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.
Spirit Solar Inc.
From: Corey Shalanski [mailto:co...@joule-energy.com]
Sent: Thursday, May 17, 2018 6:19 PM
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?
New Orleans, LA
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