Courtesy: Electronics Maker
Basically, street lighting is one of the important parts of a city’s infrastructure where the main function is to illuminate the city’s streets during dark hours of the day. Previously, the number of streets in the town and city is very small.
Therefore, the street lamps are relatively simple but with the development
of urbanization, the number of streets increases rapidly with high traffic density. There are several factors need to be considered in order to design a good street lighting system such as night-time safety for community members and road users, provide public lighting at cost effective, the reduction of crime and minimizing it is effect on the environment.
Today, street lighting commonly uses high-intensity discharge lamps, often HPS high pressure sodium lamps. Such lamps provide the greatest amount of photopic illumination for the least consumption of electricity. However when scotopic/photopic light calculations are used, it can been seen how inappropriate HPS lamps are for night lighting. White light sources have been shown to double driver peripheral vision and increase driver brake reaction time at least 25%. When S/P light calculations are used, HPS lamp performance needs to be reduced by a minimum value of 75%.
A study comparing metal halide and high-pressure sodium lamps showed that at equal photopic light levels, a street scene illuminated at night by a metal halide lighting system was reliably seen as brighter and safer than the same scene illuminated by a high pressure sodium system.
New street lighting technologies, such as induction or LED lights, emit a white light that provides high levels of scotopic lumens allowing street lights with lower wattages and lower photopic lumens to replace existing street lights. Formal specifications around Photopic/Scotopic adjustments for different types of light sources enables municipalities and street departments to test, implement and benefit from this new technologies.
General benefits of high quality LED street lights
Below listed benefits of LED street lighting are related only to latest, high quality LED street lights:
Less energy consumption
LED street lights use 40-80% less electricity and have at least 5 times the life expectancy than regular High Pressure Sodium (HPS) fixtures. LED lamps are 7 times more energy efficient than incandescent and twice as efficient as fluorescent lamps.
Higher efficiency and low light pollution due to directional light
LED street lights with a lower lumen output can replace conventional lamps with a higher output. For example, a 30W LED street light can often replace an 80W High Pressure Sodium lamp. The reason for this is directionality. LED street lamps are very directional and the light output is much more even then by other street lamps. Also there is little or no hot spot under the LED lamp. The light emitted from the LED lamp is directed downwards, spread throughout the entire area it covers. This means that a lower amount of light is needed to properly illuminate the area. This also dramatically reduces light pollution, which affects the mood of human beings, navigation in birds and insects, mating behavior in animals and flowering in plants.
Long life – up to 100.000 hours
LED street lights last much longer than conventional lamps (3 to 8 times longer). This results in less expense in replacing the lights themselves but also the labor to replace the lamp is needed less often. This provides a great cost savings by itself.
Also the loss of brightness or lumen depreciation is slower over the life of an LED lamp than that of a sodium or other lamp. So not only does the LED have a longer life span than the conventional lamp, but it stays brighter longer than other lamps. The long life span reduces maintenance expenses and makes these bulbs particularly suitable for difficult to reach locations and for streetlights where maintenance costs can be significant.
Great operating characteristics
LED operate at lower temperatures, are not sensitive to low temperature and unaffected by on off cycling. This makes them safer, more efficient in cold environments (outdoor lights, refrigerator lights and cold room lights) and better for applications requiring frequent switching on and off lights. These bulbs are shocks and vibrations resistant making them the best choice for places like bridges.
Reducing carbon footprint
The carbon footprint of LED street lights is smaller than other lights due to lower energy usage. Moreover LEDs last 4 to 10 times longer than any other bulbs, further reducing the carbon footprint of manufacture over the life time.
Because of the directional light, light is carefully distributed exactly where it is meant to go and therefore there is no or little light which is wasted by illuminating the night sky. This is a considerable plus especially if the local community has a Darksky Initiative.
Natural light specter – Color Rendering Index
LED street lamps with color temperature 3.500-4.200 K are rendering more natural light than the yellow of sodium lamps or green of flourescent streetlights. Also no UV or IR radiation is emitted from the LED street lamps. Color rendering index (CRI) is high (80-90) and displays natural colors of illuminated objects.
Free of harmful substances and lower environmental impact when used up
LED luminaires contain no harmful substances, like mercury, lead or other hazardous chemical and gasses. Spent LED lamps can be thrown away without any special handling or disposal requirement, since they are recyclable and environmentally friendly. Other light often have hazardous materials such as lead and mercury which require special handling and waste management procedures which have both economic and environmental costs.
The light is easily controllable with intelligent systems. The light can be turned on and off instantly and can be dimmed for added energy savings at dawn, dusk, and also during hours of low traffic. Switching on-off and dimming does not affect the life-time of the luminaire as in the case of CFL lights.
Present Problems with LED lights
ONE: Although the LED lights comparison with traditional street has some advantages, but due to the development of led street lighting Manufacturer for a short time, is still in testing and exploratory stage , there are a number of LED lights there have been many problems in the demonstration application, did not live up to the desired effect.
