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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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