Daylighting PDF

DAYLIGHTING What is Daylighting? 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 t...

DAYLIGHTING What is Daylighting? 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. - From Wikipedia, the free encyclopedia Daylighting 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. Daylighting 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 midwinter, it tends to be highly directional light that casts deep shadows. This may be partially ameliorated through light diffusion and somewhat reflective internal surfaces. Why Daylighting? ENERGY ISSUES: Two (2) rules to maximize the opportunity to reduce energy consumption in both electric lighting and the building’s HVAC system: 1. Minimize direct solar gain during cooling months 2. Distribution of the daylight within the building Why Daylighting? ENERGY ISSUES: Larger glass areas will likely provide a negative effect on a building’s winter load as well as solar heat gain during summertime may also be critical. Analysis of a daylight delivery system from an energy perspective requires complex energy modeling of both the lighting system and the building’s heating and cooling (HVAC) systems. Climate, both in terms of cloud cover and exterior thermal and humidity conditions, will have a significant impact on total energy consumption and a design that is efficient in one climate may be inefficient in another. Why Daylighting? ENERGY ISSUES: It is possible to operate a building with less energy using daylight to illuminate interior spaces than is possible using electric lighting alone. Electric lighting system may be either dimmed or switched off partially or entirely to provide energy savings in the operation of the electric lighting system. Changes made to the building envelope to deliver daylight to interior spaces, such as an increase in the window area or an increase in daylight transmittance, may affect a building’s heating and cooling loads, thus, does not guarantee a reduction in the building’s total energy use. Why Daylighting? ENERGY ISSUES: Minimize direct solar gain during cooling months This involves controlling east and west exposures wherever possible, and avoiding penetration of direct sunlight into interior spaces use of overhangs and other exterior devices. Minimizing glass area and maximizing daylight transmittance can help to minimize both winter heat loss and summer heat gain while providing sufficient quantities of daylight. Why Daylighting? ENERGY ISSUES: Distribution of the daylight within the building Daylight must be efficiently distributed to those areas where it is needed at levels that are near the desired illuminance for that space. Overlighting should be avoided since this will result in excessive heat gain. Why Daylighting? Color Quality of Daylight Due to its spectral composition, it provides excellent color rendering and is generally preferred over electric light sources. The spectral qualities affect both the color temperature and color rendering properties of daylight. Color temperature of daylight Color rendering of daylight Why Daylighting? Color Quality of Daylight Color Temperature of Daylight Most other times of the day, both direct sunlight and sky light are cool in relation to most electric light sources. Only at sunrise and sunset is daylight decidedly warm or red in appearance Noontime sunlight has a color temperature of approximately 5000 K, while north sky light has a color temperature in the range of 10,000 K or higher. Incandescent lighting at 2800 K, warm fluorescent at 3000 K while cool fluorescent is at 4100 K. Why Daylighting? Color Quality of Daylight Color Rendering of Daylight Daylight, except at the extremes of sunrise and sunset, renders color naturally because it possesses all of the wavelengths of the visible spectrum in a relatively uniform mix. Tinted windows and glass can alter the spectral quality of the daylight that is transmitted into a building and should be evaluated properly when selecting glass, if daylight is to play an important role. Color of daylight coming through a window will be influenced by a roof or other large surface that is of a highly saturated color and located outside the window. Surfaces outside a window may also affect the color quality of daylight that is reflected into an interior space and the color of paint on reflecting surfaces within the space will also affect the color quality of daylight Aesthetics Why Daylighting? It can provide visual interest and a dynamic character to a space because of its variability. Daylight, by its very nature, is highly dynamic and therefore adds visual interest to interior spaces making the appearance different depending on the exterior sky conditions that are present. Daylight provides interesting luminance ratios and patterns. Because of the intensity of direct sunlight, and the large amount of energy associated with it, low transmittance glass may be necessary to achieve comfortable luminance ratios and to avoid excessive solar heat gain when direct sunlight is admitted. Most daylight designs, limit the penetration of direct sunlight to eliminate the associated problems of heat gain and glare. 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. 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. Why Daylighting? Psychological Benefits The view out a window also adds additional aesthetic qualities to a space that are preferred by the occupants and allows a connection to the exterior and provides distant focal planes when workers take a short break from the near-field demands of most work Tasks. 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. 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. Skylights Skylights - horizontal windows or domes placed at the roof of buildings are 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, making all DOE skylight information libelous. Go to Walmart or Costco and look up to see what and how skylight energy efficiency is achieved. Skylights admit more light per unit area than windows, and distribute it more evenly over a space. They are, therefore, the best 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 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. Skylights The amount of light inefficient transparent glazing 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 attempt to validate 'daylighting expert' value. Like automated controls, attempts to achieve more value through expensive additional systems rarely, if ever, validate 'daylighting expert' worth. 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. 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. 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. Sawtooth Roof Another roof-angled glass alternative is a "sawtooth roof" (found on older factories). Vertical roof glass faces 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 lighting concept partially reduces the summer "solar furnace" skylight problem, but it still allows warm interior air to rise and touch the exterior roof glass in the cold winter, with significant undesirable heat transfer. Solarium In a well-designed isolated solar gain building with a sun room, solarium, 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 you see outside immediately when you enter most rooms. Halls should be minimized and 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. Solarium 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. They are particularly useful on west walls, where you would prefer to have no windows at all. 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, you can use an R-13 foam-filled solid energy-efficient exterior door. It may have a glass storm door outside. Opening the inner door allows light in. East/west glass doors and windows should be fully shaded top-to-bottom, or a spectrallyselective coating can reduce solar gain by more than three quarters. Exterior shade is better. Translucent Insulation Aerogel is a new alternative to inefficient windows, if all you need is daylight in a room. It has the lowest density of any solid material. It is translucent and has excellent insulating properties, but limited structural strength. Aerogel can be placed between two pieces of structurally-strong translucent fiber-reinforced plastic to create a translucent wall that allows 20% of available light to come through, but has an R-20 resistance to undesirable heat transfer. Images cannot be clearly seen through it, but diffuse daylight does pass through, without sacrificing good passive solar building thermal performance. Transparent insulation can be used effectively on many vertical walls, or on a sawtooth roof, to provide low-cost natural daylight with reduced heat transfer over window glass. Exterior-or-interior shading, shutters, drapes, or shade screens may be useful in some applications. Fiber Optic Concrete Wall Another way to make a secure structural concrete wall translucent is to embed optical fiber cables in it. Daylight (and shadow images) can then pass directly through a thick solidconcrete 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 Lighting Oak Ridge National Laboratory (ORNL) has developed a new alternative to skylights called Hybrid Solar Lighting, which 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. 2006-2007 field tests of the new HSL technology were promising, but the lowvolume 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. Hybrid Solar Lighting 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. End of Daylighting