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Electrical shadings: Module Layout

The Module Layout tool is aimed at the detailed calculation of the Electrical shadings mismatch loss.

It requires a description of the position of each PV module in the 3D scene, and the module interconnection as strings according to the inverters defined in the "System" part.

General description

The evaluation of the electrical shading loss requires the calculation of the I/V characteristics of the PV array, by the addition of the voltage (I/V curve) of each module in a string, and then the addition of the current of each string.

The I/V curves depend on the partial shading on each PV module. Therefore your variant should properly define the exact geometric position of each module, and the string and sub-array it belongs to.

Therefore, defining the "Module Layout" necessitates:

  • the shadings 3D construction for the positioning, and
  • the electrical definitions of the sub-arrays (PV modules and inverters) in the "System" option.

These two parts of the project should be well defined before starting to work with the Module Layout tool. Any later modifications of these parameters (shadings and system definitions) may have consequences on the Module Layout definitions, which you may then need to review once again. This is why the Module Layout is usually the last step for the study of a PV system.

Procedure

The Module layout window displays a 2D representation of all PV tables that have been defined in the 3D scene. Each 3D sub-field element (for example a fixed tilt table or a tracker) is named a Table.

If you have defined several orientations, these may be displayed separately. A dropdown menu allows to switch from an orientation to another.

The blue panel at the top right gives you some information about the current state of the module layout definitions, and advice for the next action to be done.

There are 2 main steps:

  • The "Mechanical" tab shows a rough 2D representation for all tables from the 3D scene, and requires to position all modules as defined in the "System" part, for each orientation.
  • The "Electrical" tab is meant for the attribution of each module to a specific String (according to the sub-array definitions in "System").

In addition to these two tabs, there are also two tabs containing pedagogical tools, not necessary for the Module Layout definitions that will then be used in the simulation:

  • "Shadings 3D" shows the real shadings on all tables of a selected MPPT input, and the corresponding I/V curves for each shaded PV module.
  • "I/V curves" shows the detailed I/V curves for this MPPT input, as addition of voltages all modules in series in each string, and addition of currents for each string in the MPPT array.

The electrical calculations are done taking the beam and diffuse irradiance components into account. Even when a sub-module is completely shaded (in terms of beam component), the remaining diffuse irradiance ensures a minimum current in this string. Since the diffuse incident irradiance is coming from all directions, we assume that it shines homogeneously on the PV table, that it is only affected by a constant shading factor, calculated only once for the whole simulation. The albedo contribution is evaluated in a similar manner, also using a constant shading factor.

The Module Layout definitions are mainly used within the simulation, for the detailed calculation of the shading electrical mismatch losses.

Module Layout definitions output

Incidentally, this tool may be also useful to accompany the design of the module's cabling within your real PV system.

The Module Layout definitions may be printed independently, or may be optionally shown in the final simulation report, in different pre-defined ways.

Limitations

Thin film modules

The Module Layout tool is suited for crystalline modules only, with usual rectangular cells.

It is not applicable to thin film modules, where each cell is a strip of typically 10 mm width, along the length of the module. In this case the electrical mismatch loss is null if the shades are perpendicular to the cells, i.e. if all cells are identically illuminated (Cells in portrait in a row arrangement). However the shading loss will be maximal if the shade is parallel to the cell, as one only cell in series may be shaded and block the current for the whole module. This behavior cannot be currently represented in the module layout tool.

Thin shadows

Due to the way shaded sub-modules are recognized, the module layout will underestimate the impact of long and thin shadings (e.g. poles). If your 3D scene has such objects, you should rather use the partition model.

Very large systems

The Module Layout is only useable with systems of the order of few MWp at most. Either because of the complexity of defining all the PV module locations if these are not regularly interconnected, or because of to the increased computing time during the simulation. PVsyst fixes a "reasonable" limit of around 1 MWp, and an upper limit (that you can modify in the advanced parameters) of 5 MWp. The limits can be modified via the advanced parameters "Power limit for Module Layout warning" and "Power limit for Module Layout error".

For very large systems, we advise to define a representative sub-system (for example corresponding to a single central inverter), execute the simulation and evaluate the Electrical shading loss with both tools: Module Layout and partitions. This will allow you to evaluate the "Fraction for electrical effect" representative of your system (usually close to 100% for regular systems).

Then you can simulate your full system using the option "According to module-strings", i.e. the partition model, applying this pre-evaluated factor. This latter calculation requires about the same computing time as the linear shadings option.

Relevant advanced parameters

The module layout model is parametrized by several advanced parameters:

  • Module Layout: Min. shade area to count as shaded : default value 0.024 m².
  • Module Layout: Min. shading fact. for electrical calc. : default value 1%.
  • Module Layout: Max. shading fact. for electrical calc. : default value 98%.
  • Module Layout: Frac. bottom cell for no shading : default value 50%.

Their use is explained in the model details.

Other than these, the following parameters control the warnings and errors against using the module layout for too large systems or when the 3D table sizes have changed:

  • Size diff. tolerance between Module layout and 3D tables : default value 2%.
  • Power limit for Module Layout warning : default value 1000 kW.
  • Power limit for Module Layout error : default value 5000 kW. Forbids the use of the module layout above this system capacity.