Energy-Efficient LED Street Lights Improve Visibility, Safety

Street lighting is an important asset in any urban area, contributing towards safer roads and greater security in residential, business, or municipal areas. According to research referenced in an Echelon Corporation White Paper 1, traditional high-pressure sodium streetlights can account for nearly 40 percent of a city’s electricity costs. Electricity costs continue to rise, and as existing infrastructure ages, already high maintenance costs are increasing further.

In addition, regions, national governments, utilities, city corporations, industries, and individual consumers, are feeling the pressure to reduce energy consumption, particularly energy deriving from fossil fuels, in order to reduce CO2 levels. Many nations are working towards objectives agreed to in the Kyoto Protocol and more recently 20/20 targets to reduce CO2 emissions. Low power initiatives abound. In Europe, there are EU directives, such as 2009/125/EG ErP, with similar programs running in the US (Energy Star) and elsewhere.

Meanwhile LED lamp technology has become eminently suitable for lighting applications, and today represents a more cost effective alternative to traditional street light bulbs. An LED lamp’s cost effectiveness is partially due to less power usage and longer lifetimes. Although these are important cost saving factors, the more significant savings are realized because LEDs can be networked and monitored, thereby improving maintenance efficiency, and because they can be controlled remotely.

Smart cities are safer and more secure

It’s not surprising that this combination of forces is driving an increased focus on upgrading street lighting, making it a major target application to help reduce energy consumption as well as save costs. Cities throughout Europe, particularly in the UK, France, Germany and Scandinavia, have launched ‘smart city’ initiatives. LED street lighting is at the heart of many of these ongoing, long term, multi-faceted projects, and is inspiring public/private partnerships between city corporations, utility companies and service providers.

Figure 1: LED street lighting provides a more natural white light (left) than conventional high-pressure sodium lights (right), improving visibility and reducing dazzle, as well as using less energy (Photo source: OSRAM).

Not only is the switch to LED street lighting proving that it can save money and energy, but it also helps improve safety, security, and aesthetics in the city. Road safety is improved, as there is less stray light and dazzle, and faults are communicated more effectively for swifter repair or bulb replacement. It is more cost effective to install lighting for parking areas and places that are less frequented when motion detection sensors can be incorporated to switch on or brighten the lighting when people are present, and dim or even turn off lights when they are not needed, to save energy. CCTV cameras render much higher quality images under whiter LED lighting schemes and will work more effectively if light levels become higher when movement is detected. Security personnel can be easily directed to areas where there is an intruder alert. While light pollution can be reduced if street lighting can be dimmed during the night, this has to be balanced against findings that street crime and fear of street crime is greatly reduced as street lighting is improved.

Other advantages of LEDs for street lights are directional control, resulting in less glare, greater efficiency (more lumens/watt), and more efficient fixture design. A lesser noted, but no less tangible benefit, is that LED lights are less attractive to insects than traditional high-pressure sodium designs.

Network benefits

The ability to monitor and control smart street lighting through the networked communication infrastructure allows for many new possibilities. The lights in an entire sector may be turned off, for example during a firework display, or turned on in case of exceptional weather conditions. Plus the network infrastructure can be exploited for other non-lighting applications, which appeals widely to city planners. Lighting gantries are ideal for locating sensors and other devices that can use the same power sources and networking technology to send data. The monitoring of ambient light levels, weather, noise levels, and air pollution are obvious candidates as well as traffic monitoring.

Data can be made available to city services, such as utilities or emergency services, and may even be sold to external service providers. The emergence of enhanced automotive information systems, providing navigation and traffic status services, for example, could use information on traffic levels and congestion. The key is to ensure the system is designed with this potential in mind. It is essential to use scalable, expandable, efficient, and reliable wireless sensor networks based on appropriate, industry-standard, communication protocols that ultimately can be linked to the Internet.

Considerable cost savings

The average lifetime of a traditional high pressure sodium street light bulb is 12,000 hours or four years (assuming the lights are on for eight hours a day). A city of 100,000 people typically needs around 16,000 streetlamps. Total cost will vary from country to country, but will depend on the cost of running a maintenance operation, the cost of replacement bulbs, the price of electricity, the wattage of the bulbs and the ballast and the number of hours they are illuminated.

A high brightness LED has a typical lifetime of 50,000 hours. Clearly, a direct plug-in bulb replacement alone would result in considerable cost savings. Add to that the benefits of the new generation of electronic, dimmable, and communicating LED-based streetlights, and the figures become compelling. There are many European case studies¹ demonstrating, in one case, a 30 percent reduction in energy costs. Another logged a 50 percent reduction in maintenance costs simply because bulb failure locations can be communicated by standard protocol over the power line instead of sending out maintenance crews at night to find failed bulbs.

New installations

However, most manufacturers and solutions providers are addressing the market for new installations, rather than LED-based plug-in replacement units. Cities looking to update their lighting systems are more likely to seek the highest potential cost savings, plus the additional benefits, albeit for a greater initial investment, that can only be gained by installing a fully networked, intelligent street lighting scheme.

In terms of technology, LEDs and their associated electronic ballasts have only become suitably robust replacements for high-pressure sodium bulbs in the last five years. Other technical challenges have included protection against over voltage transients, ensuring a consistent (cool white) color, and improving luminous efficacy to give an equivalent brightness to traditional solutions. The technology continues to evolve rapidly, and its increasing uptake is helping to bring prices down.

