Road lighting is public lighting and is controlled by the government. Due to its high efficiency, environmental protection, energy saving and long life, LED light source can be the first to be applied in road and tunnel lighting.
Lamps used for road lighting are called street lights. A road lighting fixture using LED as a light source is called an LED street light. The core of the LED street light is the LED light source. LED street light source is made up of many high-power white LEDs connected by a hybrid method. In addition to the LED module, the LED street light fixture includes a driving power source, an optical component, and a heat sink.
First, the development status of road lighting
When people have the need to travel at night, road lighting appears. Modern road lighting began with the emergence of incandescent lamps. Road lighting has developed with the development of the times, the advancement of science and technology and the continuous improvement of people's living standards. From the situation where people only require the lighting road to detect the road, to help people identify whether the road is a pedestrian or an obstacle, and then help the motor vehicle and the non-motor vehicle driver to distinguish the characteristics of the pedestrian.
The fundamental purpose of road lighting is to provide drivers and pedestrians with good visual conditions and guide them to move in order to improve traffic efficiency, reduce night traffic accidents and crimes, and help pedestrians to see the surrounding environment and identify the direction. With the development of social economy and the continuous improvement of people's living standards, people are more and more active at night, outdoor shopping, shopping, sightseeing, etc. Good road lighting also plays a role in enriching life, prospering the economy and enhancing the image of the city.
According to the lighting point of view, roads can be divided into four categories: car-only roads, general streets, commercial streets and sidewalks. Generally speaking, road lighting refers to road lighting for automobiles. Among the many purposes of road lighting, it is the first to provide safe and comfortable visual conditions for motorists.
The road lighting source was the first street lamp. Later, high-pressure mercury lamps, high-pressure sodium (HPS) lamps and metal halide lamps, high-efficiency energy-saving lamps, electrodeless lamps and LED lamps appeared. Among the more mature street light sources, HPS lamps have the highest luminous efficiency, generally 100~120lm/W. In the current Chinese road lighting market (about 15 million baht), high-pressure sodium lamps account for more than 60%. . The lighting sources on some community and country roads are mainly electronic energy-saving lamps (CFLs), accounting for about 20% of the road lighting market. Traditional incandescent lamps and high-pressure mercury lamps are gradually being phased out.
LED lighting is an energy-saving and environmentally friendly lighting. LED road lighting demonstrations are being launched globally, with mainland China becoming a pioneer and Taiwan being second. On April 28, 2009, the Ministry of Science and Technology officially issued a document [Guo Ke Fa Gao (2009) No. 189] in Tianjin, Shijiazhuang, Dalian, Shanghai, Hangzhou, Xiamen, Nanchang, Chengdu, Xi'an, Chongqing, Shenzhen, Dongguan and 21 cities such as Weifang in Shandong launched the pilot project of “Ten Cities and Ten Thousand Milesâ€. LED street lights have become a highlight of various semiconductor lighting technologies with their outstanding demonstration effects.
Second, the advantages of LED street lights
Compared with traditional street lamps represented by high-pressure sodium lamps, LED street lamps have many obvious and potential advantages.
(1) High light efficiency and low energy consumption
At present, the high-power white LED has a luminous efficacy of more than 100 lm/W. The laboratory level of high-power white LEDs of Cree and Osram Opto Semiconductors reached 160 lm/W as early as 2009. According to the US Department of Energy (DOE) forecast, by 2020, high-power LED commercial products can achieve 165 lm / W, and laboratory samples can achieve 230 lm / W. After 2020, LED light efficiency will no longer be greatly improved, and will enter a lag period.
In terms of road lighting, high-pressure sodium lamps dominate because of their luminous efficacy of 120 lm/W. However, with the continuous improvement of LED light efficiency, the monopoly position of high-pressure sodium lamp street lamps is in jeopardy. At present, LED street lights below 150W used on secondary roads have shown obvious energy saving advantages.
