A fan, at the ridge of a greenhouse, ducting into the ground, or under a water or dirt container, will store the heat, and eliminate the need for a window( which is more heat loss, even when not in use, and is expensive). Perhaps lining the duct with insulation could get more of the heat into the heat sink, and less of it wasted in the ground.
I have some numbers on the needs of any vent system and for my system specifically.
For a passive ventilation system, the vents need to be 20% of the size of the square footage of the growing space. so a 200 sq ft space would need 40 sq ft of ventilation (eg. wide open windows or doors).
Though not full-system temp management, automatic vent openers are a good option for venting tunnels and greenhouses in no-elec situations. Search around online - most seem to fall within the $50-100 range. We've had good success with them in helping prevent midday high temp extremes.