Analysis on the Status Quo and Problems of LED Street Light Technology Development in China

Countries actively promote the implementation of energy-saving and emission reduction projects. In particular, China's LED street lamps have been valued by LED street lamp manufacturers due to their huge business opportunities. Street lamps are an important part of urban lighting. Traditional street lamps often use high-pressure sodium lamps. The low-efficiency of high-pressure sodium lamps as a whole has caused huge waste of energy. Therefore, it develops new high-efficiency, energy-saving, long life, high color rendering index and environmental protection. Street lights are of great significance for urban lighting energy conservation.

Today's LED street lights features:

The high-power LED street lamp is different from the conventional high-pressure sodium lamp street lamp. The high-power LED street lamp uses a low-voltage DC power supply, a high-efficiency white light diode synthesized by a GaN-based power type blue LED and a yellow phosphor, and a light emitting diode (LightEmittingDiode, abbreviated as LED). It is a high-efficiency solid-state light source based on a semiconductor PN junction that emits light with weak electrical energy. Under a certain forward bias voltage and injection current, the holes injected into the P region and the electrons injected into the N region diffuse into the active region. After the radiation is combined to emit photons, the electric energy is directly converted into light energy.

LEDs are the source of street lights, and they have many advantages over traditional street light sources.

1. The LED street lamp itself has directivity of light, and there is no diffusion of light to ensure light efficiency;
2. The LED street lamp has a unique secondary optical design, and the light of the LED street lamp is irradiated to the area of the required illumination, that is, the road surface, thereby further improving the illumination efficiency and achieving the purpose of energy saving;
3, LED light source efficiency has reached 100lm / W, and there is still a lot of room for development, the theoretical value can reach 250lm / W. The luminous efficiency of the high-pressure sodium lamp increases with the increase of power. Therefore, the overall luminous efficacy LED street lamp is stronger than the high-pressure sodium lamp;
4, LED street light color rendering is higher than high-pressure sodium lamp, high-pressure sodium lamp color rendering index is only 23, and LED street lamp color rendering index can reach more than 75, from the visual psychological point of view, to achieve the same brightness, LED street light illuminance average It is more than 20% lower than that of high-pressure sodium lamps (refer to British road lighting standards); moreover, at the intermediate visual level, the human eye is more likely to distinguish things in a high color temperature environment than in a low color temperature environment, avoiding certain dangerous states;
5, LED street light's light decay is small, the light decay of less than 3% in one year, still meets the road use illumination requirement after 10 years of use. Therefore, the design of LED street light in use power can be lower than that of high pressure sodium lamp;
6, LED street lights can be automatically dimmed, can achieve the lighting requirements in different time periods, the maximum possible power reduction, save energy;
7. The LED is a low-voltage DC device that drives the voltage of a single LED to be a safe voltage, so it is a safer power source than a high-voltage power supply, and is especially suitable for use in public places. The characteristics of DC make LED suitable for combination with solar energy and wind power;
8. Each unit LED die has a small volume, so it can be fabricated into various shapes of devices, and is suitable for a volatile and irritating environment;
9, long life: theoretical life can be used more than 50,000 hours;
10, LED can be frequently switched without worrying about its damage;
11. Easy installation: directly install the LED street lamp head to the lamp post to replace the original sodium lamp head;
12, excellent heat dissipation control: can lower the LED junction temperature below 70 °C, with the development of technology, the junction temperature is still further reduced, the lower the junction temperature, the longer the LED chip life in theory;
13, reliable quality: circuit power supply all use high-quality components, each LED has separate over-current protection and series-parallel protection, even if a damage, it will not cause other LED chips to fail;
14. The LED light source does not contain harmful metal mercury, unlike the high-pressure sodium lamp or metal halide lamp, which causes harm to the environment when it is scrapped;

However, many experts and scholars have recently heard the opinion that LED as a street lamp to replace high-pressure sodium lamps is still not mature in technology. The main reasons are as follows:

1. LED luminous efficiency cannot be compared with high pressure sodium lamps
2. LED illumination is too concentrated, and the uniformity of illumination is poor
3. LED street light has serious light decay and long life
4. LED street light itself is too heavy, not conducive to installation, windproof
5. LED street lamps are too expensive to be popularized

We analyze them one by one based on the above points:

