Daylighting is the practice of placing windows or other openings and reflective surfaces so that during the day natural light provides effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved either from the reduced use of artificial (electric) lighting or from passive solar heating or cooling. Artificial lighting energy use can be reduced by simply installing fewer electric lights because daylight is present, or by dimming/switching electric lights automatically in response to the presence of daylight, a process known as daylight harvesting.
Daylighting is a technical term given to a common centuries-old, geography and culture independent design basic when "rediscovered" by 20th century architects.
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 of the globe south of the Tropic of Capricorn. Traditionally, in these parts with largely overcast skies, houses were designed with minimal windows on the polar side but more larger windows on the equatorial-side. 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. Even so, during mid-winter, light incidence is highly directional and casts deep shadows. This may be partially ameliorated through light diffusion and through somewhat reflective internal surfaces.
- Main article: Window
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:
- 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 windows.
Once used extensively in office buildings, the adjustable light reflector is seldom in use today having been supplanted by a combination of other methods in concert with artificial illumination. The reflector had found favor where the choices of artificial light provided poor illumination compared to modern electric lighting.
- Main article: Architectural light shelf
Light shelves are an effective way to enhance the lighting from windows on the equator-facing side of a structure, this effect being obtained by placing 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. Low winter sun (see Sun path) reflects off the snow and increases solar gain through equator-facing glass by one-to-two thirds which brightly lights the ceiling of these rooms. Glare control (drapes) may be required.
Template:Also Skylights are horizontal windows or domes placed at the roof of buildings, often used for daylighting. White translucent acrylic is a 'Lambertian Diffuser' meaning transmitted light is perfectly diffused and distributed evenly over affected areas. This means, among other advantages, that light source quality standards are measured relative to white acrylic transmission. White acrylic domes provide even light distribution throughout the day. Skylights admit more light per unit area than windows, and distribute it more evenly over 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 4-8% 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 with transparent glazings will cause internal heat problems, which is best treated by placing white translucent acrylic over or under the transparent skylight glazing.
With proper skylight design, there can be significant energy savings in commercial and industrial applications. Savings from daylighting can cut lighting energy use by up to 75 - 80 percent according to the US Department of Energy's Federal Energy Management Program. In terms of cost savings, the DOE reported that many commercial buildings can reduce total energy costs by up to one-third through the optimal use of daylighting.
Poorly constructed or installed skylights may have leaking problems and single-paned skylights may weep with condensation. Using modern designs with proper installation will eliminate issues with leaks and provide greater energy efficiency.
- Main article: Light tube
Another type of device used is the light tube, also called a solar tube, which is placed into a roof and admits light to a focused area of the interior. These somewhat resemble recessed ceiling light fixtures. They do not allow as much heat transfer as skylights because they have less surface area.
Tubular Daylighting Devices (TDDs) use modern technology to transmit visible light through opaque walls and roofs. The tube itself is a a passive component consisting of either a simple reflective interior coating or a light conducting fiber optic bundle. It is frequently capped with a transparent, roof-mounted dome 'light collector' and terminated with a diffuser assembly that admits the daylight into interior spaces and distributes the available light energy evenly (or else efficiently if the use of the lit space is reasonably fixed, and the user desired one or more 'bright-spots').
- Main article: Clerestory
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.
Another roof-angled glass alternative is a "sawtooth roof" (found on older factories). Sawtooth roofs have vertical roof glass facing away from the equator side of the building to capture diffused light (not harsh direct equator-side solar gain). The angled portion of the glass-support structure is opaque and well insulated with a cool roof and radiant barrier. The sawtooth roof's lighting concept partially reduces the summer "solar furnace" skylight problem, but still allows warm interior air to rise and touch the exterior roof glass in the cold winter, with significant undesirable heat transfer.
