Adding new panels to old?

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modernhistorian
modernhistorian Registered Users Posts: 2 ✭
in Advanced Solar Electric Technical Forum #1
I designed and built a grid-tied PV system back in 2009, using 14 KyoceraKD210GX-LP panels in series, feeding a Solectria PVI3000 inverter. The system is still producing, but output has declined significantly, and I'm considering adding panels to the string. Here are the specs:

Each panel stated with a Vmpp of 26.6 and Impp of 7.9, producing 210 watts. When new, in full sun the panels produced 372 VDC and 2942 watts, but they now only produce about 304 VDC and about 2400 watts, max. The inverter is rated for a max of 600 VDC and 3600 watts.

I am thinking of adding four 24 VDC nominal panels, one each at beginning and end of string, and two in the middle of the string. If these each added a max of 26.6 VDC, then the panels would be restored to slightly more than new specs and be well within the inverter's capabilities.

Are there any problems with adding new panels to old, as I've described, and are there recommendations for brand and type of panels? Thanks for any help.
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,044 ✭✭✭✭✭
    #2
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    I posted a reply on SPT to your querry

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    The general recommendation is to match Vmp of panels (series strings) within 10% or better (i.e., 340 volt parallel string in parallel 373 volt string)--Closer is better.

    For the series string, match Imp of panels within 10% or better. (i.e. you can match a 30 volt 10 amp 300 Watt panel with a 17 volt 10 amp 170 Watt panel (matching 10 Amp Imp panels).

    I have not checked my array lately (cannot do it for another month), but I seem to remember seeing Imp-array falling as the array ages. The Vmp-array voltage did not fall that much. You may have 2-3 "weak/failed" panels.

    The quick test is to measure the Voc (voltage open circuit) with a meter, and the Isc (short circuit current) if full sun (basically test each panel quickly in full sun, middle of the day at same conditions (same weather, no moving clouds, at roughly the same time).

    You have 14 panels to measure and compare... Either they will all be nearly the same, or a couple will stand out as "failed/different".

    I suggest the testing as placing "good panels" in series with failed/failing panels is not usually a great solution.

    The failed panels will have bypass diodes which does help prevent issues with a dead (high resistance) panel (bypass downs send current "around" failed cell(s) in solar panel). But bypass diodes can and so fail on their own too (high temperature, etc.). And mixing "weak/failed" panels with new good panels--You have a good chance of the bad panels failing open down the road.

    In my case, I had a bad batch of panels (design error as I recall) and if I looked closely, I could see a "coffee colored stain" over the J-Box from overheating bypass diodes. Had to look closely, the brown stain was similar to the "brown water marks from leaves" on the glass over panel.

    You should also look for other signs of failure (water stains inside panel, cracked cells, browning at on electrical connections and such).

    Solar panels are pretty cheap these days--At some point, if you have several failed panels--You might think about simply buying a new set of panels--Hopefully the new ones will not have the same flaws as the panels that failed (if they did fail).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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