Shade on One Panel in a Large String

a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
I would like to learn how shade on just one panel in a large string affects the performance of the complete string.

For example, lets assume 13 panels wired serially, each rated (Wp) at 225 W, and each panel's Imp is 7.86 A and Vmp is 28.66 V, at 1000 W/m2 irradiation.

First, is indeed the string's Wp = 2925 W, operating at Imp = 7.86 A and Vmp = 372.6 V (assuming here a theoretical set up, where it's just one string connected to a GT inverter with its MPPT operating)?

Second, what happens when one panel becomes partially shaded, at lets say 600 W/m2?

Does the partially shaded panel operate at its short circuit current (Isc), at 0 V, for the 600 W/m2 irradiation level?

If this is correct, and if Isc is 5.0 A, does the whole string now operate at Imp = 5.0 A and approximately Vmp = 12 (one less panel) * 28.66 V = 343.9 V?

Which would mean the string's power is now 5.0 A * 343.9 V = 1720 W? Which is a 41% reduction of total string power?

And thus, is it reasonable to conclude that a 40% shading on just one panel (i.e. going from 1000 to 600 W/m2 irradiance) translates to about the same reduction (41%) of the string's total power output (i.e., going from 2925 to 1720 W)?

Thank you for the help here understanding this fundamental behavioral concept of panels connected in a large string to a GT-only inverter.

Best regards,

Bill

Comments

  • Solar GuppySolar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Shade on One Panel in a Large String

    Solar panels have bypass diodes installed, usually 3-4 per panel so in partial shading condition when the cell would restrict current the bypass diode kicks in and the remaining panels in the string produce at full power, this assume the inverter has good tracking for these conditions. Any shade by the way is more like a 99% reduction in current from a cells output, not 40% are your posting proposed in the 1000 vs 600 wM2.

    For your roof, you need to assume that when more panels are shaded that would support the minimum operational voltage the string is dead from a production stand point. Otherwise its pretty linear the reduction, but this is only for single strings. When you parallel, its get very complicated as now you have basically unequal voltages for vmp and it gets ugly fast on the harvest loss

    For your home, I would give some serious thought to taking out the offending trees as over time, they will grow and shading will only get worse
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Shade on One Panel in a Large String

    Thank you very much, Solar Guppy.

    At the moment I just want to understand the theory.

    I've just learned from your response 2 key things I didn't know: (1) The presence of diodes in a panel significantly affects the subject here; and (2) any amount of reasonable shading translates to a much more drastic drop in irradiance level.

    Thanks!

    OK, so then, how do I determine if my panels have bypass diodes in them? Astronergy 225 W monocrystaline panels, Model CHSM6610M ( http://www.astronergy.com/cpyfw_danjdczj.html ).

    I found this statement in the Owner's Manual: "Avoid installing modules under shadows, even module factories use bypass diodes to decrease the loss of energy. Shadows will result in the loss of power output."

    Does this mean that these panels indeed have bypass diodes?

    And if so, if one of (these) 13 panels is shaded, theoretically, can I simply substract 225 W from the string's total output?

    I.e., 12 panels * 225 W = 2700W, down 8% from what 13 panels would produce (2925 W) (assuming theoretical 1000 W/m2 irradiance on the remaining 12 panels)?

    If I understand correctly, and if these panels indeed have the diodes, then it looks like each shaded panel, in a string of 13, will reduce the total string output by 225 W. Is this a reasonable conclusion?

    Thanks.

    Best regards,

    Bill
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Shade on One Panel in a Large String

    Example:
    original.jpg

    Best regards,

    Bill
  • BB.BB. Super Moderators, Administrators Posts: 32,924 admin
    Re: Shade on One Panel in a Large String

    What time of day?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Shade on One Panel in a Large String

    10:30 AM, CT.

    I'm hoping to understand the theory, applicable to more than just this example.

    Based on what I've learned so far, 13 panels total in a string, at theoretical 1000 W/m2 irradiance, with one panel shaded like in the example, and with the string being the only string connected to an MPPT GT-only inverter, (theoretically):

    String Vmp = 12 panels * 28.66 V = 343.9 V
    String Imp = 7.86 A
    String Wmp = 2703 W DC

    If the panels have diodes.

    For the time period the one panel is shaded.

    Thanks.

