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.
My first reaction is to put the two panels in series, then use a off the shelf buck regulator chip. If the two panels can't even put out 6 V or so in series (3 V per panel), then there is so little power available that it doesn't matter if the output is just shut off.
Best Answer
The percentages used below are "out of my head" based on experience with PV panels. Your panel may vary somewhat from my assumptions.
Depending on technology used the ratio of Vmax_power to Voc will vary but using 0.8 is a good first guess. So V_max_power ~= 8V x 0.8 = 6.4V.
Imax_power will be somewhat less than Isc - probably about 90% or better, so Imax_power ~= 650 mA x 90% = 585 MA -> say 600 mA.
So actual max power in bright light will be V x I = ABOUT 6.4V x 600 mA = 3840 mW.
Say about 3.8W.
This will be at about 6.4V so to run with best efficiency you want to convert it to 5V with a switching regulator. A switching regulator will be typically 80% to 90% efficient if you use a good one - and rather less with lower quality designs.
3.8W x 90% =~ 3.4W
3.8W x 80% =~ 3W
3.8W x 70% =~ 2.6W
If output is at 5V then Iout = Pout/Vout =
680 mA at 90% conversion efficiency
600 mA at 80%
520 mA at 70%
Building a buck converter with an MC34063 will produce an OK result but efficiency will be lower than cxan be achieved with modern IC.
An IC with synchronous rectification is required to maximise efficincy.
The Richtek RC8286 looks ideal
Datasheet here