All greenhouses are built to absorb solar energy. The glazed plastic or glass allows sunlight to pass through, heating the air inside the space and allowing for longer growing seasons. The problem with that simple design is that it works only in the daytime, when sunlight is hitting the glass or plastic. To keep your crops warm overnight or in cold weather, you must find a way to store the heat energy and use it later.
A passive heat sink solar greenhouse system is a natural heating process that has been utilized for centuries. Passive means that there are no active parts of the solar heating system, and if built correctly, this system should work on its own forever. A heat sink is a storage space that one fills with heat-absorbent material such as pumice stone, concrete blocks, water or fire bricks. There are also materials called phase change materials designed to store and release heat energy the same way these less expensive materials do. The absorbent materials are placed within or used to build a main wall of the greenhouse that gets the most sunlight. As the sunlight hits the wall, it warms the absorbent material within. At night, when the greenhouse becomes cool, the material will still give off residual heat, keeping the greenhouse warm. This system works both ways, also keeping the greenhouse cool in times of extreme summer heat.
An active system is more effective, but it uses electricity and therefore can become too expensive for use in a small home greenhouse. These systems are more appropriate in large greenhouses, where a significant number of crops are being raised. Similar to the passive system, active heat sinks also utilize absorbent materials such as water or brick. A large hole is dug out beneath the greenhouse and filled with the absorbent material. A fan, possibly powered with a rooftop solar panel, is used to pump air through the heat sink, warming the material. The heat is stored in the chamber for nighttime and cold weather usage. Because this system allows the user to store the heat in chambers rather than against the wall, much more heat can be stored for later use. This type of system can also be accomplished with water in the place of air. In this type of system, water is pumped through outdoor pipes and allowed to collect solar heat energy. Then it is pumped through the chamber, giving heat to the absorbent material. At night, the water is again pumped through the heated chamber to gather heat and then through pipes in the greenhouse, distributing the heat energy.
Solar space heaters can be used to bump the temperature in a greenhouse significantly. A simple efficient heater can be made using aluminum cans, Plexiglas and a wooden frame. These heaters are only useful during times of direct sunlight, but they are especially nice during cold winter days with a lot of sun. The cans are painted black to absorb the maximum amount of heat. They are glued into the wooden frame in a fashion that exposes as much of them as possible to direct sunlight. Plexiglas or another material that allows light and heat to pass but not air is glued into the frame to prevent air from escaping. The air enters the solar heater at the bottom of the frame; as it passes through and around the cans, it is heated by the solar energy being collected there like a convection oven. The now warm air passes out through the top of the frame either directly into the greenhouse or through a short tube that directs it into the green house. If movement of air is a problem, a rooftop solar panel can be used to run a small fan that will help to move the air through the heater or throughout the greenhouse.
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