[RE-wrenches] Add-On to DGT, string inverter, w/Different Modules on 2nd string

[jay]

HI Ash,

Backwoods solar sells 190 watt 72 cell modules that might give you some options.

jay

Peltz power
> On Jun 21, 2016, at 8:31 PM, Brian Teitelbaum <bteitelbaum at aeesolar.com> wrote:
> 
> Hi Ash,
>  
> Are you sure that there are 12 of the SolarWorld 175’s on a single string?
>  
> Those were 72-cell modules, and I would think that having 12 in series in CO would see open-circuit voltages over 600 VDC at least on some occasions. That’s 864 cells in series, with would be about equal to having 14.4 typical 60-cell modules in series. That sounds too high to me for CO.
>  
> One option would be to reduce the number of 175’s to 10 (720 cells in series) and match them with 12 of the 60-cell modules (also 720 cells in series). The older modules are 5” cells, so their amperage would be lower, but as long as you get the voltage as close as possible it should be fine. Of course, this doesn’t account for degradation, so you may want to do some actual voltage measurements on the old modules.
>  
> One thing though – since twelve 280W modules is about 3360W, and you have a 3kW inverter, there really isn’t enough room to add. The older Sunny Boys had a much narrower MPPT voltage range, so I doubt that you could use strings shorter than nine 60-cell modules. I don’t see how you would be able to add much to the old inverter anyway. Adding a second inverter system may be your only option, unless you can find some used 175’s. 18 or 20 of the 175’s, in two strings, should work fine on the 3kW Sunny Boy.
>  
> Brian Teitelbaum
> AEE Solar
>  
>  
> From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org <mailto:re-wrenches-bounces at lists.re-wrenches.org>] On Behalf Of Ash Bowersock
> Sent: Tuesday, June 21, 2016 12:48 PM
> To: re-wrenches at lists.re-wrenches.org <mailto:re-wrenches at lists.re-wrenches.org>
> Subject: [RE-wrenches] Add-On to DGT, string inverter, w/Different Modules on 2nd string
>  
> Good day RE Wrench crew,
> I’ve got a client whom installed a 2kW array with a 3kW SunnyBoy inverter a number of years back. Due to the age of the system I assume it is single MPPT. The client oversized his inverter in the hopes of using it for this future expansion (and unfortunately his installer went along with it). The original modules were 12 SolarWorld 175’s I believe, and we would be looking at adding SolarWorld 280’s more than likely. To me it is clear that this is poor design practice and not something I am willing to do. My recommendation is a micro-inverter system that we tie in w/his existing using an AC Combiner. But the client wants to know exactly why we don’t want to just add a second string of 12 SolarWorld 280’s to his 6 year old SunnyBoy inverter. 
> Can someone please give me a technical reason why mixed modules on a string inverter is “poor design practice”?
> The more technical and dry an explanation, the better ;) 
> If you can’t convince them, bore them to tears with the details. 
>  
> Ash Bowersock
> Senior System Designer
>  <http://www.rgsenergy.com/>
> 833 W. South Boulder Road | Louisville, CO 80027
> tel 303.222.8387
> RGSEnergy.com <http://www.rgsenergy.com/>
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