While the sun isn't warming your greenhouse, thermal batteries or a heat sink can be used to transfer thermal energy when the ambient temperature cools. This dispersal of thermal energy is known as the 'flywheel effect,' in which excess warmth is released into the cooler air as it comes into contact with the warmed batteries.
Heat pumps can push cool night air underneath the ground to be warmed by the ambient soil. A few feet underneath the surface of the Earth, the dirt and soil maintains a fairly regular temperature. Air warmed by the Earth is then pushed back into the greenhouse to heat the ambient temperature.
