Hello,
I'm planning a small project to measure with Arduino the maximum power that a small solar panel can produce in different positions and moving it around, and also log the values along several hours.
My knowledge of electronics is very basic and I have several doubts, but I have been investigating and I think that I could base it in a current/voltage sensor like the INA226 or INA219, a potentiometer as a variable load device and a small solar panel of about 0.5W.
I'm not sure if this would work.
I have started with a design mainly based in the next pages:
And this is what I plan to do as a first approach:
I understand that the potentiometer would consume the load and dissipate it in heat. Will it consume up to the maximum that the panel can produce all the time in the optimal voltage position?
I think that I would need to adjust the potentiometer to test different voltages and find the optimal value for the panel. What I don't know if it is dependent of the panel or depend on other things also.
It would be nice to use a potentiometer or a kind of variable resistor that I could manage from Arduino to be able to find the optimal point athomatically. I don't know if that exists and is easy to integrate.
For simplicity I have skipped for the moment the part for the display or logging of the values.
Any hint, correction or dead warning would be appreciated!
Ok, thank you, this is what I need to know. It would be also a variable resistor, right?
But how many watts should it have? If the solar panel is small, about 0,5W, something like this would be sufficient?
You are right, I think that a rheostat is better. Also because I need low resistance, about 10Ω.
I've been doing some maths and checking the specs of some solar panels.
The I/V in the panels follows this curve:
For a small cell the max current can be about 300mA and the voltage goes up to 3V, but the MP (maximum power) voltage is around 1.5V that produces 0.65W.
Doing some calculations a rheostat of 10Ω or 20Ω would be sufficient.
What happens if I put the rheostat to zero? Would it be better to add resistor of 1Ω in serie to avoid to short circuit the cell?
Short circuiting a solar cell will not damage it, even in full sun at noon, so don't worry about that.
The maximum power point current (jagged line below) is typically 10-20% lower than the short circuit current, which you can measure directly just using the current setting of your multimeter.
Finally I've got some of the parts and had some time to do some tests. It works but the results don't look like the cell panes curves posted above, so I suppose I'm doing something wrong.
I'm using the INA226 power sensor (by the way, I don't find the parts file for Fritzing anywhere) and a small solar cell of 1.5V and 650mW (8x6 cm).
I still don't have a variable resistor so I've been testing with four R combinations: 4.5Ω, 5Ω, 6.5Ω, 11.2Ω.
For each resistor value I've been moving the solar panel under the sun with different orientations. It was about 10am, sunny with no clouds.
These are the results.
Volts / current for each resistor value:
The power in mW is what the sensor reports, not calculated out the the V/I values afterwards.
The voltage is the 'bus voltage', I understand that it's the load voltage. I have connected the VBUS pin to IN- in the sensor.
I guess that it looks different than the V/I curves of the previous posts because those charts are based in the same solar power but changing the load, right? Then, what I should do is to put the solar cell static under the sun, move the variable resistor and record the values.
Each of the individually colored curves in post #6 represent the solar cell output (A versus V) for one value of sunlight intensity, for a bunch of different settings of a variable load resistance (ranging from zero Ohms to infinity).
So it is difficult to compare the curves you posted with those on that chart.
Finally I got a potentiometer of 50Ω. I added 2 resistors of 2W to reduce the load, one of 1Ω in series and another of 25Ω in parallel, so I can move from about 1Ω to 25Ω.
Anyway I think that it gets hot quickly under strong sun and it's not very precise. But I just wanted to do a few small tests to get an idea of how it looks and what can I expect from this panel.
So far I did only a few tests.
Moving the solar panel in different positions and angles:
And finally I let it running about 3 hours in the afternoon, from about 14:00h to 17:00h. It was mainly sunny but with some guerrilla clouds covering the sun every few minutes.
In the first graph the dots are the mW produced at that moment in time,one sample per minute. And the line is the smoothed average production along the time. The other graphs are the same for mA and V.
I mean the potentiometer, because in the specs it says max power 0.25W. And under strong sun the solar panel delivers almost 0.5W. That's why I added some 2W resistors.
Actually it should deliver 0.65W, maybe in august at 12:00h in the middle of the Sahara
Yes, you are right. But it's what I have at the moment.
I do each test quickly in a few seconds and it works. When I test along the day I remove the potentiometer and put 3 resistors of 10Ω and 2W in parallel to get a fix load. Then it works fine and its safe.
