Electrical – Solar Motor Circuit Design

First of all, are those numbers for the pump all from the same model? Because the range of the numbers is rather large: you say that it starts with 2 watts, nominally runs at 15 watts(?), might sink 4.5 amps at 12 volts, which gives 54 watts. And even larger numbers are calculated in your question, which probably aren't relevant.

Looks to me that even the 120 watt panel is overkill, but it can be connected directly to that pump if the pump can inded handle 24 volts, which is more than the open circuit voltage, which is the maximum voltage the panel can ever generate.

The thing to realize here is that it's the amps that turn the motor, not the volts. Volts are something that have the capability of giving amps to a load. And pushing amps through a load generates volts across the load. In your case, there's a solar panel, which is more like a constant current source, unlike, say, a battery, which is more like a constant voltage source.

When you connect the solar to the motor and there is sun, the panel pushes amperes to the load. The 120W panel looks like it might push something like 6 amps to the motor. That would perhaps generate something like 16 volts over the motor, although this number is only based on a rough feeling. That would mean that some 96 watts are dissipated somewhere in the pump - and that seems quite a lot for a pump that nominally takes 15 watts. I don't know; maybe the pump is electronically regulated somehow so that it never draws so many amps...?

All in all, I have a bit uneasy feeling about this. It would help if you could provide some more info about the pump. A link to a datasheet or a product page, maybe?

Solar panels act a little differently than you might think. For example, all "12 volt" solar panels actually output 18 volts, as high as 20 volts Voc in bright sunlight. You have to remember this is Voc and there really is no situation in which Voc is output when the panel is actually attached to a load and providing current.

I have read a couple of theories on why panels are made this way. One is that under lower light conditions you will still get the minimum voltage required for your load. This is the most common answer that I have read, but it doesn't make sense to me because under low light conditions amperage drops considerably making the panel not very useful.

The other more logical answer has to do with the power curve of the solar panel. If you graphed the current versus voltage curve of the panel you would find that for most cells, you get the maximum current (given the same sunlight) at about 75% of the max voltage of the panel or approx 14/18 volts.(when charging 12 volt lead acid batteries will rise to 14 volts and even a little higher)

So how does this apply to you? Well solar panels are interesting creatures and as soon as you hook it up to your 24 volt fan in bright sunlight, it will drop to 24 volt output and problem solved. No need for extra electronics. I am not sure of the correct terminology, but I believe that the panel's resistance is incredibly low, so that anything you hook it up to has a higher resistance. As a result the voltage of the panel drops to the voltage of the load.

For example my 12 volt panel that reads 20.5 volts in the bright sunlight drops immediately to 10.5 volts, to when I connect it to my depleted lead acid battery (which is 10.5 volts when completly depleted). I would think that the same should happen to your fans.

However, sunlight comes and go, birds, clouds, etc cause the panel's output to fluctuate and thus your fans would likely stop and start repeatedly. Have you considered hooking the panel to a battery first and the fan to a battery? It could be a relatively low capacity battery that could feed the fans a constant voltage and amperage and the panel could feed the battery. You would want a cheap charge controller for the battery. Two 12 volt lead acid batteries in series would work well. I do that with my wife's water fountain. It would shut down for a few seconds when a cloud flew over or the dog ran in front of the solar panel. I got a free lead acid battery that was essentially dead (measured capacity less than an alkaline C battery!). The panel charges the battery while the battery feeds the fountain. Works well even though the battery is almost dead, the battery only really providing full power for a few seconds at a time.

Hope this helps.