Matching PV strings

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System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
I was curious about the idea of adding diodes in series to bring a lower string voltage "in line" with a higher string voltage - rather than having the higher string bleeding off. It might be overkill, but I'm looking at having a second set of panels 50 foot away from the first set. I could accept the loss or minimize the loss - I was thinking of using a higher gauge wire for the longer run, but then thought of adding diode voltage drops to match.
Any thoughts?
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #2
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    Re: Matching PV strings

    Don't do it ............
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  • System2
    System2 Posts: 6,290 admin
    #3
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    Re: Matching PV strings
    Solar Guppy wrote: »
    Don't do it ............
    Is your position the added diodes would reduce reliability, without enough potential gain?
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
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    Re: Matching PV strings

    Why waste a diode.

    Pointless. You will loose power (as dissipated heat) and not "improve" anything.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    #5
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    Re: Matching PV strings

    If the strings are even close (10%? 20%?) I don't think it makes too much difference, as MPPT will find a good operating point. For a reason that would take a while to explain, I had to remove one panel temporarily from one string, so I had 9 panels in one string, and 8 in the other, wired directly in parallel. The inverter found an operating point that was 2% lower in voltage than before, but the power was almost the same as what you would expect just by counting panels.

    If I do a thought-experiment, I would expect less current to flow through the lower-voltage string if diodes are added in series. Diodes are not voltage sources, unfortunately - they don't give a voltage drop for free. If they did, I could take a 0.5V, 5A solar cell, put it in series with diodes with a forward voltage of 200 volts, and get 200 volts at 5A (1000W). Doesn't make sense to me. (If it actually does work like this, I think we've just solved the energy problem. We can stop drilling for oil...)
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
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    Re: Matching PV strings

    i don't believe it was his intent to use a diode to up the voltage, but to lower the voltage to equal the other. i wouldn't bother using them either.
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