STREET LIGHTING OPTIMISATION

By design, street lighting systems span hundreds of kilometres of road and therefore presents with a vast potential for energy performance improvement/ system optimisation. However, many existing street lighting systems are outdated and inefficient in their performance.  Just how do we optimise this lighting feature?

First off, we need to know if we even need it in the first place; some streets already benefit from the light pollution/ overspill from lit buildings in its vicinity, while others have the wrong lighting solution inappropriately used. Ultimately, it is in the lighting design and applied technologies that the optimisation is achieved. Street lighting systems can be the most optimised lighting system if due consideration is taken at design, sourcing and installation stages.

To start with, understanding what type of road it is you are providing with a lighting system is imperative as is factoring in the sun-hours of a specific location in designing your street lighting solution.  Figure below provides a sunset and sunrise profile for Lusaka, Zambia:

In this instance, the operating duration for the street lights is averaged at 12 hours per day (6am – 6pm), 365 days a year (4 380 hours per annum)

What Institutions such as Road Development & Maintenance Agencies including the Municipal Councils ought to do is develop a blueprint for all future new street lighting designs supported by a green sourcing policy for best available technologies (BAT), conduct extensive on-site measurements to gather the pre-design data as well as consult / reference any existing International as well as local technical requirements and standards when designing for new build or retrofitting existing street lighting systems.

In order to optimise the performance and operational efficiencies of street lighting solutions whether existing or proposed, one needs to understand what an optimised street lighting solution should look like, how much it should cost to run and how it should perform. Arguably, a Light-emitting diode (LED) system that is intelligently controlled and solar - powered is the ultimate optimisation we should seek. But just why LED lighting versus the traditional High-intensity Discharge (HID) systems we are used to? What about the supposed performance and application limitations of LED for street-lighting? Can LED lights provide the illumination levels required for visibility on a street? What about the total cost of ownership/ Lifecycle costing (LCC) of the LED street lighting system?

The financial savings realised from an efficient LED street lighting system are based on the underlying low wattage lighting technology and the related reduction of energy used as well as the avoided routine relamping and maintenance costs (Longer lamp-hours), relative to older street lighting HID systems. The majority of costs associated with street lights stem from the operation of the lighting system and not from the initial capital outlay/ investment itself. Typically the total cost of a traditional HID street lighting installation over a period of 25 years is approximately 85% on operations and maintenance (including energy costs) and 15% is for the initial capital outlay. Therefore an optimally performing system is what is desired over time.

Though an LED street lighting system may require additional pole interspacing to allow for narrower Pole spacings to meet the recommended practice for illuminating roads, the cost of additional poles can be significantly reduced with the type of pole sourced (e.g. fibreglass in preference to steel poles) or simply procuring LED lights of comparative performance as the traditional HID light (refer to table 1 below) and hence maintain the usual pole spacing without the need for additional poles for interspacing. The increased cost of the higher performing LED light will then be mitigated with voided relamping and maintenance costs over time.

It should be noted however that the application of LED lighting is not a one-size fit all; as such very elaborate and specific design principles have to be exercised in a case-by-case manner.  This is where Lighting design expertise should be sought. As an example, a motorway designed for high speeds and low pedestrian conflict along with one-way traffic will require a different lighting design than a residential road with a high pedestrian conflict.

In LED street lighting design, it is imperative that a broad-spectrum LED light is considered as a match for an HID light. Technological advancements have afforded us with broad-spectrum LED lights that can be applied in these street lighting applications with comparative results to the traditional HID lights. The LED Lights present additional benefits and an optimisation prospects for peak performing street lighting systems.

