Re: [RE-wrenches] PV Cell Temperature Variation
A dampening of the knee of the IV curve would be indicative of higher series resistance. The higher series resistance would also increase cell temperatures as shown in the thermo image. The cooling off of the cells under open circuit conditions would also correlate to the higher series resistance, whereas there would be no current flowing generating heat through the cell resistance under open circuit conditions. The question then becomes why would some cells have high resistance. Could be cell fractures, albeit odd that they would be random within the module, and also random in the group of modules shown. I’m guessing it was only this one string, perhaps this crate of modules was banged up in shipping or something. Here is a link to an article about the relationship between series resistance and fill factor. http://www.pveducation.org/pvcdrom/solar-cell-operation/series-resistance Mike Kocsmiersky Principal Spirit Solar Inc. (413) 734-1456 From: Corey Shalanski [mailto:co...@joule-energy.com] Sent: Thursday, May 17, 2018 6:19 PM To: RE-wrenches@lists.re-wrenches.org Subject: [RE-wrenches] PV Cell Temperature Variation On a recent utility-scale PV installation we noticed a strange phenomenon on a single string on a single inverter. An IV curve (attached) reveals that the measured performance characteristics for this string (solid line) are significantly less than the expected values (dotted line). For reference, the ambient temperature was 31°C and the irradiance was 1010 W/m². The phenomenon that more so caught our attention is apparent on a thermal image (also attached) of the modules in this string. For lack of a better term I would describe the distribution of cell temperatures as resembling a "checkerboard" or "scattershot" (random) pattern, ranging between roughly 55°C and 70°C. Interestingly this phenomenon was only apparent while the inverter was operating, i.e. with the inverter turned off the modules revert to a much more uniform temperature distribution nearer to 55°C, instead varying by only a couple degrees across the entire module/string. There was no apparent physical damage to the modules. Can anyone offer any suggestions about what might be causing this phenomenon? -- Corey Shalanski Joule Energy New Orleans, LA ___ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
Re: [RE-wrenches] PV Cell Temperature Variation
Corey, I have been studying IR diagnostics quite a lot, but I haven't had a whole lot of opportunity to "get my hands dirty" with it, so I'm not sure how much help I'll be. I would like to know how many modules are in each string. I'm going to share some random thoughts on this, but nothing here is at all conclusive, and if anyone can correct some flaws in my logic, I'd love to hear it! It doesn't look like a simple, obvious problem. Your curve trace has a predicted Isc, with a dampening current to the knee, it has a sharp knee, then it has a low voltage at Vmp, and it stays really low all the way to Voc. A high series resistance would explain the dampening current to the knee, but it wouldn't explain the low voltage at all. If there was high resistance in the cells, you should see signs of diodes activating unless all of the cell groups had relatively uniform resistance. Generally, the hot cells in the checkerboard pattern are underproducing compared to the cooler cells. Because they can't pass on as much current, they are actually wasting some of the energy that the other cells are producing. This wasted energy turns into heat. The often times, they waste more energy than they are producing themselves. Alternatively, there could be other issues (or damage) at the hot cells. The fact that the checkerboard pattern goes away when you shut off the inverter may be helpful. If you short out a module, it will almost always look like a checkerboard in an IR shot, and if a diode shorts out, that third of the module will generally look like a checkerboard. This is the confusing part... Because the checkerboard appearance goes away, that would most likely indicate that it's not an issue of a short circuit failure (such as short-circuited diodes, or short-circuited modules). On the other hand, your curve trace looks somewhat like what you'd expect if you had some short-circuited diodes. It looks like you have a calculated Voc of around ≈860, and a measured Voc of around ≈725. That's about 84%. My initial first guess based on this curve trace alone would have been that perhaps roughly 16% of the cell groups might be shorted out. So if you had 20 modules per string, that would be 30 cell groups (assuming 3 diodes per cell group). With these hypotheticals, you would have somewhere around 5 shorted cell groups. Now that I'm looking at this closer, the knee of the curve is very quite sharp. I wonder if this could be caused by a higher resistance short somewhere out in the array (or in a conduit somewhere). (Perhaps this could be from compromised insulation or water intrusion somewhere??) Assuming that this isn't an intermittent problem, this would be easy to test under Voc. Just take a good DC amp clamp into the array while the inverter is shut down, and test the PV wires several places to try to find current in the array. Under this hypothesis, as the curve tracer starts pulling current, and the measured voltage falls from Voc, it gets to a point where suddenly, the fault isn't passing as much current. This would cause the sharp knee on the curve trace. As the voltage gets lower, the current at the fault would also get lower and the measured curve would increasingly get closer to the predicted curve. This model would also explain why the