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I am new to the field of power electronics so please bear with me.

From what I understand, photovoltaic cells are often connected to a DC/DC converter and a MPPT power system that allows for (1) maximum power to be extracted from the device given a particular G and T and (2) a relatively constant voltage can be outputted to the load/device of interest.

Let us say I attach a device which has a specific current-voltage graph. If we attach this device to the photovoltaic/DCDC/MPPT system, the voltage becomes fixed, but the current may not be at an operating point. There are three possible cases here:

  1. The current applied to the load \$I_o\$ is too high. How will this system respond? Are there other mechanisms, perhaps a feedback to the power electronics MPPT/DCDC that can modulate the current appropriately?
  2. Say the current is too low. How will the system respond?
  3. If it is just right, then I assume everything is ok and the system will work accordingly.

Any resources and help is greatly appreciated.

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    \$\begingroup\$ MPPT can only work effectively at full bore if the load can absorb all power produced, say uncharged batteries, or a stiff grid. If not, then it depends on the programming of the MPPT, which as it's software, is anything the programmers like. Read the manual for what your particular one does in those circumstances. \$\endgroup\$
    – Neil_UK
    Jul 1, 2022 at 14:51

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  1. If you try to withdraw too much current from the system, reality hits you hard in your face. The voltage at the load will drop, it can't stay at the set level. There's no energy to maintain the voltage.

  2. In this case, some of the current producing capacity of the solar cells will be unused. This causes the voltage at the solar cells to increase (you are building up charge faster than you're depleting it), which causes the unused current to recombine at the p-n junction due to increasing voltage increasing recombination. So the MPPT controller in this case fails to maintain the "optimal" maximum power point voltage, but that doesn't affect you in any manner: you still get the power you want.

  3. In this case, the voltage at the load and the voltage at the solar cells will both be at the set level.

Consider adding a small battery to your system. There can be cloudy weather. About 10-20 watt-hours of lead-acid is optimal for one watt of panel capacity. For lithium ion, somewhat less might be acceptable as lead-acid rapidly degrades if below 40% state of charge so you can only use 60% of the capacity you have, but with lithium-ion you can nearly fully deplete the batteries.

Actually with batteries you don't necessarily need MPPT, if the panel voltage is matched to the battery voltage. You can use PWM charge controller. PWM is about 80% efficient (assuming optimal panel to battery voltage matching) whereas MPPT is about 95% efficient. Today solar panels are so cheap that MPPT often doesn't make sense, you get more bang for your buck by installing more panels than a better controller. However, if your panel capacity is surface area limited, and you want to maximize power production, in this case MPPT is mandatory (and you should only use monocrystalline cell panels in this case).

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  • \$\begingroup\$ Thank you. These fluctuation dynamics where the voltage increases or decreases accordingly, are there any models that can be used? I am trying to get some ball park approximations for how much the voltage / current might change given the fluctations within a day. \$\endgroup\$ Jul 3, 2022 at 11:43
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"I want to run a load off solar, but I don't want to involve a battery" gets asked a lot. It is a grail, but not a very workable idea IMO.

You are dealing with hypothetical MPPT controllers here. The problem is, when you actually try to go onto the marketplace and buy them, you will find they are virtually all designed to feed into a battery pack. This might work with hypothetical loads... but you get into the real world, and you find loads behave badly. That is a certainty if an inverter is involved.

(though certain loads will work; off-gridders frequently hook their "Dump" terminals to a resistive water heater).

Therefore, the right way to design such a system is with a battery of any size, really... and then use ordinary battery state-of-charge monitoring to decide when to drive the load.

For instance if it needs to drive a sump pump, on a cloudy day the pump may simply sit there stalled for lack of sufficient power to start the motor (a huge problem with motor loads). That is bad and useless. Whereas if a battery is involved, the battery manager will turn on the sump pump when the battery reaches 90% SOC, and turn the pump off when the battery reaches 60% SOC. Obviously it doesn't take much of a battery for this to be useful, if it is running 2 minutes every 20 minutes, you only need 7 minutes of battery capacity to make that work.

And the battery is able to do what the solar can never do (with any sane sized solar system) - start the motor.

On the other hand something like a dehumidifier may not do much if only allowed to run for 2 minutes, so it may need a larger battery pack so it can have a 10 minute runtime when it runs. But still, in extremely marginal conditions where you would never have any hope of running it directly, you are able to run 10 minutes every 100 minutes with the battery.

So while I know it is tempting to say "the best part is no part", the fact is, with batteries, that's not true.

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I know limiting the load current to a specific value can optimize the solar panel power output. I can manually adjust the load via PWM. And also observe the power output on the solar panel side. Finding that best power producing point is trial and error, but can be done quickly (even if only manually adjusted). This is how MPPT controllers are designed (in part) except they use a microcontroller to adjust the load current.

In many applications, you just want the most power possible for any sun condition. The battery charger part isn't required for all situations. I think the solution is to design a system from scratch that tests the power produced by incrementally raising the load current, see what that does to the power (goes up or down) if up, continue increasing load current until power drops, then decrease load and test if power increased or not.

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