The solar greenhouse differs from conventionally designed and operated greenhouses in that it does not rely on outside sources of energy for winter heating and summer cooling. A conventional greenhouse is usually all glass and pays no attention to direction of the sun. The solar greenhouse tries to get as much solar energy as possible by using glazing on surfaces with southern exposure to permit the entry of heat and light. The northern walls are insulated to reduce heat loss at night. Vents promote natural circulation to help keep the interior cool. A storage mass, usually in the form of steel drum filled with water, helps to reduce the difference between day and night temperatures. The south side of the solar greenhouse is calculated to permit the winter sun, but exclude the summer sun.
The solar greenhouse relies on passive solar energy. The advantage of passive solar heat is that it can be built right into a freestanding or attached solar greenhouse. The passive design will use very little mechanical equipment, extra piping or special maintenance, as active systems often do.
In more general terms, any solar greenhouse must contain the following parts to be considered a complete passive solar heating system:
A. A collector, such as the double layer of greenhouse window glazing (glass or plastic).
B. An absorber, usually the darkened surfaces of the walls, floors, and water-filled containers inside the greenhouse,
C. A storage mass, normally the concrete, brick, and/or water that retains the heat after it has been absorbed.
D. A distribution system, which is the means of getting heat into and around the house—fans, natural circulation flows.
E. A control system (or heat regulation device), such as movable insulation used to prevent heat loss from the greenhouse at night. Roof overhangs that block the summer sun and thermostats that activate fans are also controls. Some controls may be operated by the occupant.