Passive Solar; When, Where, How & Why

“Passive solar technologies are (a) means of using sunlight for useful energy without the use of active mechanical systems.” from Wikepedia, the free encyclopedia.

In simple terms, passive solar works like this; heat moves from warmer materials to cooler ones until there is no longer a temperature difference between the two.

In order for energy to be distributed throughout a space, passive solar design makes use of the following heat-movement and heat-storage mechanisms:

*Conduction – refers to the way heat causes molecules close to a heat source to vibrate vigorously. These vibrations then affect molecules close by, transferring warm energy.

*Convection – refers to the way heat circulates through liquids and gases. Warmer fluid is lighter so it rises, and cooler liquid is denser so it sinks.

*Radiation – radiant heat moves through the air from warmer objects to cooler ones. When radiation strikes an object, it is absorbed, reflected, or transmitted. For example; dark clothing absorbs warmth from the sun, while light colored clothing reflects that warmth.

The same rule applies in passive solar design; dark colored surfaces absorb more warmth, while light color surfaces reflect 80%-98% of incoming energy from the sun.

Clear glass allows 80%-90% of the heat radiation to pass through while it absorbs the infrared radiation. In this way, glass traps the warmth entering the home, which is absorbed by the interior surfaces and then radiated from these same surfaces.

*Thermal Capacitance – refers to the ability of materials to store warmth. And thermal mass refers to the materials that store that warmth, i.e. concrete, stones, brick, tile and water. The more thermal mass, the more heat can be stored for each degree rise in temperature.

Every passive solar design needs five elements:

1. Aperture or Collector – for instance a large glass area through which sunlight enters a building.

2. Absorber – a dark surfaced element (floor or wall) that absorbs the sun’s warmth.

3. Thermal Mass – the material that stores the absorbed warmth, i.e. concrete, stone, brick or water tank.

4. Distribution Method – Using conduction, convection and radiation until there is no longer a temperature difference.

5. Control Mechanism – A way to regulate the amount of sunlight entering the aperture, i.e. roof overhangs, window coverings, awnings and vents.

And there are three basic passive solar designs for heat regulation:

1. Direct Gain – the sun’s warmth enters a building through an opening, usually south-facing windows, it is absorbed by the dark colored thermal mass and at night as the building cools, heat stored in the thermal mass warms the spaces.

2. Trombe Wall or Indirect Gain – a dark colored wall that absorbs the sun’s heat and stores it, is placed between the south-facing windows and the living or working space. At night this Trombe wall releases the heat stored when the indoor temperature falls below that of the wall’s surface.

3. Sunspace or Isolated Gain – the use of a separate room such as a sun room to store the sun’s warmth. This stored heat is distributed throughout the building through vents, windows, doors and sometimes the addition of fans.

In cold climates this type of design takes advantage of the sun’s warmth and in hot climates it mitigates the heat. This type of climatic design tends to work best in smaller buildings and in climates with clear skies during the winter heating season. However, the principles of passive solar can be applied to buildings in almost any part of the United States.

For more information check out

U.S. Department of Energy.