1. Luminous efficiency problem

At present, the luminous efficiency of LEDs is not as good as that of high-pressure sodium lamps. The current luminous efficacy of LEDs is generally 65~75lm/w, and with the development of technology, the luminous efficacy of LEDs is increasing every year, while the high-pressure sodium lamps can reach 125lm/w. However, the spectrum of the high-pressure sodium lamp is concentrated in yellow, its color temperature is relatively low, only 2000~2500K, and the color temperature of the LED is higher, which can reach more than 3500~4500K. In addition, the light of the high-pressure sodium lamp is emitted everywhere, and a large part of the light cannot reach the road surface. Also, the color rendering index of the high-pressure sodium lamp is only about 23, which is dim; and the color rendering index of the LED is high, which can reach 75-80. The road surface is bright and comfortable. Therefore, from the actual luminous effect, the LED can be much higher than the high-pressure sodium lamp. The 135W LED can replace the 250W high pressure sodium lamp or the 300W mercury lamp. The 135W LED has an output luminous flux of only about 7020 lumens (which will be lost through secondary optics design). The lumens on the road surface is still 6500 lumens, while the average illumination of the road surface can reach 16 Lux (12 m high pole). The output luminous flux of a 250W high pressure sodium lamp is 20,000 lumens. But the number of lumens reaching the road is only 7000 lumens. The illuminance of the road surface is about 30~40 Lux. Due to the difference in color rendering coefficient, the illumination correction factor of the LED is 2.35 times, and the correction coefficient of the high pressure sodium lamp is 0.94 times. Therefore, after the 135W LED is corrected, the illumination of the ground is 37.6Lux, and the corrected illumination of the high-pressure sodium lamp is 28.2~37.6. The two are equivalent. Therefore, the 135W LED can replace the 250W high-pressure sodium lamp, the LED can save energy by 1.85 times, and the energy-saving efficiency is about 50%.

2. Irradiation uniformity problem

Indeed, if the secondary optical design is not carried out, the illumination of the LED is relatively concentrated, so the secondary optical design must be carried out so that the light intensity distribution map is bat-shaped.

3. LED light fade problem

Some merchants now use hundreds of φ5 low-power LEDs in order to reduce costs. However, the light decay of this low-power LED is very serious, and its lifetime of light decay to 80% is only 1000 hours. Therefore, as a street lamp that requires long-term use, it is absolutely impossible to use such a low-power LED. As a high-power LED, its light decay is much better.

4. Self-weight problem of LED street lamps

In the replacement process of the LED street lamp and the high-pressure sodium lamp, in order to prevent the LED street lamp head from being overweight, some LED street lamp manufacturers are trying to reduce the self-weight of the LED lamp head as much as possible. From the original single lamp head about 30 kg, it has dropped to more than 10 kg now. The weight reduction has a further downward trend.

5. LED street lamps are too expensive to be popularized

LED street lights are made up of many parts, and the most effective component of the light is the chip, and the most expensive part is also the chip. The current chip's efficacy is 65~75lm/w, but major chip manufacturers are developing chips with more light effects. Imagine, with the advancement of technology, once the light efficiency has reached a high level, the number of chips used will be less than the current one, and the price of the chip will also drop. By then, the overall price of LED street lights will also drop.

Future LED street light features:

Personally believe that the replacement of high-pressure sodium lamps with LED street lamps is a trend in energy conservation and emission reduction in the future. LED street lights should be developed, I think we need to pay attention to the following aspects:
1. Improve the light
2. Make secondary optics
3. Solve the heat
4. Modular

A. The improvement of optical communication needs to be further improved from the basic level of high-power LED epitaxial technology and chip technology. At present, the method for producing white LEDs at home and abroad is to first place the LED chips on the package substrate, bond them with gold wires, and then apply YAG phosphor powder around the chip, and then encapsulate with epoxy resin. The resin acts both as a protective chip and as a concentrating mirror. The blue light emitted from the LED chip is reflected and absorbed by the scattered phosphor layer in the surrounding phosphor layer, and finally emitted to the outside. The peak of the spectral line of the LED (blue) is at 465 nm and the half value width is 30 nm. A part of the blue light emitted by the LED excites the yellow YAG phosphor layer to emit yellow light (peak value is 555 nm), and a part of the blue light is directly or reflected and then emitted outward, and finally the external light is blue-yellow light, that is, white light. FlipChip can get more effective light output than traditional LED chip packaging technology. However, if a reflective layer is not added under the electrodes of the light-emitting layer of the chip to reflect the wasted light energy, about 8% of the light is lost. Therefore, a reflective layer must be added to the substrate material. The light on the side of the chip must also be reflected by the mirror surface of the heat sink to increase the light extraction rate of the device. Moreover, a layer of silica gel should be added between the sapphire substrate of the flip chip and the light-guide bonding surface of the epoxy resin to improve the refractive index of the chip. Through the improvement of optical packaging technology, the light extraction rate (light flux) of high-power LED devices can be greatly improved.

