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LED Street Lighting:

 

 A Design Based Approach For Maximum Light Efficiency And Energy Saving

 

As opposed to traditional approach wherein a customer demands specific wattage from manufacturers, in a design based approach, customer demands light, i.e. Lux levels. Eventually, from a lighting product, what matters is light and not watts.

 

Public Lighting amounts to over 1% of total energy consumption and roughly 4,400 MW of connected load in India. With India’s urban population set to increase from 400 million in 2011 to about 600 million or more by 2030, energy consumption for street lighting is set to shoot up significantly as well. New technologies like Light Emitting Diodes (LEDs) present an excellent option to reduce energy consumption in street lighting by as much as 50-60%, as demonstrated in many parts of the world. An LED is a semi-conductor device that emits visible light of a certain colour. High power LEDs are capable of producing higher light output using lesser electricity as compared to conventional light sources.
In addition, the co-benefits of LED include longer life, better quality lighting, lower maintenance costs, enhanced public safety and improved user satisfaction. LEDs have a relatively long useful life, when compared to conventional sodium vapour or metal halide lamps, i.e. 50,000 – 100,000 hours versus 15,000 hours.
 

 

LED Lighting is changing the landscape of outdoor lighting and today, outdoor lighting plays a crucial role in the safety and security of our streets and public places. Thus, Street lighting has to be smarter to comply with new legislation, environmental challenges and requirements to reduce energy usage.

 

  The LED market in India has been growing at a rate of over 50% for the last five years (2009-10 to 2013-14) and is expected to sustain this growth rate in the next five years (2014-15 to 2018-19).

 

  The Government of India has announced a lot of incentives and measures in order to prioritise the use of LEDs in the country. It is clear that the Street lighting application leads India’s LED lighting market, due to various state governments’ initiatives to replace conventional sodium lights with LED lights.

 

  This article on LED Street Lighting endeavours to demonstrate how a design based approach can be adopted to maximise both light and energy efficiency and save on the total project cost for the customer.

 

What is a design-based approach?

 

  As opposed to the traditional approach, wherein a customer demands specific wattages from manufacturers, in a design-based approach, the customer demands light i.e., Lux Levels, which may be as per the National Lighting Code or customer specific and does not specify watts at all. Eventually, from a lighting product, what matters is light and not watts.

 

  The customer provides the site details like the Pole Height, Pole Spacing, Road Width, Boom Length etc. to the manufacturer – and based on these details, manufacturer simulates designs in special lighting softwares by using their own IES files to achieve average lux level and uniformity ratio as per National Lighting Code or IS 1944 or as demanded by the customer.

 

  The National Lighting Code or IS1944 classifies the type of roads as given under:

 

 

 

 

Objective of the customer:

 

  To achieve the required illumination level and uniformity ratio with least wattages (Power Consumption) and eliminating dark patches...

 

  Thus, taking into consideration the above table and choosing the appropriate type of road, the customer floats, an RFQ based on illumination level and uniformity ratio and not on power consumption. He/she also takes into consideration the energy cost over the lifetime of LEDs and to minimise that.

 

  Design-based approach is the most practical approach for LED Street Lighting as it takes into consideration not only the initial investment but also the energy cost during the lifetime of LED Street Light.

 

Case study: Traditional approach

 

  Let us understand the difference between two approaches.

 

  In the old traditional approach, the customer demands specific wattages i.e. 80W LED Street Light to replace the existing 150W HPSV. The customer asks for Quotation of 80W LED Street Light from three different sellers. He/ she has not specified any lighting levels, and so there are lot of differences in specifications of all different manufacturers as under:

 

 

 

  If we analyse the table above, Manufacturer C offers his 80W LED Street Light at Rs. 9000/-. The customer would opt to buy the Street Light from Manufacturer C considering lowest price. However, post installation, the illumination levels and uniformity would definitely disappoint the customer.

 

  Manufacturer A has a price of Rs. 11000/- for his 80W LED Street Light and he is offering an Avg. Lux of 15, which is almost double that of C. The system efficacy is also 100 lm/W and uniformity ratio is also > 0.5, which is excellent. Had Manufacturer A to offer the same specifications as offered by Manufacturer C, he would have offered just 40W instead of 80W. This way, the customer would not only save on the initial capital investment, but also save additional energy of 40W per street light during its lifetime.

