Solmetric PVA-1000S, User Manual

Page: 144 / 152

7 Interpreting Measured I-V Curves
7-16
5. Steeper slope in horizontal leg
An example of this deviation is shown below.
Figure 65. An I-V curve showing steeper slope in the horizontal leg of the I-V curve.
The horizontal leg of the I-V curve may exhibit a steeper slope than the PV model
predicts.
Potential causes of this deviation are summarized below, and then discussed in more
detail.
Potential causes located in the array include:
•
Tapered shade or dirt dams
•
Module I
sc
mismatch
•
Shunt paths exist in PV cells
Shunt Paths Exist in PV Cells or Modules
Shunt current is current that bypasses the solar cell junction without producing power,
short circuiting a part of a cell or module. Some amount of shunt current within a solar
cell is normal, although higher quality cells will have a higher shunt resistance and hence
lower shunt current. Shunt current can lead to cell heating and hotspots appearing in the
module encapsulant material. Shunt current is typically associated with highly localized
defects within the solar cell, or at cell interconnections. Infrared imaging of the PV
module can usually identify minor shunt current hot spots since a temperature rise of
20 ºC or more is common.
A reduced shunt resistance will appear in I-V curves as a steeper (less flat) slope near I
sc
.
As the cell voltage increases from the short circuit condition, the current flowing in these
shunts increases proportionally, causing the slope of the I-V curve near I
sc
to become