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Information
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.
I've numbers that say passive venting requires an area 20% of your square footage to be open during the day. So if I have 200 sq ft of growing space, I would have to have 40 sq ft of vents, which is a lot. I think these would be awesome for the air intake portion of the system. Now I just need to figure out the exhaust side of things.