 

  By just specifying watts, the customer is losing on an excellent quality product.

 

Why should you adopt a design- based approach?

 

  As seen in the case study above, there are differences in specifications in luminaires of different manufacturers. The major differences may be in:

 

Luminaire Design - Thermal Management

Type & Efficiency of LEDs used

Quality & efficiency of optics employed with reference to the type of load Driver’s efficiency and quality.

 

  Thus, these differences may lead to adoption or procurement of poor quality street lights, which would not deliver light as the customer would have originally thought of. This would disturb the very purpose of LED Street Lighting.

 

  For the customer, what matters eventually are lux levels and the uniformity ratio obtained from least wattages. Thus, inferring from the above table, price of a street light does not depend on watts but on lux levels and uniformity ratio.

 

Design-based approach in LED street lighting – a case study

 

  In this case, the customer demands an average lux of 15, Uniformity Ratio > 0.4 and provides the design details to the manufacturer: The Pole height is 9 m, Road width is 8 m on two sides with a 1 m median in the centre, Boom Length is 1 m and the degree of tilt is 5°. The manufacturer has to design wattages.

 

  Let us see and analyse the designs provided by three manufacturers to achieve the required illumination & uniformity ratio:

 

  Looking at the table below, Manufacturer A provides the desired illumination and uniformity with 70W, Manufacturer B with 90W & Manufacturer C with 110W.

 

 

  Manufacturer C offers highest wattages i.e., 110W. At the same time, for the customer, the initial capex if he/she opts for Manufacturer C is the lowest i.e., Rs. 10000. But if we look at the overall cost which the customer will incur over the lifetime of the Street Light, i.e., Product Cost + Energy Cost, Manufacturer A’s LED Street Lighting system would be the most economical. The total savings by opting for Manufacturer A as compared to B & C is substantial and therefore the intial capex can be justified.

 

  If we consider 100 Street Lights in the project, the customer would incur a total cost of Rs. 34.5 lacs by installing Manufacturer A’s lights, Rs. 42 lacs for Manufacturer B & Rs. 49.5 lacs for Manufacturer C. Now whose lights would be the most ideal and beneficial to install? Obviously Manufacturer A’s.

 

  This is just a hypothetical example to show that even if the initial investment incurred by the customer is high, the total cost by opting for Manufacturer A’s product would be substantially low. It may also be the case that the customer would incur same capex for 70W & 110W.

 

  The difference in wattages of all three manufacturers is on account of the type of LEDs used, adoption of optics/lenses, efficiency of driver and thermal management.

 

  Similarly, for a completely new installation with new poles, the customer can save a lot on capital costs and energy costs by providing road dimensions to manufacturers and asking them to design the Wattages as well as Pole Spacing. With a longer pole span, the no. of poles and luminaires can be drastically reduced.

 

Physical verification of average lux and uniformity ratios - 9 point method measurement

 

  Average Lux and uniformity ratio can be easily verified by installing 3 LED Street Light samples on 3 Poles in case of single side installation or 6 LED Street Light samples on 3 poles in case of a median.

 

  Light is to be measured at 9 points as shown in the diagram with help of a simple lux meter.

 

Average Lux= (P1+P3+P9+P7) / 16 + (P2+P4+P8+P6) / 8 + P5 / 4

 

Uniformity Ratio = Min Lux / Avg Lux. (Generally, the minimum lux would be at P8 since it is the farthest point from both the poles.)

 

  Thus, the lighting quality can be ensured by a mock sample installation.

 

 

 

 

 

 

Benefits of design-based approach

 

  By adopting a design-based approach, the customer is taking into consideration the total project cost over the lifetime of the product, which is very practical.

 

  At the same time, the lighting levels and the uniformity ratios achieved are as per IS 1944 or as per customer’s requirement. There is no compromise on the quality of light.

 

  Importantly, the customer is saving maximum energy by going for least wattages from the available LED Street Lights in the market.

 

  There may be a drastic decrease in the overall capital cost when the number of poles and luminaires are reduced as a result of longer pole span in case of new installation. In case of replacements too, saving in energy costs will be substantial.

 

Source: Lighting India,

           Climate Change Association

 

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