    Best regards,

    Bill
  • BB.BB. Super Moderators, Administrators Posts: 32,924 admin
    Re: Shade on One Panel in a Large String

    Yep, that is how I would look at it (basically, shaded panel(s) are shorted by the diodes and act like a wire--dropping Vmp by the number of panels shaded).

    You can use the PV Watts program with the detailed output... 365 days of hour by hour power output information and see how much you lose for 9am-11am (for example) on a west facing roof.

    Of course, during winter, low sun angles will make shading worse. Can you get a sun tracker [actually a Solar Pathfinder] plot on that corner of the array and see what the chimney takes out of the day?

    And, if you have tree shading, younger trees can grow surprisingly quick.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Shade on One Panel in a Large String

    Bill, thanks! I'm sorry for the questions, but I'm learning quite a bit and am most appreciative.


    Diodes:
    • how do I determine if my panels have bypass diodes in them? Astronergy 225 W monocrystalline panels, Model CHSM6610M ( http://www.astronergy.com/cpyfw_danjdczj.html ).
    • From the Owner's Manual: "Avoid installing modules under shadows, even module factories use bypass diodes to decrease the loss of energy. Shadows will result in the loss of power output."
    • Does this mean that these panels indeed have bypass diodes?

    "Sun Tracker Plot:" what are you referring to / what is this?


    (I am familiar with PVWatts - ver 1.)


    Many thanks!

    Best regards,

    Bill
  • BB.BB. Super Moderators, Administrators Posts: 32,924 admin
    Re: Shade on One Panel in a Large String

    Sorry, I should have said Solar Pathfinder... I am dexslexic, dislexic, dyslecix, dyslexic... Oh heck, I can't spell and I confuse terms and lexically impaired. :blush:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Shade on One Panel in a Large String
    a0128958 wrote: »
    how do I determine if my panels have bypass diodes in them?

    Ok, I have never done this, but I think this test method should work: Take a multimeter, set it to Ohms, make sure panels is laying face down so it has no light hitting it, connect meter to solar panel. Measure resistance, then switch polarity (flip wires around) and measure it again. If measurements differ a lot between polarities, then you have bypass diodes.

    But any solar panel meant to be used in series should have them anyway.
  • BB.BB. Super Moderators, Administrators Posts: 32,924 admin
    Re: Shade on One Panel in a Large String

    If you have a DMM (Digital Multi-Meter), do as AntonX says--except set it to "DIODE" setting.

    The Ohms setting on many meters is very low voltage and may not forward bias a diode (it will look like a high resistance).

    When a DMM is set to Diode Test mode, it will output up to a volt or two, and show the forward bias voltage across the diode (typically around 0.2-0.7 volts, depending on diode type).

    Regarding Astronergy's panels, there here is a manual for another series that describes (your quote on factory diodes may have been from this manual):
    Bypass Diodes
    The type, voltage rating, current rating of the diodes are SL1515, 40V, 16A, respectively. The plastic material has an UL classification of 94V-0.

    Partial shading of an individual module can cause reverse voltage to occur across the shaded module. The electrical current is then forced through the shaded area by other modules.

    When a bypass diode is wired parallel with the series string, the forced current will flow through the diode and bypass the shaded module minimizing module heating and array losses of current.

    Diodes used as bypass diodes:

    Set a Rated Average Forward Current of 12A, above maximum system current at highest module operating temperature. Set a Rated Repetitive Peak Reverse Voltage 40V, above maximum system voltage at lowest module operating temperature.

    There are three bypass diodes per module and 20 cells per bypass diode

    I do not think that they can make a solar panel for use in a GT system that does not have bypass diodes... Without the diodes, a "dark" panel would be damaged by the rest of the "lighted" panels in a series strings (solar cells go "high resistance" if there is no light.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Shade on One Panel in a Large String

    Bill / AntonX, thanks very much.

    I have a DMM, with a Diode Test function.

    Indeed, it reads 0.4 with pos/neg DMM leads connected to the panels +/- wires respectively. When I reverse the leads, the DMM reads 1.4. I don't know what the units are.

    My guess is the fact that the measurement differs depending on polarity of test connections means that my panels have internal diodes.

    I have the same (panel) manual. Earlier today I didn't have enough solar expertise (yet) to understand the material you quoted, and/or the Chinese to English translation prevented me from understanding what's being said.

    Your note, along with some additional research, leads me now to believe there are 3 diodes for each panel, one per 20 cells. And the diodes are located in the Tyco Electronics Solarlok Interconnection System junction box on the back of each panel.

    I've learned a lot today, and have appreciated everyone's contributions. Summary:

    1. My panels have diodes in each panel's junction box.

    2. The presence of diodes causes a shaded panel to effectively be bypassed, i.e., if one of 13 in a string panels is shaded, and each panel is 225 watts, then subtract 225 watts from the strings total power.

    3. On a numbers basis, the strings Imp stays constant, and the string's Vmp declines by an amount equal to an individual panel's Vmp.

    Many thanks!

    Best regards,

    Bill
  • BB.BB. Super Moderators, Administrators Posts: 32,924 admin
    Re: Shade on One Panel in a Large String

    Typically, the DMM in diode mode reads volts with a bias current... So the 0.4xx volts is when the diodes are forward biased (direction of current flow). The 1.4xx volts would be the "diode" turned off, and possible the result resistance of the solar cells themselves (or if there is a little light bleeding through from the back or sides).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Shade on One Panel in a Large String

    In the process of continuing to digest the comments made on my other thread, regarding parallel strings, I've learned a lot more about what happens to a large string with shade on one panel, the continuing subject of this thread.

    I again thank the many who have contributed so far. I've learned a lot and hopefully at some point can return the favor.

    Here's what I believe will happen for a 13 module string, with Astronergy panels characterized as Isc=8.48 A, Voc=37.33 V, Imp= 7.86 A, Vmp=28.66 V, Pm=225, connected as the only string to a GT-only inverter with good MPPT capabilities:

    No shading on any module:
    • (From specs)
    • String Imp = 7.9 A
    • String Vmp = 28.7 * 13 = 373 V
    • String Pm = 2928 W
    0.8 Sun shading on one module (8% of string):
    • (from I-V chart)
    • Imp = 6.0 A
    • Vmp = 30.0 * 13 = 390 V
    • Pm = 2340 W (delta = -588 W, or -20%)
    • I'm assuming bypass diodes do not 'kick in' at 0.8 Sun
    0.6 Sun shading:
    • (from I-V chart)
    • Imp = 4.1 A
    • Vmp = 30.2 * 13 = 393 V
    • Pm = 1610 W (delta = -45%)
    At this point I'll assume the shaded module's bypass diodes 'kick in,' and the MPPT control logic works, such that instead of the -45% string power loss, it's:
    • Imp = 7.9 A
    • Vmp = 27.2 V * 12 = 327 V (assuming 95% of 27.88 V)
    • Pm = 2568 W (delta = -12%)
    0.4 Sun and less irradiance:
    • Since bypass diodes are 'kicked in,' results don't change any further.
    • Imp = 7.9 A
    • Vmp = 27.2 V * 12 = 327 V
    • Pm = 2568 W (delta = -12%)
    Summary:
    1. For an 8% loss of modules (1 of 13) due to considerable shading, worse case instantaneous power loss is 12%.
    2. A very small amount of shading can cause considerably more string power loss until shading increases, depending on when bypass diodes 'kick in.'
    3. If the shading is on the panel for 1 'sun-hour' out of 5 winter time 'sun-hours' (insolation), then total energy loss for this string is 20% * 12% = 2.4%.
    Conclusion:
    • An approximately 2.4% Winter time string energy loss, which improves in Summer time, seems reasonable versus the alternative of reducing the module count in all strings just to avoid shading of one module.
    I'm continuing to work to understand the parallel example in the other thread. Comment/questions coming soon.

    Again, many thanks for the help.

    Best regards,

    Bill
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Shade on One Panel in a Large String
    BB. wrote: »
    I do not think that they can make a solar panel for use in a GT system that does not have bypass diodes... Without the diodes, a "dark" panel would be damaged by the rest of the "lighted" panels in a series strings (solar cells go "high resistance" if there is no light.

    Agreed. In fact, I believe this is a fact. I learned today that damage to a module is probable without the diodes.

    In my case, for my Astronergy panels, there are 3 bypass diodes built into the connection box on the back side. Every 20 'cells' are bypassed (60 cells total). I.e., shading on less than one-third of a module (assuming the shading is all concentrated on either end, or the middle) will only result in losing one-third of the module.

    Best regards,

    Bill
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