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Solar Panel Shading and MPPT


DrLoLCat
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Hi,

 

I am planning a 4 x 4 Solar PV array installation on my east facing low slope roof. The array will be located on the roof in such a way that each parallel string of the array will more or less be sequentially cast in shadow as the sun goes down. 

 

For example: @3:30pm 1 out of 4 strings will be cast in shadow, @4:00pm 3 out of 4 strings will be cast in shadow etc.

 

The idea is to keep shaded an unshaded panels in separate strings to optimize on power generation during sunrise and sunset. I realise the gains from this will be small but it does not affect my PV panel installation.

 

From what I have read on-line and in research papers it seems as if the MPPT charge controller will harvest the most energy with this kind of shading as opposed to having shaded panels in each string. 

 

I think this results from the fact that that a higher power point exists on the I-V curve of an array where shaded an unshaded panels are kept in different strings. And this would then allow the MPPT charge controller to harvest more energy because of the higher power point that exists.

 

Does anyone have a similar understanding of this?
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Hi DrLOLCat,

 

My system uses a built in MPPT, so I can't comment too much on the MPPT side.

 

However I have learn't alot from building my own solar panels and comparing them with factory produced panels.

With my panels I did in a single string of solar cells. Downside to this in that covering just one cell means a drop in the entire panels output.

 

However the manufacturers are quiet clever in this regard. I opened the junction box on the back on my Jinko 235w panels to see how they connect them all together.

My findings are that they split the panels into 3/6 different sections. So basically it's 6 strings combined into one with diode bypasses connecting them all together.

This means that shading of on cell effects only that string, so only a 6th of the output is effected. The panel will just bypass the string that is suffering from shading.

 

How the MPPT's use this to produce max power is a bit above my pay grade

I'll attach a pic of the back of a junction box so it makes better sense.

post-604-0-56468900-1425291082_thumb.jpg

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Hi Slayer69,

 

I have indeed taken into account the sub-module PV panel architecture in my design when I decided on the orientation of my panels. Here is a picture showing the sub-module sections of which you speak:

http://image.slidesharecdn.com/solarshadinganditseffects-121010073500-phpapp01/95/solar-shading-and-its-effects-23-728.jpg?cb=1349872605

 

My understanding is that if a dynamic shadow (for example sunset) approaches the top or bottom of the panel in the picture it will immediately cause all 3 bypass diodes to engage thus short circuiting the entire panel. However if the panel is mounted so that a dynamic shadow approaches from the left or right it will sequentially take out the panel sub-modules in this order:

 

0 Bypass Diodes activated: 100% generating capacity remaining

1 Bypass Diodes activated: 66% generating capacity remaining

2 Bypass Diodes activated: 33% generating capacity remaining

3 Bypass Diodes activated: 0% generating capacity remaining

 

Essentially some sub-modules of the PV panel can keep on generating power even when other sub-modules are already cast in shadow if the orientation is correct.

 

In my PV array configuration I would like to optimise on both points:

1.) Array Configuration (Parallel strings separated in terms of shading) - first post 

2.) Panel Orientation (Sub-modules separated in terms of shading)

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While reading up on Power optimizers in another topic, they did make some interesting reading.

If you suspect you will have problems with shading, you could look at seperate MPPT controllers for each individual string.

So instead have having a single 100Amp MPPT, you could have 4 x 30Amp MPPT's.

So, once one panel in a string goes down, it will not pull the other strings down too.

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