What's wrong with running them at less than 7V? I've ran 24VDC fans on as little as 6V. (But not for very long as yet.) I've also ran AC fans (little square ones, like they used in computers in the olden days) on a small fraction of the nominal voltage, to get a bit of air with no noise. I've done that for YEARS on some fans and none seem to have been damaged. Sometimes I used a transformer to lower the (AC) voltage, sometimes a capacitor in series (around 1 uF, depending on fan size, must be a high-voltage non-electrolytic type). For DC fans: a 24VDC model directly on a 12VDC system, a 12VDC fan on a small PV panel designed for charging a 6V battery, or a 12V fan off a 12V battery via a resistor (around 30-100 ohms).