Has anybody tried using a solar powered fan, or maybe a 12v fan, to help ventilate a polytunnel? I have a 14' x 28' polytunnel, mesh covered window at bottom end (with roll-down plastic sheet for a curtain). I've been wondering about buying a solar powered fan or a 12v fan to run off a battery, but ones I've seen online don't look as though they'd make much of a draught and the solar powered ones are quite expensive. (Mains electricity not really an option.) Has anyone used one? Do they actually shift much air?