I've saved thousands of dollars by building quality homemade greenhouses. In order to retain that savings, I find simple and inexpensive ways to heat them. That means I count on the sun to meet nearly all of my needs for heat.
Passive solar energy is one of the most cost-effective ways of heating a greenhouse and retaining heat during times when the sun isn't shining. Of all the solar energy techniques, passive solar is probably the oldest, least costly and most reliable.
The term passive means that there are no active component in the system of energy collection, storage and release. This means no pumps, fans, heaters or louvers that require electricity or fuel. By design, the system works all by itself without any help from you - except initial design and construction.
Before we look at passive methods of using solar energy, let's discuss four key principles of thermal energy (heat) that will help to explain how these solar heating systems work. Here they are in a nutshell.
The first design uses six 55-gallon black steel drums to hold water. I fill each drum to near the top, so that provides about 300 gallons of water to act as a heat absorber for the greenhouse during the day. At night, the drums release their thermal energy back into the greenhouse slowly.
The drums are located on the north wall where they get best light from the low angle winter sun. The backs of the drums are painted white to reduce the amount of heat they give off to the cold north wall. The black sides face the south where they release heat to the plants I am growing.
Water is one the most effective materials for thermal mass. The most challenging aspect of water is keeping it contained, and keeping it from freezing. No common item will hold heat quite like water.
The second design is 55-gallon steel drums cut in half widthwise so each makes two 1.5 foot high miniature raised beds. The half-drums are arranged in the greenhouse for growing taller plants like peppers and tomatoes, and filled with soil. Since they are round, there is always air space between the drums that help them warm up quickly as they are surrounded on all sides by warm air during the day.
The steel drums are burned clean inside and out, and left to oxidize as they naturally will. This provides a dark brown surface for thermal energy absorption and emission. The challenge to using half-drums is cutting them in half. I use a plasma torch, and this makes the job a snap. A regular torch will work, or a circular saw with a metal cutting blade would also work well.
Caution: the drums must be free of flammable or explosive materials before you cut them. I burn them out in a deep pit for a couple of hours to make certain the flammable material is gone before I cut into them.
The drums are an excellent way to retain heat in the greenhouse because they are filled with moist soil. Next to water, I can't think of a better material to use as thermal mass than moist soil. The rust colored drums emit energy readily, but the moist soil doesn't give it up too quickly.
Passive solar energy is one of the most cost-effective ways of heating a greenhouse and retaining heat during times when the sun isn't shining. Of all the solar energy techniques, passive solar is probably the oldest, least costly and most reliable.
The term passive means that there are no active component in the system of energy collection, storage and release. This means no pumps, fans, heaters or louvers that require electricity or fuel. By design, the system works all by itself without any help from you - except initial design and construction.
Before we look at passive methods of using solar energy, let's discuss four key principles of thermal energy (heat) that will help to explain how these solar heating systems work. Here they are in a nutshell.
- Energy is always trying to move to a lower state; hot things cool down by giving off their thermal energy to cooler things.
- Dark colored objects absorb heat readily; light colored objects absorb heat much more slowly.
- Dark colored objects emit (give off) energy readily; light colored objects emit heat much more slowly.
- We need good thermal mass to store and release energy. The material must have high density (heavy for its size) and low conductivity (doesn't warm up or cool down quickly). Water, concrete, brick, rock and moist soil are ideal to use as thermal mass.
The first design uses six 55-gallon black steel drums to hold water. I fill each drum to near the top, so that provides about 300 gallons of water to act as a heat absorber for the greenhouse during the day. At night, the drums release their thermal energy back into the greenhouse slowly.
The drums are located on the north wall where they get best light from the low angle winter sun. The backs of the drums are painted white to reduce the amount of heat they give off to the cold north wall. The black sides face the south where they release heat to the plants I am growing.
Water is one the most effective materials for thermal mass. The most challenging aspect of water is keeping it contained, and keeping it from freezing. No common item will hold heat quite like water.
The second design is 55-gallon steel drums cut in half widthwise so each makes two 1.5 foot high miniature raised beds. The half-drums are arranged in the greenhouse for growing taller plants like peppers and tomatoes, and filled with soil. Since they are round, there is always air space between the drums that help them warm up quickly as they are surrounded on all sides by warm air during the day.
The steel drums are burned clean inside and out, and left to oxidize as they naturally will. This provides a dark brown surface for thermal energy absorption and emission. The challenge to using half-drums is cutting them in half. I use a plasma torch, and this makes the job a snap. A regular torch will work, or a circular saw with a metal cutting blade would also work well.
Caution: the drums must be free of flammable or explosive materials before you cut them. I burn them out in a deep pit for a couple of hours to make certain the flammable material is gone before I cut into them.
The drums are an excellent way to retain heat in the greenhouse because they are filled with moist soil. Next to water, I can't think of a better material to use as thermal mass than moist soil. The rust colored drums emit energy readily, but the moist soil doesn't give it up too quickly.
Clair Schwan is an experienced gardener with three quality homemade greenhouses of his own design and construction. See detailed how-to discussions, vegetable gardening tips and photos of his greenhouses and vegetable gardening success at http://www.frugal-living-freedom.com
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