Traditional street light sources generally illuminate the entire space, but it is only the roads of pedestrians and pedestrians that need to be illuminated by street lamps. Therefore, in the design of street lamps, in order to make the lights evenly and uniformly projected onto the road surface, a curved reflector is needed. Collect light to illuminate it in the desired direction. In the process of light propagation, due to the blockage of the light source and the absorption of the reflective surface, the light-emitting efficiency of the street lamp is only 65%~70%. In contrast, LED street lights, because of their good directionality, when using secondary optical lenses, the efficiency of the lamps can reach about 80%. If the optical design is performed three times, the luminous efficiency of the luminaire can reach 85%~90%.
Table 1 shows the comparison of parameters such as the efficiency and road utilization of a 400W high-pressure sodium lamp and a 250W LED street lamp.
Table 1 Comparison of 400W high pressure sodium lamp and 250W LED street lamp parameters
It has been proved that replacing the 250W high-pressure sodium lamp street lamp used on the secondary trunk with the 100W LED street lamp with reasonable scientific light distribution can not only achieve the average illumination required by the road lighting standard, but also the uniformity is greatly improved. The average illuminance of high-pressure sodium lamp street lamps may be higher, but the uniformity is poor. It can be seen that LED street lamps have considerable energy saving space.
High-intensity discharge (HID) street lamps can only be dimmed in a small range, while LED street lamps can achieve dimming control from 0% to 100%, and can flexibly adjust the light output according to ambient lighting and traffic conditions. Reduce unnecessary energy consumption while ensuring lighting needs. It can be seen that for road lighting, which accounts for about 15% to 20% of the total lighting power consumption, the large-scale implementation of LED street lamps is of great significance for energy saving and carbon reduction.
(2) Long service life
The life of the street light directly affects the maintenance cost of the entire road lighting. At present, the life of high-pressure sodium lamps is generally about 20,000 hours, and the service life of high-pressure sodium lamps for road lighting is only about 5,000 hours, while the life of high-power LEDs is generally 50,000 to 70,000 hours.
The average life expectancy of high pressure sodium lamps and LEDs is different. The average life of a high-pressure sodium lamp is defined as “the time when a batch of test light source samples are ignited to 50% of the amount of damageâ€, while the average life of the LED is “to ignite a batch of test samples to 50% of the light decay. Up to 70% of the time."
If the LED is measured according to the definition of the life of the high-pressure sodium lamp, the LED with a life of 50,000 hours can reach 100,000 hours; if the high-pressure sodium lamp is measured by the life definition of the LED, the high-pressure sodium lamp with a life of 20,000 hours is only about 10,000 hours.
Of course, the life of LEDs is not the life of LED street lights. The life of an LED street lamp is determined by the life of the LED light source and the life of the driving circuit. The life of the LED and its driving circuit depends on various factors such as the heat dissipation capability of the lamp, the topology of the circuit, and the selection of electronic components. From the current technical level, the life of LED street lamps can exceed the high-pressure sodium lamp street lamps.
(3) good color rendering
Although the high-pressure sodium lamp has the highest luminous efficacy in the conventional light source, its color rendering is the worst, and the color rendering index Ra is only about 20. Such poor color rendering can only help people to detect the road conditions and does not help pedestrians to clearly distinguish. The color rendering index of LED can reach 80 or so, which is basically close to natural light, and the color is more realistic, which can reflect the color of the object itself. The high color rendering of LEDs undoubtedly helps drivers and pedestrians identify targets and provide better access conditions at the same road surface brightness.
(4) Quick start
The incandescent light is on, but the actual start-up time is about 0.1 to 0.2 seconds. Gas discharge lamps such as high-pressure sodium lamps and metal halide lamps require several tens of seconds or even ten minutes from start-up to stable light output. After shutdown, wait for 3 to 6 minutes for cooling before starting. The startup time of the LED is only a few tens of nanoseconds (ns), and there is no waiting time for restarting. The LED luminaire can work normally in the continuous on/off state.
(5) Easy optical design
The LED is small in size and can be illuminating in a half plane. It can be approximated as a point source when designing the luminaire. It is very suitable for optical design with a lens or reflector to achieve better light distribution and achieve higher light. Lamp efficiency.
(6) The shape is plastic and strong, and the structure is firm
The LED street light fixtures are plastically strong and can be used to embody decorative and local humanities, adding aesthetic and urban image added value.
