At present, the light source used for road lighting is mainly high-pressure sodium lamp, and the initial light efficiency of the light source is above 100 lm/W, while the high-power LED device on the market has a luminous efficiency of about 90-100 lm/W, which is slightly lower than that of the high-pressure sodium lamp. However, LEDs have the characteristics of high color rendering index and color temperature selection, which can provide higher quality road lighting effects. With the development of technology, LED light efficiency has been increasing year by year, and it has become more powerful to replace high-pressure sodium lamps as the main source of road lighting.
In practical applications, the design and development of LED street lamps are mainly focused on optical, thermal, power and mechanical design. This paper takes the LED street lamp project of Wuxiang South Road in Wujiang City as an example to analyze the characteristics of this technology in terms of energy saving and light efficiency:
It is understood that the street lamp of Wujiang Township South Road is a single pole double lamp mode, and the original single pole is equipped with 2 ç› 250 watt sodium lamp. The sodium lamp needs to be equipped with a ballast, so the entire system consumes about 320 watts and consumes 640 watts per pole. The sodium lamp has a color rendering index of about 21 and a color temperature of 2100K.
After replacing the LED configuration, the 200 watt LED street light is used in the motor vehicle lane, and the 120 watt LED street light is used in the non-motor vehicle lane. The driving power is placed in the lamp cap, and no additional driving power source or ballast is required, so the system power is only 200. Tile and 120 watts, single pole power consumption 320W, color temperature of about 3500K, color rendering index greater than 70.
Actual measurement The actual illuminance of the ground before and after installation can be seen in the following table. The average illuminance of the motorway before and after installation is increased (28.2-22.8)/22.8=23.7%, and the non-motor vehicle illuminance is increased (25.4-13)/13=95.4%, single pole Power consumption is reduced (640-320) / 640 = 50%. Not only has the illuminance been significantly improved, but it is calculated by replacing 124 ç› 250 watt sodium lamps, saving more than RMB 68,000 per year.
In addition to illuminance improvement and energy saving effect, the advantages of using LED instead of high pressure sodium lamp include the improvement of color rendering index. The following figure is an example. The left picture shows the use of sodium lamp as the illumination source. Because the color rendering index is only 21, the color of the color ball cannot be Clearly distinguish. The light source used in the right picture is an LED light, and the color of the colored ball can be clearly distinguished.
Why is there such a good effect, but also have to analyze the main points of LED street light design:
Point 1: Optical
At present, the design of domestic road lighting needs to be based on the CJJ45-2006 urban road lighting design standard specification. However, in order to achieve the required illuminance in the long area of ​​the actual road surface, the light pattern of the LED device must be flooded through the secondary optical design. Converted to the light distribution of the batwing type. The secondary optical design to achieve the light distribution of the batwing type is mainly lenticular and mirror type, and both optical designs have their own advantages and disadvantages. The LED street light used in this example is a mirror-type patented system for light distribution, using a high reflection coefficient material as a reflective surface to reduce light loss, and the surface uses tempered glass to provide sufficient strength to block the impact of foreign matter and avoid the use of optical plastics. The problem of lens aging.
In addition to the heat dissipation from the pn junction to the epitaxial layer of the chip, and the use of eutectic soldering or flip-chip packaging to increase the heat dissipation efficiency of the package to the substrate, the selection of the substrate and the design of the heat dissipating fins should also be considered in the system design. How to achieve the best heat dissipation design with the least amount of fin volume.
Point two: power supply
In order to enable the LED to achieve the claimed long life, the LED drive power source using the constant current output is used. If the LED driving power supply with constant voltage output is used, it will cause difficulty in controlling the current difference of each LED unit, which causes one of the currents to rise and fail, which leads to the failure of the entire LED lamp. Even if it is also a constant current power supply, there are key technical indicators to determine its pros and cons:
1. Power efficiency: The efficiency of converting from AC to DC. Since the purpose of using LEDs is to save energy, the efficiency of AC conversion DC is naturally higher as possible. Moreover, the higher the efficiency, the less heat is emitted from the power supply, and the system is naturally affected by heat, which indirectly reduces the light decay of the LED.
2. Power factor: The driving power source with high power factor can reduce the harmonic pollution of the power grid and improve the stability and load of the power grid.
3. Drive current stability: LED is a diode with diode characteristics, which is quite sensitive to the drive current. Therefore, the unstable drive current will cause the current received by the LED to fluctuate greatly, causing serious or even failure of light.
Key point three: institutional design
Since LED is called the new generation of light source, the change brought to the road lighting is not only the change of the light source, but also its exterior design should bring the impression of modernizing the road landscape. At the same time, the appearance of the LED street light must also achieve the function of system heat dissipation, in order to maintain the normal operation of the LED for a long time. The LED street light used in this case uses a modular design that can replace a single module, simplifying the maintenance work. And its open architecture, in addition to improving the heat dissipation efficiency of natural convection, also has a sense of design elements of the times.
to sum up:
Due to the limitation of physical properties, the color temperature of the sodium lamp is limited to about 2000K, and there is no sodium lamp with high efficiency and high color temperature to choose from. However, human eye photoreceptors can be divided into two major categories according to their shape, namely rod cells and cone cells, the visual perception of the cones, and the perception of the main dark vision of the rod cells. Rod cells are more sensitive to light at blue-green wavelengths (generally light with higher color temperatures). At the same time, according to the research, using the high dark/bright visual ratio light source at the same brightness, and having better depth of field and resolution, it is suitable for a place like the road environment that needs a bright response to environmental changes. Therefore, from this point of view, a high color temperature light source is more suitable for road lighting than a low color temperature sodium lamp.
With the rise of energy prices and the awareness of environmental protection, LED lighting has received more and more attention from the market due to its high light efficiency and non-polluting materials. In terms of LED street lights, it has gradually been applied in pilot roads. Although the current price is still high, with the breakthrough of key technologies, the light efficiency of LEDs has been continuously broken, and the price has decreased with the increase of market acceptance. It is believed that in the near future, LED lighting will definitely replace the traditional light source and become a true new generation lighting source.
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