Monday , 27 May 2019

Energy efficient street lights

Road Lighting consumption accounts for 30-40% of total electricity consumption in the city. As the energy prices are fast moving northward, the gap between its supply and demand is widening day by day. But by adapting to recent technologies and using lighting controls with energy efficient sources like LED or induction lamps, the consumption of energy can be reduced to a considerable level.

Objective of Road Lighting

• Improving the visibility to facilitate the easy flow of traffic and pedestrians

• Reducing night accidents

• Providing visual cues and revealing obstacles

• Preventing crime and aiding in police protection

• Enhancing the appearance of city at day and night

• Promoting business

The quality of road lighting is divided into four categories – a) Illuminance Level, b) Uniformity, c) Glare Limitation and d) Optical guidance. Uniformity as per the specified standard has to be maintained for all road lights to avoid dark patches on the road. It can be achieved by using proper optics, mounting height and type/wattage of the lamp. If good uniformity is maintained on the roads, it can achieve five times the result of a well mounted light by using LED luminaire designed with good optics and with proper spacing between the poles.

There are mainly two types of Glares: Discomfort Glare and Disability Glare. By selecting the proper optics, lamp wattage and mounting height, glare can be limited within the values mentioned in the standard.

Optical guidance is very important in road lighting. It can be achieved by:

• Designing the road junction with higher illuminance level compared to the connected roads. For example, if the connected roads are designed with average illuminance level of 15-20 lux, then the junction can be provided with 30 lux.

• Using different colour of light for road junctions and connected roads. In case the connected roads are provided with sodium luminaires which are golden yellow in colour, the junction can be designed with white colour with the use of metal halide or induction luminaires.

• Using luminaires of different types and arrangements for the main and secondary roads. Today different shapes of luminaires are available. By designing the main road and secondary roads with different shapes / types of luminaires and different shapes of brackets, the roads can be easily identified and differentiated. Likewise, we can have different types of mounting arrangements for the roads. For example, the main roads can be designed with opposite arrangements and the secondary roads can be designed with staggered or single sided arrangements. High mast is the ideal choice for the junction lighting which is easy to maintain. It avoids cluster of poles which further helps to reduce the energy cost.

Road Lighting Design

Steps involved:

1. Determining roadway classification

2. Selection of the lighting level and uniformity as per the standard

3. Preliminary selection of several luminaires and light sources

4. Preliminary selection of one or more lighting system geometries, including mounting heights and lateral luminaire positions

5. Selection of the light loss factors

6. Calculating the pole spacing for various luminaires and lamp combinations

7. Selecting the scheme based on optimal design

8. Final selection by considering safety, aesthetics, cost and maintenance

Selecting the Luminaire

Reflector system, compactness, light source e / t, mechanical strength, degree of protection, aesthetics, ease in installing & maintaining and Economy are the parameters to be taken into account while selecting the luminaires for road lighting.

Selecting the mounting height

Though the uniformity improves by increasing the mounting height, the lux level on the road goes down. In the case of single sided arrangements, the mounting height is to be maintained almost equal to the road width and it can be decreased for opposite or staggered type of arrangement. As the mounting height increases, the glare reduces. So, in case the trees are present on the sides of the roads, the mounting height has to be maintained below the level of the tree branches to avoid the shadows of the trees from falling on the roads. In order to access the luminaire for the maintenance, the mounting height is generally considered below 14m unless the luminaire has the lowering or rising type of arrangement like High Mast.

T5 Street Light

T5 lamp has got good efficacy and longer life. It emits white light which has better visibility as against the yellow colour sodium vapour lamps and is suitable for secondary roads, colonies and boundary lighting. T5 luminaires with different wattages and combinations are now available in the market.

LED Street Lights

LED Street lights are becoming popular in India and have many advantageous over the conventional street lights. At present, the LED street light of low wattages which is suitable for lower light levels is more popular since the payback for higher wattages (required for main roads or Class A type of roads) is not attractive. The problem of ‘hot spots’ below the luminaire, which is common in HID luminaires, is absent in the case of LED street lights. LED street lights give very good uniformity (more than 60%) and better quality lighting. LED lights have more than 60,000 hours of life. When used with solar power, they save enormous amount of energy and lower the operating and maintenance costs. LED Street lights are the right answer for remote areas like villages where the power supply reaches with difficulty as they give uninterrupted light during the night hours. The laying of power cable and cable trenching, which are necessary in the conventional lighting system, are done away with in LED lighting. By using the proper lenses, the optics of LED luminaires can be better controlled and they will give more optical efficiency when compared with conventional luminaires. The biggest challenge in LED luminaire, however, is the designing of the heat sink which will determine the lamp life and lamp lumen output. Since there is no mercury in LED, it is ideal for Green Building Lighting.

Induction Street Lights

Induction lamps have a number of advantages over the conventional lamps like sodium and metal halide lamps. Induction lamps have a lamp life of more than 60,000 burning hours and re-strike instantly unlike metal halide lamps which need 10-15 minutes to re-strike during power failures. Induction lamps are the best solution for cold area applications since there is no lumen depreciation in them at cold temperature and they can start instantly. Electronic Control Gear of induction lamps ensures lower losses and reduces total system consumption. Induction lamps help in up to 50% energy saving as compared to the conventional lamps. Better quality of light gives higher perceived Illumination. Higher Photopic Lumen ensures that illumination perceived by human eye is higher in induction lamp. It effectively operates in temperatures ranging from 400C to + 500C and can also operate in areas prone to vibration. It is very suitable for street lighting application.

Master Plan for Road Lighting

A master plan has to be drawn at the initial stage of the project. The following points are to be considered while preparing the master plan:

• Discussion with different utilities such as drainage and water supply.

• Lighting simulation for selected stretch of road at day and night time.

• Considering the importance for visual guidance and position of pole.

• Studying the illumination of other structures related to road lighting.

Today, street light luminaires, better known as street furniture, are not considered merely as light providers but are taken as an integral part of the city beautification too. LED and induction luminaire will play a major role in future road lighting applications. Energy efficient products with reasonable payback period are well accepted in the Indian market now. Quality products meeting international standards (CE / ROHS / UL / CSA) are available at reasonable prices in India. Lighting controls and supply voltage dimming arrangements which will save enormous amount of energy are getting popular in municipalities.

K Naveen, General Manager
Lighting Design Luminaires BU
Bajaj Electricals Limited

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