How are managed the new "twin modules" with half-cells ?

How are managed the new "twin modules" with half-cells ?

Postby André Mermoud » Sat Apr 29, 2017 12:55 pm

A new technology for the PV modules design has appeared on the market, with the aim of reducing the shading elecrical losses.
These modules involve half-cells strings, mounted in the following way:

Half-cell_Twin_Module.PNG
Twin-module with half cells architecture
Half-cell_Twin_Module.PNG (204.31 KiB) Viewed 1715 times


Now how can this new module configuration be taken into account in the PVsyst shading calculation ?

First of all, we will only consider "regular" shading situations, when the shades affect all lower submodules at a time, i.e. mutual row shadings in sheds arrangement (portrait disposition).
In this case as soon as the lower half-cell row is shaded the lower submodule becomes completely inactive, and the string current will be half the "normal" current (upper sub-module). This is the key point of the benefit of this configuration.

This should be taken differently into account in the different ways PVsyst evaluates the electrical shadings:

- In the "Unlimited sheds" 2D calculation, where the electrical shading loss is considered complete as soon as the first bottom cell is shaded, we can simply consider loosing half the module instead of the full module width.
This option asks for the number of strings in the width of the row. We can simply double this value for twin modules. This doesn't require any program nodification.

- In the calculation "according to module strings", the rectangle-string becomes inactive when the rectangle is hitten by a shade. In the present time there is already a correction for sheds arrangement: when the bottom cell is partially shaded, the remaining current should be proportional the the enlighted part of this cell. In practice, PVsyst accounts for electrical shadings only when half this cell is shaded (correct on an average over numerous situations). Now we could extend this correction to the half-module. However this will probably not be so accurate as the amount of energy (and shading loss) is probably not the same for the higher half-submodule as for the lower submodule. If we implement such a correction, we should analyze the real energy distributions involved in the shading losses during the full year, and reduce the "half module" correction width accordingly.

- With the "Module layout" calculation, this will require a deep modification of the I/V curve of such a module. This represents probably an important development, that we put on our ToDo list.

Other "general" situations with any shade shape would be very difficult to take into account, and the benefits should be very low.
As a general rule, please observe that the overall benefit of taking this configuration into account will only allow to gain a (little?) part of the electrical shading loss calculated for a "normal" case by PVsyst. Therefore the electrical shading loss amount (on the loss diagram) represents an upper limit to the potential benefit of such a configuration with respect to the "normal" case.

Cells temperature
Now some people are wondering about the cell's temperature, and if it should be lowered in the model due to half the current in each sub-module.

I can't imagine why, physically, the cell temperature should be lower than in normal PV modules. The current is half, but in half a cell. Therefore the current per cell area is the same. Now if you have scientific papers with reliable cell-temperature measurements (differential between normal and half-cut modules), please send them to me.

Moreover I don't know the exact relation between the currents in the cells and the module temperature. This should be very low as to my understanding, the temperature elevation would be due to the power loss Rs * Ioper². Now Rs is around 0.3 ohm for a usual PV module, therefore the power loss is 24 W for Imp = 9A (and 6 W when operating at half-irradiance). If we assume an irradiance of 1000 W/m2, this module of 1.6 m2 will receive 1600 W, the current loss is 24/1600 = 1.5% of the irradiance, i.e. roughly a contribution of 1.5% to the U-value (and 0.75% under 500 W/m2). PVsyst desn't pretend to provide default U-values with this kind of accuracy.
By the way, this Rs loss is already accounted in the thermal balance equation, as this equation accounts for the efficiency.

Now if this is really the case, the benefit will probably be exactly the same in hot and cold climates, as the temperature correction wiil be added to the module temperature whatever the original temperature, and therefore lead to similar power differences.
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André Mermoud
 
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