Daylight
From Swikipedia
Introduction
With the solar heating movement, a less known but highly regarded design science has stepped to the fore—“daylighting,” the use of natural light to illuminate building interiors. Of course, this concept is as old as the window itself, but relatively recent advances in lighting research, window and glazing technology, and lighting controls have opened up new horizons for daylighting.
Daylight is nothing but the visible part of global solar radiation. It is the natural light which can be used in a very efficient manner in buildings and without compromising on the aesthetics or human comfort, huge amounts of energy can be conserved. A well designed building maximizes the use of natural lighting without compromising energy efficiency.
Brief Description
Decrease energy use:
Daylighting is the use of natural light, through skylights and windows, to complement or replace interior electric light. This is different than sunlight, which is a direct, often glare-producing beam. With appropriate controls, one can modulate how much daylight enters one’s space.
Benefits of daylighting:
• Pleasant, attractive spaces that people feel good about. Daylight in retail shops can encourage customers to visit more often and to purchase more. In day lit classrooms, students concentrate better. Workers in day lit commercial buildings experience improved morale and can become more productive
• Reduced energy costs for electric lighting
• Reduced energy costs for cooling, due to the reduced use of heat-generating electric lights
• Day-lighted buildings may lease at better rates, have reduced tenant turnover and greater resale value
Use of daylight for Energy Efficient Buildings:
The following measures can be adopted:
1. Locate Windows with Daylighting in Mind
Good lighting begins with the effective use of sunlight. Consider the size, location and primary function of the windows. Will the window frame a view, capture prevailing breezes, allow adequate light, or provide architectural balance? There is a tradeoff between the benefits of windows, such as daylighting and good views, and the major drawback, reduced insulation value. Because even new windows have a much lower insulation value than walls, they should not exceed 15% of the wall area, unless the building is specifically designed to take advantage of passive solar heating. With proper window placement, there should be more than enough daylighting in the building with 15% or less window-to-wall area.
2. Low-E Glazing
Low-emittance (low-E) windows keep the heat inside in winter and outside in summer and they also protect furnishings from fading. They have a thin film applied to the glass that improves the performance of the window. The two most important indicators of a window's performance are the U-factor and the Solar Heat Gain Coefficient (SHGC). The U-factor measures how well the window insulates. The lower the U-factor, the better is the insulation value. The SHGC measures how well the window blocks heat from the sun. The lower the SHGC, the less solar heat the window lets in. The better the window in these aspects, the more comfortable and more energy efficient is the building.
3. Seasonal Window Shading
In warm climates, sunlight can overheat the building through windows on the south and west facing sides of the buildings. Roof overhangs can provide shade for these windows as well as improve the building's long-term ability to withstand rain. They also allow the occupants of the building to open windows on a rainy day. Where overhangs will not throw shade, like first floor windows in 2-storey buildings, architectural features like awnings can offset any potential heat gain that windows on southern and western facades might be subject to during the cooling season. Deciduous trees and vine shading can also help keep the building cool. Window treatments like window films can also be added to windows to keep heat from the sun from entering the building, although they have the disadvantage of darkening the building throughout the day. Shades and blinds can also be closed during peak solar hours to prevent overheating.
4. Tubular Skylights
Where windows are not practical due to elevation or a room's layout, tubular skylights can provide natural light while minimizing the size of the roof penetration. Tubular skylights have a roof-mounted light collector that reflects light through a metal or plastic tube with a highly reflective interior coating. The reflective tube guides the sunlight to a diffuser lens mounted on the interior ceiling surface that spreads light evenly throughout the room. They are frequently used in windowless bathrooms and closets where natural light is desirable and where skylights aren't possible because of attic space above the ceiling. They are also often installed in other dark spots throughout the house, such as hallways and stairwells. In single-storey buildings they can direct natural light to basements.
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