checkerboard phenomenon goes away when you turn off the inverter... it would be because the current going across the fault would be much lower than Isc, so when it's resting, there isn't enough current flowing to produce the checkerboard effect. Using this hypothesis, the checkerboard phenomenon might not be indicating a problem in the modules at all. If there is a high resistance short somewhere in the array, then that would cause the actual current in the modules to be above the predicted Imp because the current is being lost before it gets to the inverter. Because nearly all modules get the checkerboard phenomenon at Isc, it wouldn't be surprising if many normal modules would also get the phenomenon if they're operating between Imp and Isc. Again, take these thoughts with a grain of salt. If anyone sees somewhere that I'm going wrong, I'd love to hear from you! Hopefully, something here helps! Thanks, Kienan Maxfield Solar maxfieldso...@hotmail.com<mailto:maxfieldso...@hotmail.com> (801) 477-0-SUN (477-0786) (801) 631-5584 (Cell) From: RE-wrenches on behalf of Corey Shalanski Sent: Thursday, May 17, 2018 4:19 PM To: RE-wrenches@lists.re-wrenches.org Subject: [RE-wrenches] PV Cell Temperature Variation On a recent utility-scale PV installation we noticed a strange phenomenon on a single string on a single inverter. An IV curve (attached) reveals that the measured performance characteristics for this string (solid line) are significantly less than the ex
Re: [RE-wrenches] PV Cell Temperature Variation
I've checked several PV modules under load in past years and they all seem to do this. Just cell mis-match I was guessing. boB On 5/17/2018 3:19 PM, Corey Shalanski wrote: On a recent utility-scale PV installation we noticed a strange phenomenon on a single string on a single inverter. An IV curve (attached) reveals that the measured performance characteristics for this string (solid line) are significantly less than the expected values (dotted line). For reference, the ambient temperature was 31°C and the irradiance was 1010 W/m². The phenomenon that more so caught our attention is apparent on a thermal image (also attached) of the modules in this string. For lack of a better term I would describe the distribution of cell temperatures as resembling a "checkerboard" or "scattershot" (random) pattern, ranging between roughly 55°C and 70°C. Interestingly this phenomenon was only apparent while the inverter was operating, i.e. with the inverter turned off the modules revert to a much more uniform temperature distribution nearer to 55°C, instead varying by only a couple degrees across the entire module/string. There was no apparent physical damage to the modules. Can anyone offer any suggestions about what might be causing this phenomenon? -- Corey Shalanski Joule Energy New Orleans, LA ___ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org ___ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
Re: [RE-wrenches] PV Cell Temperature Variation
Corey, What make of panel please. Forgive if I missed it... Dave Angelini Offgrid Solar "we go where powerlines don't" http://members.sti.net/offgridsolar/ e-mail offgridso...@sti.net text 209 813 0060 > Corey, > > I have been studying IR diagnostics quite a lot, but I haven't had a whole > lot of opportunity to "get my hands dirty" with it, so I'm not sure how > much help I'll be. I would like to know how many modules are in each > string. > > I'm going to share some random thoughts on this, but nothing here is at > all conclusive, and if anyone can correct some flaws in my logic, I'd love > to hear it! > > It doesn't look like a simple, obvious problem. Your curve trace has a > predicted Isc, with a dampening current to the knee, it has a sharp knee, > then it has a low voltage at Vmp, and it stays really low all the way to > Voc. A high series resistance would explain the dampening current to the > knee, but it wouldn't explain the low voltage at all. If there was high > resistance in the cells, you should see signs of diodes activating unless > all of the cell groups had relatively uniform resistance. > > Generally, the hot cells in the checkerboard pattern are underproducing > compared to the cooler cells. Because they can't pass on as much current, > they are actually wasting some of the energy that the other cells are > producing. This wasted energy turns into heat. The often times, they waste > more energy than they are producing themselves. > > > Alternatively, there could be other issues (or damage) at the hot cells. > > The fact that the checkerboard pattern goes away when you shut off the > inverter may be helpful. If you short out a module, it will almost always > look like a checkerboard in an IR shot, and if a diode shorts out, that > third of the module will generally look like a checkerboard. > > This is the confusing part... Because the checkerboard appearance goes > away, that would most likely indicate that it's not an issue of a short > circuit failure (such as short-circuited diodes, or short-circuited > modules). On the other hand, your curve trace looks somewhat like what > you'd expect if you had some short-circuited diodes. > > It looks like you have a calculated Voc of around 860, and a measured Voc > of around 725. That's about 84%. My initial first guess based on this > curve trace alone would have been that perhaps roughly 16% of the cell > groups might be shorted out. So if you had 20 modules per string, that > would be 30 cell groups (assuming 3 diodes per cell group). With these > hypotheticals, you would have somewhere around 5 shorted cell groups. > > > > > > > > Now that I'm looking at this closer, the knee of the curve is