Best power arrangement for solar air heater?

A cloud could easily knock the available power down to 10% to 25% of the panel rating, below the fan
requirement. Batteries will only delay the failure, but make it much more expensive.

HOWEVER, part of the problem might be your fan using current in much larger pulses than the
stated average, putting the system out of action when the panel can not supply that level. I
would try putting a capacitor across the fan so that the panel does not need to supply the peaks,
might allow the fan to run at somewhat clouded level, at reduced speed. I would try about
2000 ufd at 20V, observing polarity as a test. A scope will tell the story.

That may still be inadequate, the final fix is a 12V 50W or 100W panel that will produce enough
power even under some clouding. If voltage rises too high in best sun, you might need a regulator.

Bruce Roe

I'd suggest a 50w panel, if you can borrow one to try. Cloudy/overcast don't expect more than 10% of nameplate output. But in cloudy conditions, you won't have much hot air to move either.

I'd also recommend getting a 14V 80W zener diod to wire in with the fan to prevent the panel from zapping it with 21.6 v on a sunny day like a 1N3305, 2 in series, to get the needed wattage, and a heat sink to dump the excess power into

I realize this is an old thread, but I had no idea you could wire two zener diodes in series.

It is something like 2 batteries in series. Bruce Roe

Zeners in series. If you need 14V @ 50W, you can us two 7V @ 25W and spread the heat easier. The drawback is you have a "softer" knee as the diodes turn on and start conducting,

If you try to put in parallel, one will "hog" and burn itself out, so you have to use "swamping resistors" (ballast resistors) so the resistor helps raise the resistance of the conducting path and then another diode will kick in, and another and another......