[RE-wrenches] large array string sizing

[frenergy]

Your also going to have to consider what's going to happen to your high temp 
voltages in 5-10-15 years as the PVs derate with age plus add in dirt/dust 
etc.  You'll be losing more and more of those summer hours as time goes on. 
Seems like an extremely narrow voltage window on the inverter and/or very 
high Voc per PV.  Are you locked with the PVs you've spec'ed...and the 
inverter?  You say you're completing this system, but still trying to 
configure the strings??

How about a temperature controlled disconnect with 13 PVs.

Bill
Feather River Solar Electric
4291 Nekson St.
Taylorsville, CA  95983


----- Original Message ----- 
From: "Jeff Yago" <jryago at netscape.com>
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Sent: Monday, December 07, 2009 9:28 PM
Subject: Re: [RE-wrenches] large array string sizing


> We are completing an institutional solar project about 120 kW going on a 
> large parking deck. The large spans require some really large beams to 
> support the multiple rows of modules since support columns are as much as 
> 60 feet apart. In other words, adding more modules to each multiple row is 
> a really big structural issue and they are at the limit now.
>
> However, no matter how we design the strings, the number of modules in 
> each row can only be divided by 12 and multiples of 12 which is the only 
> combination of modules per string that also works with the number of 
> modules per row, the number of rows per system, and the number of modules 
> stacked N-S related to shading issues between rows. However,the inverter 
> manufacturer wants 13 modules per string to avoid low voltage cutoff on 
> hot summer days. if I go to 13 modules per string, I am really hitting 
> voltage max during average cold days and believe me, this site will see 
> more cold and snow days (reflections) with temperatures far below average 
> winter temperatures, then it will have hot summer days. The summer average 
> temperature is 83 degrees and I calculate the minimum array voltage is 
> still over 316 volts, on a 480 VAC 3 phase inverter.
>
> Since by both structurally and module count, there is no way we can 
> increase row lengths to provide 13 module strings, and we cannot reduce 
> modules per row to achieve 13 module strings as this would make the system 
> far below what the client expects in system capacity.
>
> Weather data for this location indicates it exceeds our summer design 
> temperature by onlyv60 hours total per year, and no doubt most of these 
> hours occur near the late afternoon at the end of a solar day.
>
> Whats the best way to deal with this less than ideal combination of string 
> sizes since we are getting nailed at both ends - too few modules per 
> string can shut down inverters when hot summer days drop module voltages, 
> and too many modules per string can do some real high voltage damage on a 
> very cold sunny day with a foot of snow on the ground.
>
> What say yea?
>
> Jeff Yago
>
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