very quite > sharp. I wonder if this could be caused by a higher resistance short > somewhere out in the array (or in a conduit somewhere). (Perhaps this > could be from compromised insulation or water intrusion somewhere??) > Assuming that this isn't an intermittent problem, this would be easy to > test under Voc. Just take a good DC amp clamp into the array while the > inverter is shut down, and test the PV wires several places to try to find > current in the array. > > Under this hypothesis, as the curve tracer starts pulling current, and the > measured voltage falls from Voc, it gets to a point where suddenly, the > fault isn't passing as much current. This would cause the sharp knee on > the curve trace. As the voltage gets lower, the current at the fault would > also get lower and the measured curve would increasingly get closer to the > predicted curve. This model would also explain why the checkerboard > phenomenon goes away when you turn off the inverter... it would be because > the current going across the fault would be much lower than Isc, so when > it's resting, there isn't enough current flowing to produce the > checkerboard effect. Using this hypothesis, the checkerboard phenomenon > might not be indicating a problem in the modules at all. If there is a > high resistance short somewhere in the array, then that would cause the > actual current in the modules to be above the predicted Imp because the > current is being lost before it gets to the inverter. Because nearly all > modules get the checkerboard phenomenon at Isc, it wouldn't be surprising > if many normal modules would also get the phenomenon if they're operating > between Imp and Isc. > > > > Again, take these thoughts with a grain of salt. If anyone sees somewhere > that I'm going wrong, I'd love to hear from you! Hopefully, something here > helps! > > Thanks, > Kienan > > > Maxfi
Re: [RE-wrenches] PV Cell Temperature Variation
Corey I consulted and did some independent tests with a module manufacturer who had some modules with low Power output and curve traces similar to yours. They however could detect no abnormal cracks in the modules with EL testing. I also detected no issues on the thermal image at no or low power, but when loaded I saw a similar pattern and temperature difference. Remember that the curve tracer is applying a linear load from 0 volts and no load at Isc to MPPT ( knee of the curve). From MPPT the load goes exponential to infinity at Voc (open circuit) or full load. Also No cells are perfectly Isc matched and some variation is common, especially with high power perc cells and seldom will effect the curve trace as much as your case or the one that I had. I do not believe that it is a series resistance issue because the slope of the curve from the MPPT to the Voc has not changed. The majority of the slope change in the curve is between Isc and MPPT, which is indicative of a Shunt resistance. in my case I determined that the modules at the plant had mixed different cells from different power bins and so the lower Isc cell passes some power forward bias through the cell, but cannot pass the full current through that route so it reverse bias a portion of the cell to pass the remainder of the difference in Isc. This causes a hotter cell. So in many ways it is acting like a shaded cell getting hotter (classic Isc mismatch). Because a cell in either reverse bias or forward bias is like a diode ( a leaky switch) this may or may not be enough of a difference to trigger a diode ( junction boxes do not appear to be warmer indication a diode triggering), or maybe it has and the continuous heat over time has caused the diode to fail. At least by the patterns I see in your modules that does not appear to be the case. So my first guess would be that you are seeing something similar to what I saw, that is that the module maufacturer has produced a series of modules with different power rated cells and the temperature difference is from some level of reverse bias. This should have been detected in the binning of the modules unless these had a large +/- 5% tolerance ( which I would never ever purchase). Regardless it has impacted your power output and depending on the age and the nature of the module manufactures warranty I would think that you have a warranty claim. Caution as always it is hard to determine how much temperature difference is enough to be an issue, especially if you did not adjust for emmissivity and reflectivityin your images. If you do not know how to do that you should take a Thermal Imaging Level I class, otherwise you may freak out whenever you see minor checkerboarding!. I again think the curve trace is pretty good indication at the irradiance that you were tracing at. Bill On Thu, May 17, 2018 at 3:19 PM, Corey Shalanski wrote: > On a recent utility-scale PV installation we noticed a strange phenomenon > on a single string on a single inverter. An IV curve (attached) reveals > that the measured performance characteristics for this string (solid line) > are significantly less than the expected values (dotted line). For > reference, the ambient temperature was 31°C and the irradiance was 1010 > W/m². > > The phenomenon that more so caught our attention is apparent on a thermal > image (also attached) of the modules in this string. For lack of a better > term I would describe the distribution of cell temperatures as resembling a > "checkerboard" or "scattershot" (random) pattern, ranging between roughly > 55°C and 70°C. Interestingly this phenomenon was only apparent while the > inverter was operating, i.e. with the inverter turned off the modules > revert to a much more uniform temperature distribution nearer to 55°C, > instead varying by only a couple degrees across the entire module/string. > > There was no apparent physical damage to the modules. > > Can anyone offer any suggestions about what might be causing this > phenomenon? > > -- > Corey Shalanski > Joule Energy > New Orleans, LA > > ___ > List sponsored by Redwood Alliance > > List Address: RE-wrenches@lists.re-wrenches.org > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches. > org/maillist.html > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > www.members.re-wrenches.org > > > -- William Hoffer 420 SE Wyers Street P.O. Box 1823 White Salmon, WA 98672-1823 suneng...@gmail.com ___ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wre
