Daylighting
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Daylighting is the practice of placing windows, or other transparent media, and reflective surfaces so that, during the day, natural light provides effective internal illumination.
Within the overall architectural design of a building, particular attention is given to daylighting when the aim is to maximize visual comfort, productivity, or to reduce energy use. Energy savings from daylighting are achieved in two ways--either from the reduced use of electric lighting, or from passive solar heating or cooling.
Electric lighting energy savings can accrue because occupants choose not to switch their lights on, or because an automatic lighting control system ("photocontrol system") switches the lights off or dims them to a lower level.
In passive solar technique, buildings are designed such as to account for local climate, in particular the luminance of the sky. For instance, in cooler parts of the globe with largely overcast skies, a house will be designed with minimal windows on the polar side but more and larger windows on the equatorial-side. This is because there is no direct sunlight on the polar-side wall of a building from the autumnal equinox to the spring equinox in parts of the globe north of the Tropic of Cancer and in parts south of the Tropic of Capricorn. Equatorial-side windows receive at least some direct sunlight on any sunny day of the year, so they are effective at daylighting areas of the house adjacent to the windows. One disadvantage of relying on conventional window space for daylighting is that, especially during mid-winter, it tends to be highly directional light that casts deep shadows. This may be partially ameliorated through light diffusion and somewhat reflective internal surfaces.
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[edit] Windows
Windows are the most common way to admit daylight into a space. Their vertical orientation means that they selectively admit sunlight and diffuse daylight at different times of the day and year. Therefore windows on multiple orientations must usually be combined to produce the right mix of light for the building, depending on the climate and latitude. There are three ways to improve the amount of light available from a window.[1]
- Place window close to a light colored wall.
- Slant the sides of window openings so the inner opening is larger than the outer opening.
- Use a large light colored window sill to project light into the room.
Different types and grades of glass and different window treatments can also affect the amount of light transmission through the window.
[edit] Light reflectors
Once in extensive use in office buildings, the adjustable light reflector is seldom seen, having been supplanted by a combination of other methods in concert with artificial illumination. The reflector found favor where the choices of artificial light provided poor illumination compared to modern electric lighting.
[edit] Light shelves
An effective way to enhance the lighting from windows on the equator-facing side of a structure is to place a white or reflective metal light shelf outside the window. Usually the window will be protected from direct summer season sun by a projecting eave. The light shelf projects beyond the shadow created by the eave and reflects sunlight upward to illuminate the ceiling. This reflected light can contain little heat content and the reflective illumination from the ceiling will typically reduce deep shadows, reducing the need for general illumination.
In the cold winter, a natural light shelf is created when there is snow on the ground. As the outside temperature drops below freezing, moisture in the atmosphere precipitates out, often in the form of snow (or freezing rain). This makes the ground highly reflective, and the skies have few clouds. Low winter sun (see Sun path) reflects off the snow and increases solar gain through equator-facing glass by one-to-two thirds, brightly lighting the ceiling of these rooms. Glare control (drapes) may be required.
[edit] Skylights
Skylights are often used for daylighting. Skylights admit more light per unit area than windows, and distribute it more evenly over a space. They can therefore be a good choice when daylight is being used to illuminate a space. The optimum number of skylights (usually quantified as "effective aperture") varies according to climate, latitude, and the characteristics of the skylight, but is usually 1-10% of floor area. The thermal performance of skylights is affected by stratification, i.e. the tendency of warm air to collect in the skylight wells, which in cool climates increases the rate of heat loss. During warm seasons, skylights also can cause internal heat problems, which is usually treated by placing a shade over the skylight, or by opening it if it is openable.
The amount of light skylights deliver peaks around midday, when the additional light and heat it provides is least needed. Some skylight designs use domed or pyramidal shapes along with prismatic or other light-redirecting glazings to achieve more even light levels through the course of a day. Poorly constructed or installed skylights may have leak problems and single-paned ones may weep with condensation. Using skylights with at least two panes and a heat reflecting coating will increase their energy efficiency. Skylights may also be more prone to breakage than vertical windows.
The skylight topic is controversial. Although roof-angled glass provides good daylighting, it can also create a solar furnace in the summer (when the sun is nearly perpendicular), and then lose more solar gain than it captures (when the low winter sun reflects off it, and warm interior air rises to touch the cold glass on winter nights). Heating and cooling costs for a building are normally greater than the value of skylight daylighting, especially when modern energy-efficient lighting systems are employed.
Skylight Drawbacks:[2]
- Significant source of heat loss or heat gain
- Can constrain design of building shape and orientation
- Difficult\Complicated to specify
- Point of condensation
- Uncontrolled, uneven illumination
- Susceptible to water leakage
- Susceptible to ventilation leakage
- Not appropriate for low ceilings
- Difficult to relocate or reconfigure
- Suitable for downlighting only (i.e. N/A to directional lighting or uplighting)
- Does not maximize the use of available sunlight
- A source of light pollution at night
- Cannot be easily turned off
- Security concerns
[edit] Light tubes
Another type of device used are light tubes, also called solar tubes, placed into a roof and admitting light to a focused area of the interior. These somewhat resemble recessed light fixtures in the ceiling. They do not allow as much heat transfer as skylights because they have less exposed surface area. It is also easier to retrofit light tubes into existing buildings, especially those with deep roof constructions.
[edit] Clerestory windows
Another important element in creating daylighting is the use of clerestory windows. These are high, vertically-placed windows. They can be used to increase direct solar gain when oriented towards the equator. When facing toward the sun, clerestories and other windows may admit unacceptable glare. In the case of a passive solar house, clerestories may provide a direct light path to polar-side (north in the northern hemisphere; south in the southern hemisphere) rooms that otherwise would not be illuminated. Alternatively, clerestories can be used to admit diffuse daylight (from the north in the northern hemisphere) that evenly illuminates a space such as a classroom or office.
Often, clerestory windows also shine onto interior wall surfaces painted white or another light color. These walls are placed so as to reflect indirect light to interior areas where it is needed. This method has the advantage of reducing the directionality of light to make it softer and more diffuse, reducing shadows.
[edit] See also
[edit] References
- ^ Sun/Earth Buffering and Superinsulation page 68 ISBN 0960442243
- ^ Hybrid Lighting Applications. Retrieved on 2008-01-15.
