Lighting Energy
Another important finding by Lam (2000) was the significance of reducing the lighting load in office buildings in Hong Kong, as the study showed that this load has a much higher sensitivity coefficient than the building envelope, indicating greater influences. The total energy consumption went down by 15 % after lowering the lighting load from the 20 to 12.5 W/m2 with an indoor illuminance of 500 lux. It was recommended to maximize reliance on daylighting as daylight can result in significant energy savings in cooling-dominated commercial buildings in hot climates. That is because: (1) less artificial lighting is used, and (2) daylight has a higher luminous efficacy (110 lumens/W in Hong Kong) than most electric lighting systems (60 lumens/W), thus generating less heat per lighting level provided.
Since lighting control integrated with daylighting is recognized as an important and useful strategy in energy-efficient building design and operation, it is believed that proper daylighting schemes can help reduce the electricity demand and contribute to achieving environmentally sustainable building development. Li and Lam (2001) conducted a study on lighting performance in fully air-conditioned office building facing opposite orientations with day lighting controls in Hong Kong. Electricity consumption by the fluorescent luminaries, indoor illuminance levels, and the room parameters affecting daylighting designs were recorded and analyzed. The results showed substantial lighting energy savings (up to 50 % for the perimeter offices) if proper daylighting schemes are incorporated. Further electricity savings can be realized as a result of the reduction of heat dissipation from artificial lighting, and hence, lower cooling load. The study recommends attending to the subtle interactions of a large number of design features as it could help to improve the quality and quantity of daylight entering into an interior space. To have a more complete picture of the indoor visual environment, more on-site measurement of the actual visual condition is required.
Lighting accounts for the most energy consumption (39 %) in office buildings in the U.S. Reducing artificial lighting loads requires an integrated design approach aiming to optimizing building configuration to admit natural light and using more energy-efficient artificial lighting installations such as efficient low energy lamps, better electronic ballasts, and high quality fittings. Energy-efficient lighting design contributes greatly in improving sustainability levels, especially with regard to energy efficiency and indoor environmental quality. For instance, 75-W incandescent lamp with an 18-W compact fluorescent lamp will avoid emitting the equivalent of 4300 kg of carbon dioxide and about 10 kg of sulphur dioxide from a typical generating plant in the USA (Yeang 1999; MacKenzie 1997; Zeiher 1996). Lighting switching systems can contribute in achieving significant energy savings. This can be coupled with the building management system (BMS), or by using local controls and ambient light sensors to adjust artificial lighting based on the amount of natural light entering the building. An office with simple daylight strategies (such as sidelights and light shelf) and fluorescent lighting system can achieve 60 % total reduction of lighting energy and 51 % annual electric energy savings (Guzowski et al. 1994).
The potential of daylight in saving energy in buildings in the eastern coast of Saudi Arabia was investigated by Alshaibani (2001). The study used a room prototype with certain size and glazing properties (width: 4 m, depth: 5 m, height: 2.8 m, reflectance: 0.47, transmittance: 0.7). The method was based on estimating the percentage of occurrence of the vertical illuminance of 10,638 lux during a full year based on daylight requirement of 500 lux inside the room prototype. The results showed that this requirement of vertical illuminance was available at the four orientations for more than 75 % of the working year; and hence, this indicated a potentiality of 75 % savings in artificial lighting consumption.