LEDs are solid-state light sources that do not contain fragile components such as glass and filaments. LED lamps can be made very solid in structure through reasonable design.
(7) Low carbon environmental protection, especially suitable for solar photovoltaic technology
Gas discharge lamps such as high-pressure sodium lamps contain harmful substances such as mercury, which cause long-term serious pollution to the environment and water sources after the light source is discarded. The LED chip and its package can meet the requirements of the EU RoHS Directive and contain no harmful substances such as mercury and lead. After the LED is discarded, it can be recycled without polluting the environment.
The characteristics of LED DC drive are especially suitable for the application of solar photovoltaic technology and wind-solar complementary technology to road lighting, thus realizing energy-saving and environmentally-friendly lighting in the true sense.
(8) has the potential total cost advantage
At present, the price of LED street lamp is 4 times higher than that of high-pressure sodium lamp. A 250W LED street light is equivalent to a 400W high-pressure sodium lamp street lamp, but the former price is 6000~8000 yuan, and the latter is only 1500 yuan. However, as long as the high luminous efficiency and long life characteristics of the LED street lamp can be fully utilized, the difference between the initial cost and the maintenance cost can be recovered. As the LED street light technology matures and the initial cost continues to decrease, its potential advantage in total cost will not be apparent in a long time.
Third, the current problems of LED street lights
Although LED street lamps have some advantages compared with traditional street lamps, due to the short development time of LED street lamps, they are still in the stage of trial and exploration. Some LED street lamps have experienced many problems in the demonstration application and have not achieved the expected results.
The current level of problems with LED street lights is:
(1) The actual luminous efficiency is lower than that of high-pressure sodium lamp street lamps. On the lighting effect, the road surface brightness is low and the uniformity is poor. The “zebra effect†is obvious, and can only meet the road with relatively low lighting level, such as the light pole below 8 meters. The main road.
(2) The color temperature is too high, glare, no comfort
(3) The heat dissipation performance is poor, and the temperature rise is too high, resulting in serious light decay and poor reliability.
(4) The performance of the driving power supply is poor, and the faults emerge one after another. From the early failure of the LED street light, more than 95% of the faults are caused by the damage of the driving circuit.
(5) The cost is too high, and the one-time investment is too large, which has become a major bottleneck for promotion and application.
(6) Since there is no unified LED street lamp technical standard, the versatility of products of different manufacturers is extremely poor, and maintenance is very difficult.
There are many reasons for these problems, both technical factors and mindsets.
In fact, LED street lights are a system engineering. Since the LED light source is essentially different from the high-pressure sodium lamp, the LED street light is quite different from the traditional street lamp in the design method. In addition to the design experience of the luminaire, the designer needs a comprehensive and in-depth understanding of the performance of the LED. Some street lamp manufacturers do not know much about the requirements and characteristics of LED road lighting production, and follow the traditional street lamp design ideas to design LED street lights. They still use the conventional "snake head" shape in the shape, and apply the concept of gas discharge lamps. The discharge of a certain power LED in the lamp holder can meet the requirements of road lighting. According to such ideas, LED street lights designed and produced cannot be put into practical use.
Fourth, LED street lights need to be cracked technical difficulties
LED street lights need to be applied to large-scale applications and become the mainstream light source for road lighting. They also face some difficulties that need to be solved. To sum up, there are three main difficulties, namely optical design, heat dissipation and drive power design.
Optical design
Convenient optical design is a big advantage of LED street lights, but at the same time it is a big difficulty.
From the perspective of the light distribution of LED street lamps, the light output characteristics have practical value only if they meet the requirements of the CJJ45-2006 road lighting standard shown in Table 2.
Table 2 Road lighting requirements specified in CJJ45-2006
LED street light distribution includes primary light distribution (with package lens), secondary light distribution, and tertiary light distribution. When the secondary light distribution can meet the requirements of road surface brightness, illumination and uniformity, it is not necessary to perform three optical designs. To meet the requirements of road lighting, the light distribution of LED street lamps must be repeated after repeated design.
