Loss in Strings

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How much power is lost due to shading/dirt on a single module in a string?

I have heard that a string acts like the lowest performing module in the string.
For example, a string of 3 100W modules would normally produce 300W; however, if one module is shaded and is operating at 50% capacity, the entire string acts like that module. Thus the string would produce just 150W.

Am I correct in my assumption?

Dan

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: Loss in Strings

    As always, it is more complex... First, if you have a PWM controller--any shading that drops Vmp below Vbatt-charging + Vcontroller-wiring-drop will cut the charging to nothing very quickly.

    If this is three panels in series connected to an MPPT charge controller that only needs one panel to operate (Vmp=17 volts per panel and Vbatt=12 volts)--then substantial shade on one 100 watt panel would leave 200 watts left from the other unshaded panels to charge your battery bank (sort of a worst case guesstimate--but the simplest answer).

    Your actual losses may be less than 100 watts--but then it depends on the amount of shade, where it falls on the panel, how the panel is wired with bypass diodes, etc...

    But since shadows move across the panels during the day--doing a 10 minute by 10 minute analysis is pretty difficult to justify.

    Either move the panel, or the offending shade, or add more panels to make up for shading losses during part of the day.

    Solar PV just does not function that well in the presence of any shade as losses can be very significant with less than full shading.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • papa
    papa Solar Expert Posts: 51 ✭✭
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    Re: Loss in Strings

    Great question Breezy, and I'd be keenly interested if someone would duplicate the test that I just did using crystalline modules. I used 3 x 15w amorphous Harbor Freight modules.

    Very slightly hazy conditions, but clear enough to produce a decent shadow. Modules are labeled at 23.5 Voc and 1.15A Isc. 'Rated' 17.5V @ .86A

    Connected in series, I measured 64.6 Voc, and when short circuited (Isc), measured 652 mA.

    Covering just 1/3 of ONE module dropped the Voc to 63.1, while the Isc dropped to 540 mA.

    Please bare-in-mind, that all 3 modules have 11' each, of what appears to be, double conductor, 20 gauge wires - that equates to 66' of additional circuit which must be taken into account.
  • FreeWatts
    FreeWatts Registered Users Posts: 14
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    Re: Loss in Strings
    papa wrote: »
    Great question Breezy, and I'd be keenly interested if someone would duplicate the test that I just did using crystalline modules. I used 3 x 15w amorphous Harbor Freight modules.

    crystalline modules and the one you used in your test, amorphous, react different to shading. amorphous are better on cloudy days.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: Loss in Strings

    I have seen people write that XYZ panel (typically some sort of amorphous / thin film panel) works better than mono/poly crystalline panels on a cloudy day...

    Has anyone here actually confirmed that measurement with lab gear (other than the manufacturer testing)? For the most part, people use any of these solar cell (photo diodes) shorted output current to measure solar radiation (so that would indicate a very close coupling between output shorted current and solar radiation--regardless of cell type).

    As I understand, the electrical characteristics (sun vs I*V/P curves) are actually quite similar between the various crystalline/silicon based thin film technologies (CIGS and others may have more differences).

    And, for the most part, the variation of Vmp over temperature vs output power only applies to MPPT controllers (and does not apply to PWM controllers as long as Vmp@temp > Vbatt-charge + Vcontroller-drop). (Vmp falls with temperature and Imp rises, very slightly, with temperature and Imp changes are usually ignored).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • papa
    papa Solar Expert Posts: 51 ✭✭
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    Re: Loss in Strings
    FreeWatts wrote: »
    crystalline modules and the one you used in your test, amorphous, react different to shading. amorphous are better on cloudy days.
    Indeed, my understanding as well. It's purely a guess, but the additional module surface area is, at least, partially responsible.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Loss in Strings
    FreeWatts wrote: »
    crystalline modules and the one you used in your test, amorphous, react different to shading. amorphous are better on cloudy days.

    that point may be moot as for the same area of pv the amorphous will not outdo the crystalline output even if the crystalline would have a higher loss percentage.
  • papa
    papa Solar Expert Posts: 51 ✭✭
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    Re: Loss in Strings
    niel wrote: »
    that point may be moot as for the same area of pv the amorphous will not outdo the crystalline output even if the crystalline would have a higher loss percentage.
    I would be inclined to agree, but installation requirements are typically based on wattage needs, not surface area. So I would think that the 'cloudy' output comparison testing should be based the module's wattage rating, irrespective of actual surface area.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Loss in Strings
    papa wrote: »
    I would be inclined to agree, but installation requirements are typically based on wattage needs, not surface area. So I would think that the 'cloudy' output comparison testing should be based the module's wattage rating, irrespective of actual surface area.

    ok, then, i think i'll stick with the higher output as seen by the crystalline pvs for it makes no sense to me to get amorphous just because they do better percentage wise as this % is on an output that is about 1/2 of the crystalline and is thus quite small in comparison. i also can't see buying pvs that require a larger area unless you have plenty of room and the costs are 1/2 of those for the crystalline as that would be about the same cost per watt factoring the area. all of it is redundant to me when the actual difference of improvement is so small anyway.
    but this is all just me and my opinion as you can do and think what you want to.
    i am not prejudice against thin film either as i have some. mine are more resistant to breakage and that is a reason to buy them in some applications.:cool: