What are IV Curves
I = Current and V= Voltage
Voc = OPEN CIRCUIT VOLTAGE
We can see the point called Voc (open circuit voltage). This is the point at which there is an open circuit. This means that the PV system is turned off. As we can see, the voltage is as high as it can get, but the current is zero. Since voltage multiplied by the current is power, then:
Voc × Zero Amps = Zero power (Amps is current I)
It is important to note that when you turn off a PV system, the voltage will increase. That means you can still get shocked when PV is off.
ISC = SHORT CIRCUIT CURRENT
Short circuit current (Isc) is the upper left point on the IV curve where there is all current and no voltage.
This is an operating point that hopefully no PV system ever experiences. A short circuit occurs happens when there is a direct connection between positive and negative. This is the point of maximum current, where voltage is zero.
As we know anything multiplied by zero is zero, so Isc is not a power-producing situation.
Isc × Zero Volts = Zero Power
If conductors are installed wrongly and the insulation of the wire disappears, you will have a positive to negative short. This can cause a fuse in a combiner box to activate.
Vmp = MAXIMUM POWER VOLTAGE
At maximum power voltage, we have the point of maximum power. This means that we want to operate in this position to get the most out of our PV. It is interesting to note that “maximum” power voltage is less than open-circuit voltage.
Imp = MAXIMUM POWER CURRENT
Imp is maximum power current and this is at the same point as Vmp.
If we multiply Imp × Vmp we will get the maximum power at the maximum powerpoint.
Maximum power current is less than short circuit current.
Vmp × Imp = Maximum Power
For example, if we have a PV module where the Vmp is 40 Volts and the Imp is 5 Amps then:
40 Vmp × 5 Amp = 200W
We have a 250W PV module
It is a common mistake to think that Vmp or Imp is the higher voltage or current, since m is for maximum, however, this is not the case. It is a higher current than Imp, but at Isc there is no power.
Isc is not a power-producing place, and the same goes for Voc.
Another common mistake people make is multiplying Voc and Isc. This does not get you power!
If we look at Isc at the upper left part of the IV curve, and see how the line gradually slopes down to the maximum power point (MPP) we see places where power is made, but not as much as the power made at the MPP. It is interesting that at the point of the most current, there is no power at all.
Most of the inverters can keep the PV working at the MPP and we call this maximum power point tracking or MPPT.
Summary:
Voc × Zero Amps = Zero Power
Isc × Zero Volts = Zero Power
Vmp × Imp = Maximum Power
Voc × Isc: PV cannot operate here
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