Passive solar design uses the way heat naturally functions (whereby heat moves from warmer materials to cooler ones until there is no temperature difference) in order to generate energy for a home.
There are two types of radiation important to passive solar design: solar radiation and infrared radiation. When radiation strikes an object, it is absorbed, reflected or transmitted, depending on certain properties of that object.
Opaque objects absorb 40-95 percent of incoming solar radiation from the sun, depending on their color-darker colors typically absorb a greater percentage than lighter colors. This is why solar panels, for instance, tend to be dark colored. Bright-white materials or objects reflect 80-98 percent of incoming solar energy.
Clear glass transmits 80-90 percent of solar radiation, absorbing or reflecting only 10-20 percent. After solar radiation is transmitted through the glass and absorbed by the home, it is radiated again from the interior surfaces as infrared radiation. In this way, glass traps solar heat entering the home.
Five Elements of Passive Solar Home Design
The following five elements constitute a complete passive solar home design. Each performs a separate function, but all five must work together for the design to be successful.
Usually a window through which sunlight enters the building. Typically, the collector, or apertures, should face within 30 degrees of true south and should not be shaded by other buildings or trees from 9 a.m. to 3 p.m. each day during the heating season.
The hard, darkened surface of the storage element. This surface, which could be a masonry wall, floor, panel or water container, sits in the direct path of sunlight. Sunlight hits the surface and is absorbed as heat.
The materials that retain or store the heat produced by sunlight. The difference between the absorber and thermal mass, although they often form the same wall or floor, is that the absorber is an exposed surface whereas thermal mass is the material below or behind that surface.
The method by which solar heat circulates from the collection and storage points to different areas of the house. A strictly passive design will use the three natural heat transfer modes-conduction, convection and radiation-exclusively. In some applications, however, fans, ducts and blowers may help with the distribution of heat through the house.
Roof overhangs can be used to shade the aperture area during summer months. Other elements that control under- and/or overheating include electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; and awnings.
Direct gain is the simplest passive solar home design technique. Sunlight enters the house through the aperture (collector) – usually south-facing windows with a glazing material made of transparent or translucent glass. The sunlight then strikes masonry floors and/or walls, which absorb and store the solar heat. The surfaces of these masonry floors and walls are typically a dark color because dark colors usually absorb more heat than light colors. At night, as the room cools, the heat stored in the thermal mass convects and radiates into the room.
Some builders and homeowners have used water-filled containers located inside the living space to absorb and store solar heat. Water stores twice as much heat as masonry materials per cubic foot of volume. Unlike masonry, water doesn’t support itself. Water thermal storage, therefore, requires carefully designed structural support. Also, water tanks require some minimal maintenance, including periodic (yearly) water treatment to prevent microbial growth.
The amount of passive solar depends on the area of glazing and the amount of thermal mass. The glazing area determines how much solar heat can be collected. And the amount of thermal mass determines how much of that heat can be stored. It is possible to undersize the thermal mass, which results in the house overheating. There is a diminishing return on oversizing thermal mass, but excess mass will not hurt the performance. The ideal ratio of thermal mass to glazing varies by climate.
Another important thing to remember is that the thermal mass must be insulated from the outside temperature. If the thermal mass is not insulated, the collected solar heat can drain away rapidly. Loss of heat is especially likely when the thermal mass is directly connected to the ground or is in contact with outside air at a lower temperature than the desired temperature of the mass.
Even if you simply have a conventional home with south-facing windows without thermal mass, you probably still have some passive solar heating potential (this is often called solar-tempering). To use it to your best advantage, keep windows clean and install window treatments that enhance passive solar heating, reduce nighttime heat loss and prevent summer overheating.
Indirect Gain (Trombe Walls)
An indirect-gain passive solar home has its thermal storage between the south-facing windows and the living spaces.
Using a Trombe wall is the most common indirect-gain approach. The wall consists of an 8/16-inch-thick masonry wall on the south side of a house. A single or double layer of glass is mounted about 1 inch or less in front of the wall’s surface. Solar heat is absorbed by the wall’s dark-colored outside surface and stored in the wall’s mass, where it radiates into the living space.
The Trombe wall distributes or releases heat into the home over a period of several hours. Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening. When the indoor temperature falls below that of the wall’s surface, heat begins to radiate and transfer into the room. For example, heat travels through a masonry wall at an average rate of 1 hour per inch. Therefore, the heat absorbed on the outside of an 8-inch-thick concrete wall at noon will enter the interior living space around 8 p.m.
Isolated Gain (Sunspaces)
The most common isolated-gain passive solar home design is a sunspace. A sunspace-also known as a solar room or solarium-can be built as part of a new home or as an addition to an existing one.
Sunspaces may often be called and look a lot like “greenhouses.” However, a greenhouse is designed to grow plants while a sunspace is designed to provide heat and aesthetics to a home. Many elements of a greenhouse design that are optimized for growing plants, such as overhead and sloped glazing, are counterproductive to an efficient sunspace. Moisture-related mold and mildew, insects and dust inherent to gardening in a greenhouse are not especially compatible with a comfortable and healthy living space. Also, it is difficult to shade sloped glass to avoid overheating, while vertical glass can be shaded by a properly sized overhang.
Passive Solar Home Design for Summer Comfort
It makes little sense to save money on winter heating just to spend it on summer cooling. So in most climates, a passive solar home design must provide summer comfort as well. The solar heat in the summer must be blocked by an overhang or other devices, such as awnings, shutters and trellises.
The physical dimensions of an overhang are an important element because overheating will occur unless the overhang provides enough shade. Many variables-including latitude, climate, solar radiation transmittance, illuminance levels and window size and type-need to be considered for properly sizing an overhang in a specific locale. Therefore, it’s best to have an experienced solar designer or builder calculate the proper overhang dimensions.
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