What are the problems with LED street lights?

The application of LED (light-emitting diode) can be said to be everywhere. As a semiconductor luminescent material, compared with other luminescent materials, LED has the advantages of being green, efficient, reliable and durable, which makes other luminescent materials pale in comparison. With the continuous reduction of the cost of high-power LED , it is possible to gradually replace traditional lamps. The application of LED light source in road lighting has become a hot spot in the semiconductor lighting industry in recent years. What are the special requirements for the technology of LED device products applied to street lamps? What are the advantages of LED street lamps compared with ordinary street lamps? What are the shortcomings? How to solve them? Industry experts have explained from different aspects.

Formulating national standards is a powerful means of popularization and promotion

LED as a street light has the following advantages or characteristics: high luminous efficiency , low reflection loss of lamps, 70% energy saving; with digital control brightness function, more power saving; no high voltage required, high safety; with software, the brightness can be automatically controlled remotely; provide ultra-high brightness and high color rendering lighting source in special circumstances such as accidents, foggy days, rainy days, etc.; easy installation and maintenance; module installation, no redundant wiring; no light pollution or waste; long life, which means that it does not need to be replaced frequently, thus reducing the potential for traffic interruption and the maintenance costs paid for it. Some people have calculated that the use of LED street lights is much better than high-pressure sodium lamps in terms of laying cost, power consumption cost and life, except that the cost of a single lamp and the initial installation cost are higher than high-pressure sodium lamps.

The main technical problems faced by LED street lights are: output power and luminous flux , secondary optical design, heat dissipation design and power supply system design. The improvement of output power and luminous flux also requires further improvement from the basic levels of epitaxial technology and chip technology of high-power white light LEDs . In terms of secondary optical design, the radiation forms of LEDs include Lambertian, side-emitting, bat-wing and concentrating types. In the field of road lighting, according to design experience, Lambertian and bat-wing types are more suitable. Through secondary optical design, the illumination range and photometric curve of LEDs meet the needs of road lighting. Heat dissipation is one of the key issues that LED street lights need to solve. Due to the high brightness requirements and high heat generation of LED street lights, and the harsh outdoor use environment, poor heat dissipation will directly lead to rapid aging of LEDs and reduced stability. In addition to the need for good heat dissipation of power-type LEDs themselves, since street lights are used outdoors at night, the heat dissipation surface is located on the side, which is conducive to natural convection heat dissipation of air. The power supply system of LED street lights is also different from that of traditional light sources. The low energy consumption and low operating voltage of LEDs are well matched with the low output power and low output voltage of solar cell modules . How to make a group of tightly compressed LEDs work safely and reliably is also an indicator for evaluating LED street lights.

Based on the above analysis of the market prospects and technical issues of LED street lights, it can be seen that the factors affecting their application are:

First, the high cost of LED street lamps is not conducive to their popularization. Although the heat dissipation problem of LED street lamps has been initially solved, the high price and heavy lamp heads still hinder the popularization of LED street lamps.

Second, the reliability of LED street lamp modularization still needs to be strengthened. Due to reasons such as the drive circuit, LED street lamps may also fail.

Third, LED street lamps, whether single-point or multi-point light sources, have the problem of heat dissipation. A street lamp using a 250W high-pressure sodium lamp, due to its mature technology and good heat dissipation control, has a small light decay even after working for 5,000 hours, and the luminous efficiency can reach more than 70%. With current technology, under the same conditions, high-power LED street lamps have a large light decay and a luminous efficiency of 30% due to the difficulty in solving the heat dissipation problem. If you want to increase the light efficiency by increasing the voltage and current, it is easy to cause damage to the LED chip.

Fourth, LED street lights have a short range, so LED street lights need to further strengthen research on how to work at higher altitudes. In addition, formulating national LED road lighting standards is also a very powerful means to popularize and promote LED street lights.

Heat dissipation and reliability are the main factors affecting applications

There are two ways to use LED light sources. One is to use traditional low-power LEDs as a combination, usually up to hundreds or even hundreds of them, and the power supply design is complex. The other is to use high-power tubes as light sources, which are relatively expensive. Both methods inevitably take heat dissipation design and working reliability as the main design considerations. Domestically, they are mostly used in government demonstration projects, and there are few truly market-oriented projects. There are many application examples in this regard abroad, but their biggest disadvantages are still reliability, light output lumens and price. Many projects do not show the advantages of long life well due to the poor quality of LEDs. In addition, from the perspective of cost and market, whether LED is used in combination with solar energy as a lighting source requires different design routes, and it cannot be completed by developing it as a light source alone.

When LED device products are applied to street lamps, the special technical requirements are mainly to combine the LED light intensity and the emitting angle to design. In addition, due to the combination of multiple LEDs, the light output design must take into account the irradiation area, and the lamp needs to focus on the effectiveness of heat dissipation.

The advantages of LED street lights over ordinary street lights are: energy saving, environmental protection, easy to adapt to low voltage, and can be directly matched with solar energy systems without the need for additional inversion and conversion processes, thus achieving maximum energy utilization.

