Natural sunlight consists of three portions, UV, visible light and the near infrared portions of the spectrum. All three portions result in heat when sunlight is absorbed. As UV can cause sunburn, cancer and fading of furniture, it should be stopped from entering indoor environment. Visible light, between the wavelengths of 380-780 nm, is a free and green source of illumination that can reduce artificial lighting. U.S. Department of Energy reported that many commercial buildings can reduce total energy costs by up to one-third through the optimal use of daylighting. However, excessive visible light can create glare problem. Eye-friendly environment should be provided in computer workspace area.
Solar elevation and azimuth, and solar intensity vary from sunrise to sunset and vary from spring to winter. Sunshine duration varies throughout the year. Solar intensity varies during sunny, cloudy or rainy situation. As a whole, daylighting intensity entering building envelop is not constant.
To control inlet of daylight into a building, interior sun shading system, tinted/low-e glass, and/or exterior sun shading system are applied nowadays.
(1) Interior sun shading system
Solar transmittance of common interior sun shading systems (for example, roller blind, curtain…etc.) are fixed after installation. To allow optimum inlet of strong visible light at noon (highest sunlight intensity), interior sun shading system with low visible light transmittance is usually selected. The consequence is insufficient inlet of visible light before or after noon. When peripheral visible light intensity is low, interior sun shading system is in “OPEN” or “Up” status. On the other hand, solar heat absorbed by interior sun shading system has to be removed by cooling load in summer. If interior sun shading system is installation between glasses, mechanical ventilation can remove the absorbed solar heat.
Solar transmittance of glazing glass is also fixed after installation.
(3) Exterior sun shading system
When applying exterior sun shading system, wind load, installation and maintenance cost, and safety are important concerns. To allow optimum inlet of strong visible light at noon (highest sunlight intensity), exterior sun shading system with low visible light transmittance is usually selected. Motorized louver, a popular exterior sun shading system that can withstand gust, can only tilt the slats, but not fully retract the slats. When peripheral visible light intensity is low, slats reduce visible light transmittance. Exterior sun shading system has an outstanding feature. Absorbed solar heat absorbed is dissolved by natural air movement.
(1) Sun shading system should have adjustable daylight transmittance according to changing daylight intensity.
(2) Reflected daylight has not been utilized as tenants avoid daylight reflected from the sun shading system installed in opposite building.