> What is Passive Solar? - Page 2

You can also combine all these features into one system, such as a trombe wall, see below.

The 3 types of passive solar system:

1. Direct Gain
The actual living space is a solar collector, heat absorber and distribution system. South (or North if Southern hemisphere) facing glass admits solar energy into the house where it strikes masonry floors and walls, which absorb and store the solar heat, which is radiated back out into the room at night. These thermal mass materials are typically dark in color to absorb as much heat as possible. The thermal mass also tempers the intensity of the heat during the day by absorbing energy. Water containers inside the living space can be used to store heat, but unlike masonry water requires carefully designed structural support, and thus it is more difficult to integrate into the design of the house. The direct gain system utilizes 60-75% of the sun’s energy striking the windows.

2. Indirect Gain
Thermal mass is located between the sun and the living space. The thermal mass absorbs the sunlight that strikes it and transfers it to the living space by conduction. The indirect gain system will utilize 30-45% of the sun’s energy striking the glass adjoining the thermal mass.

There are two types of indirect gain systems: thermal storage wall systems (Trombe Walls) and  roof pond systems

Trombe Walls
Trombe walls are the most common indirect gain approach. The thermal mass, a 6-18 inch thick masonry wall, is located immediately behind South (or North) facing glass of single or double layer, which is mounted about 1 inch or less in front of the wall’s surface. Solar heat is absorbed by the wall’s dark-colored outside surface and stored in the wall’s mass, where it radiates into the living space. Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening. When the indoor temperature falls below that of the wall’s surface, heat is radiated into the room.

Operable vents at the top and bottom of a thermal storage wall permit heat to convect between the wall and the glass into the living space. When the vents are closed at night, radiant heat from the wall heats the living space.

Important guidelines to remember:

    * The space between the thermal mass wall and the glass should be a minimum of 4 inches;
    * Vents used in a thermal mass wall must be closed at night;
    * Thermal wall thickness should be about 10-14 inches for brick, 12-18 for concrete, 8-12” for adobe or other earth material and at least 6 inches for water.

Roof Pond Systems
This system can provide both heating and cooling. 6-12 inches of water are contained on a flat roof, usually stored in large plastic or fiberglass containers covered by glazing. During the cooling season, an insulated cover is removed at night to expose the water to cool night air. The water absorbs heat from below during the day, and radiates it out at night. During the heating season, the insulated cover is removed during the day. The water absorbs heat from the sun, and radiates it in to the building below. In cold climates an attic pond beneath pitched glazing is more effective than a flat roof pond.

Roof ponds require somewhat elaborate drainage systems, movable insulation to cover and uncover the water at appropriate times, and a structural system to support up to 65lbs/sq ft dead load.


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