LED lights currently exist in varying degrees of problems as follows:
1. The actual luminous efficiency is lower than the high-pressure sodium lamps , lighting effect on low surface brightness , uniformity is poor, ” zebra ” effect is more obvious and can only meet the relatively low level of road lighting , such as light poles at 8 m below the times roads .
2. Color temperature is too high, glare -free comfort
3. poor thermal performance , high temperature , resulting in a more serious light fades , reliability worse.
4. drive power performance is poor, failure after another, from the case of failure of the LED street lamp early , more than 95% of failures are caused by the driving circuit is damaged.
5. the cost is too high, too much time investment to become a major bottleneck in promotion applications.
6. Since there is no uniform standard LED lamp technology , products from different manufacturers versatility poor maintenance very difficult.
The reason for these problems are many factors on both the technical level, but also affect other aspects of mindset.
Indeed, LED lights are a systems engineering. Since the LED light source and high pressure sodium essentially different in the design of LED lights and traditional street is different. In addition to design experience designer lamps , LED performance also need to have a comprehensive and in-depth understanding. Some street lights manufacturer still not well understood in the production of LED street lighting requirements and characteristics of the case , follow the traditional lamp design ideas to design LED lights, in the shape still using conventional ” snakeheads ” shape, apply the concept of gas discharge lamps , as long as a certain power LED emissions in the socket on the road to meet the lighting requirements . According to this line of thought the design and production of LED lights, impossible in practice.
SECOND, LED street lights need to crack the technical difficulties
Large-scale application of LED lights in order to become a mainstream source of road lighting , but also face some difficulties need to be addressed . To sum up, there are three main difficulties, namely, optical design, thermal design problems and drive power.
1. An optical design
Facilitate optical design was originally a big advantage of LED lights , but they are also a major difficulty .
From the perspective of LED street light , the light output characteristics only meet CJJ45-2006 standards for road lighting requirements specified in Table 2 only has practical value.
LED street light including a light ( using encapsulated lens ) , the second with light and three light distribution . When light meets the road through the secondary brightness , illumination and uniformity requirements, then you do not need three optical design. To meet the requirements of road lighting , LED street light must pass through several design iterations can be achieved.
Currently a lot of people come up LED street light program , such as a LED light ( ie lens package ) plus secondary light fixtures , LED secondary light (using a secondary lens ) plus three lamps with light and reflectors light , etc. However, no matter what kind of light distribution scheme , it can not be scientific and reasonable, there is a low overall optical efficiency , there is a big road glare and brightness , uniform illumination and poor varying degrees . How to choose the light way , how to choose lenses, how to improve the structure of street lights with light and heat to reach the requirements , there are still needs more sophisticated solutions.
2. Heat problem
Cooling capacity of LED lights directly affect the actual LED luminous efficiency and lifetime.
LED is an electroluminescent device , in the course of their work , only 15% to 25 % of the energy is converted into light , almost all of the rest of the energy is converted into heat, so that the LED temperature, resulting in a sharp decline in luminous efficiency . If the LED junction temperature exceeds the maximum allowable temperature ( typically 125 ℃), LED will damage due to overheating . Therefore, LED lighting design , the most important work is the thermal design .
LED lamps including LED module cooling system cooling and lighting structures ( such as housing ) on the heat sink cooling , the latter of which is now common cooling mode , the LED lights work environment is bad , easy to accumulate dust and insects , susceptible to corrosion of the radiator surface oxidation, which will affect the heat within the LED lighting and appliances , resulting in led street lighting Manufacturer life is shortened.
In order to solve the heat problem of LED lights , mini fans can be configured to be active on the mandatory heat lamps , but the cooling fan life in harsh outdoor environments is difficult to meet the requirements.
LED street light weight is mainly determined by the weight of the heat sink. With the increasing power LED lights, radiator size and weight also requires a corresponding increase. To reduce the high power LED street light weight heat sink, you need to cooling mode, heat and other aspects of the structure and materials to optimize the design.
LED street lamps requires a good dustproof and waterproof (IP) protection ( eg IP45 and above ) , but a good IP protection is often hinder cooling of the LED. How to resolve this contradiction is an important direction for the design of LED lights can not be ignored.
3. Driving power supply design
Currently almost all use LED lights frequency mains supply. LED lamp drive circuit topology in general are off-line switching power supplies (SMPS). From the case in recent years, LED street lamp demonstration application point of view, failure occurs almost exclusively focused on the drive power.
Since the LED lights work environment is very bad, so drive power design, all aspects of the selection circuit topology selection and protection of electronic components circuits, heat and water resistance, etc. , are very demanding.
According to the current level of technology , the average life expectancy of conventional off-line switching power supply is usually 30,000 hours or less , life LED drive circuit LED street lamp life decisions . The life of the LED driver power supply has a decisive influence is electrolytic capacitors, followed by power semiconductor devices. Therefore, the choice of electrolytic capacitor is extremely important.
In LED SMPS design, not only to pay particular attention to its reliability , but also must meet AC input current harmonic content , the line power factor, electromagnetic interference (EMI), lightning and water and energy efficiency and other aspects of performance requirements and safety requirements. Explore low-cost, high reliability and high performance solution that is a difficult drive power LED street light design.