PC fan, powering directly by small 12V solar panel - adding more panels to help working with less sun

Question

So I have a 12 V solar panel and a fan (size of a PC fan, assume 12 V and 0.15 A) that if connected directly work great in a full sun. Its located in a shady environment and if I kick-start it with my hand - it will keep working until dark, but has trouble starting on its own when there is not enough light. I have another panel (approximately 65% less in size) also 12 V nominal and would like to add a second one.

I assume, motor is not starting because there is not enough current generated by panel while in shade. At the same time, not much of a voltage too obviously.

What I need advice on is: should I connect these panels in series or parallel?

1. If I connect parallel, voltage in shady environment would stay the same, but current may increase and that would be enough to start for the motor? Hopefully, in full light (direct sun), current won't exceed max for motor in the fan.

2. If I connect in series, (current same(?), voltage increase) in shady times, will have better voltage, more close to 12-16 V. Would it help the motor to start? In direct sun, voltage would go up to 24 V, so probably need voltage regulator to leave it around 12 V.

This is to help with vent underneath a deck and just for fun. Appreciate an advice or if I am missing something.

Open circuit voltage - on full Sun is 12-14V and in shady 4-6V.

Answer 1

The fan your describing has a minimum voltage required to get the fan going. You can figure this out easily with an adjustable power supply: Start at zero volts, and sweep the voltage up until the fan kicks on. It may be 9 volts.

After you get the fan going, turn the voltage down. What you will notice, the fan will continue to spin even at a much reduced voltage. This point is called the under-voltage lockout (UVLO) point. This is similar to your solar panel getting shaded.

Since you say that you can get the fan spinning by hand, and it stays spinning until dusk, this implies you already have enough current with one panel. So putting 2 panels in parallel won't help. But, if you put them in series, you have an over voltage problem. So what I would do, even if you can't make the measurements I suggested, get a wide input buck convertor and put the panels in series. Then, hook the fan up to the output of the convertor. This will do 2 things: protect the fan from over voltage in bright conditions. And you get a little more current capacity in darker conditions because you are starting with 2 panels in series. Your setup should look something like this:

^{simulate this circuit – Schematic created using CircuitLab}

The rectifiers are probably optional but recommended as good practice; use them if you got them but don't sweat it if you don't (helps if one panel is shaded and the other is lit. They prevent "starving" the complementary panel of current).

Answer 2

Side by side. In series will be massively inefficient, and 40V is too much for a 12V fan.

Side by side, 12V panels won't go above about 22V worst case, and since we already know they are underpowered, considerably less than that.

Anyway, you should start by measuring the Open Circuit voltage of the panels, and the short circuit current. How big are your panels? It's possible, of course, that you have a cracked panel, and that's artificially lowering the available current.

Answer 3

It is worth measuring the open circuit (i.e. no load) voltage of the panel where you want it to go. If you have a variable power supply you can then see if that's enough to start the fan. If it is, there are potential solution. The fan draws more current when stopped, causing the panel voltage to sag.

If the open circuit voltage is high enough to start the fan, charging a capacitor directly off the panel, and switching on the fan only when the capacitor is charged. I have an idea that would be adequate for a hobby project, but for a product you'd need to protect it from undervoltage as well