The following principles briefly explain the basics of understanding how a solar greenhouse operates:
A. The sun shines through the clear areas in short waves.
B These waves strike objects in the greenhouse and are reradiated as long waves, The long waves do not readily return through the glazing. This is known as the greenhouse effect. The greenhouse effect is similar to hot air trapped in a car on a sunny day with the windows closed. The inside air becomes warmer than the outside air (Figure 5).
C. Massive (heavy) objects in the greenhouse such as masonry walls, rocks, water drums, concrete, etc., absorb heat during the day and return heat to the structure at night. Pound for pound, the most efficient heat storer you can get is enclosed water. It is necessary that the greenhouse have considerable mass in order to perform properly (about 2 gallons of water or 80 pounds of concrete per square foot of glazing). If this is done, the greenhouse will maintain temperatures as high as 30°F above outdoor lows in winter.
D. The warm air (80-90°F) from the greenhouse goes directly into the adjoining structure (Figure 6). This works best if there are high and low openings. The vents establish a natural air circulation system that benefits the home and the greenhouse. At night the openings can either be left open or closed, at the occupant’s option, If open, the greenhouse will draw on some home heat and will keep higher temperatures.
E. The partially shaded and insulated greenhouse roof will keep it warmer in winter and cooler in summer. Notice that the south face of the unit is tilted 60° according to the latitude on which the greenhouse is located. The tilt maximizes winter sun and reflects a large percentage of the summer sun off the front of the greenhouse. Thus, overheating is less of a problem (Figure 7).
F. How well the greenhouse keeps warm is largely determined by how well it is constructed and sealed. All cracks and joints in the greenhouse must be insulated and caulked to prevent “infiltration’ heat losses. The greatest heat lose area for the greenhouse is through the clear wall portions. A moveable insulator to cover these areas would greatly increase the winter performance of the greenhouse.
