Electrical shading losses: partition model

[Michele Oliosi]

Version 7.4 features novelties in the way the electrical shading losses are computed in the partition model. Here is a short recapitulative summary of the different ways the model worked in the past.

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Up to version 7.2, the partition model has been implemented as an all or nothing model. Tables were split into partitions, i.e. PV areas that would be electrically affected by a shadow, via mismatch effects. Shaded partitions would produce power only up to the diffuse irradiance, and their direct irradiance contribution would be considered naught.

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Version 7.3 introduced the notion that if shades were small, then these mismatch shading losses would be mitigated. However the implementation was only well adapted to very regular arrays, with a single orientation. In other situations, the electrical shading losses were underestimated.

The following post details the possible issues with this particular model: https://forum.pvsyst.com/topic/3085-electrical-shading-losses-in-versions-73x/

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Version 7.4 takes the idea of small shading mitigation further, in that the shade on each PV table is evaluated to estimate whether or not the electrical shading losses will be mitigated or not. In this way, the cases that were not well treated in version 7.3 (underestimation in case of irregular shadings, or multiple orientations) are now handled properly.

In practice, the partition model in version 7.4 splits each partition into three areas:

• A central area that follows the all or nothing model: if shade reaches the central area, the direct irradiance contribution is considered lost.
• Two lateral one-cell-wide strips, over which the full shading losses are proportional to the shaded area.

In the case of regular shadings that affect an entire row of modules, this translates well into the notion that whenever a single cell per submodule is partially shaded, the production of the submodule is lowered proportionally to the shaded portion of the cell (current limitation in a string of cells in series).

More details on the model itself can be found here: https://www.pvsyst.com/help/near_shadings_partition.htm

For more details on how to configure that partition model in a PVsyst project, see this help page: https://www.pvsyst.com/help/shadings_partitioninstrings.htm

 

[Michele Oliosi]

Version 7.4.6 further improves the evaluation of electrical shading losses by correcting the bottom cell strips positioning for NS-horizontal-axis trackers, tilted-axis trackers, and EW-frame trackers.

In version 7.4.0 to 7.4.5, the bottom (and top) one-cell-wide strips were drawn incorrectly along the short side of trackers. Version 7.4.6 corrects this and draws the one-cell-wide strips along the longer edge of the trackers.

This correction will drive electrical shadings down slightly for projects with electrical shading losses, trackers, and an irregular topography. This mitigation of the losses comes on top of a conservative model, thus moving towards more accurate values, in general. As a consequence, especially for projects with NS-axis SAT (and other less common trackers noted above), it is advised to update to 7.4.6 and run key simulations to obtain more accurate results.