System Performance degradation

Hi All

Last week I was chatting with my supplier and he pointed out that my system in not performing well. To be honest I was aware that the KHW numbers went down but just convinced myself that this year, we have more clouds and the Summer is hotter (which is true after all). However my supplier insisted that the system is not performing well and was asked to make somechecks for him.

The table below is the average daily production (KHW) for 2011 and 2012, as you can notice 2012 is lower for all months. The system has been in operation in December 2010.

Month 2011 - 2012
Jan 5.22 - 4.10
Feb 6.42 - 5.51
Mar 8.20 - 6.87
Apr 9.93 - 7.80
May 10.84 - 8.83
June 9.94 - 8.71
July 9.80
Aug 9.48
Sep 8.05
Oct 6.03
Nov 4.14
Dec 4.11

My system is grid tie, nine IBC 230W solar panels for a total of 2070W (configured as one string) and the grid tie inverter is the transformerless SMA SB 2100TL.

I Checked the Short Circuit current & Open Voltage for the nine panels at 11:00 local time (Panels should peak at about noon / 13:00...):
Today the thermometer reached 33 degrees Celcuis with a heat stress of about 40 degrees Celcuis (according to Met Office).

Panel 1 - 25.5 Voc / 6 Asc
Panel 2 - 27.5 Voc / 5.97 Asc
Panel 3 - 26.8 Voc / 5.92 Asc
Panel 4 - 29.0 Voc / 5.92 Asc
Panel 5 - 26.1 Voc / 5.81 Asc
Panel 6 - 27.8 Voc / 5.83 Asc
Panel 7 - 32.4 Voc / 6.39 Asc
Panel 8 - 32.2 Voc / 6.37 Asc
Panel 9 - 31.9 Voc / 6.3 Asc

The readings look fine at least in my opinion. Panels 1-6 are mounted in parallel to the building and are not facing truly South, in fact their output before noon is slightly less than the other three panels. Panels 7-9 are facing South.

I took the Open Circuit Voltage of all nine panels in series (261v) while the Short circuit current is 6.78Amps. These reading were taken on the roof.

I also took the Open circuit voltage of all nine panels in series (262v) while the Short circuit current is 7Amps at the Inverter input. (This was taken after the roof measurements in a time span of 10mins and there is a length of about 12 meters 10mm copper cable). From this I can conclude that the wiring is OK and there is no voltage drop from the panels to the inverter?

Once I connected the Inverter, as expected, the voltage went down to 200v while the current 5.42, the inverter exporting 989W. (All these last 3 readings taken directly from the inverter display)

At 16:00 I took more readings: 264Voc / 7.67Asc while the inverter was exporting only 1047W, Vpv 189. The inverter has been all day on MPP, no errors/ warnings and the max exported wattage has never exceeded 1200W.

I'm tempted to point my finger at the inverter MPPT... what's your opinion and if anyone has any recomendations for me to follow.

Regards
Brian

Comments

  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: System Performance degradation
    Brianellul wrote: »

    I also took the Open circuit voltage of all nine panels in series (262v) while the Short circuit current is 7Amps at the Inverter input. (This was taken after the roof measurements in a time span of 10mins and there is a length of about 12 meters 10mm copper cable). From this I can conclude that the wiring is OK and there is no voltage drop from the panels to the inverter?

    If there is no current flowing there can be no voltage drop, so it wouldn't show up in a Voc measurement.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: System Performance degradation

    if the voc was measured at the pvs (isolated that is not connected to anything else) themselves with full sunshine even at 11am then i would say the pvs are out of spec. you should ask your installer to verify what you've found and if this is the case then get your pvs replaced under warranty.

    the isc can vary with conditions, but voc should be in the ballpark and you are showing it way down in all cases.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: System Performance degradation
    niel wrote: »
    if the voc was measured at the pvs (isolated that is not connected to anything else) themselves with full sunshine even at 11am then i would say the pvs are out of spec. you should ask your installer to verify what you've found and if this is the case then get your pvs replaced under warranty.

    the isc can vary with conditions, but voc should be in the ballpark and you are showing it way down in all cases.
    I agree, mostly.

