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 <co...@joule-energy.com> 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 <bhof...@sunergyengineeringservices.com>
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