Passive Solar Design

Passive solar design refers to a number of intelligent building design techniques that reduce or eliminate the use of fossil fuels and electricity for heating, cooling and lighting buildings (during the day). The modern version of this traditional approach to building design was developed beginning in the 1970s and applied to a wide variety of building types throughout the US, with a focus on the West and Southwest.115 The term "passive solar design" was chosen to contrast with the more prevalent — and far more expensive — active solar systems that used expensive copper or aluminum rooftop collectors and lots of fans, pumps and controls to heat and cool spaces. The idea behind passive solar design was to incorporate sunlight and natural ventilation into the basic design of the building, minimizing the need for mechanical systems. In many of the hot, arid climate zones of the US, this is an excellent design strategy. In hot, humid zones, more focus needs to be given to ventilation and less to heating.

In 1980 I built a passive solar adobe home in the San Francisco Bay area. In a climate that typically ranged from cool winter nights (in the 30s) to hot summer days (in the low 100s), the three-bedroom post-and-beam design featured a heavy-mass structure (adobe clay), plenty of south-facing glass, a well-insulated roof and abundant cross ventilation between low north-facing windows and high south-facing clerestory windows. The long axis of the home was east-west, with small windows on those façades. The south-facing windows were shaded with overhangs to prevent the high summer sun from entering the house, but did allow the lower-angle winter sun to shine directly on the mass of the structure, heating the home not only during the day, but also well into the evening. The home had abundant daylighting, and temperatures in the space stayed 20°F above outside levels in winter and 20°F below outdoors in summer, comfortable almost year-round. Only a small woodstove in the living room was used for supplemental heating, and no air conditioning was used. Even on hot summer days in the Bay area, there are cooling breezes most evenings that lower temperatures quickly.

Applied to commercial buildings, passive solar design looks for opportunities to orient buildings with a long axis east-west, to have a more rectangular than square structure (called "massing" by architects) and to provide shading and overhangs on the south and west faces using a window design that allows for abundant daylighting with minimal glare.116 In smaller buildings, it makes design sense to have direct solar penetration into a space, such as an atrium, to warm it in winter. Such buildings can also use radiant heating systems to supplement solar heating in winter, and many use concrete or other forms of thermal mass to absorb heat during the day in summer, radiating back slowly at night when people have left the building. Some approaches also use a strategy known as "night-flush cooling" in which fans slowly circulate cooler night air through the building (and exhaust warmer air) to lower temperatures to comfortable levels before morning. This approach saves a lot of energy compared with conventional air conditioning.

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