[ecomodded]

You could have a car battery on the panel and put the fans on a timer.
I say timer so you can control the vent times to turn on after the dewy mornings and end shortly after dusk, to vent the warm day air that contains the higher moisture, once it cools.

I found from my last houses garage that the sunlight warmed the garage till the night air would cool and condense moisture in the warm garages air, leading to mildew/mold.
I put a vent in so it naturally convected and that solved the heat build up with moist air issue, perhaps you could make a side vent chimney to encourage ventilation.