Table below provides LED - HID equivalence based on lumen output range and maximum wattage:

Sourced from the City of San José Public Streetlight Design Guide

Table below is an example of an LED technical specification;

In optimising a Street lighting system, a prioritisation/ rankling approach has to be followed starting with a traditional system to a fully optimised LED system that is intelligently controlled and solar – powered. Ranking should be based on a number of criteria that includes ease of implementation, cost and perceived disruption to the operations of the existing system (if a retrofit). In Prioritised ranking, the implementation has to be in the order as illustrated below:

1.   Start with applying low-cost controllers to all lights

2.   Followed by removing high wattage lamps and replacing them with low wattage lamps (e.g. 400W to 250W)

3.   Complete retrofit of all high wattage HID lights with Lower wattage LED lighting technology (400/250W to 70/60W)

4.   Change to solar-power from grid-power by incorporating solar PV panels to existing lower wattage LED lights

5.   And finally do a complete system over-haul of providing state-of-the art, best available technology (BAT) in integrated solar-powered street lighting. This becomes the new design and green sourcing imperative for the Council/ Municipality of providing off-grid solar powered street lighting for all new build road networks and public spaces.

In further detail, the prioritised/ ranked implementation is as given below:

1.   Start with applying low-cost controllers to all lights:

Application of Intelligent Lighting controls to the existing and new retrofits/ designs results in energy consumption reduction/ energy performance improvement.

The current and most prevalent lighting control regime is the use of timers that switch the lights on and off at a pre-set time and for a pre-determined duration irrespective of obtaining natural and artificial light conditions.

Example of type of intelligent controllers is given below:

Proposed Intelligent controllers use an integrated timer with daylight harvesting/ Crepuscular features of twilight dimming where the light is dimmed depending on whether it's a cloudy day or clear sky day or gradual dimming at twilight or when sunset or dawn starts to set in. Typically, a good control system provides both light control and time control functionality for operator own preference.

There are however other control systems that either come as standalone add-on to an existing system or come integrated into a lighting fitting or energy storage system.

e.g. the lithium battery intelligent control system illustrated below is an integrated control system within a energy storage/ battery unit:

 Image sourced from Anhui Longvolt Energy LTD Product catalogue

The inteliLIGHT? control system offers lamp-level management capabilities that include Individual dimming and ON/OFF switching of the street lighting fixtures. Below is an image of an inteliLIGHT Control module (indicating where it is housed within the lamp head):

 The application of this intelligent controller provides a dimming feature as illustrated in the 24-hour profile below:

The intelligent control of the lights has an added benefit of elongating the lamp life of the lights and reducing on early lamp failures and unplanned re-lamping.

2.   Followed by removing high wattage lamps and replacing them with low wattage lamps (400/ 250W to 70/60W)

Retrofitting the existing High wattage 250W HPS lights complete with the Luminaire/ light fitting with the low wattage 60W LED light results in reduced energy consumption at maintained operating levels.

It should be noted that LED wattage should be matched to lamp height for optimal performance, e.g. 60W LED with 9m pole and 80W LED with 10m pole.

Here below is an illustration of a transition from a high wattage to a low wattage light

The replacement of 250W HPS for 60W LED may require re-adjustment of pole spacing with the possibility of inter-spacing additional poles between the 50m spans to reduce them to 25m spans. This has an effective 50% increase in number of LED street lighting poles. This is however subject to a rigorous onsite measurement and compliance testing of the street lighting design philosophy. Additional cost of poles as earlier explained can be reduced with use of alternative cheap pole material or simply finding the matching LED light without need for inter-spacing.

3.   And finally do a complete system over-haul of providing state-of-the art, best available technology (BAT) in integrated solar-powered street lighting. This becomes the new design and green sourcing imperative for the Council/ Municipality, of providing off-grid solar powered street lighting for all new build road networks and public spaces.

Total utility energy cost saving is finally achieved with a complete migration from grid-power to a Solar-powered lighting system of the retrofitted LED lights or a complete redesign of the street lighting system installation of a fully integrated all-in-one solar-powered LED lights that come complete with solar panel, LEDs, battery and controller.

The complete removal of the street lighting system from Grid connection to 100% solar-powered system, results in zero utility billing and cost. There is an additional cost benefit of avoided recurring re-lamping costs for at least +10 years given the 50 000 lamp hours for the LED lights.

Image sourced from Solarsol Product Catalogue

This becomes a fully optimised street lighting system in all aspects (reduced total cost of ownership, increased light output/ performance, improved system durability, enhanced installation aesthetics, et cetera).