Passive Solar Designs

All-passive solar systems utilize south-facing glass or transparent plastic for solar collection. The low winter sun puts out 90 percent of its energy during the period from 9:00 a.m. to 3:00 p.m. Where other buildings or tall trees block access to the sun during this critical period, solar energy systems are not practical. The area of the glazing amounts to 30 to 50 percent of the floor area in cold climates, and 15 to 25 percent in temperate climates, depending on the average outdoor winter temperature and projected heat loss. Glazing materials must be resistant to degradation by the sun's ultraviolet (UV) rays. Double-glazing and insulation are used to minimize heat loss at night.

A second essential component of a passive solar design is the presence of thermal mass for heat collection, storage, and distribution, oriented to receive the maximum amount of solar exposure. Concrete, brick, stone, tile, rammed earth, sand, water, or other liquids can be used. The thicknesses necessary for effective thermal storage are significant: for concrete, 30 to 46 cm

(12-18 in.); for brick, 25 to 36 cm (10-14 in.); for adobe, 20 to 31 cm (8-12 in.); and 15 cm (6 in.) or more of water. Some systems use materials that hold and release energy through phase changes (changing from liquid to gas, for example), like eutectic salts and paraffins. Dark-colored surfaces absorb more solar radiation than lighter ones. Vents, dampers, movable insulating panels, and shading devices can assist in balancing heat distribution.

When designing a building to take advantage of solar heating, provisions must be made to prevent overheating in warm weather. Roofs provide a barrier to excess summer solar radiation, especially in the tropics where the sun is directly overhead. The transmission of solar heat from the roof to the interior of the building can result in high ceiling temperatures. Surfaces that reflect most infrared (IR) rays heat up very little in the sun. High ceiling temperatures can be reduced with thermally resistant materials, materials with high thermal capacity, or ventilated spaces in the roof structure.

Orienting building entrances away from or protected from prevailing cold winter winds, and buffering entries with airlocks, vestibules, or double entry doors dramatically reduces the amount of interior and exterior air change when people enter. Locating an unheated garage, mudroom, or sunspace between the doors to a conditioned interior space is a very effective way to control air loss in any building.

The interior layout of a passively solar heated build- 3 ing should be designed at the same time as the building's siting, rough building shape, shading, and orientation for maximum compatibility. Spaces with maximum heating and lighting needs should be located on the building's south face. Buffer areas, such as toilet rooms, kitchens, corridors, stairwells, storage, garage, and mechanical and utility spaces need less light and air-conditioning, and can be located on a north or west wall. The areas with the greatest illumination level needs, for accounting, typing, reading, or drafting, should be next to windows and have access to natural lighting. Conference rooms, which need few or no windows for light and views, can be located farther away from windows. Spaces that need a lot of cooling due to high internal heat gains from activities or equipment should be located on the north or east sides of the building.

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