Hi,
These are the type of fan I've been playing with, two 40mm fans with just two wires and an electronic commutator for just over a Pound. However, the (generic) specification is rather pessimistic, they actually consume about 100mA each from 12 volts. There is little difference between the start and stop voltages (about 4 volts), probably because their inertia and friction are small compared with the detenting produced by the magnetic field. They seem to run comfortably from 5 volts, consuming 50mA (250mW) each, but obviously the air flow is not great at this power.
Personally, I don't plan to just connect the fan(s) directly to a solar panel. My (partly tested) system will charge a capacitor of about 10,000uF from the PV cell until a threshold of typically 10 volts is reached and then "fire" the fan with a circuit like a Schmitt Trigger. With reasonable illumination the fan should then keep running, but the driver electronics will cut the supply before the motor stops, to ensure that it doesn't drain all the available power without turning.
But I must add that I don't have any great confidence in the viability of simple solar aspiration. There may be a need for aspiration when there is no sun (or even daylight), and IMHO the amount of energy available from (small) solar panels is over-rated. Also, my (un-related) attempts to transfer low grade heat using (mains-powered) fans (e.g. from a hot conservatory to an adjacent room) have been disappointing. It seems necessary to move a
large volume of air for effective heating/cooling when the temperature difference is relatively small.
Cheers, Alan.
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