Civil Engineering Reference
In-Depth Information
FIGURE 8-7
Current versus voltage (i-v) characteristics of the pv module in sunlight and in dark.
8.5
i-v and p-v Curves
The electrical characteristic of the pv cell is generally represented by the
current versus voltage (i-v) curve.
Figure 8-7
shows the i-v characteristic of
a pv module under two conditions, in sunlight and in dark. In the first
quadrant, the top left of the i-v curve at zero voltage is called the short-
circuit current. This is the current we would measure with the output ter-
minals shorted (zero voltage). The bottom right of the curve at zero current
is called the open-circuit voltage. This is the voltage we would measure with
the output terminals open (zero current). In the left shaded region, the cell
works like a constant current source, generating voltage to match with the
load resistance. In the shaded region on the right, the current drops rapidly
with a small rise in voltage. In this region, the cell works like a constant
voltage source with an internal resistance. Somewhere in the middle of the
two shaded regions, the curve has a knee point.
If the voltage is externally applied in the reverse direction, say during a
system fault transient, the current remains flat and the power is absorbed
by the cell. However, beyond a certain negative voltage, the junction breaks
down as in a diode, and the current rises to a high value. In the dark, the
current is zero for voltage up to the breakdown voltage which is the same
as in the illuminated condition.
The power output of the panel is the product of the voltage and the current
outputs. In
Figure 8-8
,
the power is plotted against the voltage. Notice that
FIGURE 8-8
Power versus voltage (p-v) character-
istics of the pv module in sunlight.
