How Does Passive Solar Work?
Winter Operation
Passive solar architecture is a building design system that uses solar energy directly to reduce heating and cooling costs. The passive solar building operates in two modes, one during the winter heating season, the other during the summer cooling season.
During the winter, the sun is relatively low in the sky. Windows and skylights are oriented to admit as much of the solar energy as possible. This radiant energy, or insolation, shines on the building's floor and walls, its thermal mass. During the day, the sun's energy is absorbed (figure 1). In the evening, as the interior air cools, the thermal mass radiates this stored heat back into the building (figure 2).
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Even during cloudy days, there is considerable insolation and subsequent heating of the thermal mass. Furthermore, in many climates, the coldest weather occurs during periods of high atmospheric pressures that are typically accompanied by clear, cloud free days.
Summer Operation
In the summer, the sun is higher in the sky. Roof overhangs and shading devices for skylights prevent the sun from shining directly into the building (figure 3). The large thermal mass is shaded, and not warmed by the sun's energy. Remaining cool, the building's thermal mass absorbs excessive heat during the day and evening (figure 4).
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Design considerations
The primary design elements in passive solar architecture include the orientation of the structure on the building site, correctly sizing windows and skylights, detailing roof overhangs with regard to summer and winter sunlight, the inclusion of thermal mass and careful detailing of insulation and drainage elements.
Solar design begins by considering the solar path and sun angles for the latitude of the building's location. Placement of the windows and skylights determine when and how much light is allowed into the structure. Solar angles determine the location and sizing of roof overhangs and shading devices, which can be manual or automatic, limiting solar gain during summer operation. Thermal mass, the building's walls and floor, is used as a heat sink. In the winter, it absorbs heat during the day, and radiates it back into the structure, offsetting heating costs. In the summer, as the interior of the building warms, the thermal mass absorbs the heat build-up, helping to cool the interior and reducing mechanical system costs for cooling. The layout of the floor plan is carefully considered to optimize the passive heating and cooling characteristics of the building.
Thermal Mass
The key to the system is the thermal mass of the passive solar structure. Thermal mass is the ability of a body to store thermal energy, or heat. All objects have thermal mass (even air has thermal mass), however for passive solar structures we are interested in using materials that have a high thermal mass. Objects with high thermal mass are slow to heat up, and once warmed they are slow to cool down.
The thermal mass of a passive solar building is typically located in the floor slab and exterior walls, and usually constructed of concrete or other types of masonry. Water also has a high thermal mass and can be used successfully in some design aspects, such as an internal Trombe wall.
In the winter, the large thermal mass allows the building to absorb solar energy during the day and release it back into the building as the structure cools. Conversely, in the summer the large thermal mass is shaded from direct sun and absorbs excess heat in the building, helping to cool the structure. Due to the large thermal mass of the passive solar structure, the interior is less susceptible to temperature fluctuations making the building more comfortable to inhabit in both summer and winter.
The ideal material for creating thermal mass is dense and takes a relatively long time to heat and cool. Water, brick, adobe, earth, stone, and concrete are all examples of materials fitting these criteria, and thus are excellent for creating thermal mass in passive solar structures. In modern building construction, concrete is a readily available. It is a ubiquitous building material with well-known structural and operational characteristics. It has a proven track record both in conventional and passive solar construction. It is the material most commonly used as the thermal mass in contemporary passive solar structures.
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