Re: [RE-wrenches] PV Cell Temperature Variation
Hi Corey, I recently commissioned a large system and saw something very similar. Is it possible that there is a minor amount of shading on those modules? Maybe weeds growing up or powerlines? When I had noticed that cell pattern, it took a little bit to realize that there were powerlines just south of the array casting a thin shadow. The hot cell pattern did not specifically follow the shade pattern on the front of the module. Cheers, Dave *--* *Dave Tedeyan* *Senior Engineer* *Taitem Engineering, PC* 10 Verizon Lane, Lansing, NY 14882 Voice: (607) 930-3481 x6 www.taitem.com On Fri, May 18, 2018 at 6:53 PM, Bill Hoffer wrote: > Corey > > I consulted and did some independent tests with a module manufacturer who > had some modules with low Power output and curve traces similar to yours. > They however could detect no abnormal cracks in the modules with EL > testing. I also detected no issues on the thermal image at no or low power, > but when loaded I saw a similar pattern and temperature difference. > Remember that the curve tracer is applying a linear load from 0 volts and > no load at Isc to MPPT ( knee of the curve). From MPPT the load goes > exponential to infinity at Voc (open circuit) or full load. Also No cells > are perfectly Isc matched and some variation is common, especially with > high power perc cells and seldom will effect the curve trace as much as > your case or the one that I had. I do not believe that it is a series > resistance issue because the slope of the curve from the MPPT to the Voc > has not changed. The majority of the slope change in the curve is between > Isc and MPPT, which is indicative of a Shunt resistance. in my case I > determined that the modules at the plant had mixed different cells from > different power bins and so the lower Isc cell passes some power forward > bias through the cell, but cannot pass the full current through that route > so it reverse bias a portion of the cell to pass the remainder of the > difference in Isc. This causes a hotter cell. So in many ways it is > acting like a shaded cell getting hotter (classic Isc mismatch). Because a > cell in either reverse bias or forward bias is like a diode ( a leaky > switch) this may or may not be enough of a difference to trigger a diode ( > junction boxes do not appear to be warmer indication a diode triggering), > or maybe it has and the continuous heat over time has caused the diode to > fail. At least by the patterns I see in your modules that does not appear > to be the case. > > So my first guess would be that you are seeing something similar to what I > saw, that is that the module maufacturer has produced a series of modules > with different power rated cells and the temperature difference is from > some level of reverse bias. This should have been detected in the binning > of the modules unless these had a large +/- 5% tolerance ( which I would > never ever purchase). Regardless it has impacted your power output and > depending on the age and the nature of the module manufactures warranty I > would think that you have a warranty claim. > > Caution as always it is hard to determine how much temperature difference > is enough to be an issue, especially if you did not adjust for emmissivity > and reflectivityin your images. If you do not know how to do that you > should take a Thermal Imaging Level I class, otherwise you may freak out > whenever you see minor checkerboarding!. I again think the curve trace is > pretty good indication at the irradiance that you were tracing at. > > Bill > > On Thu, May 17, 2018 at 3:19 PM, Corey Shalanski > wrote: > >> On a recent utility-scale PV installation we noticed a strange phenomenon >> on a single string on a single inverter. An IV curve (attached) reveals >> that the measured performance characteristics for this string (solid line) >> are significantly less than the expected values (dotted line). For >> reference, the ambient temperature was 31°C and the irradiance was 1010 >> W/m². >> >> The phenomenon that more so caught our attention is apparent on a thermal >> image (also attached) of the modules in this string. For lack of a better >> term I would describe the distribution of cell temperatures as resembling a >> "checkerboard" or "scattershot" (random) pattern, ranging between roughly >> 55°C and 70°C. Interestingly this phenomenon was only apparent while the >> inverter was operating, i.e. with the inverter turned off the modules >> revert to a much more uniform temperature distribution nearer to 55°C, >> instead varying by only a couple degrees across the entire module/string. >> >> There was no apparent physical damage to the modules. >> >> Can anyone offer any suggestions about what might be causing this >> phenomenon? >> >> -- >> Corey Shalanski >> Joule Energy >> New Orleans, LA >> >> ___ >> List sponsored by Redwood Alliance >> >> List Address: RE-wrenches@lists.re-wrenches.org >> >> Cha
