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Passive Solar Design

Passive solar design is a term that almost exclusively applies to the building of stationary structures. It is a common misconception that passive solar design only applies to maximizing heat collection, but that is not true. Passive solar design incorporates all aspects of a building design from heating to cooling to lighting. (Note that the ideas discussed are in reference to building in the Northern Hemisphere. For the Southern Hemisphere, the ideas are the same, but orientation changes.)

History

This history of solar energy is more extensively covered elsewhere, but it is worth mentioning that the basic fundamentals of passive solar design are centuries old. In Europe, the Greeks were among the first to apply such design practices not just to homes, but to entire cities. The need for passive solar heating arose from the fact that growing urban centers did not have access to abundant forest and thus had to maximize use of fuel for heating and cooking. To that end, the Greeks built their homes with all windows facing south and with large awnings. The awnings were designed to block the sun in the summer when it was relatively high overhead and thus keep the interior of buildings cooler. By contrast, in the winter when the sun was low the awnings did not block the direct entry of sunlight through windows, which allowed for increased light and heat.

The principles mentioned above where also practiced by native peoples in America. The Cliffside homes of the Anasazi faced south and were carved in such a way that cliff overhangs provided shade in the warm summer months but did not block light entry into homes in the winter.

Modern Solar Design

Modern solar design makes use of the same basic principles employed by the Greeks and the Anasazi. The only thing that has really changed in that time is the materials available for building. Insulation, double-paned insulated glass, and other materials allow modern builders to make even better use of the sun’s energy and retain heat longer or prevent its entry more easily. Below are discussed some the elements of design incorporated into buildings to achieve maximum solar efficiency.

Basic Rules of Thumb

Though each application will have unique needs, there are nonetheless overarching principles that can be applied to almost any structure. One of those principles is the east-west axis. The long axis of a building should always be oriented east to west or parallel to the Equator. This ensures that maximum exposure to sunlight occurs throughout the day.

Adding to the above rules, the south face of a building should receive sunlight from 9 AM to 3 PM during the heating season. The heating season refers to the season in which indoor heat is desired. As an extension of this rule, the most used rooms in a building and those requiring the most heat should also face south. Open floor plans help to optimize passive heating.

Shading should be strategically placed to prevent sunlight from entering the structure in the summer. Overhang calculations and foliage placement are both determined by latitude.

Types of Gain

Gain simply refers to the loss or accumulation of energy. Positive gain is when energy in a system is increased and negative gain is when energy is lost. When the term “gain” is used on its own, it is almost always referring to positive gain.

In referenced to solar energy, there are three types of gain as explained below. The types of gain are divided based on how the sunlight impacts the physical environment. Direct light into an area is direct gain. There are also indirect and isolated gains.

To understand solar gain it is important to understand several of the concepts used and terms used in reference to solar gain. The first of these is thermal mass. In solar building, thermal mass refers to any substance that resists changes in temperature. A good example of thermal mass would be concrete on a driveway. It heats up slowly, but also cools down slowly. A concrete driveway will be warm long after the sun has gone down and the grass has cooled because it has a large thermal mass or thermal inertia. Thermal mass will absorb heat in the day and give off heat at night because thermal energy always travels from warm to cool environments.

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