    The Voc for the module is 36.8V. The ambient temp was 33 degrees, and if his modules are flush mounted they could be as much as 35 degrees C hotter. The temperature coefficient for Voc is -121mV/degree C. If his cell temp is 68 degrees C, he should expect Voc to be (68-25)(.121) = 5.2V down from 36.8V, or about 31.6V. Three of his modules showed Voc of about that, but the other six were substantially lower.

    Statistically speaking, it looks like a bimodal distribution with three of them grouped around 32V and the other six around 27V; if his problem were something random, I would not expect that. Also, the group of six was all together (Panels 1-6) as was the group of three (Panels 7-9). That all looks a little odd and implies to me that there might be something else going on. What that could be, though, I dunno. Maybe he should look at the module nameplates and see if all the modules are the ones he thinks he has.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: System Performance degradation
    ggunn wrote: »
    I agree, mostly.

    The Voc for the module is 36.8V. The ambient temp was 33 degrees, and if his modules are flush mounted they could be as much as 35 degrees C hotter. The temperature coefficient for Voc is -121mV/degree C. If his cell temp is 68 degrees C, he should expect Voc to be (68-25)(.121) = 5.2V down from 36.8V, or about 31.6V. Three of his modules showed Voc of about that, but the other six were substantially lower.

    Statistically speaking, it looks like a bimodal distribution with three of them grouped around 32V and the other six around 27V; if his problem were something random, I would not expect that. Also, the group of six was all together (Panels 1-6) as was the group of three (Panels 7-9). That all looks a little odd and implies to me that there might be something else going on. What that could be, though, I dunno. Maybe he should look at the module nameplates and see if all the modules are the ones he thinks he has.

    If the Voc of the panels (rated) is 36.8 and the maximum measured Isc is around 6.4A, I have a hard time getting a 230 Watt panel out of that given that Vmp and Imp will have to both be lower.
    For the low 6 panels it looks even worse, but they are not getting perpendicular insolation.

    Also, the one measurement which I see only once, as measured by the inverter itself is Imp. We can indirectly compute a value for the other time by dividing the exported power by the displayed Vmp and then fudging for assumed inverter efficiency. 1047w divided by 189v is about 5.5A. That seems consistent with the measured Isc for the string.

    Since three panels are close to South and six panels are not as close, the differences between the sets seems as expected, but the design using one MPPT inverter for both sets in a series string is questionable.
    SMA SB 3000, old BP panels.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: System Performance degradation
    inetdog wrote: »
    If the Voc of the panels (rated) is 36.8 and the maximum measured Isc is around 6.4A, I have a hard time getting a 230 Watt panel out of that given that Vmp and Imp will have to both be lower.
    For the low 6 panels it looks even worse, but they are not getting perpendicular insolation.

    Also, the one measurement which I see only once, as measured by the inverter itself is Imp. We can indirectly compute a value for the other time by dividing the exported power by the displayed Vmp and then fudging for assumed inverter efficiency. 1047w divided by 189v is about 5.5A. That seems consistent with the measured Isc for the string.

    Since three panels are close to South and six panels are not as close, the differences between the sets seems as expected, but the design using one MPPT inverter for both sets in a series string is questionable.

    From the data sheet (link in post #1): Vmp = 29.6V, Imp = 7.78A. (29.6V)(7.78A) = 230.3W

    Like modules won't have different Voc values simply because they have different orientations. Maybe the low reading ones were hotter? But they would have to be at 106 degrees C to depress Voc from 36.8V to 27V @ -121mV/degree C. Curiouser and curiouser.

    I agree that having modules in a string pointed in different directions is bad design; the string current will always be limited to that of the worst performing module in the string. That could explain a lot, the screwy looking measurements notwithstanding.
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: System Performance degradation
    ggunn wrote: »
    If there is no current flowing there can be no voltage drop, so it wouldn't show up in a Voc measurement.

    You're right! Without any current flowing there will be no voltage drop. I should have worded differently... basically I concluded that there are minimal losses in the 12meter run.

    Thanks
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: System Performance degradation
    niel wrote: »
    if the voc was measured at the pvs (isolated that is not connected to anything else) themselves with full sunshine even at 11am then i would say the pvs are out of spec. you should ask your installer to verify what you've found and if this is the case then get your pvs replaced under warranty.

    the isc can vary with conditions, but voc should be in the ballpark and you are showing it way down in all cases.

