data. These losses will be available in the results:

ShdLossGlobal linear shading loss  (or irradiance shading loss), total of the 3 contributions,
ShdBLssLoss on the beam component,
ShdDLssLoss on the diffuse component,
ShdALssLoss on the albedo component.

 

Electrical shading losses

The Electrical shading losses is a result of electrical mismatches when interconnecting the PV modules as an array.

Remember that the global current in a string of modules (or cells) is driven by the cell producing the lower current. And if the current imposed in the string is higher than the max. current of a shaded cell, the by-pass diode of the concerned sub-module will be activated.  

In PVsyst, we have 2 ways to compute the electrical losses:

-a  calculation "According to module strings", which is an approximation giving an upper limit of the potential mismatch loss. As this represents a maximum, PVsyst proposes a parameter  "Fraction for electrical loss"  for taking the partial effect into account, if any. For sheds arrangement (regular mutual shades), this factor will be 100%.
-a detailed calculation named "Module Layout", which gives a more accurate evaluation involving the exact position of each PV module in the 3D construction, as well as in its electrical system.

During the simulation, the electrical loss is accumulated in the variable named ShdElec.

NB:The electrical losses only apply to the Beam component. The diffuse irradiance comes from all directions of the sky, and the irradiance inhomogeneities are not sufficient to create significant mismatch losses (and we don't have any mean for evaluating them).
NB:The only effect of the module orientation (portrait or .landscape) is the electrical loss. In the module layout tool, you have to explicitly define it. With the option "according to module strings", you will define the "rectangle-strings" sizes accordingly.

Recovery by Optimizers

Optimizer at the module level (or better at sub-module level) may recover a part of the mismatch losses due to shades on a string.

However this recovery does not represent the full electrical losses: when a few cells are shaded in a sub-module, the by-pass diode will be activated for short-circuiting this sub-module in the string. Therefore:

- The production of the remaining illuminated cells in the sub-module, as well as the production of the remaining diffuse part, is lost,

- The voltage across the diode (< 1V), times the current in the string, represent an additional power loss, the diode will heat-up.  

Reflexions

Gains due to eventual reflections on near reflective surface, usually specular, cannot be calculated by PVsyst.

However, though they are sometimes spectacular, these effects have negligible energetic consequences: they are in general involved only for very special hourly periods, and in the presence of the beam component. Moreover, their effect on the real output of a sizeable PV field remains negligible, when considering that to benefit from it, a complete string of cells in series should be uniformly illuminated by this supplementary supply (the production of a chain is indeed limited by the production of the weakest cell). In the same way, the reflection on the back of a shed, on the one hand only intercepts a small efficient part of the beam component, and on the other hand only illuminates a non-homogenous band in the lower part of the collector.