pv panels in series.

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Lefty Wright
Lefty Wright Solar Expert Posts: 111 ✭✭
in General Solar Power Topics #1
When wiring pv panels in series, if one is shaded is the current blocked through the string?

How about when partially shaded?
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
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    Re: pv panels in series.
    Lefty Wright wrote: »
    pv panels in series, if one is shaded is the current blocked through the string? How about when partially shaded?

    What about zebra stripes ?

    It depends on the panel. Some have a single bypass diode, and some have several and stand partial shading better than others. Some, even a shadow of a rope across the panel, will be enough to shut it down, then you have to rely on the bypass diodes to let the other panels feed the system. Of course, your voltage is going to drop by the value of 1 panel.

    Or I'm all wrong, and some one will straighten me out on how this works.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #3
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    Re: pv panels in series.

    Basically, solar panels go "high resistance" when dark... So, there are "bypass" diodes in most panels that let the current go around dark panels. Some are one per panels, some are several bypass diodes per panel.

    If you are running panels in series and parallel (say 4 series x 4 parallel)... Generally, almost any shadow blocking just a part of one panel will pretty much zero out the current from that entire string (especially assuming an MPPT charge controller).

    Think of 4 batteries in parallel, but one of the batteries has less voltage--the amount of current it can supply is virtually zero because it cannot match the voltage/current of the other 3 batteries in parallel.

    Pretty much, if you have any shadowing (tree, power lines, etc.), assume that that panel/string will produce almost zero power during the time it is under partial/full shade. Try to avoid shading between 9am-3pm--the prime sunlight times.

    If you have shading (say overhead power lines) that you cannot do anything about--you might wish to consider going with solar thermal collectors (hot water/hot air). These are much more tolerant of partial shading--and saving money on gas/fuel oil/electric heat is still saving money.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
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    Re: pv panels in series.

    even with bypass diodes the voltage can drop far enough to stop all charging unless you opt for a higher voltage design pv system that can step down to the lower one or be on the high end for the same system voltage. for example if you have 2 bypass diodes on a 12v pv means half of the pv could be shaded and the other half allowed full current output, but at half the voltage. will you charge anything with half the voltage? even uping the voltages used may be negated depending on the amount of shade and the number of bypass diodes used in the pvs. simply put shade=enemy.
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  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
    #5
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    Re: pv panels in series.

    That's what I was afraid of.

    Since I live in heavy timber shade is inevitable. I've built a tower for my panels and cut down a couple of trees but there are times when one of my panels gets partially shaded.

    I was considering installing an mppt controller and rewiring my panels in series. It looks like I'm better off leaving them in parallel. The loss of output from one panel in parallel should be less than losing the output from a whole series string.

    The reason for my proposed change was to raise the voltage (and lower the current) to cut down the losses from my 75 foot run from the tower to the house.

    I could move my inverter to the tower and cut voltage drop that way. But I like to switch off the inverter when I don't need AC to stop the idle current loss. Relocating the inverter would complicate that.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #6
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    Re: pv panels in series.

    If the string was only one series connected (no parallel strings)... and the voltage of (X panels - 1 x Vmp) was still Battery Voltage + 2 volts or better, you would loose (roughly) a maximum of just the shaded panel rated wattage.

    The "loss of a string's worth of power" is only when you have series/parallel strings. If you have just one series string, shading of one panel would be no worst than if all the same panels were in parallel (again, assuming that X-1 panel voltage is greater than Battery Bank + 2 volts voltage).

    And, depending on your temperatures and battery voltages, it is still possible that you would do better overall with a good MX-60 type controller.

    You pretty much have to do that calculations to see if a new controller is worth the costs (wiring loss, Vmp vs Vbatt * current of MPPT vs PWM type controllers, Vmp over temperature, etc., loses of MPPT controllers with smaller <200-400 watt arrays, and such)...


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
    #7
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    Re: pv panels in series.

    If I understand you correctly Bill, if one panel in a series string is shaded it will not block the current flow from the unshaded panels. (I have KC130's with factory installed bypass diodes)

    Is this correct?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #8
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    Re: pv panels in series.

    LW,

    Yes, the diodes will bypass the current around a shaded panel... As long as the rest of the set of panels still have a high enough working voltage to still charge the batteries. There is still a 0.5-1.0 volt drop caused by the diode drop...

    Say Vmp=17.5 volts, batteries need 15.5 volts per 12 vdc (equalization) + 2 volts across controller and a 24 VDC battery bank.

    3 panels in series = 52.5 vdc > 15.5 v * 2 + 2 volts > 33.5 volts in 24 vdc bank

    2 panels + 1 blocked = 2*17.5v - 1 v = 34 VDC > 33.5 volts...

    Make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
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    Re: pv panels in series.

    you must evaluate everything and bill is right that only the shaded part is deducted in that case and the rest free to pass current to the controller. now if more pv area is shaded that won't allow enough of the voltage to pass then yes consider keeping a parallel arrangement. i would think the best answer is either trimming that tree or cutting it down. I also would assume you know that and would've already done so if possible.

    we'll use bill's example of 3 pvs and i don't know without looking it up, but for example's sake we'll use 2 diodes per pv. each pv actually has 17.6v, but for keeping with bill's example it'll be 17.5v. that will be about 8.75v per diode. if one section is shaded we lose the 8.75v of that section, plus the voltage drop of the diode (about .5v per diode) for a total loss of 9.25v. 52.5vtotal-9.25v=43.25v. lose 2 sections of any of the pvs (equal to 1 whole pv) and you get 17.5v lost along with the loss of 2 diodes it passed through for another 1v drop gives 52.5v-18.5v=34v just as in bill's example and can still charge the 24v battery bank. now have 3 sections shaded and you lose 26.25v and the loss of 3 diodes for another 1.5v so 52.5v-27.75v=24.75v. this won't be sufficient even in a float charge so this is the point of too much shade at least for a 24v battery system. for that connected to a 12v battery system you can still have a viable charge up to having all but 1 pv shaded and that's lossy being left with 1 being the diodes take some voltage from the charge with their voltage drops.

    you must weigh if the parallel arrangement will work better, less, or equal to this and only you can see how much shading is impending upon your pvs.
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  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
    #10
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    Re: pv panels in series.

    That clears it up. Thanx.
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