- Main article: Sunroom
In a well-designed isolated solar gain building with a solarium, sunroom, greenhouse, etc., there is usually significant glass on the equator side. A large area of glass can also be added between the sun room and your interior living quarters. Low-cost high-volume-produced patio door safety glass is an inexpensive way to accomplish this goal.
The doors used to enter a room, should be opposite the sun room interior glass, so that a user can see outside immediately when entering most rooms. Halls should be minimized with open spaces used instead. If a hall is necessary for privacy or room isolation, inexpensive patio door safety glass can be placed on both sides of the hall. Drapes over the interior glass can be used to control lighting. Drapes can optionally be automated with sensor-based electric motor controls that are aware of room occupancy, daylight, interior temperature, and time of day. Passive solar buildings with no central air conditioning system need control mechanisms for hourly, daily, and seasonal, temperature-and-daylight variations. If the temperature is correct, and a room is unoccupied, the drapes can automatically close to reduce heat transfer in either direction.
To help distribute sun room daylight to the sides of rooms that are farthest from the equator, inexpensive ceiling-to-floor mirrors can be used.
Building codes require a second means of egress, in case of fire. Most designers use a door on one side of bedrooms, and an outside window, but west-side windows provide very-poor summer thermal performance. Instead of a west-facing window, designers use an R-13 foam-filled solid energy-efficient exterior door. It may have a glass storm door outside with the inner door allowing light to pass through when opened. East/west glass doors and windows should be fully shaded top-to-bottom or a spectrally-selective coating can be used to reduce solar gain.
Fiber-optic concrete wallEdit
Another way to make a secure structural concrete wall translucent is to embed optical fiber cables in it. (See Concrete casts new light in dull rooms.) Daylight (and shadow images) can then pass directly through a thick solid-concrete wall. The only drawback is an inability to put insulation on either side of such a fiber-optic concrete wall. One possibility is to insulate it with aerogel after concrete wall construction, for natural daylight with the highest-possible structural security and no glass windows.
Hybrid solar lightingEdit
Oak Ridge National Laboratory (ORNL) has developed a new alternative to skylights called Hybrid Solar Lighting. This design uses a roof-mounted light collector, large-diameter optical fiber, and modified efficient fluorescent lighting fixtures that have transparent rods connected to the optical fiber cables. Essentially no electricity is needed for daytime natural interior lighting.
Field tests conducted in 2006 and 2007 of the new HSL technology were promising, but the low-volume equipment production is still expensive. HSL should become more cost effective in the near future. A version that can withstand windstorms could begin to replace conventional commercial fluorescent lighting systems with improved implementations in 2008 and beyond. The U.S. 2007 Energy Bill provides funding for HSL R&D, and multiple large commercial buildings are ready to fund further HSL application development and deployment.
At night, ORNL HSL uses variable-intensity fluorescent lighting electronic control ballasts. As the sunlight gradually decreases at sunset, the fluorescent fixture is gradually turned up to give a near-constant level of interior lighting from daylight until after it becomes dark outside.
HSL may soon become an option for commercial interior lighting. It can transmit about half of the direct sunlight it receives.
- Active daylighting
- Passive daylighting
- Passive solar building design
- Daylight harvesting
- Sun path
- Transom (architectural)
- Roof lantern
- ↑ Sun/Earth Buffering and Superinsulation page 68 ISBN 0960442243
- ↑ Muhs, Jeff. "Design and Analysis of Hybrid Solar Lighting and Full-Spectrum Solar Energy Systems". Oak Ridge National Laboratory. http://www.ornl.gov/sci/solar/pdfs/Muhs_ASME_Paper.pdf. Retrieved 2007-12-23.
- U.S. Department of energy page on passive daylighting
- Daylighting, Chapter 2 of the SynthLight Handbook, Low Energy Architecture Research Unit, London Metropolitan University, April 2004
- Sun Light Redirecting Devices - examples of geometrical set-up of light shelves etc.
- Solar control façades and Daylighting façades, University of California, Berkeley
- MIT, Building Technology Program, Daylighting Lab
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