The supply chain in the street lighting sector is diverse and developing as the ecosystem grows. In this market, the end customer might be a utility company or a city council or a construction company. The decision makers could be architects, builders, or town planners. The trend, as in other sectors where the end customer is not necessarily an electronics OEM, is for suppliers to move up the supply chain by developing ‘solutions.’ As a result, partnerships have been struck, and vendors initially known for their LEDs, drivers/controllers, power management circuitry, or wireless networks, are now offering broader based platforms or building blocks for the street light system builders.

LEDs suitable for streetlights are available from a number of sources, including Cree and OSRAM. Arguably, Cree’s XLamp LEDs were among the first to be bright enough for general-purpose illumination applications, including streetlights. Its latest range, the cool white XLamp XP-G LED provides up to 148 lumens and 141 lumens per watt. In addition, the company has acquired Ruud Lighting and its BetaLED range specifically designed for street lamps. These are also available in street lamp housings with a number of options available. Cree works with a range of partners to develop systems solutions.

OSRAM, meanwhile, offers the Golden DRAGON and OSRAM OSTAR ranges of LEDs, targeted specifically at street light applications. The Golden DRAGON single chip packages provide a typical luminous flux of 82 lm at 350 mA, and 105 lm for the Golden DRAGON Plus range. See Figure 2 below. The OSRAM OSTAR-Lighting LE UW E3B series consists of multiple LED chips (with either 4 or 6 die) contained in a single package, providing a typical luminous flux of 895 lm at 700 mA for the 6 die version. An OSRAM application note on street lighting using LED sources is available on the OSRAM website³.

Figure 2: Street lamp implementing OSRAM’s Golden DRAGON LEDs with Oval lens (LW W5JM) (Photo Source: OSRAM).

Future proofing systems

A smart lighting system includes not only the smart LED lamps which effectively form a wireless sensor network, but may require additional circuitry at the node level, such as controllers, drivers, power supplies, circuit protection, and further sensors, for example. A central control system, or ‘dashboard,’ is also essential. When designing such systems, it is important to select system components that support appropriate, and suitably scalable (a street lighting system might require 5,000 nodes or more) communication protocols, and, where necessary can be easily reprogrammed to deliver new features in the future. Some suppliers use dedicated proprietary protocols (such as Echelon’s LonWorks) that then connect to industry standard communications and IP networks.

Dimmable drivers

NXP Semiconductors has recently introduced its GreenChip smart lighting solutions, combining wireless IP connectivity with low energy, high efficiency lighting and power conversion technologies. The range of solutions is continuing to evolve. System components include the GreenChip iSSL chipsets that function as highly efficient, dimmable drivers for smart solid-state lamps. A 2.4 GHz IEEE 802.15.4 standard-compatible wireless microcontroller with a transmit/receive current below 17 mA complements the drivers, while an ultra-low-power standby supply controller, available with 10 mW no-load capability, is ideal in applications where standby power is critical.

Low-power, IP-based wireless connectivity is enabled via NXP’s JenNet-IP network layer software, providing a highly robust 6LoWPAN mesh-under tree self-healing network supporting IPv4 and IPv6 with over-network upgradeability. Although initially targeted at residential home automation networks, NXP claims that its GreenChip smart lighting solutions are scalable to large-scale commercial and industrial installations (already proven with more than 500 nodes) where IP-connectivity facilitates the seamless integration of the lighting network into central management systems.

Power management

Power management and circuit protection for LED lighting applications are the subjects of other articles. But it is useful to note here that there are a number of companies now specializing in this area with components aimed at the fast growing street lighting market.

Power Integrations, for example, already a leader in high-voltage integrated circuits for energy-efficient power conversion, has introduced its first high-power product, designed to control AC-DC converters from 80 to 600 watts. It is said to be ideal for street lighting applications, and the company has produced a Design Reference document² that describes a 150 W reference design power supply for 230 VAC input LED street lights(see Figure 3). Based on the HiperPLC PLC810PG, the device combines and synchronizes both PFC and LLC controllers into a single IC, thereby reducing the size of the PFC magnetics required. The IC provides up to a 0.99 power factor.

Figure 3: 150W street light power supply using PLC810PG from Power Integrations (Photo source: Power Integrations).

Multichannel approach

Taking a different approach, National Semiconductor (now part of TI) offers the multichannel LM3464 high-brightness LED driver, a member of its PowerWise® energy-efficient product family. It is also claimed to maximize system efficiency and reduce system complexity and cost in LED-intensive applications such as industrial and street lighting. In this case, the LED driver features four channels to drive up to 20 LEDs per channel. Its dynamic headroom control feature dynamically adjusts the LED supply voltage through the power supply feedback to the lowest level required for optimum efficiency. It operates in conjunction with external N-channel MOSFETs and sense resistors to accurately drive current to each LED string. By directly controlling the output of the offline regulator, the device can eliminate the second stage switching regulator commonly required for offline LED power supplies. The company offers additional LED drivers and ancillary devices for street lighting applications.

Summary

This article has outlined the technological, economic, and political drivers that have created a compelling case for the move to LED-based street lighting. This is a relatively new market that is expected to grow fast. As a result, the supply chain is evolving as vendors jostle for position to take advantage of its potential. This provides a great deal of choice and innovation for systems builders looking to build solutions.

References:

1. Echelon Corporation, Monitored Outdoor Lighting White Paper
2. Power Integrations, Streetlight Design Reference document
3. OSRAM, Street Lighting with LED Sources Application Note