Adding larger modules

solarix · January 2011

Check me on this. I need to add capacity to a customer's array that currently has two strings of 9 - 190w modules. We originally installed an SMA GT 4000 as they wanted to add on more at a later time like - now. Can't find the original 190w modules as they are out of production (at least not at a reasonable price). It would actually be cheaper to add 3 of our current 235w modules to each string and take the efficiency hit than to pay $$$ trying to match the old ones. Am I right that the new modules will just operate at the old modules lower current but at a somewhat higher voltage? Will the efficiency of the old modules be affected?

mike95490 · January 2011

Re: Adding larger modules

need to know the volts and amps of what you are trying to match,

if you can match the volt max power of the existing strings, with new panels, the parallel string will just add amps. All voltages will be pulled to the lowest strings voltage.

niel · January 2011

Re: Adding larger modules

you need to match the vmp within 5% even though others may say 10% and if you can do better then that's all the better. this will wind up being a 3rd string and as such all strings will need fused and the use of a combiner box that can often house those fuses. be sure the inverter can handle the expansion you plan on going with.

solarix · January 2011

Re: Adding larger modules

Existing modules are Canadian Solar CS6P-190 6.60A, 28.8Vmp.
I can't find any modules equivalent to these - let alone at a competitive price.
I'm thinking that going to 2 strings of 12 is better than adding another string of 9.
Would the new modules be limited to the 6.6A or would the bypass diodes in the smaller modules kick in and the strings operate at the higher current levels of the new modules?

BB. · January 2011

Re: Adding larger modules

Series connections should match Imp within 10%.

Parallel connections should match Vmp within 10%.

The farther your mismatch--the less of rated array wattage you will harvest (for various reasons).

Matching existing panels with new panels a few years later can be very difficult. At times, about the only options are to dump the original array (try EBay) or install a second controller to manage the new array.

Depending on the existing array's Vmp/Imp (both panel rating and whole array rating) with adding new panels (need their Vmp/Imp ratings) and the capabilities of the 4kW GT inverter--Need to really go over the specifications and the various possible configurations.

It may end up better adding a second inverter for the new string, or even look at the Enphase units to provide panel fill-in...

No easy answers without doing to pencil work on the various specifications and options available.

-Bill

stephendv · January 2011

Re: Adding larger modules

If this helps here are a bunch of 190W panels: http://www.posharp.com/photovoltaic/database.aspx?sort=wp&dt=asc&pg=285

Just need to match the Vmp (and find them at a reasonable price in your are

).

SteveK · January 2011

Re: Adding larger modules

stephendv wrote: »

If this helps here are a bunch of 190W panels: http://www.posharp.com/photovoltaic/database.aspx?sort=wp&dt=asc&pg=285

Just need to match the Vmp (and find them at a reasonable price in your are ).

Wow, thanks! Great list!

solarix · January 2011

Re: Adding larger modules

I'm not sure I'm getting my question across.
I realize that the added 235w modules are going to take a big efficiency hit. I'll be happy if they put out just the 190W (stc) the old ones do. But I can get them for less than $2.00/w where the 190's are over $2.75/w if you can find them. As long as they don't cause some other problem because of bypass diodes or whatever, it seems better to add in the less expensive 235's.
I think I'll stick a couple mismatched modules out in the sun today and see what happens.

ggunn · January 2011

Re: Adding larger modules

solarix wrote: »

I'm not sure I'm getting my question across.
I realize that the added 235w modules are going to take a big efficiency hit. I'll be happy if they put out just the 190W (stc) the old ones do. But I can get them for less than $2.00/w where the 190's are over $2.75/w if you can find them. As long as they don't cause some other problem because of bypass diodes or whatever, it seems better to add in the less expensive 235's.
I think I'll stick a couple mismatched modules out in the sun today and see what happens.

I have done this experiment. If you connect them in series, the Voc of the pair will be sum of the Voc's of the two modules and the Isc will be equal to the lower of the two module's Isc. If you connect them in parallel the Isc of the pair will be the sum of the Isc's of the modules but the Voc will be equal (or nearly so) to that of the lower of the two.

solarix · January 2011

Re: Adding larger modules

OK, just tried connecting a 200W, 6.78A module and a 235W, 7.8A module in series to an 8ohm load. The smaller module's voltage collapses as it's bypass diodes turn on. Back to searching for 190's

BB. · January 2011

Re: Adding larger modules

What are you using for a load? With a mppt gt inverter, I would expect the array to produce something in the 400 watt range... Not perfect, but certainly better than you saw.

-Bill

solarix · January 2011

Re: Adding larger modules

Got to thinking about the load I used as being not very similar to a MPPT load so went ahead and installed more modules for this customer. Figured it was worth the effort to satisfy my curiosity.
Added three 235w modules to a string of nine 190w modules after disconnecting the other string. The GT inverter showed 1180 watts at 5.0A and 236V at mid morning, then 1600w at 4.9A and 325V about 15 minutes later after rewiring for the added modules. The current only slightly decreased showing that the existing modules were not affected. The power from the larger modules added was just a little more than the smaller ones and we won't be seeing their full capability, but so what, they are a lot less expensive to buy than the out of production smaller ones.

BB. · January 2011

Re: Adding larger modules

Sounds like a reasonable solution...

Sorry I missed the 8 ohm load part... But, yes, because of the MPPT "optimizing" array output, you should not have the voltage collapse because of the current mismatch like you did with the fixed load.

I am certainly not the expert in how an MPPT controller works--but pay attention to start-up and shut down to make sure that it does what you need (does not get "stuck" at low power while starting up in the morning, etc.).

Edge conditions can be a real killer in electronic systems design.

-Bill

Adding larger modules

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