A greenhouse can be passively and sustainably heated through harnessing solar or geothermal energy, and directing it into the greenhouse. Solar energy is naturally taken in by the greenhouse during the day time. Thermal energy is trapped within the greenhouse's glass walls, and heats the ambient internal temperature much faster than it would outdoors. This power can be preserved and saved for later through the use of heat sinks, or thermal batteries. Some kinds of mass are particularly happy to absorb solar/thermal energy. This includes dense, dark materials, like water barrels painted black. This is a popular option for thermal batteries since they are so cheap and easy to make on your own.
The Earth itself stores a massive amount of energy, much of that in the form of heat beneath the surface. Around 6 feet under the Earth's surface, the soil maintains a steady temperature of abourt 64 degrees, regardless of it being hotter or colder above. Greenhouses can use this radiant, geothermal heat to warm their interiors. Sometimes, this is done through direct exposure, like in the case of sunken greenhouses or those with an earthen wall. Other times, air is pumped out of the greenhouse and through a series of pipes underground. Here, the air matches the temperature of the ambient soil, and is then pushed back into the greenhouse. This geothermal heat pump is incredibly sustainable.
