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 <suneng...@gmail.com> 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 <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 >> -wrenc...@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> > > > _______________________________________________ > 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 > > >
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