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|Greenhouse Solar Heat Absorption|
The two most critical factors affecting the amount of solar heat a greenhouse is able to absorb are:
* The position or location of the greenhouse in relation to the sun
* The type of glazing material used
Since the sun's energy is strongest on the southern side of a building, glazing for solar greenhouses should ideally face true south. However, if trees, mountains, or other buildings block the path of the sun when the greenhouse is in a true south orientation, an orientation within 15° to 20° of true south will provide about 90% of the solar capture of a true south orientation. The latitude of your location and the location of potential obstructions may also require that you adjust the orientation of your greenhouse slightly from true south to obtain optimal solar energy gain.
Slope of Glazing Material
In addition to north-south orientation, greenhouse glazing should be properly sloped to absorb the greatest amount of the sun's heat. A good rule of thumb is to add 10° or 15° to the site latitude to get the proper angle. For example, if you are in northern California or central Illinois at latitude 40° north, the glazing should be sloped at a 50° to 55° angle (40° + 10° or 15°).
Glazing materials used in solar greenhouses should allow the greatest amount of solar energy to enter into the greenhouse while minimizing energy loss. In addition, good plant growth requires that glazing materials allow a natural spectrum of photosynthetic active radiation (PAR) to enter. Rough-surface glass, double-layer rigid plastic, and fiberglass diffuse light, while clear glass transmits direct light. Although plants grow well with both direct and diffuse light, direct light through glazing subdivided by structural supports causes more shadows and uneven plant growth. Diffuse light passing through glazing evens out the shadows caused by structural supports, resulting in more even plant growth.
Many new greenhouse glazing materials have emerged in recent decades. Plastics now are the dominant type of glazing used in greenhouses, with the weather ability of these materials being enhanced by ultraviolet radiation degradation inhibitors, infrared radiation (IR) absorbency, anti-condensation drip surfaces, and unique radiation transmission properties.
The method used for mounting the glazing material affects the amount of heat loss. For example, cracks or holes caused by the mounting will allow heat to escape, while differences in the width of the air space between the two glazes will affect heat retention. Installation and framing for some glazing materials, such as acrylics, need to account for their expansion and contraction with hot and cold weather. As a general rule, a solar greenhouse should have approximately 0.75 to 1.5 square feet of glazing for each square foot of floor space.
This information is courtesy of ATTRA - National Sustainable Agriculture Information Service