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Electrical shading mismatch loss

Qualitative behaviour

Mismatch loss between cells

When a PV array is shaded, there is a inhomogeneity of irradiance on the different PV cells, and therefore a disturbance of the Electrical yield ("mismatch loss").

Remember that in a set of PV cells connected in series (a string), the current of the less irradiated cell will govern the current within this whole string. Therefore, as soon as one cell is shaded the full string does not produce electricity anymore, or just the amount corresponding to the remaining irradiance on the shaded cell (mainly diffuse light).

In PVsyst the diffuse is considered as uniformly distributed on the array (for simplicity). Therefore, the electrical mismatch loss will only be applied to the beam component.

Sub-modules and By-pass diodes

Mismatch losses may be mitigated by the use of by-pass diodes.

Any PV module can be protected against "Hot spot" phenomenon by by-pass diodes. We name "sub-module" a set of cells protected by a by-pass diode (usually 20 - 24 cells). Within an array, as the shaded cells are unable to produce the current imposed by the rest of the array, they become reverse-biased. The by-pass diodes derive this imposed reverse current, recovering a part of low-current deficit of the shaded cells.

These by-pass diodes may recover a part of the lack of current in shaded cells. However, if the number of shaded sub-modules is too high, they cannot recover all. See the explanation of the by-pass effect. In particular in an array arrangement, as soon as the bottom cells are shaded, a string becomes inactive for the beam component.

Electrical shading mismatch calculation

PVsyst has 3 different calculation modes:

  • In the unlimited sheds (or trackers) orientation, the electrical effect may be evaluated in a simple and reliable way, using a 2D analytic calculation.
  • When defining a general 3D scene, there is an approximate simple calculation named "Electrical shadings according to modules strings". This calculation allows a reliable evaluation in the case of mutual shading in regular rows of fixed tilt tables or regular rows of trackers. It is not so well suited for irregular shades.
  • The most reliable calculation is the Module Layout model, which requires a detailed description of the PV array and the exact positioning of the modules in the 3D scene.

Linear and Electrical shading losses

We name "Linear shadings" the loss due to the real shades cast on the PV array, i.e., the "visible" shades on the array. These Linear shadings correspond to the deficit of irradiance.

The "Electrical shadings" are an extra loss, representing the loss during the transformation of this irradiance into electrical power. This is caused by the mismatch losses due to the inhomogeneity of irradiance, as explained above.

On the loss diagram: while the linear shadings are taken into account in the "Irradiance" part of the simulation, the electrical losses are applied to the "Electrical" part of the simulation.

Beam and Circumsolar

Both Linear and Electrical shading losses are related to the beam component.

Up to version 6, PVsyst used the usual interpretation of the transposition models: in the original weather data, a circumsolar component (strengthening of the irradiance in a crown around the sun) is assimilated to the measured diffuse irradiance. During the transposition, this circumsolar component is evaluated by the model and treated as the beam component, i.e. submitted to the sun's position. But in the shading calculations, this circumsolar was treated within the Diffuse, i.e. as an isotropic contribution.

From version 7 of PVsyst onwards, the circumsolar contribution may (optionnally) be distinguished from the Diffuse and Beam components. For the calculation of the shading losses, only the isotropic diffuse part is considered as diffuse, and the circumsolar is assimilated to the beam. We have therefore an increased contribution of the Linear shading loss, and a diminution of the diffuse shading loss. But as these values are often rather equivalent, these contributions compensate each other.

For the electrical contribution, this is not the case: the electrical loss is fully proportional to the beam, and the circumsolar is added as a "new" contribution.

NB: You will find the option to associate the Circumsolar contribution to the sun position in the main menu Preferences > Physical Models > Use new transposition.