At present, many LED street light distribution schemes have been developed, such as LED primary light distribution (ie, lens packaging) plus secondary light distribution of lamps, secondary light distribution of LEDs (using secondary lenses), and three-time light distribution and reflectors for lamps. Light distribution, etc., however, no matter which kind of light distribution scheme, it can not be scientific and reasonable, and there are problems such as low overall optical efficiency, large glare and road surface brightness, and poor illumination uniformity. How to choose the light distribution method, how to choose the lens, how to improve the structure of the street light to meet the requirements of light distribution and heat dissipation, and now need more mature solutions.
2. Heat dissipation problem
The heat dissipation capability of the LED street light directly affects the actual luminous efficiency of the LED and its lifetime.
LED is an electroluminescent device. During its operation, only 15%~25% of the electrical energy is converted into light energy, and the rest of the electrical energy is almost converted into thermal energy, which causes the temperature of the LED to rise, resulting in a sharp drop in light efficiency. If the LED junction temperature exceeds the maximum allowable temperature (typically 125 ° C), the LED will be damaged by overheating. Therefore, the most important task in the design of LED lamps is the heat dissipation design.
The heat dissipation system of the LED lamp includes the heat dissipation of the LED module and the heat dissipation of the heat sink on the lamp structure (such as the outer casing), wherein the latter is a common heat dissipation mode, and the working environment of the LED street lamp is relatively bad, and it is easy to accumulate dust sand and flying insects. The surface of the radiator is easily corroded and oxidized, which affects the heat dissipation of the LEDs and electrical appliances in the lamp, resulting in shortened service life of the LED street lamp.
In order to solve the heat dissipation problem of the LED street lamp, a micro fan can be arranged on the lamp for active forced cooling, but the service life of the cooling fan in the harsh outdoor environment is difficult to meet the requirements.
The weight of the LED street light is mainly determined by the weight of the heat sink. As the power of the LED street lamp increases, the size and weight of the heat sink also need to increase accordingly. In order to reduce the weight of the high-power LED street lamp heat sink, it is necessary to optimize the design of the heat dissipation method, heat dissipation structure and materials.
LED street light fixtures require good protection against dust and water (IP) (such as IP45 and above), but good IP protection often hinders the heat dissipation of the LED. How to solve this contradiction is an important direction that can not be ignored when designing LED street lamps.
3. Drive power supply design
At present, almost all LED street lamps are powered by power frequency mains power. The LED street light driving circuit topology is generally an off-line switching power supply (SMPS). From the situation of LED street lamp demonstration applications in recent years, almost all of the faults occurred on the drive power.
Because the working environment of LED street lamps is very harsh, in the design of the driving power supply, the selection of the circuit topology, the selection of electronic components, the protection of the circuit, the heat dissipation and the waterproof performance are all very demanding.
According to the current state of the art, the average life of a conventional offline switching power supply is usually less than 30,000 hours, and the life of the LED driving circuit determines the life of the LED street lamp. Electrolytic capacitors are the decisive influence on the life of LED driver power supplies, followed by power semiconductor devices. Therefore, the choice of electrolytic capacitor is extremely important.
In the design of LED street lamp driving power supply, not only must pay special attention to its reliability, but also must meet the performance requirements of input AC current harmonic content, line power factor, electromagnetic interference (EMI), lightning protection and waterproof and energy efficiency. Safety requirements. Exploring low-cost, high-reliability and high-performance solutions is a difficult point in the design of LED street lamp power supplies.
V. Conclusion
LED is a new generation of green light source, and LED has many advantages compared with traditional street lamps. Road lighting is a kind of public lighting that is paid for by the government, so LEDs can be the first to be used in road lighting in the field of general lighting. However, since the design scheme of the traditional street lamp is not applicable to the LED street lamp, there are still many problems in the optical design, the heat dissipation problem, and the driving power supply design, and the problem has not yet been solved. Therefore, there is still a long way to go to promote the application on a large scale. The LED street lamp technology is still in the exploratory stage, and it cannot be rushed, rushed and blossomed everywhere, but must adhere to the pilot and re-demonstration to the final implementation route.
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