[RE-wrenches] large array string sizing

[Phil Schneider]

Mind giving away your selected module?  Or at least module Voc and Vmp with
temp coefficients?

Phil Schneider
Creative Energies



On Mon, Dec 7, 2009 at 10:28 PM, Jeff Yago <jryago at netscape.com> wrote:

> 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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