Partitions with optimizers.

[Peter.W]

How should the partitions be set up in the near shade for the following scenario? We are using half cut cells with optimizers connecting every two modules. We are importing our shade scene so the tables each represent a singular module in landscape.  Based on the guidance for partitions systems with "little" tables partitions should be 1x1, but for the section "Defining rectangles as one module/ module optimizers" indicates that rectangular strings should represent 1 to 2 modules(2 in our case). Additionally the section before "String Optimizers" mentions that for landscape the best model is two partitions per row. So i believe there are two options

• Partitions of a 1x1 per table which defines a whole module (this is problematic because the optimizers connect every 2 modules).
• Partitions of a 1x3 per table which cuts a module into three landscape based rectangles  (this is problematic because the optimizers connect every 2 modules).

[Michele Oliosi]

It is almost never problematic to split partitions across tables, especially when the two connected tables are side by side. This is because often the shadings on the two tables are similar, so it makes no difference whether it is a single table or two tables.

Here I would recommend to act as if each table was a 1L string (the fact that there is a single module doesn't matter much). In this case I would put 2 partitions in height per table. I agree that intuitively 3 partitions (corresponding to the submodules) could seem to work, but when considering the details of the model over typical DHI/GHI ratios, we found that 2 partitions works better.

[Peter.W]

@Michele Oliosithanks! I do need a little clarification though. When you are saying put 2 partitions in height, do you mean that in the settings it would be 1x2 creating 3 long rectangle?or are you saying essentially cut the module in half with a partition?

 

thanks,

[Michele Oliosi]

Ah I meant two rectangles in height, basically cutting the module in half. Sorry I notice that "partition" is not very explanatory by itself.

[Peter.W]

Oh interesting @Michele Oliosi! I would have thought based on the guidance for tables (shown below) two high that it would have had partitions defined as the whole module or as three rectangles. Is it due to the optimizers or the modules being defined individually that we would cut it in half (see the second image?

[Michele Oliosi]

Yeah basically it is rather the following case:

No this is not really related to optimizers, even if you had no optimizers I would recommend something similar. Optimizers will help if you have shadings that are different from (pair of) module to (pair of module) module in the same string of optimizers, and this is already handled by having separated tables and the partition model.

Even though there are three submodules, since we apply the partition model only on the direct irradiance, we have to compensate for the diffuse that produces electrical energy in full in this approximation. In the end 2 partitions works well for this reason.

[Peter.W]

@Michele Oliosi. That’s interesting I have noted that the more partitions defined the less shading and electrical losses due to Shading effect our systems. I’m a little turned around if we are trying to compensate for diffused light due to the electricity that could still be generated from non direct light wouldn’t we want more partitions not less? This approaches feels more conservative than the how the submodules already act.

[Michele Oliosi]

Actually it's the other way around, with the partition model the diffuse is accounted separately it does not suffer from the partitions electrical shading losses. However in reality, when the current is limited on 1/3 of the submodules, by using the bypass diodes you lose both direct and diffuse on those submodules.
Therefore we need to increase the electrical shading losses in some way in the partition model (at least for this scenario). Currently we found that setting 2 partitions works quite well.

[Peter.W]

@Michele Oliosi I think that makes a lot of sense, but this did muddle my understanding of the Fraction for Electrical effect. I take it when this percentage is used at below 100% it isn't "adding" diffused light than. For example I would have thought with a tree where light is still seeping through branches there would be an increase diffused light making it to the modules, but based on the guidance you have given and my review of this section in the help guidance my understanding was incorrect. Does this fraction only affect the the direct light and its effect on those partitions? I am not entirely understanding the use of "regular" in the guidance. 

[Michele Oliosi]

You can use the fraction for electrical effect for several purposes. One would be to mitigate the effect of shadings overall. Indeed if for some reason light is seeping through, the electrical shading losses may be mitigated you can use the fraction for that. That would work for a semi-opaque shading object. However for trees the shading patterns will be very sparse, I am not sure what the impact would be. If one of the cells is fully covered for example, then it the light seeping through to other cells won't increase the production.

The main reason to consider the fraction for electrical effect is if your shadings are not due to row-to-row mutual shading (geometrically regular across tables) but impacts only some submodules in a row. E.g. a pole will cast a shadow on a couple submodules only, which should have less impact in terms of electrical shadings than mutual shadings, for which a whole row of submodules is impacted at a time.

[Peter.W]

Thanks @Michele Oliosi, This has given me a pretty new and deeper understanding of partitions. I did have one more case study based on this guidance, but it is isn't related to Rooftop systems and is instead related to Single Axis Tracker systems, using half-cut modules. This would be for a 1 in "portrait" with strings around 25-30 long. I currently define 2 partitions for each string essentially cutting the strings in half into two electrical zones.  Is this what you would recommend? I have attached an image for 1 string. If this were two in length, I would change the inputs to "2" in length and "2" in width. 

[Michele Oliosi]

Yeah that's the gist of it !

[Peter.W]

Hello Michele, 

 

I wanted to ask a follow up question regarding partitions as we would like to evaluate First Solar's Cadmium telluride modules. As these don't have internal diodes similar to the case studies we mentioned above, I was curious which modeling practice would be more accurate to have the modeled according to linear shading or to have them defined as partitions for the whole string. For example a SAT system with 108 modules but strings of 6 would look like this. My assumption would be that we would still use partitions, but any guidance you have would be greatly appreciated. 

[dtarin]

First solar modules have a linear response when shading is perpendicular to the cells. For flat sites and uniform shading, select linear shading. If there is terrain shading or other non-uniform shading, it may be appropriate to select a partition according to the string size with a low electrical effect, but ultimately depends on your site conditions and installation type.  

Edited December 14, 2023 by dtarin