Blocking diodes for parallel strings?

Elke
Elke Solar Expert Posts: 28 ✭
in Solar Beginners Corner #1
Is it advisable to install a blocking diode on each parallel string of modules?

I have four parallel strings of three 165 Watt panels. Each panel came equipped with its own bypass diodes. The combiner box is equipped with a 10 Amp breaker for each parallel string.

I recall reading somewhere, that one should also install a blocking diode for each parallel string. This would prevent current flowing from one parallel string into a lower-current string (possibly due to shading) and therefore helps minimize mismatch losses in parallel connected arrays.

Would the installation of these blocking diodes help improve efficiency?
What size of blocking diodes would I need?
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Blocking diodes for parallel strings?

    Unless it's been too long since my last nap, almost any panel you buy today has a blocking diode built-in already.

    Solar Guppy would know for sure. :D
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  • Elke
    Elke Solar Expert Posts: 28 ✭
    #3
    Re: Blocking diodes for parallel strings?

    The specs for these older (2004) Shell Siemens 165 Watt modules did mention only the pre-installed bypass diodes. However, there is no word about pre-installed blocking diodes. There was only a note about 'space being available' to install a blocking diode if wanted.

    I know there is something like a blocking diode all the way back at the charge controller to prevent backfeeding to the modules at night.

    Now I am wondering, whether to install a blocking diode at each individual parallel string (inside the combiner box) to maximize each strings current output. Without individual blocking diodes I would lose current from a higher performing string to a lower current-output string. (Similar to not uniformly wearing parallel connected batteries, I guess.)

    But maybe this is just not so important. I don't know.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #4
    Re: Blocking diodes for parallel strings?

    I think that all of the larger solar panels only have bypass diodes and no blocking diodes...

    The blocking diodes are really only needed on panels that connect directly to a battery bank (and the bank is 24 volts or larger)... Charge controllers block any current leaks back to the panel. And for low voltage setups (like 12 volts) an old Wind-Sun faq says that a blocking diode really does not save much power anyway.

    For larger/grid tied systems, blocking diodes have significant voltage drop (0.2 to 1.0 or more volts). And if you are putting 5-10+ amps through a diode, they will waste upwards of 10 watts of power--needing a heat sink, electrical insulation, etc.

    For an average array, shading will just cut current flow to near zero amps--it will not cut voltage to zero volts--as there is still back-scatter from the sky to keep the voltage near Vmp/Voc.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Elke
    Elke Solar Expert Posts: 28 ✭
    #5
    Re: Blocking diodes for parallel strings?

    Thanks!

    I read the explanations and will be satisfied without the blocking diodes.

    After the initial problems with our installation, see:
    http://forum.solar-electric.com/showthread.php?t=8281

    I decided to gradually look over the entire system step by step, starting with the PV array. I know it will take a while for me to learn about all the details, but I just don't quite trust the workmanship.

    Our system has been up and running for a month by now. The installer eventually supplied a new MPPT-60 and this time used the correct wire gauge. No smoke, no melting components, no fire so far!

    Thanks for all the help and information!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #6
    Re: Blocking diodes for parallel strings?

    Glad to hear that things are working out now for your home!

    Assuming the system is "working"--Start concentrating on learning about how to take care of, and monitor, your battery bank.

    Everything else is pretty much going to work no matter what you do (short of shorting something out)--except the battery bank (obviously, bad wring, perhaps panels not bolted down well, leaks around anchors, proper programming for controllers, and wire entry points, etc. all do need to be looked at too).

    Deep Cycle Battery FAQ
    www.batteryfaq.org

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