Passive Solar Energy

The sun's energy provides the light and heat that is essential to people’s thermal and visual comfort. But at different times of year, the amount of energy provided directly by the sun is either too much or too little for us to be comfortable. A central function of buildings is to use this solar energy when it meets our needs, and protect us when it is inadequate or too harsh.

Passive solar design refers to a building that effectively uses the sun’s energy, simply through the way the structure is oriented, designed, and constructed. Buildings that use passive solar design can use fewer or smaller-scale active technologies to meet the remainder of their heating and lighting needs. Because the sun's energy is free, maximizing use of natural solar energy before adding active technologies can significantly reduce ongoing energy costs.

Passive solar design has a long history, stretching back to early indigenous cultures and the Greeks and Romans, whose written records indicate thoughtful use of passive solar design to accommodate sunlight and solar heat. Modern passive solar design use some of these same techniques but add newer ones that depend on advanced structures and materials.

Here are some key aspects of passive solar design:

Building Orientation

The correct orientation of a building is the foundation of passive solar design. Because of the Earth's tilted axis and its orbit around the sun, sunlight does not hit the Earth's surface at the same angle in everywhere or at all times of the year. A building must be oriented in relationship to this range of angles in order to maximize the use of solar energy for interior lighting and winter heating.

In the Northern hemisphere, this means focusing on the southern side of buildings. In the summer, the sun passes nearly directly over our heads. But in the winter, it follows a path lower in the sky and to the south. To take advantage of this winter sun for heat, and to take advantage of the sun year round for light, a building must be designed to allow the entry of sunlight from the south.

Using Natural Light

Once a building is oriented to the south, a means of accommodating sunlight is needed. At a basic level, a building's windows serve this purpose, passing light through while protecting the building from other natural elements. Advanced passive solar design takes this concept further to receive specific amounts of light at different times of year, and even channel sunlight deeper into the parts of buildings that cannot receive as much direct sunlight. These designs use elements like atriums, skylights, light shelves and reflective panels to bring large amounts of light inside and reflect it into less well-lighted areas.

These methods are generally categorized as daylighting techniques. Daylighting has many benefits, both reducing electric lighting costs and providing a more pleasant indoor environment. However, daylighting can also have drawbacks, most notably in its potential to capture large amounts of solar energy inside a building, creating more heat than the building needs. Finding the right balance between maximizing natural light and minimizing overheating presents a significant design challenge.

Blocking Heat While Using Light

The level of heat produced inside a building as a result of solar energy is called solar gain. In the winter, solar gain is desirable as it reduces the need for active heat production. But in the summer, solar gain needs to be minimized to keep buildings cool.

One method for allowing heat at certain times of year and blocking it at others is the use of well-designed overhangs or sunshades. These shading devices are sized and placed to block sunlight and prevent overheating in the summer, when the sun is above the building, and to accommodate sunlight and create heat during the winter, when the sun is lower in the sky.

Overhangs have been used in buildings for centuries, and are a practical design element for small scale buildings whose roofs can be extended to shade windows below. Sunshades are a newer way to provide shading and can be used on larger buildings where overhangs may be impractical.

Storing Solar Heat

Though the majority of sunlight, and therefore heat, enters buildings directly through windows, other building materials can be used to capture sunlight and transfer controlled amounts of heat into a building.

One such approach is known as a Trombe wall, named after Felix Trombe, a French scientist who developed techniques for using this heat-storage concept. At its most basic, a Trombe wall is simply a dark-colored building wall constructed from concrete, adobe or stone, all good materials for storing heat. When hit by sunlight, the wall converts the solar energy into heat which is slowly transferred through the wall and into the building, often timed to reach the interior late in the day when direct solar energy is no longer available.

Another heat storage method is called a double facade wall, typically made of glass and used in larger-scale buildings. This design uses two layers of windows with an air barrier in between that accommodates fresh air. As sunlight hits the outside layer of windows and passes through the second layer into the building, it heats the air in between. This air can then be vented into the building during the winter and out of the building during the summer, allowing for significant control over solar gain by reducing heat in the summer and allowing it inside in the winter.

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