A. Site Location
The mayor consideration in designing solar greenhouses is the availability of direct sunlight. In order to learn about the availability of direct sunlight, it is necessary to understand how the sun moves across the sky. The sun rises in the east, follows an arc through the southern sky, and sets in the west. The sun travels a different path in the winter than in summer, thus solar radiation falls on the earth’s surfaces (and on your greenhouse) at different angles during each season (Figure 2). During the cold part of the year the Earth’s tilt causes the sun to appear low in the southern sky, rather than overhead as in summer. Since the sun rises and sets sky the southern sky, east and west walls can see very little sunlight in the dead of winter. The north wall loses out completely. But a double-glazed south-facing window gains more solar heat in eight hours than it loses over the entire 24-hour day.
An attached solar greenhouse must be constructed on the south aide of the existing building. A glass-exposed south wall area will have adequate daily exposure to the sun and a southern sky. However, shading obstruction to the south will affect your site. Nearby buildings, trees and hills can block the southern sun and limit a greenhouse’s efficiency, During the winter months, there must be few obstructions when maximum solar exposure is critical.
B. Latitude, Altitude, and Azimuth
The length of the days and the intensity of the sunlight is mainly determined by the latitude. Connecticut is approximately 40° north latitude, so corresponding figures have been chosen. Different angles are needed for other latitudes.
To avoid shade covering the solar greenhouse before costly contraction begins, it is important to know two particular solar angles: altitude and azimuth (Figure 3). The importance of knowing these two angles cannot be overestimated, to insure adequate site selection.
The altitude is the angle formed by the sun and the horizon. In Connecticut, the noon sun is at the low altitude of’ 26,1/2° on December 21, the shortest day of the year. On June 21, the longest day of the year, the sun la at a high 73, 1/2° altitude. During the Spring/ Fall equinox the altitude angle is 48,1/2°.
The azimuth is the angle made between the horizontal direction to the sun and a true north-south line. A 20° variance east or west is acceptable. It is important to remember that in Connecticut the magnetic north is 13° west of true north.
An effective solar greenhouse must be designed with the full knowledge of the sun’s path during all the seasons (Figure 4). Since the coldest weather occurs during January, the sun’s path on January 21 most directly determines the solar greenhouse design.
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-On January 21 in Connecticut
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Altitude
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Azimuth
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the sun rises at 7:00 a.m.
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0°
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S 65° E
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-At 10:00 a.m. the sun is at
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25°
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S 32° E
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-At 12:00 noon the sun is at
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30°
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0°
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-At 2:00 p.m. the sun is at
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25°
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S 32° W
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-At 5:00 p.m. the sun seta at
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0°
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S 65° W
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This table points out the symmetric nature of the sun’s path.
A solar greenhouse should be ideally placed due south, that is, facing azimuth angle 0°. In addition, the south glass or glazing should be at right angles to the incoming sunshine on the coldest day. It is the noon altitude angle on the coldest day that determines the most efficient angle for the south wall of the greenhouse. The altitude angle in Connecticut on January 21 at noon is 23,1/2°. In order to accept as much sunlight as possible, the angle of the south glass on the greenhouse should be 66,1/2°: 66,1/2°+23,1/2°=90°,
These angles are not critical. A 20° variation results in less than a fifteen percent difference in received solar energy. When clean snow covers the ground, the reflected radiation favors a vertical glazing.