B. LED lighting equipment is optimized to improve the quality of LED use. Therefore, it is especially urgent to study the secondary optical light distribution design of high-power LED light source to meet the needs of large-area floodlighting and floodlighting. Through the secondary optical design technology, the design of the additional reflector cup and multiple optical lens and aspherical surface can improve the light extraction efficiency of the device. The traditional light source illuminates 360°. The luminaire relies on the reflector to reflect most of the light to a specific orientation. Only about 40% of the light passes directly through the glass cover to the road surface. The other light is projected through the luminaire reflector. The efficiency of the reflector of the lamp is generally only 50%~60%, so about 60% of the light output is in the lamp, and it is projected on the road after losing 30%~40%. A large portion of the light output of the source is limited to the internal heating of the luminaire. Most of the light of LED lights is front light, which can achieve >95% light effect. This is one of the important characteristics that LEDs are different from other light sources. If this feature is not used well, it will make LEDs more advantageous. discount. Most high-power LED lamps are assembled with multiple LED chips. To illuminate so many light sources in different directions, we fully utilize the characteristics of the overall package of the chip, solve them with lenses, and design different convex curves according to different needs through optical design. Relying on the lens to distribute the light to different directions, to ensure that the light angle can reach 120°-160°, and the small light can concentrate the light within 30°. Once the lens is shaped, the same kind of lamp is guaranteed under the premise of the production process. The light distribution characteristics are also consistent. It is possible to continually sum up experience through a number of trials and trials, and to enable LED street lights to meet the batwing light type required by road lighting standards. At present, tunnel lights, street lights and general lighting have reached the lighting requirements of their respective applications.

C. Heat dissipation is a key problem that LED street lights need to solve. As we all know, LED is a photovoltaic device, in which only 15% to 25% of the electrical energy is converted into light energy, and the rest of the electrical energy is almost converted into thermal energy, so that the temperature of the LED rises. In high-power LEDs, heat dissipation is a big problem. For example, if a 10W white LED has a photoelectric conversion efficiency of 20%, 8W of electric energy is converted into thermal energy. If no heat dissipation measures are taken, the core temperature of the high-power LED will rise rapidly, when its junction temperature (TJ) When the temperature rises above the maximum allowable temperature (typically 150 ° C), high power LEDs may be damaged by overheating. Therefore, in the design of high-power LED lamps, the most important design work is the heat dissipation design. Due to the high brightness requirements of LED street lamps, the use environment is harsh. If the heat dissipation is not well solved, the LED will be aging quickly and the stability will be reduced. As for the street light with 250W high-pressure sodium lamp, due to the mature technology, the heat control is very good. Even if it works for 5,000 hours, the light decay is still very small. With the current technology, high-power LED street lamps under the same conditions, if the heat dissipation is not good, the light decay will be great. At present, the heat dissipation methods of LED street lamps mainly include: natural convection heat dissipation, forced cooling of the installed fan, heat pipe, heat dissipation of the loop heat pipe, and heat dissipation of the uniform temperature plate. The fan is forced to dissipate heat. The system is complicated and the reliability is low. The heat pipe and the temperature equalization plate have higher cost.

D. LED street lights will eventually choose modular installation and maintenance. Nowadays, the most high-pressure sodium lamps used on the road surface, internal ballasts and other components are not easily damaged. Most of the reasons for the non-lighting are the damage of the light source. The maintenance method can be replaced by replacing the light source. A skilled operator can perform high-level operations on his own. However, LED street lights have many internal components. Except for the light source (chip), damage to other parts will cause the chip to be off. Therefore, at the scene, it is impossible to immediately determine the cause of the LED street lamp damage. If the LED street light is not bright, the LED street light needs to be taken off and transported back to the factory for various tests. This way of replacing LED street lights is very cumbersome. The final version of the development of LED street lights is to develop into modularity. Light source, electrical, etc. are all replaced according to the plug, so that a skilled worker can also independently determine the cause of the damage and carry out on-site maintenance.
in conclusion

Recommendation: Unify the LED standard, improve the quality stability, establish a market competition coordination organization, avoid vicious competition, vigorously promote the quality concept of LED, avoid the influence of bad examples, let people accept the LED from the idea can actually achieve its long life. The advantages of improving the level of selection and use of LEDs. In summary, the replacement of high-pressure sodium lamps by LED street lamps is an inevitable trend of road lighting in the future.

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