    Neil,thanks for your feedback! I was not aware the voc should not vary with conditions. I was under the impression that both the voc and isc will vary, both reaching their maximum when the sun is perpendicular to the panels... Is this what you're telling me?
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: System Performance degradation
    ggunn wrote: »
    I agree, mostly.

    The Voc for the module is 36.8V. The ambient temp was 33 degrees, and if his modules are flush mounted they could be as much as 35 degrees C hotter. The temperature coefficient for Voc is -121mV/degree C. If his cell temp is 68 degrees C, he should expect Voc to be (68-25)(.121) = 5.2V down from 36.8V, or about 31.6V. Three of his modules showed Voc of about that, but the other six were substantially lower.

    Statistically speaking, it looks like a bimodal distribution with three of them grouped around 32V and the other six around 27V; if his problem were something random, I would not expect that. Also, the group of six was all together (Panels 1-6) as was the group of three (Panels 7-9). That all looks a little odd and implies to me that there might be something else going on. What that could be, though, I dunno. Maybe he should look at the module nameplates and see if all the modules are the ones he thinks he has.

    In fact you're right, Panels 1-6 are grouped and will reach their maximum output 30-45mins after panels 7-9 (that's why the difference in output for the panels). Yes the panels are identical, I've rechecked the nameplates.

    Something which I should consider after reading all your comments is that I should allign ALL 9 panels. Panels 1-6 are totally fixed and can't be moved/tilted, however panels 6-9 can be tilted to match the first 6 panels, here I'm talking just a couple of degrees. In this way all panels will peak at the same time.

    Having said all this, last year (i.e. installation year), I used to see peaks > 2KW output from the inverter and it's only this year which I'm noticing a decrease in output.
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: System Performance degradation

    Guys, thanks to all for your feedback. I'm currently still waiting for the installer to comeback with feedback from his supplier... will let you allknow what's the outcome.

    Regards
    Brian
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: System Performance degradation
    Brianellul wrote: »
    Neil,thanks for your feedback! I was not aware the voc should not vary with conditions. I was under the impression that both the voc and isc will vary, both reaching their maximum when the sun is perpendicular to the panels... Is this what you're telling me?
    No. Voc will not vary much (if any) with the angle of the sun if insolation is the only thing considered. However with heating effects you may very well see Voc be at its lowest when the sun is perpendicular to the modules. As cell temperature goes up, Voc goes down. See the nameplate temperature coefficient for Voc for how much; it's usually somewhere around -0.35% or -125mV per degree C for crystalline Si modules.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: System Performance degradation
    Brianellul wrote: »
    You're right! Without any current flowing there will be no voltage drop. I should have worded differently... basically I concluded that there are minimal losses in the 12meter run.

    Thanks
    With no current flowing you will see no losses in voltage unless you have a serious problem with wiring insulation.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: System Performance degradation
    Brianellul wrote: »
    Neil,thanks for your feedback! I was not aware the voc should not vary with conditions. I was under the impression that both the voc and isc will vary, both reaching their maximum when the sun is perpendicular to the panels... Is this what you're telling me?

    yes, for pvs reach their voc with very little light to start and it is fairly steady from sun up to sun down without a load on it. there's a point though that the smallest amount of light like around sunrise and sunset that the voltage will drop below the voc and with that little bit of light solar is fairly useless, but once the voc is reached all that can be realized after that would be more current output capability. now when the pv gets loaded it will pull down the voltage and when it is at a point that it delivers its max power the voltage is noted and you see that as the vmp or volts max power. that max power point does vary a tad with solar intensity though.

    i keep rambling here no doubt confusing you, but know that the voc is pretty steady throughout the day and that is without a load on it as it stands for voltage open circuit.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: System Performance degradation
    niel wrote: »
    yes, for pvs reach their voc with very little light to start and it is fairly steady from sun up to sun down without a load on it. there's a point though that the smallest amount of light like around sunrise and sunset that the voltage will drop below the voc and with that little bit of light solar is fairly useless, but once the voc is reached all that can be realized after that would be more current output capability. now when the pv gets loaded it will pull down the voltage and when it is at a point that it delivers its max power the voltage is noted and you see that as the vmp or volts max power. that max power point does vary a tad with solar intensity though.

    i keep rambling here no doubt confusing you, but know that the voc is pretty steady throughout the day and that is without a load on it as it stands for voltage open circuit.

