Results 1 to 7 of 7

Thread: Mis-match V or remove panels?

  1. #1
    Join Date
    Jan 2012
    Location
    CA
    Posts
    502

    Mis-match V or remove panels?

    Just added 12 panels to an old 34-panel system. New and old are different make and size panels. Also, old Poly vs new Mono.

    Despite my calcs before i started job - after installation the actual measured V is quite different from expected.

    I just measured 467VOC on both the original circuits. (2 @ 17 modules, 180W each) 6,120W
    I also measured 438 VOC on the new circuit. (1 @ 12 modules, 310W each) 3,720W
    old vs new string: 6.6% higher Voc TOTAL: 9,840W

    They all parallel to an old transformer based SMA SB US inverter. Single MPPT.
    Was expecting matching Voltage.
    Actually i only focused on Vmp matching, not Voc, for efficient MPPT tracking and operation.
    (unfortunately have not measured operating V yet, all I know is actual Voc is off more than expected)

    Questions:
    1. Is this V diff. workable? Is this a problem? How much efficiency loss?
    2. Is it possible that the old modules will drop at a higher rate from Voc to Vmp, than the new modules? This could close the gap between V-operating of old and new a bit.
    Using the values from their spec sheets, the ratio of Voc/Vmp does indicate factors of 1.26 and 1.21 respectively.
    3. What would you do?
    4. Perhaps remove one module each from the old strings of 17?
    2 strings of 16 at 440 Voc would result. (16/17 x 467Voc)
    Not happy about nixing two good panels.
    Here, the question is: what produces more power? All panels intact but producing the disparate V above
    vs
    removing 2 modules and losing the 2 @ 180W.
    Adding a 13th new module to the new string of 12 is not an option. No good room for it.

  2. #2
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
    Posts
    19,131
    A very crude rule of thumb tells us that if the Voc (and correspondingly Vmp) are within 5% on the two strings the power loss compared to the sum of the exact Vmp-Imp powers of the two strings is going to be 1% or less. The reason is that the output power from each string is changing very slowly with voltage in the vicinity of the MP point.
    At a 6+% difference you will not be doing quite as well, but i think that you can be confident that the resulting total power will be greater than if you were to remove panels from the old string to make a better match.

    If you do not trust this approximation, it would be easy enough to compare the total power input to the inverter as is and with two panels shorted out. There is no need to change the wiring to take them out of the string.

  3. #3
    Join Date
    Jan 2012
    Location
    CA
    Posts
    502
    Quote Originally Posted by GoldDigger View Post
    A very crude rule of thumb tells us that if the Voc (and correspondingly Vmp) are within 5% on the two strings the power loss compared to the sum of the exact Vmp-Imp powers of the two strings is going to be 1% or less. The reason is that the output power from each string is changing very slowly with voltage in the vicinity of the MP point.
    At a 6+% difference you will not be doing quite as well, but i think that you can be confident that the resulting total power will be greater than if you were to remove panels from the old string to make a better match.

    If you do not trust this approximation, it would be easy enough to compare the total power input to the inverter as is and with two panels shorted out. There is no need to change the wiring to take them out of the string.
    thank you. Very useful.

    I will also measure operating Voltage to see how much closer the V's get to each other once I turn it on.

  4. #4
    Join Date
    Oct 2009
    Location
    Austin, TX, USA
    Posts
    9,288
    Quote Originally Posted by Zee View Post
    thank you. Very useful.

    I will also measure operating Voltage to see how much closer the V's get to each other once I turn it on.
    That won't tell you anything. Once the system is turned on all the string voltages will be the same no matter how badly mismatched they are.

  5. #5
    Join Date
    Jan 2012
    Location
    CA
    Posts
    502
    Quote Originally Posted by ggunn View Post
    That won't tell you anything. Once the system is turned on all the string voltages will be the same no matter how badly mismatched they are.
    yes. excellent point. was unsure about that.

    I was going to turn the strings on independently and measure.

  6. #6
    Join Date
    May 2011
    Posts
    4,124
    Measuring current would tell you a bit more, both running the strings separately and together.

  7. #7
    Join Date
    Jul 2009
    Location
    Mesa Arizona
    Posts
    263
    As manufacturers improve the solar cells, the fill factor (ratio of maximum power to the product of Voc x Isc) gets higher. This means that at operating voltages higher than Vmp the current (and power) drops off sharply. In this case the newer strings will likely show more of the mismatch loss than the older strings.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •