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What is Passive Solar Heating?
Passive solar homes make use of building components, the orientation of the house, and various design features to provide passive solar heat to the building. Windows that face the sun as much of the day as possible are used to bring solar radiation into the building, where it can be absorbed by the floor, walls and other such building components, where it is converted to thermal energy (heat). In a completely passive solar heating system, all of the energy (heat) flow is by natural means (conduction, natural convection, and radiation.) By contrast, an active solar heating system uses pumps, blowers and/or fans to move heated fluid from the solar collectors to the heated space, from the collectors to solar heat storage area, and from heat storage to the heated space.
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The Components of a Passive Solar Heating System
Either an active or a passive solar heating system must have six basic components, but they are arranged quite differently and do their job quite differently in the two types of solar heating systems. The six components of a solar heating system are an aperture, absorber, solar heat storage mass, distribution system, controls, and a backup heating system.
In a passive solar heating system, the aperture is windows or other glazing that the sun shines through to heat the house. These windows should be south-facing in the northern hemisphere or north-facing in the southern hemisphere. The windows should not be shaded by trees or other buildings during most of the day throughout the heating season. The functions of absorber and solar heat storage mass are both handled by floors, walls, and/or other building parts that are in the path of the sunlight shining through the windows. The absorber/heat storage mass is typically made of some type of masonry that can store significant amounts of heat as it absorbs solar energy from the sunlight during the day and releases it slowly during the night. The functions of the aperture and absorber/heat storage mass in daytime and nighttime in a simple passive solar heating system are illustrated in the diagrams in this section.
The distribution of heat in a strictly passive system will include heat flow like conduction of heat into the walls etc in the daytime, conduction of heat from the inside of the wall to the surface and radiation into the heated space at night, and heat flow by natural air flow (natural convection). Fans and/or blowers are sometimes used to help distribute heat throughout a house in otherwise passive solar homes. Controls include items like moveable window insulation, operable vents or dampers, and roof overhangs or awnings that shade windows during the summer. The backup heating system can be any type of non-solar heating system.
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Basic Types of Passive Solar Heating Systems
The simplest type of passive solar heating system, direct gain, is illustrated in the diagrams in the previous sections. The floor and walls are heated during the day by the incoming solar radiation and they give off heat to the room at night. A couple of other options for passive solar homes, the thermal storage wall and attached sunspace, are described briefly here.
A passive solar heating system with a thermal storage wall is shown in the figures at the left and one with an attached sunspace is shown in the figures at the right. In both cases the sun shines in onto a masonry wall (or in some cases water filled containers) that is/are heated during the daytime and give off heat at night. Vents at the top and bottom of the wall are kept open during the daytime to aid in air and heat flow by natural convection. The vents are closed at night to reduce heat loss to the space between the window and the wall.
The thermal storage wall is sometimes called a “Trombe wall” after Felix Trombe, an engineer who popularized the use of this type of solar heat storage in the 1960's.
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Passive solar homes use windows and walls and floors to provide passive solar heat to the building. Several configurations are possible, but there must always be adequate window area for the sun to shine through and adequate solar heat storage in the mass of the walls and floor. Additional articles in this series will detail some calculations and use of available online data to make performance estimates for passive solar heating systems.
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About the Author
Dr. Harlan Bengtson is a registered professional engineer with 30 years of university teaching experience in engineering science and civil engineering. He holds a PhD in Chemical Engineering.
Passive Solar Design Calculations
- Principles of Passive Solar Heating Systems and How They Work
- Heating Degree Days for Passive Solar Heating Systems in the U.S.
- Estimation of Heat Loss/Heating Needs from Your Utility Bills
- Solar Insolation Data for Passive Solar Heating at Your Location in the U.S.
- Estimating Passive Solar Heating System Performance