Reliable Street Lighting: Thermal Stability with Application-oriented Thermal Management
- Published: Tuesday, 06 December 2016 14:14
LEDs are efficient light sources that are being used more and more as the standard technology for street lighting.
The advantages are obvious. Provided the LED lighting system is carefully engineered, LEDs offer long life and can considerably reduce energy consumption, maintenance requirements and therefore running costs. Thermal management tailored to the particular application plays a crucial role in road safety.
Even though LEDs are not thermal radiators like incandescent lamps or halogen lamps, they do produce heat as electrical energy is converted into light. For LEDs to operate safely and efficiently this heat must be reliably dissipated via the board and the luminaire housing. High temperatures have an adverse effect on lifetime and luminous efficacy. Manufacturers' specifications on luminous flux and lifetime therefore always relate to a defined operating temperature range.
Potential risks in a simple solution
A simple way to operate LEDs exclusively in the recommended temperature range is to monitor the temperature of the driver and LED module using NTC components (Negative Temperature Coefficient). Compliance with this operating temperature ensures that the expected lifetime and required illuminance are met. If, however, the temperature is outside the operating range the illuminance is often automatically reduced by dimming for the purposes of thermal protection. This does not take into account the fact that the luminaires are planned and designed for a particular illuminance and a certain lighting level must be achieved in order for the application to comply with relevant standards. The temperature monitoring system with integrated feedback puts the luminaire in an unreliable state, with risks to users. Even though many tenders specify that active thermal protection is required this simple solution should not be the first choice.
Focus on light for greater safety
The main task of street lighting is to keep all road users – pedestrians, cyclists and drivers – safe from harm in the dark. Street lighting that is sufficiently bright and tailored to the particular traffic situation makes an important contribution to road safety. European standard EN 13201 defines quality criteria for street lighting such as illuminance levels, freedom from glare, colour rendering and the distribution of luminance. The greater the risk to the safety of road users, the more stringent the quality demands placed on street lighting. The risk is determined by the traffic situation and the number and speed of the road users. Inadequate lighting increases the risk of collisions significantly.
Sensible light planning will always ensure that street lighting complies with relevant standards. At night the various traffic areas are illuminated based on demand, tailored to the different users and the volume of traffic. Lighting planning must include the now common practice of night-time power reduction which can provide additional energy savings. This requires a detailed analysis of the traffic situation at night and must not be at the expense of safety.
A rapid reduction in the level of light following thermal stress, without regard for the volume of traffic, jeopardizes safety and increases the potential for accidents. This is at the expense of road users and, in the event of injury or damage, also of the responsible decision-makers and planners.
The alternative to reducing lighting levels
For safety reasons, lighting experts at Tridonic do not favour dimming as a way of protecting against overheating if operating temperatures get too high. They point out the option of designing components and the entire lighting system and adapting them on site in such a way that overheating cannot occur in the first place. In Europe this is always possible thanks to the moderate range of ambient temperatures.
Extensive application support
Luminaire manufacturers design their luminaires for the relevant temperatures and luminous intensity, tailored to the planned site and the conditions to be found there. For these parameters Tridonic offers manufacturers an extensive range of test services in Tridonic's own laboratories and will then make the results available. This transparency enables every lighting task to be adapted precisely to the requirements and the best combination of individual components to be determined.
Even so, it is still worthwhile equipping electronic components with temperature sensors. Their main task is to capture data. This data provides information on the actual temperatures in the luminaire head and enable the performance quality to be assessed and maintenance measures to be initiated in good time so that the specified lifespan can be achieved.
The data is also useful when it comes to upgrading the luminaires – for example for integrating them in the Internet of Things. This would require further electronic sensors that also generate heat.
Excessive temperatures can be controlled
If as a result of unusual circumstances there is a brief increase in the operating temperature the wide operating temperature range and the defined reserves of electronic components ensure that this excess temperature dos not lead to luminaire failure. Piotr Dudek, Segment Manager Outdoor and Industry: “Before we risk an unsafe lighting state we design our components to be robust enough to withstand temporary thermal stress. The possible reduction in lifetime is in the single-figure percentage range. But the entire lighting system and the road users benefit from much greater safety.”
Recommended course of action
Communities, municipal authorities and lighting planners can make a great contribution to the reliability of their lighting systems because safety in public spaces is the top priority. This starts with comparing the thermal data provided by the various luminaire manufacturers and includes special maintenance routines which have to be carried out in fixed or demand-led intervals depending on the situation. The recommendation is to include in the tender specifications the illuminance levels and temperature ranges that the lighting system must comply with throughout its entire life. In addition to the power consumption or dimming level, the temperature per light point also needs to be recorded and regularly evaluated in future.