Daylighting is the complete process of designing buildings to utilize natural light to its fullest. It includes all of the activities given below:
• Siting the building—that is, orienting it for optimum solar exposure
• Massing the building—that is, presenting the optimum building surfaces toward
• Choosing fenestration to permit the proper amount of light into the building,
taking into account seasons, weather, and daily solar cycles
• Shading the façade and fenestration from unwanted solar radiation
• Adding appropriate operable shading devices, such as blinds and curtains, to permit
occupant control over daylight admission
• Designing electric lighting controls that permit full realization of the energy savings
benefit of daylighting
INTEGRATING DAYLIGHTING AND ELECTRIC LIGHTING
To harvest the energy-saving benefits of daylighting, electric lights must be switched off or dimmed. This can be designed in several ways.
•Adequate manual switching or dimming to encourage the user to turn off or dim electric lights.
• An automatic photoelectric device in each daylighted zone that either switches off lights during daylight periods or dims lights in proportion to the amount of daylight.
• An automatic photoelectric system that dims or switches off lighting systems throughout a building in response to daylight.
• An automatic time-of-day control system, preferably with astronomic time functions, that switches or dims lights according to a fixed solar schedule.
Each of these ways are acceptable. It is generally agreed that switching lights is least expensive but dimming lights is most desired. Step dimming has been found disruptive in many situations. The use of both electric light and daylight often raises the question of whether the electric light source should match the natural light. In most cases, choosing an electric light source that is appropriate independent of daylight is probably best. To match daylight, a light source of a very high color temperature is needed.
Toplight Daylighting Methods
One of the most common ways to introduce daylight is through skylights and other means of top lighting. Top lighting behaves as direct electric lighting does—by radiating light downward. Principles commonly used for designing electric lighting systems can also be used for top lighting, which is the easiest form of daylighting and is relatively unaffected by site orientation and adjacent buildings.
Here are several classic prototypes for top lighting:
Sidelight Daylighting Method
Side lighting employs vertical fenestration (usually windows) to introduce natural light. Unlike top lighting, side lighting tends to introduce light that can be too bright relative to the room surfaces, sometimes causing glare. However, the desirable view provided by windows usually makes glare an acceptable side effect.
Many modern commercial windows employ low-e glazing. Low-e glazing employs two or more panes of glass, one of which is coated with a relatively clear material that reflects infrared energy while passing visible portions of the
sun’s energy. In any building with a cooling season, low-e glass is essential in minimizing solar heat gain. Reflective coatings can also be used; these make the building look mirrored while further decreasing solar penetration. Tinted glass can also reduce solar penetration and glare. Glazing selection is always a compromise between clarity and energy efficiency.
One way to increase both efficiency and clarity is to employ solar shading other than within the glass. Solar shading uses building elements to prevent direct solar radiation from entering the space during the cooling season. Overhung soffits, canopies, and awnings are the most common forms of external solar shading, while blinds, curtains, and shades are the most common forms of interior shading. Solar shading is difficult to design for east- and westfacing façades because preventing direct solar penetration very early or very late on a summer day is impossible without blocking the view. Interior shading devices should have a reflective surface to reflect unwanted light back outdoors. Dark shades prevent glare but absorb solar energy and become warm, heating the space. Adjustable exterior shading devices are probably the best means of shielding windows, but architectural and/or construction cost limitations may prevent their use.
An additional problem caused by side lighting is the limits of penetration into the space. Generally, the effect of the daylight is lost at a distance from the windows about 2.5 times the window’s height. For example, in a room with windows having a maximum height of 8′, the maximum useful penetration of natural light is about (8 x 2.5) or 20′ (assuming, of course, that no walls are in the way). High windows increase the usable daylight area but can introduce glare.
Many modern buildings employ a light shelf to shade the lower part of the window, or view glazing, permitting clearer glass. The top of the shelf is reflective, intended to bounce light inward and onto the ceiling, which provides for deeper light penetration and improved interior light quality. The daylight glazing is generally darker or more reflective than the view glazing to prevent direct solar radiation and glare from a bright sky.
A light shelf is designed to scoop direct solar radiation into the room and onto the ceiling, where it becomes diffuse indirect light, one of the best types of light for both work and comfort. A light shelf can increase the depth of penetration of daylight by 100% or more, but only when it captures and redirects direct rays of the sun. A light shelf has limited benefit with diffuse light, as from sky without sun and on cloudy days. In general, light shelves work best on the south side of the building.
DESIGN AND AWARENESS PRINCIPLES OF DAYLIGHTING
While daylighting design can be relatively technical, you can use the following basic principles to develop designs that address daylighting opportunities:
1. Begin by planning the building such that every regularly occupied work or living space has access to a window, skylight, or other source of natural light. Give high priority to windows that provide a view. Remember that the effective daylighted area extends into the building only about 2 times the width of a window and about 2 to 2.5 times its height.
2. Minimize the size of the east and west sides of the building and maximize the south and north sides of the building. Because of the seasonally varying paths of the sun in the sky, it is difficult to design east- and west-facing windows. North-facing windows in the northern hemisphere present no solar heating problems, and south-facing windows are the easiest to protect with passive elements like overhangs, awnings, and light shelves.
3. If a large area of the building is not near a window, investigate top-light skylights in one-story buildings or the top floor of multistory buildings. Simple top-light skylights should occupy 3% to 5% of the total roof area in order to provide adequate levels of interior lighting.
4. Protect the interior from too much natural light—2.5 times or higher the level of ordinary electric light—by employing appropriate window glass, exterior shading devices, interior shading devices, or a combination of these.
5. Provide an electric lighting system and/or automatic lighting controls to permit harvesting of the energy savings. The best way is to dim the electric lights rather than switch them on and off. Modern fluorescent dimming systems allow daylighting controls and fundamentally energy-efficient fluorescent
and compact fluorescent lighting.
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