Adding panels and matching voltages to existing strings

[Zee]

Here is an interesting one:
Customer wants to expand system. Turned on NOV, 2010...6 years ago.
He has an oversized inverter, so I can add a string, no problem.

He has 3 strings of 9 each Schuco 185's.
9 modules x 36.5Vmp = 328.5 V rated string voltage.
So I am basically looking for any number and size of currently available mod.s to add up to close to that V.
But...would I also factor in fading of panels AKA Voltage degradation, of... say ....0.5% annually to the existing voltage?
===>97% or about 319V

 

[jaggedben]

I think you'll find that worrying about the degradation is a luxury. Unless you find some random old stock somewhere, I think you're going to find that you're in between 8 and 9 panels with available 60cell options. Presumably those Shuco's were 72cell with 5inch cells so unless you can find some old stock of that format I believe you're going to end up off by a bigger factor. I would say aim for higher voltage but it may depend on the particulars of the inverter MPPT.

 

[BillK-AZ]

You need to double check the module labels. Maybe measure Voc and cell temperature. The limited data I have on Schüco from 2010 indicates a Vmax of 24.2 to 25.4 volts for 180 or 200W modules. See attachment.

If the new subarray will be higher power rated than the existing modules, use new modules with a equal or slightly lower Vmax than the existing array. Operating output falls off quicker for operating points above Vmax, so the lower power subarray should have the higher Vmax.

 

[Zee]

Thank you. Yes they are the 72 cell modules.
The (E) modules are in fact 36.5 Vmp as stated....agreed, always good to check actual installed equipment

I found a great match of (N) 60 cell 280W modules by QCell.
10 of these add up to 317 string Vmp ......vs the (E) Vmp of 319! :thumbsup:

I'll be overloading inv. a bit with (E) 5 kW array + (N) 2.8 kw = 7.8 kW on a 6 kW inverter.
I know 7.2 kw is no prob in this climate. What's a little more?:roll:

 

[Zee]

What I am really curious to see: will the AHJ enforce 2014 RAPID SHUTDOWN req's on this addition?

 

[GoldDigger]

AFAIK the degradation for silicon cells affects current more than voltage.
And if you are adding a new string in parallel to the same MPPT input there are really only two factors to consider:
1. There should not be partial shading of the new string that is different from the old string. A different orientation of the whole string is OK.
2. The voltages (VOC and Vmp) should match the old string within 5%. The Maximum Power Point is broad enough that there will be little (1%?) loss in overall power.

mobile

 

[Anode]

Thank you. Yes they are the 72 cell modules.
The (E) modules are in fact 36.5 Vmp as stated....agreed, always good to check actual installed equipment

I found a great match of (N) 60 cell 280W modules by QCell.
10 of these add up to 317 string Vmp ......vs the (E) Vmp of 319! :thumbsup:

I'll be overloading inv. a bit with (E) 5 kW array + (N) 2.8 kw = 7.8 kW on a 6 kW inverter.
I know 7.2 kw is no prob in this climate. What's a little more?:roll:

I would like to throw my hat in the ring and say that from a design perspective, and from an o&m perspective long term with mixed and matched modules (even if they have different electrical characteristics), adding dc optimizers to the system would be a great move. Then you don't need to worry about mix and matching. Tigo also works with 3rd party inverters very well. We did a lot of tigo before switching to Solaredge for our optimized systems.

Of course, this is adding more costs. But if you are in an AHJ that has rapid shutdown in effect, you may have to comply with that too upon renovating this system. Then you are killing more than two birds with one stone.

 

[jaggedben]

It would be quite a lot cheaper and easier to add a rapid shutdown box than 37 optimizers, though.