Thermal energy, or heat, can be stored in a greenhouse by adding "thermal batteries," like barrels of water or a heat sink. Sunlight warms and "charges" these batteries during the day. When the sun sets, and ambient temperatures drop, excess heat is released from the batteries into the air.
Red brick is another popular choice in geothermal greenhouses, the deep color and density of the brick absorbs lots of heat, and serves as an excellent thermal battery when the nights get cool.
