Greenhouses are structures designed to absorb the sun's heat in the day for plant use. It not only gives your crops the light they need, but also the heat. This system is great in the warm season, but when you grow crops in a greenhouse to extend the growing season into the cold months, you need a way to keep the grow room warm at night. Although electric greenhouse heaters exist, you also have available an inexpensive method of collecting thermal mass for later use: filled water bottles without lids.
Collect the largest water bottles you can find. Gallon jugs and bigger containers are excellent. Your goal is to line the north-facing wall of your greenhouse with containers of water. In the day, the water absorbs the sun's heat. At night, it releases it. The more water is inside the greenhouse when it's sunny, the warmer the room will be at night.
Paint one side of the bottles black and the other white, going halfway around the jugs with each colour. This step is optional, but "Vegetable Gardening and Greenhouses," a website that provides information on vegetable gardening, explains that when the black side of the bottles faces south, heat absorption occurs more easily. Having the white side pointing north keeps the heat from escaping in that cold direction, where you won't have any plants growing.
Place the jugs in their locations in the greenhouse and fill them with water. If you painted them, have the black side face south and the white facing north. Refill them with water as the liquid evaporates.
Put a thermometer in your greenhouse and monitor the temperature in it for a few days (if you're up for being out and about in the middle of the night). Or look at your plants. If their vigour seems reduced, increase the amount of water sitting in the greenhouse. Some people use 55-gallon drums to collect the sun's energy. How much water to use depends on how large your grow room is and how intense the sun gets in the day. You'll learn from experience.