Disadvantages: small and uneven lighting angle, low color rendering index , complex optical and heat dissipation design.

Technical deficiencies: The luminous flux under current technology is not enough, the luminous efficiency is too low, and the quality is difficult to guarantee.

Solution: LED products used in street lamps require special designs and standards (using test data of traditional light sources to evaluate LED light sources is often not objective).

LEDs have inherent advantages and disadvantages in street lighting. The advantages are: first, as a point light source, if the design is reasonable, LED can directly solve the secondary light collection and light loss problems that traditional spherical light sources must rely on light emission to solve to a large extent; second, the uniformity of the light irradiation surface can be controlled, and in theory, it can be completely uniform in the target area, which can also avoid the light waste in the "bright under the lamp" phenomenon of traditional light sources; third, the color temperature is optional, which is also an important way to improve efficiency and reduce costs in applications in different occasions; fourth, there is still a lot of room for technological progress.

Disadvantages (factors affecting the promotion and application of street lamps): The current price is still too high, the luminous flux is low, and the price of LED light sources with the same illumination is about 4 times that of traditional light sources (however, in street lamp products, the light source part does not account for a high proportion of the total cost, so the cost increase in engineering installation will not be too high, and there is still a lot of room for application), which is difficult to bear in civilian use. The current design and manufacturing standards are relatively chaotic, and the damage rate is high, which affects the life advantage of LED.

Suggestions: Unify LED standards, improve quality stability, establish a market competition coordination organization, avoid vicious competition, vigorously publicize the quality concept of LED, avoid being influenced by bad examples, make people accept that LED can actually achieve its advantage of long life, and improve the level of selecting and using LED.

The demand for white high-power LEDs will reach 300 million

When LED devices are applied to street lamps, the special technical requirements are mainly reflected in the following two aspects: first, good luminous efficiency stability, which should not decay too quickly. Second, the consistency and maintainability of light color should be good, and the color of white light should not change due to long-term use. Both of these points require the overall heat dissipation of the lamp to be good and the optical design to be in place.

Compared with the current ordinary high-pressure sodium lamps, LED street lamps have the following advantages:

First, energy saving: At present, the luminous efficiency of white light LED is about 80lm/W, which can save 50% to 60% of electricity compared with traditional high-pressure sodium lamps. With the rapid improvement of LED efficiency, semiconductor street lamps have shown great potential in energy saving.

Second, low maintenance cost: Based on the current technical level and test results, high-power LED light sources can be used normally for 10 years without replacement, while traditional high-pressure sodium lamps need to be replaced every 1.5 years on average. The use of semiconductor light sources can greatly reduce maintenance costs.

Third, good color rendering: LED has a high color rendering index (75-80), people react quickly, the road looks brighter, feels more comfortable, and drivers feel safer. High-pressure sodium lamps have a narrow spectrum and poor color rendering (20-40), which makes them feel dim.

The main deficiencies are as follows:

First, the luminous efficiency is not high enough: although the luminous efficiency of laboratory-level high-power white light LEDs has exceeded 100lm/W, the luminous efficiency of mass-produced, cost-effective high-power white light LEDs is still around 60lm/W, and the energy-saving advantage has not been fully utilized.

Second, the ground has low reflectivity to LED light: Due to the current white light LED spectrum characteristics, the road surface and surrounding objects have low reflectivity to it, so the brightness of the road surface is low, compared with the yellow light of the sodium lamp, this is a weakness.

The current luminous efficiency of high-power white LED light sources is still at about 70lm/W to 80lm/W, and theoretically its luminous efficiency can reach about 300lm/W, so there is still a lot of room for development in technology. At the same time, semiconductor light source street lamps are currently in the initial stage of industrial development, and there is still a lot of room for improvement in improving heat dissipation efficiency, optimizing drive control systems, and light distribution design, and there is still a lot of research and development work to be carried out in depth.

At present, an LED street lamp requires an average of about 100 1W white light LEDs. Based on the annual demand for 3 million street lamps in the country, the annual demand for white light high-power LEDs will reach 300 million.

Lack of standards and specifications prevents full promotion

Since LED street lights have two outstanding advantages: energy saving (reportedly can save more than 50% of electricity ) and long life (can reduce maintenance and extend replacement time), the development and application of LED street lights in China are developing rapidly. Many large and medium-sized cities have some roads that are trying out LED street lights, but none of them have been adopted in large quantities.

The main problem with LED street lights at present is that there are no standards and specifications, and they cannot be fully promoted. Among them, several key parameters of LED street lights, such as color temperature, color rendering index (color rendering index) and glare, have not yet been determined.

In addition, the price of LED street lights is too high. Currently, the price ranges from 3,000 yuan to 5,000 yuan per unit, depending on the power and installation height, making it difficult to promote and apply them on a large scale.

The urban management system of street lamps is multi-party responsible, which brings great difficulties to the promotion and application of LED street lamps.

It is recommended that relevant departments organize the formulation of relevant standards for LED street lamps and establish relevant testing agencies as soon as possible.