Analysis of obstacles and countermeasures for large-scale application of LED street lamps

I. Introduction

Under the background of energy conservation and emission reduction, the “Ten Cities and Ten Thousand Miles” LED streetlight projects of various local governments have been vigorously carried out. However, due to the rush of many projects, the ideal results have not been achieved. In some sections, there are problems in that LED street lights are not energy-saving, light decay is obvious, lighting effects are poor, and service life is short. Therefore, many experts believe that LED as a street lamp to replace high-pressure sodium lamps is still immature in technology, and even the replacement of traditional high-pressure sodium lamps with LED street lamps is a disaster. In this regard, LED street lamp manufacturers obviously do not agree, the two sides arguing increasingly fierce, but they did not do quantitative detailed analysis, staying at the level of qualitative analysis, subjective judgment is stronger. From the user's point of view, due to the asymmetry of information, the user hopes to lower the purchase price in order to avoid greater losses, which leads to the phenomenon of “bad money expelling good money”. The market is full of exaggerated propaganda and quality. Poor product.

From this technical point of view, this article does not evade the problems existing in the use of LEDs, analyzes them one by one, and strives to restore the obstacles of large-scale application of LED street lamps, and proposes corresponding countermeasures with scientific and rational attitude to promote the progress of the industry.

The main problem of the two LED street lights

LED street lights find various problems in use, which hinder the large-scale promotion and application of LED street lights. The main problems are as follows:

1. The luminous efficiency of LED cannot be compared with that of high pressure sodium lamp, and the energy saving effect is not obvious;

2. LED street light road surface lighting effect is not good, the illumination is concentrated, the road lighting has zebra pattern phenomenon;

3. LED street lights have serious light decay and long life;

4. The LED street lamp itself is too heavy, which is not conducive to installation and windproof;

5. The color of LED street light is white and the penetration is not strong, which is not conducive to the illumination of rain and fog;

6. LED street lamps are too expensive and the initial investment is high.

Three countermeasures analysis

Based on the actual product application test of the Maige Rui brand LED street light from Guodian Optoelectronics Technology Co., Ltd., the above technical issues are discussed as follows.

1. LED street light energy saving effect

The nominal light source efficiency of the sodium lamp is 120-140lm/W, and the light is emitted to the four places. The efficiency of the lamp is low. The light efficiency of the lamp can only reach 70%. Taking the 250W high-pressure sodium lamp as an example, the nominal power is 250W. In addition to the power supply, the input power reaches 280W, and the luminaire efficiency is 69lm/W. In addition, the high-pressure sodium lamp does not have a light distribution for the road surface. A large part of the light cannot reach the road surface, and the utilization rate of the lighting road surface is about 55%. Taking the 12-meter light pole as an example, the actual effective light output is 10780 lm. The color rendering index of the high-pressure sodium lamp is only 20 to 40, which is dim and the color of the object is difficult to distinguish under the light;

The Maige Rui brand LED street light is tested by the third-party unit Shanghai Semiconductor Lighting Engineering Center, and the whole lamp efficiency reaches 120lm/W. Taking 100W LED street lamp as an example, the light source efficiency reaches 140lm/W, the power efficiency is 92%, the optical efficiency is 95%, and the color temperature is 4000K. Taking the batwing type of light distribution, the utilization rate of the lighting road surface is 90%. Taking the 12-meter high pole as an example, the light output on the road surface is 10710 lm, which can completely replace the 250 W high-pressure sodium lamp. LED street lights have a high color rendering index and can reach 70-80, so the road surface is bright and comfortable. It is easier for drivers and pedestrians to identify the difference between objects on the road and the background, and provide better traffic conditions under the same road surface brightness.

Table 1 Comparison table of high pressure sodium lamp and LED street lamp parameters

Table 1 lists the replacement of traditional sodium lamps with LED street lamps without reducing the road illumination when the road illumination is the same. High-pressure sodium lamp with 150W high-pressure sodium lamp (actual power 170W) can be replaced by Maige Rui LED 60W street lamp, energy saving effect is 65%; 250W high-pressure sodium lamp (actual power 280W) can be replaced by 100W LED street lamp, energy saving effect is 64%; 400W can be replaced by 175W LED street lamp High-pressure sodium lamp (actual power 450W), energy saving effect of 61%. In the actual measurement of the Yulan Road section of Hefei City, the 250W high-pressure sodium lamp was replaced by 100W LED street lamp. Before and after the replacement, the road illumination was unchanged, and the energy saving ratio was 65%.

2. LED street light road lighting effect problem:

The traditional high-pressure sodium lamp has a circular optical distribution, the brightness is the brightest under the lamp, and the utilization of the road lighting is not high. For LED street lights, if the secondary optical design is not carried out, the light distribution of the LED is circular, generally 120 degrees, and the illumination is concentrated, which is not suitable for road lighting, and is easy to cause glare directly to the driver's eyes, which affects driving safety. For the first generation of LED street lights, in order to facilitate heat dissipation, a separate multi-lamp bead solution is generally adopted. A secondary optical lens is added to each lamp bead to perform light distribution. Since it is a multi-point light source, it is easy to cause objects under the lamp. Multiple images affect the safety of driving; and because it is technically difficult to simultaneously distribute light at multiple points, the road surface illumination effect is not good, and there is a zebra pattern phenomenon.

