Guesswork and theory

A project log for MPPT for low voltage solar harvesting

Microcontrollers + caps = MPPT in the time domain?

Simon MerrettSimon Merrett 09/17/2019 at 19:070 Comments

@Dan Maloney is wondering where this is going, so that now makes at least two of us. Time to put a bit more thinking into the concept. I admit I am getting a little confused about energy, charge and conservation of current between the PV buffer caps and the reservoir supercaps. In the following mental ramble, please ignore the ability of the PV panel to continue supplying current while the buffer capacitor is being discharged into the reservoir - I don't think it matters because our MPP is calculated only when charging the buffer and not the reservoir.

So, based on the calculator at this website we see the energy stored in a 100uF capacitor at 5V is 1.25 mJ (I think - you can check). If we allow that to drop to 4V, the remaining energy is 0.8 mJ. So we have transferred out 0.45 mJ of energy. The apparent charge drop is 0.1 mC.

If we then let the PV panel recover the voltage on the buffer cap to 5V and it takes 100 ms, we can assume the PV panel is capable of outputting 0.45 mJ / 0.1 s = 4.5 mW under those lighting conditions, between 4V and 5V and with a capacitor attached (which will admittedly affect the observable PV characteristics).

If we then try the same discharge between 4V and 3V (0.25 mJ / ) and then let the buffer cap recover to 4V, let's say it does it in 50 ms. The apparent power in this voltage range is 0.25 mJ / 0.05 s = 5 mW. A slight improvement on the previous voltage range. 

Am I missing something, or does this make sense, at least on the PV panel and buffer cap side? I'm ignoring the varying voltage of the reservoir cap side, which would eventually not accept charge from lower voltages, even if they were the MPP.

Moving on to implementation notes, I'm thinking of using a microcontroller that either sleeps between switching or perhaps even just runs at very low speeds and currents. Perhaps something along the lines of #ATtiny 0 Series programming on the cheap. @Sander van de Bor 's PWM LEDs in #ATtiny Super Capacitor Spider are a definite source of inspiration too.