PV panel -a current source or voltage source?

Question

I'm reading about PV behaviour and am confused on whether a PV panel/cell would be considered to be a voltage source or current source or both or neither (from the characteristic IV curve). The IV curve looks like a combination of both constant current and constant voltage. It seems that from (a) panel is unloaded to (b) panel is loaded to max power--the operating point travels from Voc to MPP(If this is the case; I'm confused as to the reason behind why a PV cell is modelled as a current source). Do we use only this region under load conditions? Under what conditions does the operating point traverse from MPP to Isc point? Why is a solar cell modelled as a current source?

Answer 1

As usual, the question is about building a model, and how well it conforms to reality.

If you connect a solar panel to a high impedance load (hence expecting a very low current in the panel), modeling the solar panel as a imperfect voltage source (ie. with a series resistor) is certainly the most pertinent.

Else, you need to understand that the physics of a solar panel implies that the current that flows through it is directly proportional to the number of photons impacting the cells. In that case, if you have a (very) low impedance load, the solar panel would be better approximated with a current source.

You can find a more mathy explanation here.

Answer 2

Unfortuneately the graph has an ambiguous y plane axis which maybe missleading to people not familliar with tech jive. IE the y axis is correctly labelled Current (Amps) but one of the curves (labelled Power) strictly requires the orthoganol axis to be labelled in Watts not Amps.

The VI curve is the curve with the flat top. So current stays pretty much the same right up to maximum voltage. This is typical of a constant current device. The Power curve can simply be derived by multiplying Volts x amps for every point along the VI curve and is overlayed here simply to confuse you.

When the sun comes out the current goes up and when darkness falls the current goes down. IE the flat part of the curve rises and falls. If your load is a resistor then the voltage will also go up and down in proportion (V = I x R).

To discover what the voltage and current across your resistor is under any conditions you can plot the resistor on the V I graph as a straight line that goes through the origin. Where it intersects with the V I curves will give you a voltage and current for that condition. The power is the product of V and I (or VxV/R or I x I x R) which will all give the same number

Answer 3

Most PV inverters have an MPPT (max. power point tracker) in them. It's purpose is to keep the panel operating at the MPP for what should be obvious reasons.

You do NOT want to load the panels so that the voltage drops (i.e. to the left of the MPP) and you are moving toward Isc. This, of course, operates away from the MPP and also causes excess current to flow which may, over time, damage the panel.

You do, often, operate to the right of the MPP since there are times when the inverter must "throttle down" the output of the panel or even switch them off. Here the current drops and the voltage approaches Voc. That rightmost point is where you are operating an unconnected panel.

The reason a PV panel is modelled at a current source is that is how they behave.