    My guess is that the only reason that the open circuit voltage ever goes below the region of the rated Voc of the panel is that some current is being drawn by internal leakage or by the voltmeter you are using to make the measurement. Some of the steps in production of a solar cell (and just the semiconductor material itself) can cause leakage current between + and -, and the magnitude of that leakage is one of the things that cell manufacturers test for.
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: System Performance degradation
    niel wrote: »
    there's a point though that the smallest amount of light like around sunrise and sunset that the voltage will drop below the voc

    are you trying to say that after sunset, and until sunrise, the volts may drop below Voc? I think that's generally true, but I've seen some pretty high voltages by moonlight in the winter. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: System Performance degradation

    vtmaps,
    i know i didn't explain it well.:roll:

    moon volts are different than sun volts and you need a special moon volt meter to detect it.:p
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: System Performance degradation
    niel wrote: »
    vtmaps,
    i know i didn't explain it well.:roll:

    moon volts are different than sun volts and you need a special moon volt meter to detect it.:p

    I've been looking for one of those meters ever since my retailer told me years ago that my panels would charge the batteries with just moonlight, due to "the UV light given off by the moon". Thanks to his enlightening information, I now use special "Moon Blocker" SPF 30 to protect my skin when I expose myself on moonlit nights, but my panels still don't charge. Would this be a warranty issue?
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: System Performance degradation
    I've been looking for one of those meters ever since my retailer told me years ago that my panels would charge the batteries with just moonlight, due to "the UV light given off by the moon". Thanks to his enlightening information, I now use special "Moon Blocker" SPF 30 to protect my skin when I expose myself on moonlit nights, but my panels still don't charge. Would this be a warranty issue?

    http://en.wikipedia.org/wiki/File:Golden_Earring_-_Moontan_US.jpg
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: System Performance degradation
    ggunn wrote: »

    The ultraviolet end of the spectrum is much greater proportionally once in a blue moon.
    SMA SB 3000, old BP panels.
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: System Performance degradation

    Hello Everyone

    Update on my situation...

    To eliminate the inverter MPPT failure probability (also it is the easiest one to eliminate!), my installer brought over another brand new SMA inverter and we just plugged it in, however as most of you said, the inverter was OK. Same behaviour with this inverter, just 1000W-1200W being generated/pumped out from a 2070W array.

    I've also swapped two panels...
    Panel 6 - 27.8 Voc / 5.83 Asc
    Panel 7 - 32.4 Voc / 6.39 Asc

    Panel 6 swapped position with panel 7, however still same Voc from the panels...

    As was also suggested here and by my installer, I've aligned the panels to all point exactly in one directly (slightly off South). Although this does not explain why last year my output was more...

    Now, we're looking at the panels and as suggested here, the panels seem out of spec! My installer is communicating with his supplier (IBC Solar) for a way forward. Unfortunately, It doesn't seem that straightforward/easy to just replace the panels!

    Regards
    Brian
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: System Performance degradation

    Hello Guys

    It me, back with some more feedback from my installer. Well it seems that the reason for my degradation is due to PID or Potential Induced Degradation. I have a document as prepared by SMA detailing this effect. I still haven't read it thoroughly and to be honest was not aware of this phenomenon! I'm still waiting for the installer to replace the panels and perform their detailed investigation to determine exactly the cause of my panel’s degradation however I thought to share it with you.
    Regards
    Brian
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: System Performance degradation
    Brianellul wrote: »
    I have a document as prepared by SMA detailing this effect.

    I clicked the link, but it doesn't show the document.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: System Performance degradation
    NorthGuy wrote: »
    I clicked the link, but it doesn't show the document.

    just google "Potential Induced Degradation". Its all over the internet. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: System Performance degradation
    vtmaps wrote: »
    just google "Potential Induced Degradation". Its all over the internet.

    Thanks vtMaps. I've never heard of that before. Looks like a case against grounding panel frames.
    Brianellul wrote: »
    I'm still waiting for the installer to replace the panels and perform their detailed investigation to determine exactly the cause of my panel’s degradation however I thought to share it with you.

    Yes, please. I'm very interested.
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