The importance of artificial lighting in todays scenario is increasing day-by-day. The proportion of the total energy that is consumed in its production varies between 3 per cent and 5 per cent, depending on the country concerned. Hence, reductions in lighting can only influence total energy consumption by a small amount. In addition, it must be borne in mind that lighting is for the benefit of people, enabling them to see, not only so that they can perform their tasks with maximum efficiency but also for their general well-being and prevention of eye-fatigue and reduction of errors and rejects. This article deals with the effective use of lighting devices in order to save energy.
Management of energy
to facilitate and accelerate the introduction of energy conservation, the experts of the field are carrying out innovations and various researches. In most buildings, especially in the tropics, the energy consumed by air-conditioning is higher than that consumed by lighting. Therefore, when thinking in terms of energy conservation, it is necessary to arrive at a design giving an efficient integration of air-conditioning with the internal heating sources, which include the lamps. In addition, it is also necessary to consider the exchange of energy between the inside of the building and the outside, as a whole. For the latter aspect, there are two important factors. The first is the need for a favourable surface-to-volume relationship of the building itself. This can be facilitated by deep-plan buildings that are more or less cubical in shape (thus having a maximum space enclosed by a minimum envelope). The second is the need for good heat insulation of the outer walls and reduced transmission of heat through windows. This insulation can be improved by reducing the proportion of the glazed (window) area to about 20 per cent of the total area of facade. This amount of window has proved sufficient for building appearance and for maintaining visual contact with the outside world while reducing solar heat-gain from outside.
Lighting system efficiency
In many cases, lamps and luminaires are one part of the total system that has to be planned as a whole to achieve the most efficient results. When planning such an installation, not only the initial costs should be looked at but also the total economics over a period of time should be looked into. This will imply that sometimes-higher initial costs should be accepted in order to achieve lower overall running costs.
The constant factor influencing energy consumption is the lighting level. Therefore, this aspect has to be specially considered. Lighting experts who want to decide on the lighting level for a specific activity or an interior can rely on : Investigations into the relations between visual performance and lighting level. The results of these investigations are important for working interiors - commercial and industrial - and give guidance on how improved lighting can lead directly to increased productivity and to improved quality and reliability of the finished product.
Assessments of the degree of visual satisfaction produced by the lighting. Visual satisfaction (which is different from visual performance) is an important additional criterion in all types of environments.
By comparing existing installations and doing on-the-spot tests in some cases, one can obtain a fairly reliable assessment of what levels are needed.
Relying on codes of practice
All the above investigations are the basis of Lighting Codes of Practice in many countries. Hence, design illuminances for different applications can be selected straightaway to form the basis of lighting design or renovation. The dominant aim of lighting design for working interiors has always been to install sufficient light to allow work to be carried on effectively, safely and in comfort. Sufficient light in this respect means illuminance and this will primarily depend on the difficulty of the task and the level of performance desired, although a workers satisfaction with his visual environment must also be considered. The visual performance criterion is not so valid in circulation areas and places intended for social contact and relaxation where the emphasis is placed almost entirely upon visual satisfaction. Thus, in working interiors, lighting for visual performance is a basic necessity while visual satisfaction can be an additional refinement.
The term visual performance is basically productivity, related to lighting. It is used to indicate quantitatively how a work is performed in terms of speed and accuracy when detecting and identifying details in the visual field. The standard of visual performance rises with increasing luminance (which is the reflected brightness which again is proportional to illuminance). Other factors are the size of the task, its distance from the eye (i.e. apparent size), contrasts in luminance and colour. Another factor is the performance of the eye itself, which decreases with advancing age. The trend towards improved visual performance, reduction of errors and minimising fatigue become optimum as illuminance increases. At low illuminances, the slope of each curve is rather steep. Above 500 lux, however, the improvements begin to level off until, above 1000 lux, there seems little to be gained form increasing the illuminance further. These curves, can therefore, be regarded as standard lighting preference curves for many common tasks taking place in working interiors like normal offices and industries. From these results and taking into account the aspects of cost and energy consumption, the range of horizontal illuminances between 200 and 1000 lux (with a frequent optimum of 500 lux) can be taken as the recommended illuminance-range for the general lighting of working interiors.
It must be kept in mind that not all lamp types are suitable (because of their specific qualities) for all application fields. A special mention must be made here of the introduction of "Triphosphor" fluorescent tubes by the lamp manufacturing industry. Prior to the Seventies, a high lumen output and a good Colour Rendering Index (CRI) were considered incompatible in a fluorescent tube. The so-called deluxe tubes had a good Colour Rendering Index but their output was always lower than the standard tubes. But in the past 10 years, both the fluorescent powder technology and lamp manufacturing techniques have so vastly improved that energy saving tubes are available with Colour Rendering Index of 85 and at the same time with a luminous efficacy of 95 lm/watt (excluding ballast losses). These tubes are available in the common 18 W (610 mm), 36 W (1220 mm) and 58 W (1525 mm). Philips calls them the Colour 80 series.
These lamps together with low-loss control gear and high efficiency luminaries enable designers to achieve an illuminance of 500 lux in office interiors at a loading of about 10 W/sq m.
Energy-effective lighting practices
1. Check the visual task in terms of difficulty, duration, criticalness and surroundings to determine the lighting need. National and international Codes of Practice for interior lighting give a list of applications vs the recommended illumination levels as well as limiting glare levels.
2. Select the most efficient and appropriate lamp for the application (office, lowbay, highbay industry, etc), taking into account the demands of colour rendering and colour impression. The use of Compact Fluorescent Lamp (CFLs) in the place of incandescent lamps can save up to 80% energy.
3. Provide high room-surface reflectances to achieve the best Utilisation Factor for the lighting
4. Integrate lighting with the air-conditioning system to save on the energy needed for cooling purposes. In open-plan offices, air-conditioning and lighting can be combined in such a way that the return-air is extracted through the luminaires. The result is less heat penetration into the space, thus increasing thermal comfort and allowing for less air changes per hour and more economic air-conditioning.
a) Provide a flexible lighting system so that sections of the interior can be turned off or reduced in lighting level when not needed.
b) Provide a flexible mounting system for the luminaires so that their positions can be changed according to changing office layouts. This can be accomplished either by using power tracks or by the use of false ceilings and electrical junction boxes, so positioned as to facilitate easy relocation of luminaires. Apart from High Frequency operation, a new generation of fluorescent lamps called TL5 has also been introduced in 1995. Due to their much superior Lumen-maintenance over life, very well-defined light-distribution, compact, convenient luminaire-size (which will fit well inside ceiling modules) and continuous mounting possibility, the TLD 18W - 36W - 58W luminaires can be substituted by the TL5 14W - 28W - 35W systems and imagine the energy savings for the system.
c) Use new High Frequency (28 - 32 kHz) electronic ballasts instead of the tradition magnetic ballasts. Then a 36 W tube consumes only 32 W and a 58 W tube consumes 50 W in the lamp. Ballast loss (for twin tubes) is about 9 W and 12 W, respectively.
d) Another modern method is microprocessor-controlled switching of different circuits by using logic units located in the ceiling, which can take pre-programmed commands and activate specified lighting circuits.
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|Posted : 10/26/2005|