With regard to fans in the summer, I have purchased a few small, batter-operated camping.
I hang those in a couple of the coops that I think need more ventilation, but I don't use fans in most of my coops. Earlier this year, the temps were in the high 90s with heat indices in triple digits, and everyone was okay.
I open the coop windows as much as possible, and they are all under shade trees. Plus, I keep water dishes filled and drop ice cubes into the water, as well as giving "cooling" treats like chilled watermelon.