Passive Solar Heating

Analyze building thermal-load patterns. An important concept of passive solar design is to match the time when the sun can provide daylighting and heat to a building with those when the building needs heat. This will determine which passive solar design strategies are most effective. Commercial buildings have complicated demands for heating, cooling, and lighting; therefore their design strategies require computer analysis by an architect or engineer.

Although passive solar heating is best incorporated into a house during the initial design, the concepts of passive solar heating can also be used when remodeling or adding to your home. Window design, and glazing choices in particular, are critical factors for determining the effectiveness of passive solar heating in a home. In heating climates, large south-facing windows are used, as these have the most exposure to the sun in all seasons.

Windows can also be located to provide solar heating in cold climates or avoid solar heating in hot climates. In cold climates, large south-facing windows allow significant solar energy into the house and also provide daylighting; properly sized overhangs can prevent overheating in the summer. In hot climates, north-facing windows can provide daylighting without heating the house.

East- and west-facing windows generally cause excessive heat gains in the summer and heat losses in the winter, and are usually sized small. Although overhangs are impractical for east- and west-facing windows, vertical shading can be used, or trees and shrubs can be strategically located to shade the windows. Landscaping has other benefits, including natural cooling and protection from the wind.

Windows can now be designed for a number of purposes. Some windows are designed to let the sun's heat in while insulating against the cold, and are ideal for south-facing windows in cold climates. Others are designed to reject the sun's heat while providing insulation, and are ideal for all windows in hot climates and east- and west-facing windows in moderate climates. See section Buying Windows For Energy Efficiency.

Thermal mass-such as tiles, masonry, or even water-filled walls-provides a means of storing the solar energy that enters through the windows. Built into the floors and walls near the south-facing windows, thermal mass will absorb solar energy during the day and keep the house from overheating. At night, the thermal mass will release the heat, keeping the house warm.

An alternate approach is to locate a thermal mass wall on the south-facing side of the house, with glazing on the exterior, separated from the wall by only a few inches. The wall absorbs heat on the sun-facing side and releases it slowly into the living space over the course of the day. Although the wall will block the sunlight, daylighting can still be achieved through narrow windows located above the thermal wall.

For thermal mass to be effective, air must circulate freely through the house to carry the heat from the thermal mass to the places where it is needed. Fans are sometimes used, but natural convection will often circulate the air sufficiently. For instance, a central staircase provides an effective means for allowing hot air to rise, and to complete the circuit, vents between the upper and lower floors along the exterior walls will allow cooler air to flow back to the thermal mass. Doors must be left open for this approach to work.

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Figure 19: Passive solar design can greatly reduce the need for heating and cooling.


Integrate passive solar heating with daylighting design. A passive solar building that makes use of sunlight as a heating source should also be designed to take advantage of sunlight as a lighting source. However, each use has different design requirements that need to be addressed. In general, passive solar heating benefits from beam sunlight directly striking dark-colored surfaces. Daylighting, on the other hand, benefits from the gentle diffusion of sunlight over large areas of light-colored surfaces. Integrating the two approaches requires an understanding and coordination of daylighting, passive design, electric lighting, and mechanical heating systems and controls.

Design the building's floor plan to optimize passive solar heating. Orient the solar collection surfaces, for example appropriate glazings in windows and doors, within 15 degrees of true south, if possible. Because of the solar path, the optimum orientation for passive solar buildings is due south. South-facing surfaces do not have to be all along the same wall. For example, clerestory windows can project south sun deep into the back of the building. Both the efficiency of the system and the ability to control shading and summer overheating decline dramatically as the surface shifts away from due south.

Identify appropriate locations for exposure to beam sunlight. Overheating and glare can occur whenever sunlight penetrates directly into a building and must be addressed through proper design. A "direct-gain" space can overheat in full sunlight and is many times brighter than normal indoor lighting, causing intense glare. Generally, rooms and spaces where people stay in one place for more than a few minutes are inappropriate for direct gain systems. Lobbies, atria, or lounges can be located along the south wall where direct sun penetrates. Choose glazings that optimize the desired heat gain, daylighting, and cooling load avoidance.

Avoid glare from low sun angles. In late morning and early afternoon, the sun enters through south-facing windows. The low angle allows the sunbeam to penetrate deep into the building beyond the normal direct-gain area. If the building and occupied spaces are not designed to control the impact of the sun's penetration, the occupants will experience discomfort from glare. Careful sun-angle analysis and design strategies will ensure that these low sun angles are understood and addressed. For example, light shelves can intercept the sun and diffuse the daylight. Workstations can be oriented north-south so that walls or high partitions intercept and diffuse the sun.

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