PV shading

[edexlab]

I haven't posted here for quite a while as I'm no longer working on PV however I'm currently in southern Germany for a couple of weeks and as can be expected nearly every building and the majority of houses even a few small barns and chicken coops here have a system installed,

I was surprised to see an almost blatant disregard to the shading of panels by chimneys, satellite dishes, ducting and other items, nearly every system has something which casts shade on parts of the arrays throughout the day
I did a PV course here in 2010 and they talked a lot about avoiding this issue ,yet it seems it's not really practiced here!
I'm thinking perhaps the array sizes are slightly oversized to compensate for this?

 

[Gavin A]

perhaps they're actually taught properly how to mitigate shading rather than just to attempt to avoid it entirely.

Use of SMA optitrac global peak / ABB full scan shading function or similar together with orientating the panels correctly for the shading enables the bypass diodes to operate correctly to bypass shaded areas of the panels, so that the shading impact is merely to result in the affected area being out of use while it's shaded, as opposed to being out of use permanently as it is if there's not a panel fitted.

IIRC Germany also has one FIT band from 0-10kWp, so less incentive to only fit 4kWp on a roof that could take more.

 

[Gnarly]

If you have multiple strings paralleled into a single input, then shading is very bad because the voltage on one string drops to below the other string, and it doesn't provide any power. I wonder how common this is?

 

[Gavin A]

If you have multiple strings paralleled into a single input, then shading is very bad because the voltage on one string drops to below the other string, and it doesn't provide any power. I wonder how common this is?

It shouldn't be very common, given that dual tracking inverters cost little more than single trackers there's not much excuse.

Then again, seeing as there's nothing in the MCS guide about how to design to mitigate shading, and it's therefore barely mentioned on the UKs 3 days courses to become a PV qualified spark, maybe it is common practice.

[/dismounts from soapbox]

 

[bxman]

Gavin A wrote

re mitigate shading

Use of SMA optitrac global peak / ABB full scan shading function or similar together with orientating the panels correctly for the shading enables the bypass diodes to operate correctly to bypass shaded areas of the panels, so that the shading impact is merely to result in the affected area being out of use while it's shaded, as opposed to being out of use permanently as it is if there's not a panel fitted.

I have manged to get my head around the SolarEdge optimisers and appreciate the way that system works

However not so the S/W systems which I understand them to be .

Is there a possibility some one could kindly give me a simple guide that would help me understand the SMA optitrac or more importantly the ABB system (as I have a Power One inverter )

or failing that to recommend a link or links that might help in this respect.

This last week the sun has dropped low enough for one of my strings to suffer from about 5.30 PM on
not that much but as they say "every little helps" .

Many thanks

 

[Gavin A]

The new OptiTrac Global Peak makes sure that partially shaded solar power plants operate efficiently

 

[bxman]

Thanks Gavin A

Read that several times not fully understanding it .

I got the impression that the OptiTrac raised the string voltages which increased the possibility of panel diodes disconnecting shaded cells .
Also watched some u-tube clips which explained just how good it was but not how it worked as far as i could see.

There was one useful comment by
databeestje

" So if 2 out of 8 panels are shaded, the inverter will find the lower Global MPP voltage of 6 panels so that the bypass diodes in the panels will trigger. Normally, the dip causes the 99.9% MPP trackers in inverters to fail to traverse the voltage range far enough down.You lose less power this way. If you have shade, check that your inverter can work with it! "

To my mind the inverter can only raise or lower the voltage across the entire string.

I cannot see how it can possibly give the improvements claimed for it in those videos .

Or how it could work any where near as well as the Solar Edge system .


I am sure I have just demonstrated how thick I am , never mind !

 

[Gavin A]

the only advantage solar edge gives is that in fully shaded panels it allows the shaded output from the panels to contribute, whereas with optitrac etc it simply bypasses the shaded panels so they contribute nothing, but they also don't impact on the rest of the system.

without the bypass diodes functioning to cut the shaded cells out the entire array output drops to pretty much the level of the shaded cells, the shaded cells act as a resistor.

 

[bxman]

Thanks

Is it raising the string voltage that triggers the bypass diodes into functioning ? that is the only way the inverter can have any influence on the bypass diodes as far as I can see.

Correct me if I am wrong but I believe there are a number of bypass diodes within each panel each operating in conjunction with a number of cells within that panel these will all act independently so I was led to believe .