Voc vmp imp isc explained

quique · April 2013

Can someone please help me understand these four terms.
I understand voltage maximum and current maximum are the maximum voltage and current, respectively, we can get from a solar panel. But I am not clear as to open circuit voltage and short-circuit amperage.Attachment not found.

Cariboocoot · April 2013

Re: Voc vmp imp isc explained

This is why we have a glossary of terms: http://forum.solar-electric.com/showthread.php?6136-Glossary

Basically you will only see Voc when there is no load on the panel (except the meter) and you will only see Isc when the output of the panel is a dead short.

quique · April 2013

Re: Voc vmp imp isc explained

Yes I know the definitions. I'm actually taking SEI course at this time. Words are great but sometimes I have trouble understanding. I was hoping there was a better way to explain what they actually mean Thanks.

Cariboocoot · April 2013

Re: Voc vmp imp isc explained

That is what they actually mean: Voltage open circuit (no load attached) and current short circuit (output shorted). It can't get any clearer than that.

quique · April 2013

Re: Voc vmp imp isc explained

Okay let me ask it this way: does the current drop as the voltage gets bigger because = a bigger voltage means too big a difference of energy that the electrons must overcome in order to flow?

BB. · April 2013

Re: Voc vmp imp isc explained

Nice post from a bit earlier today:

RCinFLA wrote: »

Trying to keep this explanation to minimum techno talk, a PV panel is an illumination based current source that is capped in voltage by the inherent diode within each individual cell. In other words its current output is based on sun's illumination. If no load is applied to the panel it will rise to maximum voltage, called Voc, for voltage open circuit, which is roughly 0.65 vdc times the number of cells connected in series within the panel.

Let's say your panel is putting out 8 amps at full sunlight. Without any load on the panel the 8 amps generated will shunt back through the panel through the cells' inherent conducting diode. The voltage at no load will be Voc. If you load the panel less then 8 amps the voltage will stay close to Voc. If a cloud goes by and drops the illumination current to 4 amps and you had a 6 amp load merrily running along at full sun, the voltage will drastically collapse to very low level, near zero, and your device load will shut down. This is why you cannot power a load reliably directly from a panel.

As long as your load current is small or you are sure the sun will not dip behind a cloud, you can use the panel as a voltage source of near Voc.

Just for info, Vmpp or voltage at maximum power point, is the operating point where just a small percentage of illumination generated current (whatever the illumination provides) is wasted down the cells' inherent diode. You have to modulate the load on panel to keep finding this sweat spot as sun's illumination varies on the panel. Vmpp is approximately 0.52 vdc times the number of series connected cells within the panel. Actual voltage depends on current supplied and on temperature of the panel.

And Vmp/Imp are not, the Maximum Voltage and maximum Current (Voc and Isc are respectively)...

They are the values that give Pmax-power:

Pmp = Vmp * Imp

If you go to the Blue Line in your I/V curve--That is V*I of the solar panel output. Note, there is a natural "peak" where V*I = peak power output from the solar panel.

And the values of Vmp/Imp are usually defined at ~25C (real panels get upwards of 20+ C rise in full sun, so Vmp is typically much lower than the spec sheet says.

-Bill

quique · April 2013

Re: Voc vmp imp isc explained

Okay I'm getting closer. One last thing, is my understanding correct that voltage is an energy differential. As such as that energy differential gets larger between two terminals, current drops because electrons stop flowing from one terminal To the other because that gap got wider?

Cariboocoot · April 2013

Re: Voc vmp imp isc explained

Panel function is a bit more difficult to understand. Under almost any amount of illumination a panel will produce Voc. But once a load is attached the Voltage will drop and current will rise. Depending on the amount of illumination and the amount of load the current and Voltage will vary; primarily the panel is a current source and will try to produce its Imp rating at any Voltage up to Vmp. Thus you have maximum power (Watts) at Imp * Vmp. Reduction of the load will allow the Voltage to rise above Vmp and the current will drop off. This is what the graph you supplied shows: current (vertical axis) in relation to Voltage (horizontal axis).

What it does not show/explain is the uselessness of Isc and Voc for application. Since Isc comes from a dead short on the output it does not represent usable current but only a maximum (which is a value needed for sizing string fusing and wiring). Likewise Voc only exists with essentially zero load, so its only practical use is determining V-max in to a charge controller.

Other things will alter a panel's output. Chiefly temperature will vary Voltage (higher temp = lower Voltage and vice versa) in respect to the power curve (which is at a fixed temperature, usually 25C) but has little effect on current. Additional insolation, such as from snow reflection or edge of cloud events (where the water vapour in the air acts as a lens and momentarily focuses sunlight on the panel) will increase current but will not alter Voltage.

Does that help?

BB. · April 2013

Re: Voc vmp imp isc explained

I think you are getting confused with "Band Gap Energy"... It is real thing, but not not a physical gap between conductors:

http://en.wikipedia.org/wiki/Band_gap

This article is about solid state physics. For voltage control circuitry in electronics, see Bandgap voltage reference.
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material. So the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have very small band gaps or none, because the valence and conduction bands overlap.

I always like water analogies (hey--got me through physics and much of control theory).

Voltage is Water Pressure (lbs per square inch) type thing. You can have a little piece of tubing or a big fire hose with both with 100 PSI... But nothing happens unit the water flows.

Current (Amperes) is Water Gallon per Minute type thing. You can have water flowing lazily down a stream with thousands of gallons per second of flow, but very little pressure. Or you can have "pressure washer" with 2,000 PSI pushing a few gallons per minute.

A 100 PSI and 5 gallons per minute from a garden hose vs 100 psi AND 100 gpm from a fire hose--One takes a lot more energy to function...

Just like voltage*current=power ... Pressure * GPM = a form of water power

-Bill

Voc vmp imp isc explained

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