Solar panel output voltage and current

[Darius Baronas]

Hi all. I'm new with solar panels. Can I calculate output voltage and current , if I know, that Voc=7V , Isc=0.33A , and load is 30 Ohm ? The source resistance i think is R=7/0,3=23.3 Ohm. Thanks !

 

[FvM]

A solar cell doesn't act as a source with internal resistance, it has a nonlinear saturation characteristc. https://en.wikipedia.org/wiki/Theory_of_solar_cells

Thus you can't calculate the operation point without knowing the exact characteristic of yout solar cell. You can however estimate it assuming a typical characteristic. I would expect an output voltage around 5 to 6 V.

 

[Darius Baronas]

For real I do not understand mppt tracking. I know that this is implemented with dc-dc converter **** as buck, and using pwm we can change output current and voltage (Pin=Pout) . But how it is related to maximum power ? How source and load impedance are matched in this way ? Sorry for my bad english skills

 

[FvM]

Review the varying solarcell I/V characteristic with different irradiation levels. https://en.wikipedia.org/wiki/Mppt
For each irradiation, there's one maximum power point. MPPT is the method to find it.

 

[Darius Baronas]

Review the varying solarcell I/V characteristic with different irradiation levels. https://en.wikipedia.org/wiki/Mppt
For each irradiation, there's one maximum power point. MPPT is the method to find it.

So, if i want to know output current and voltage from solar cell, i can find the values from I /V curves, correct ? And if i do not know I/V curve, first i have to do some measurements, lets say with solar cell and potentiometer, and get that curve, yes ?

 

[BradtheRad]

I wanted to find out how my PV cells performed.

I did measurements on small PV cells with various resistance loads. I measured volts and amps.

Then I ran a computer program to graph the data.



As you can see, with light load, V is high, A is low.
With heavy load, A is high, V is low.

In the middle or thereabouts, is an obvious peak where W is greatest. It is with a 30 ohm load. This was for small PV cells, in sunlight.

Results will be different for your PV cells.

 

[Darius Baronas]

I'm reading about MPPT now, and i'm thinkink : do MPP tracking needed only for solar panels, because they have varying impedance depending on light, or MPPT is needed for all energy harvesting methods such as thermo, wind, piezo etc. ?

 

[D.A.(Tony)Stewart]

I'm reading about MPPT now, and i'm thinkink : do MPP tracking needed only for solar panels, because they have varying impedance depending on light, or MPPT is needed for all energy harvesting methods such as thermo, wind, piezo etc. ?

We know Solar Panels are current sources with limited Voc and the MPT voltage drops with solar input. But when you use a fixed low current , say 1% of rated max, this V(MPT) usually tracks 80% of the Voc(1%I)

.
Thus my suggestion to a smart MMTP controller uses a simple photodiode to derive solar power reference Voc(1%I) and scale the panel voltage regulator to 80% of this Voc. (5% tolerance for different chemistry)


For linear voltage sources the MPT ends up being a load that matches the ESR of the power source (e.g. Vbat) at 50%Voc. But we know batteries have a memory effect after charging, so again the Voc is obtained at something like 1%Imax.

However this method of loading is not good for voltage regulation, self heating or longevity of chemistry, but is OK for energy harvisting MPT.

 

[Darius Baronas]

When i read about MPPT there are always buck, boost, or buck-boost controlled by pwm. I read but still do not understant principle of load matching. Lets say we have irradiance, when solar panel output is Voc=6V , Impp=0.2A (Lets say 1W at mpp) . And i connected straight to the battery with 3.3V voltage, what i got now ? 3.3V ,0.2Amps and 0.66W power ? How to increase extracted power with dc-dc ? Please explain in simple words..

 

[chuckey]

Power from solar cell = 1W, so if converter is 100% efficient, then 1W will go into your battery, so at 3.3V, current = .3A. If converter 90% efficient the current int battery will be .3 X .9 = 2.7 A
Frank

 

[D.A.(Tony)Stewart]

When i read about MPPT there are always buck, boost, or buck-boost controlled by pwm. I read but still do not understant principle of load matching. Lets say we have irradiance, when solar panel output is Voc=6V , Impp=0.2A (Lets say 1W at mpp) . And i connected straight to the battery with 3.3V voltage, what i got now ? 3.3V ,0.2Amps and 0.66W power ? How to increase extracted power with dc-dc ? Please explain in simple words..

PV Panel is a current source proportional to Solar power input. Power transferred is VI product to load thus MPT is a peak that spreads near 80% +/-5 of Voc (drops with input power)

Thus simple solution is choose battery that is 80% of Voc.

If not simple, optimal efficient solution is regulate voltage on either side of a inductor using PWM with low ESR Cap across PV (MPT V) while battery is like a large cap.

MPPT PWM regulates the voltage at the PV source while regulating the current and voltage at load. Energy is stored in the inductor between pulse switches. ( 1/2 LI^2) and must never saturate.

 

[Darius Baronas]

Power from solar cell = 1W, so if converter is 100% efficient, then 1W will go into your battery, so at 3.3V, current = .3A. If converter 90% efficient the current int battery will be .3 X .9 = 2.7 A
Frank

So if there are 1W (5V and 0.2A) current at mpp, and the battery is 3.3V , buck dc-dc regulator (100% efficiency) should work with 66% duty cycle, then voltage 5*0.66=3.3V , current 0.2/0.66=0.3A , and 1W goes to the battery ? And if there are no dc-dc, then we have 3.3*0.2=0.66W , right ?

 

[BradtheRad]

So if there are 1W (5V and 0.2A) current at mpp, and the battery is 3.3V , buck dc-dc regulator (100% efficiency) should work with 66% duty cycle, then voltage 5*0.66=3.3V , current 0.2/0.66=0.3A , and 1W goes to the battery ? And if there are no dc-dc, then we have 3.3*0.2=0.66W , right ?

You'll need another topology than a buck converter. It will not give you more current out than you have going in.

 

[D.A.(Tony)Stewart]

You'll need another topology than a buck converter. It will not give you more current out than you have going in.

actually Buck mode boosts DC average current from Cap discharge. So Power transfer is constant and current ratio is inverse to voltage ,

but multiply x by efficiency. (Typ 90%)

 

[BradtheRad]

actually Buck mode boosts DC average current from Cap discharge. So Power transfer is constant and current ratio is inverse to voltage ,

but multiply x by efficiency. (Typ 90%)

You are correct. I forgot that an LC input filter can be added at the supply.



By adding the input filter, continual 200mA is drawn from the PV panel. This is maximum utilization of the PV panel.

Current through the 3.3V battery is 250 mA avg. This is a net current gain as you state, even though there are some losses.

Hence a buck converter topology can be made to perform as Darius considered in post #12.

 

[Darius Baronas]

What is that app with simulation window? Looks like android app, interesting Want to do same simulation using simulink or multisim..

 

[BradtheRad]

What is that app with simulation window? Looks like android app, interesting Want to do same simulation using simulink or multisim..

This is Falstad's animated interactive simulator. Free to download and use at the link below. You need to have Java installed on your computer.

www.falstad.com/circuit