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
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