Maige Rui LED street light adopts COB integrated light source and one-time forming optical glass lens. Its light transmittance is above 93%, which replaces the current matrix-type small lens and then strengthens the glass. The multi-lens method requires two optical lenses and the light transmittance is below 80%. Centralized light distribution increases the optical efficiency of the luminaire and makes it easier to achieve the ideal butterfly-type light distribution. After field measurement, the uniformity of road illumination (illumination uniformity = average illuminance / maximum illuminance) can reach 0.82, the road illumination illumination is uniform, far higher than the national standard illumination uniformity of 0.4.

3. LED street lamp life and light decay issues:

The heat dissipation, material and protection level determine the life and light decay of LED lamps. First, the LED lifetime is 50,000 hours at a junction temperature of 85 degrees. For every 10 degrees increase in temperature, the life is halved. Conversely, for every 10 degrees decrease in temperature, the life expectancy will be doubled. At present, the luminous efficiency of LEDs has not been raised to a very high degree, and the heat dissipation can be improved as much as possible to extend the life of the LEDs. Although each small crystal of LED consumes a small amount of energy, even if it is only 1W, the heat transfer area of ​​the crystal is too small, so that the heat flow cannot be quickly exported. Traditionally, copper or other materials with high K values ​​are tried to extract heat, which still cannot be solved. The reason is that most heat transfer experts ignore the cause of the diffusion heat resistance. If many LED small crystal COBs are packaged together, the thermal interaction will be more severe. The heat flux density of LEDs is 80W/cm2. However, the heat flux density of metal materials in thermal conductivity is very limited. For example, aluminum is 4W/cm2, copper is 10W/cm2, silver is 12W/cm2, and Maigerui street lamps adopt multi-point heat. The superconducting temperature equalizing plate has a heat flux density of 170 W/cm2, which is very suitable for the heat dissipation substrate of the LED. The first generation of LED street lights, completely adopting the technology of passive heat dissipation, relies on natural conduction and convection to dissipate heat. In the case that the heat treatment is not in place, the junction temperature of some LED street lamps may reach 115 degrees, and its theoretical life is only 6000. hour. Maige Rui LED street light adopts active heat dissipation technology. It relies on the uniform temperature plate to quickly dissipate the concentrated heat source into a plane. It uses heat pipes and large-area fins to dissipate heat. It is measured by an infrared temperature camera and is at an ambient temperature of 30 degrees. The junction temperature is 75 degrees, which greatly improves the service life of LED lamps. It is estimated that after 10 years of work, its light decay is only 10%. The COB-based integrated light source is tested in 2500 times of the -40 to 50 degree temperature cycle test, and its light decay is within 1%.

Because streetlights are used in outdoor complex climates, the choice of materials is critical. The first generation of LED street lamps, commonly used organic materials such as PVC to make secondary light distribution lens, as shown in Figure 1, ordinary PC material, after 30,000 hours of indoor use, its light transmittance is only 85% of the initial value . Considering the outdoor environment, after two or three years of use of organic materials, cracking, deterioration, and yellowing will occur. Especially in the case where the outdoor temperature is minus 40 degrees, organic materials are prone to cracking. This problem has not been caused. Universal attention. The Maige Rui LED street light adopts an optical glass lens, and its characteristics and light transmittance do not change with time and temperature, eliminating the life hazard caused by the material.

Figure 1 Light transmittance of material versus time

The degree of protection also determines the life of the LED luminaire, which is a low-voltage semiconductor component that emits white light by means of a phosphor coated on the LED dies. The ingress of moisture affects the properties of the silica gel, changes the molecular structure of the phosphor, and reduces the insulation of electronic components. Light causes light decay, reduces life, and causes safety accidents such as electric shock. The first generation of LED street lights, due to the use of matrix multi-point light sources, requires a large area of ​​protective shielding, it is difficult to achieve waterproof and dustproof, and some street lights even have mosquitoes entering. The Maige Rui LED street light adopts an integrated light source, the protection level can be IP67, it can be safely lit under water, so that the lamp can be used in a wide range of applications, it can be used in extremely cold areas, safe and reliable and maintenance-free. Compared with multi-lens LED street lights, the IP protection level is higher, which eliminates the influence of mosquitoes, dust and moisture on the life of the lamps.

4. The problem of the weight of LED lamps

At present, LED lamps produced by most manufacturers are extremely heavy, usually exceeding 10 kg. Such a large self-weight often causes the pole to be overloaded, reducing its wind resistance, and the installation is laborious and dangerous. This is mainly due to the use of a huge cast aluminum radiator. The Maige Rui LED street light adopts active heat dissipation technology, and uses the uniform temperature plate, the heat pipe and the aviation aluminum fin to carry out triple heat dissipation, which greatly reduces the weight of the lamp. The 100W LED luminaire, which weighs only 5 kilograms, can completely replace the 250 watt high pressure sodium lamp. The comparison is shown in Figure 2.

Figure 2 Passive heat dissipation and active heat dissipation