What is the shading loss "according to module strings" ?

[André Mermoud]

With near shadings in PVsyst, you can define and perform simulations with 3 kinds of near shadings (to be applied to the beam component):

- The shading loss due to the irradiance deficit on the modules (optical loss), which we call "Linear shadings". The shading factor is the ratio of the effective shaded area with respect to the total field area.

- The Electrical losses, i.e. the effect of the partial shadings on the electrical response of your PV array. The calculation requires to define the position of each module, and specify its electrical connexion on the inverter input, in order to combine each I/V curve of each sub-module. This is done in the "Module Layout" part. The electrical loss will appear in the Array losses on the loss diagram.

- Simplified Electrical losses, which we called shadings "according to module strings": this is intended to the evaluation of the maximum electrical effect on the array production, in a simplified and approximate way. Remember that in a string, the poorer cell determines the current of the whole string. That is, when one cell is shaded, the entire string is affected, and is supposed to become almost inefficient (for the beam component). Therefore in this mode, the array is partitioned into rectangles, each rectangle representing (approximately) the area of a full string of modules (to be defined within the shading scene, icon on the left). Then, when a part of this rectangle is shaded, the whole rectangle is considered inactive.

The Global Shading factor is the ratio of the shaded rectangles (grey + yellow) to the total field area. The additional loss represented by the yellow parts (Global - Linear shading factor) gives indeed an evaluation of the upper limit for the electrical loss.

Now some part of the string energy is recovered by the by-pass diodes (see How to evaluate the effect of by-pass diodes in shaded arrays?). This is the reason why we have defined the "Fraction for relectrical effect". However when about one third of "sub-modules" in a string are shaded (as for example in rows arrangement), the output for beam irradiance becomes almost null, and this fraction will be near to 100%.

This evaluation of electrical losses "according to modules strings" was the only way of estimating the electrical losses up to the version 5.

It should still be used with very big systems, when the "Module layout" becomes impracticable due to calculation time. In this case you can evaluate the "Fraction for electrical effect" by comparing the "real" electrical loss calculated by the Module Layout on a reduced sample of your system, to the calculation "according to module strings". And apply this last calculation to the full system.