samlexsolar SCC-30AB, Руководство пользователя, Страница: 13 / 48

13
2
GENERAL DESCRIPTION OF PV/SOLAR SYSTEM
A Current (I) versus Voltage (V) Curve of a PV / Solar Module (“I-V” Curve) shows the
possible combinations of its current and voltage outputs. A typical I-V curve for a 12 V
Module is shown at Fig. 2.8.
The power in a DC electrical circuit is the product of the voltage and the current.
Mathematically,
• Power (P) in Watts (W) = The Current (I) in Amperes (A) X the Voltage (V) in Volts
(V) i.e. W = V X A
A PV / Solar Cell or a Panel (Module) produces its maximum current when there is
no resistance in the circuit, i.e. when there is a short circuit between its Positive and
Negative terminals. This maximum current is known as the Short Circuit Current and is
abbreviated as I
sc
. When the Cell / Panel (Module) is shorted, the voltage in the circuit
is zero.
Conversely, the maximum voltage occurs when there is a break in the circuit. This is
called the Open Circuit Voltage (V
oc
). Under this condition, the resistance is infinitely
high and there is no current, since the circuit is incomplete. Typical value of the open-
circuit voltage is located about 0.5 – 0.6 V for Crystalline Cells and 0.6 – 0.9 V for
Amorphous Cells.
These two extremes in load resistance, and the whole range of conditions in between
them, are depicted on the I-V Curve. Current, expressed in Amps, is on the vertical
Y-axis. Voltage, in Volts, is on the horizontal X-axis.
The power available from a photovoltaic device at any point along the curve is just
the product of Current (I) in Amps (A) and Voltage (V) in Volts (V) at that point and is
expressed in Watts. At the short circuit current point, the power output is zero, since
the voltage is zero. At the open circuit voltage point, the power output is also zero, but
this time it is because the current is zero.
Maximum Power Point and Rated Power of PV / Solar Panel (Module)
There is a point on the knee of the I-V Curve where the maximum power output
is located and this point is called the Maximum Power Point (MPP). The voltage and
current at this Maximum Power Point are designated as Vmp and Imp.
The values of Vmp and Imp can be estimated from V
oc
and I
sc
as follows:
Vmp
≈
(0.75 – 0.9) V
oc
Imp
≈
(0.85 – 0.95) I
sc