MPPT for multiple low voltage solar panels

Question

Hi, I am in crisis with an energy harvesting design. I'll try to explain this:

I have 6 solar panels (with about 20V and 1A at maximum power), which receive very different lighting from each other. For example, I can have 1 panel at maximum power, 2 panels at medium power and others in the dark.

I started a design with LT3652 MPPT, but I have a problem: how can I get power from all solar panels, even with different voltages?

If I put all panels in parallel, through 6 diodes, at the input of the MPPT, only the panel with the highest voltage will transfer its power to the MPPT. So, if I have other active panels with a lower voltage, they won't contribute to the system power.

If I use a different MPPT for each panel, I can get the same output voltage from each active panel (with a minimum input voltage threshold). But how can I use 6 different output, with the same voltage, summing the current, at the input of a battery charger (e.g. LT1513)? If I put a diode between each MPPT output and the battery charger input, I think only one regulator will "win" and power the charger. Correct?

Do you have some solution or suggestions?

I've been looking for similar solutions online for days, but without success.

Thanks in advance...

Simone

genebren · Answer

This is the problem that a lot of the inverter manufactures work with. A few companies (maybe more) have taken an approach that is similar to what DAB is referring to. A great approach is the inverter per panel designs, where each panel can individually reach a peak power point and the converted energy is then pushed on to the grid.

DAB · Answer

You have hit upon one of the really interesting aspects of solar panels. Most implementations use a simple strategy that tries to adjust the load to the power available using the MPPT approach. As you have found out, that approach only works if all of the panels are being illuminated the same way at the same time. By using several panels at different orientations, you will need to approach the problem with a solution for each panel. The easiest solution I can think of is to put a separate battery pack on each panel. Then you can take the more stable power from each battery pack to either go to a reconverted AC power or combine for DC power, letting the batteries be your overall power load normalizers. The panels themselves are just too specifically oriented to do this using just their voltage and current output. You are using them in a way they were not designed to support. That is why I recommend using batteries as your power equalizer. As long as each battery can be kept fully charged, the battery chemistry will regulate voltage and current for a given load. This approach will increase your cost a bit, but you get the flexibility in panel placement you want. DAB

MPPT for multiple low voltage solar panels

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