Market researchers at Northeast Group estimate that there are more than 280 million street lights currently in place globally, and that there will be nearly 340 million by 2025. Each street light uses 600 to 1,000 kWh/yr of energy, which translates to $70 to $125 in annual electricity costs (assuming an average worldwide energy cost of $0.12 per kWh). In addition, each street light is responsible for generating 330 to 1,500 kg of CO2 each year, contributing to global climate change.

Cities have complex and multifaceted street lighting control challenges, including where and how to implement wired and wireless communications. Wireless communications are difficult or impossible in some locations due to impairments such as buildings, trees, and tunnels. On the other hand, wired Power Line Communications (PLC) has trouble getting around transformers and can be cost-prohibitive in some situations. Cities and municipalities typically have had to either settle for less-than-complete street lighting control coverage or manage a patchwork of street lighting control systems, some wireless and others wired. Using advanced standards-based lighting control technology, cities worldwide are gaining the management efficiency and cost benefits of integrated wired/wireless streetlight control.

The importance of outdoor lighting control in smart cities

Adding controls to lighting systems – often in conjunction with conversions to energy-efficient LED lighting, but also without LED conversion – enables energy savings through adaptive lighting. In addition, outdoor lighting controls can help cities save operational costs through more efficient maintenance and better scheduling based on usage, failure analysis, and timely access to data. Plus, a smart lighting control system can serve as the platform for additional smart city applications, such as those for security or traffic management.

In addition to these broadly applicable rationales for adding control networking to outdoor lighting systems, other geographic- or situation-specific benefits can include peak energy management – such as happens on the East Coast in the U.S. around 7 pm in the summer, when buildings are still using air conditioning at the same time that street lights are beginning to turn on; improved public safety, based on research showing that 50 percent of automobile accidents happen within a three-hour time period at dusk; and energy optimization, which allows cities to add more lights for the same amount of energy usage.

Energy savings with adaptive lighting

Cities typically allocate and spend between 35 percent and 40 percent of their total energy budget on street lighting. Conversions to more energy-efficient lighting sources, such as LEDs, can cut energy expenditures by 30 percent to 50 percent. Adding smart controls yields an additional 15 percent to 30 percent of energy savings, over and above the contributions of the luminaires themselves.

In developed regions of the world, cities are more likely to reapply the saved energy costs to other parts of the city budget. In developing areas, cities lacking the energy resources to power all the streetlights they need are more likely to view a watt saved as a watt made. Using less energy to power a given number of streetlights frees up energy to deploy more lights or to power other productive assets.

Adaptive lighting – the alteration of the output or duration of lighting in response to demand, real-world lighting conditions, or other parameters – is an important aspect of how control networks improve outdoor lighting energy efficiency. Adaptive lighting savings result from:

• Constant Lumen Output (CLO). To compensate for deterioration over time, most lighting fixtures are overrated initially by 20 percent to 25 percent. CLO makes automatic adjustments to a lamp to lower the lumens when a lamp is young, which typically results in 10 percent less energy used over the life of a fixture as well as 20 percent longer lamp life.
• Lumens On the Road (LOR). Light bulbs come in discreet wattages. Rarely, however, do the conditions of a particular road demand precisely 150 W or 250 W per fixture. Rounding down risks under-lighting the road; rounding up to the nearest wattage means over-lighting and over-paying for energy usage. Using LOR to selectively lower wattage can save 10 percent in overall energy costs for the street lighting system.
• Better scheduling. Outdoor lighting owners can see an additional 5 percent in energy savings by using an astronomical clock to schedule switching lights on and off; overriding schedules with a photoelectric (PE) cell that adjusts lumens based on actual ambient light available; and moderating and reporting if PE cells show aberrations during daylight.
• Programmed and dynamic dimming. Dimming lights during non-peak hours can lead to 20 percent increased energy savings; dynamic dimming – also called "follow-me" lighting – can save 15 percent more.

Taken together, adaptive lighting methodologies contribute about half of the energy savings attributed to the addition of control networks.

Choosing wired or wireless controls

Wired Power Line (PL) based lighting control is easy to install, reliable, and can span long distances supporting a large number of lights per transformer segment. Wireless Radio Frequency (RF) based lighting control has the benefit of being independent from the power distribution system. It can be easy to install with a plug-in architecture based on the recently defined ANSI-standard socket (ANSI 136.41), which enables RF outdoor lighting controllers from any vendor to be plugged into a photocell socket to add controls. Both RF and PL controls require no new wires. For various historical and other reasons, Europe and Asia rely primarily on PL communications for outdoor lighting, while North America has traditionally preferred more RF communications.

Regardless of geographic location, however, cities have certain use cases in which only one or the other approach will work.

For example, RF-controlled lighting is not possible or ideal in situations such as:

• Tunnels. RF signals cannot traverse tunnels.
• Parking structures. Concrete impedes RF signals, making it inappropriate for parking garages or underground parking structures.
• Remote stretches of highway. Establishing RF connections is impractical.
• Historical or decorative luminaires. Light fixtures that lack photocell sockets but need to remain in place for aesthetic reasons cannot be brought into the control network via RF.
• Mission-critical lighting situations. PL is much more robust and reliable than RF, so it is the medium of choice for situations where lighting must operate flawlessly for safety or other reasons.

Lighting control systems have traditionally been built using only one approach or the other. As a result, cities aiming for complete coverage with both wired and wireless lighting systems have had to mix and match solutions from different control networking providers, under separate management – adding complexity and increasing operational costs.

Hybrid wired/wireless lighting control

More recently, advanced standards-based lighting control technology is enabling cities to implement hybrid wired/wireless outdoor lighting control to include all their luminaires (Figure 1). The components of such a hybrid system include:

• Segment controllers with PL and RF border routing
• RF-PL bridges
• PL and RF light point controllers
• Management software

Key to the hybrid approach is that all the components of the system be standards-based, so that they can all interoperate with one another and with luminaires and sensors from a range of suppliers.

The most important standard supporting hybrid wired/wireless outdoor lighting control is ISO 14908, an open, proven, multivendor standard (Figure 2). Its standard features include on, off, and dimming commands; lamp feedback monitoring; failure identification; electrical measures (including U, I COS PHI, E); and energy and run hours metering.

Importantly, the ISO 14908 standard supports both wired and wireless communications.

Another emerging standard of importance to hybrid wired/wireless outdoor lighting control is TALQ for integrating segments to central management systems. A consortium rather than a standard per se, TALQ aims to specify gateway standards for communication via IT standards; common databases and data sets; and single user interface to multiple gateway vendors.

Control networking and PL-based outdoor lighting control solutions company Echelon Corporation has added RF-based offerings and developed standards-based hybrid wired/wireless outdoor lighting control solutions that fit into the ISO 14908 ecosystem. With this hybrid approach, cities embarking on connected/smart city initiatives no longer have to settle for patchwork outdoor lighting solutions and separate management for PL and RF systems. Instead, they can use PL and RF approaches where each is most appropriate and attain the management efficiency and cost benefits of a single outdoor lighting control system, without having to compromise or to leave some of their luminaires "in the dark" when it comes to networked control.

Sanjay Manney is Director of Product Management at Echelon Corporation.

Vijay Dhingra is Director of Commercial Product Development at Echelon Corporation.

Echelon Corporation www.echelon.com @echeloncorp linkedin.com/company/echelon blog.echelon.com