Passive geothermal greenhouses can take advantage of physical principals like the thermal flywheel to transfer naturally occuring heat from underground via air that circulates through a length of underground pipe and exits into the greenhouse. Carefully located intake and exhaust portals encourage the air to circulate naturally without fans or other electric appliances.
Completely passive greenhouses are also likely to take advantage of passive solar heating and the valuable contribution of thermal batteries. These dense masses naturally store heat from the sun, later releasing it as the temperature cools overnight.
