Thermal energy, or heat, can be stored in a greenhouse by adding "thermal batteries," like barrels of water or a heat sink. These store energy as sunlight hits and warms the materials through the day. As ambient temperatures lower, like during the night, the stored energy is released in the form of heat, keeping the greenhouse warm. Barrels of water are cheap and can be deal-purposed within a greenhouse. Another type of thermal mass that's easy to work with is brick. Deep red bricks absorb far more heat than white cement or other white stones.
Adding additional forms of insulation, or points of contact with warmed mass, are also favorable ways to warm your greenhouse. A greenhouse cover can also be used at night, to assist in preventing heat escaping during the cooler hours.
