Solmetric PVA-1000S, User Manual

Page: 130 / 152

7 Interpreting Measured I-V Curves
7-2
I-V Curve Terminology
These abbreviations will be used in the following discussion:
•
I
sc
Short circuit current
•
I
mp
Max power current
•
V
mp
Max power voltage
•
V
oc
Open circuit voltage
•
PF Performance Factor (PF, %) = 100 * (Measured P
max
/Predicted P
max
)
•
FF Fill Factor = I
mp
* V
mp
/ (I
sc
* V
oc
)
Performance Factor
The Performance Factor is the most important figure of merit for PV array performance.
The Performance Factor is defined as the measured maximum power value divided by the
predicted (modeled) maximum power value. Its range is 0 to 100%.
The predicted maximum power value is determined by the PV model, which takes into
account the current values of irradiance and temperature. This means that Performance
Factor values taken at different irradiances and temperatures can be directly compared.
Like most array performance measurements, the comparison is more accurate if the
irradiance levels are high. Performance measurements taken at less than 400W/m2
irradiance are not good predictors of how the string will perform at high irradiance.
If a PV string or module is operating correctly and is not shaded, soiled, or age-degraded,
and is measured at high irradiance, the measured Performance Factor will typically be in
the range of 90-100%. If the irradiance is stable and the array temperature is not changing
rapidly, the measured I-V curves should be very consistent from string to string.
Fill Factor
The Fill Factor is a valuable measure of the square-ness of the I-V curve. It is defined by
three points on the I-V curve, as shown in Figure 57. The Fill Factor range is 0 to 1.0, and
the closer the Fill Factor is to 1.0, the squarer is the shape of the I-V curve. Each PV
technology has its own nominal range of fill factor values, and each module model
number typically has an even narrow nominal range.