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Need help designing load testing methodology for MPPT controller
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cbc02009:
Hello all,

For our senior capstone project, my group is designing a solar tracker / mppt controller. We've got the mppt controller finished and working, and we're nearly done with the solar tracking part. The last part is designing a scientifically rigorous test to compare the solar tracker to a stationary panel and compare the differences in power the panel generates.

So how, in general does one test power transfer over a long period of time?

I have some ideas of how to do it, like seeing how long it takes to fully charge a 12ah lead acid battery, but I'm worried the results of that wouldn't be very consistent, or that the battery charging wouldn't take long enough to be representative of a full day's worth of sunlight.

I could also attach the output to a resistive load, and use another MCU to measure current through a shunt resistor, and voltage across the load, and take the average (or the integral) to find either average power or total energy delivered. I worry though, that with the resistive load, that the energy (and therefore effeciency) output of the MPPT will depend on the value of the resistor I select.

Let me see if I can explain better. I have a 50W solar panel with a maximum power point at V = 17.5V and I = 2.86A. Say that the MPPT algorithm runs its calculations, and decides that 50% duty cycle will cause the solar panel to reach that number. The output voltage will be 17.5V * 50% = 8.75V. If we assume 80% efficiency, the output current that creates the maximum power will be (50W * 80%)/8.75V = 4.5A, which gives us an optimal load resistance of 8.75/4.5 = 1.99 Ohms. Cool that's what we use.

But say the sun goes partially behind a cloud (or something), and the MPPT redoes it's calculation, and decides it now needs 70% duty cycle to make the panel operate at it's MPPT. Our new output voltage is 17.5V*70% =14V. At 14V, the 1.99 ohm resistor from our last calculation would want to draw over 7A through it, but the maximum current we could get at the output (again assuming 80% efficiency) is (50W*80%)/14 = 2.85A. Since the system is obviously current controlled at that point, the actual voltage across the load would be 1.99*2.85A = 5.68V which gives us an output power of 5.68V*2.85A = 16.81 W out of a 50W panel just because of the value of the resistor I chose.


My last thought was to use something like a lab bench power supply as either a constant current or constant voltage load (which I don't actually know if that's a thing that's safe to do. Can those supplies sink current, or only source it? The word power "supply" kind of hints at no.) and then again use a micro to measure current and voltage. I found this https://smile.amazon.com/MakerHawk-Adjustable-Electronic-Intelligent-Resistance/dp/B07F3NHHST/ on amazon, maybe something like that? I could probably design one and build it for cheaper.

If I could somehow design and build a constant current or voltage source, which would be better? I know that solar panels tend to operate as current sources, but the output of the MPPT could be either constant current or constant voltage.

Or am I just thinking myself in circles and confusing things?

Any advice would be greatly appreciated.
Janne:
Why not just attach a dump load to the battery, for example just divert power to dummy resistors when the battery reaches rated charging voltage. Then get a pair of DC energy meters, one for the tracking panel installment and one for the stationary panel one.
fourtytwo42:
I think it would be helpful if you posted a schematic of this "mppt" controllers power circuit and told us what the load was designed to be. From your description this sounds like a simple pulse width modulator that would in reality not be able to provide an mppt function for any stable load such as a fixed voltage variable current or constant resistance. So your problem in testing appears to be you have designed an "mppt" controller with no concept of what the load actually is.
cbc02009:

--- Quote from: fourtytwo42 on October 22, 2018, 06:53:20 pm ---I think it would be helpful if you posted a schematic of this "mppt" controllers power circuit and told us what the load was designed to be. From your description this sounds like a simple pulse width modulator that would in reality not be able to provide an mppt function for any stable load such as a fixed voltage variable current or constant resistance. So your problem in testing appears to be you have designed an "mppt" controller with no concept of what the load actually is.

--- End quote ---

It is a full MPPT tracker running the incremental conductance algorithm on an atmel SAML21. The mppt algorithm varies a reference voltage that is fed into a PI controller that adjusts the PWM input of a synchronous buck converter until the desired voltage is achieved. I've attached schematic pictures to this post.

The load is meant to be 12V sealed lead acid batteries, but as I stated, I'm worried about reproducible results, so I was hoping for some ideas for a slightly more rigorous test method.

So no, the MPPT wasn't designed without a load in mind, but yes, I'm not sure the load is what I would like to use for scientifically rigorous testing for the capstone report we will have to write up.
cbc02009:

--- Quote from: Janne on October 22, 2018, 06:32:10 pm ---Why not just attach a dump load to the battery, for example just divert power to dummy resistors when the battery reaches rated charging voltage. Then get a pair of DC energy meters, one for the tracking panel installment and one for the stationary panel one.

--- End quote ---

That's an interesting idea. Thank you.
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