Getting rid of the technical barriers of light and heat, LED street light system design has made a great leap forward

LED lamps are expected to replace traditional lamps and become the mainstream of new-generation street lamps due to their advantages of power saving, energy conservation and environmental protection. However, the current market products are of varying quality, and the design technologies of optical, mechanical, electrical and thermal systems still need to be broken through. Among them, the technical difficulty of optical and thermal systems is the highest. Fortunately, major breakthroughs have been made in the current technical bottlenecks, which will help accelerate the expansion of the street lamp market in the future.

In the era of high oil and electricity prices, how to effectively improve the efficiency of lighting equipment has become an important task for energy conservation and carbon reduction. With the improvement of LED luminous efficiency and lifespan, LED lamps, which were originally only used for indication or auxiliary purposes, have gradually expanded to general lighting, and can provide nighttime traffic safety lighting and street lamp facilities for maintaining nighttime public security, improve the overall appearance of the city, and achieve the multifunctionality of power saving, energy saving and environmental protection. LED street lamps have varying quality

LED street lights must withstand the test of high and low temperature differences and the sun and rain. Some products break down within half a year of installation. LED street lights are of varying quality, resulting in a lack of confidence among users and naturally weak market demand. Some experts believe that the current luminous efficiency of LEDs is still far behind that of high-pressure sodium lamps. They are fine for decorative lighting and some environmental lighting; but if used for road lighting, they are currently only suitable for courtyard (park) area passages or secondary roads; at the same time, as functional road lighting (main roads), there are still many technical problems to be overcome, such as heat dissipation, secondary light distribution, uniformity, etc., so they are not suitable for main road lighting at present.

Optical/mechanical/electrical/thermal technical bottlenecks need to be solved

Because of the luminous characteristics of LED, more than 80% of the input electrical energy is converted into heat energy that must be discharged. In addition, LED crystals are semiconductor materials, so they are not resistant to high temperatures, and the luminous efficiency will decrease as the chip temperature rises. Heat dissipation becomes the primary consideration for LED street lights. Secondly, street light applications must comply with road lighting specifications. Depending on the region and road conditions, there will be different regulations based on illumination , illumination uniformity, etc., so optical design becomes the second biggest challenge for LED street lights. Other factors, such as LEDs require DC drive , so the arrangement method and the selection of power supplies are technical focuses, and there are also safety considerations such as wind pressure resistance, lightning resistance, and vibration resistance, which increase the difficulty of LED street light design.

Simply put, LED lamps need to consider four major parts: heat, light, electricity, and machinery. The highest technical bottleneck in the current market is heat and light. The following will discuss these two major technical parts.

Solve the heat dissipation problem by using heat dissipation mechanism/passive heat dissipation/components

The maximum temperature that LED chips can withstand is about 125-150℃. In the application of LED street lights, the ambient temperature can reach above 35℃, and even above 40℃ in the high temperature area. If the heat cannot be effectively dissipated (the junction temperature is controlled), the life of the LED will decrease rapidly. According to Cree 's data (Figure 1), if the junction temperature of the light-emitting chip can be controlled at 65℃, the overall life of the LED will reach 100,000 hours (calculated at 30% light decay).

Data source: Cree
Figure 1 LED lifespan chart

The LED lamp starts to generate heat from the chip, and then the heat is dissipated by the heat dissipation device. The whole heat transfer diagram is shown in Figure 2. The front section from the LED chip to the LED base is related to the LED package design, so the selection of temperature-resistant chips, phosphors, packaging materials and packaging design with low thermal resistance is the primary requirement. At present, the thermal resistance can generally be below 10K/W, and the better technology has reached the level of 5K/W. The back section from the LED base to the air end is related to the heat dissipation mechanism.

Figure 2 Schematic diagram of LED simple heat transfer

There are generally several forms of heat dissipation mechanisms for high-power LEDs

: ‧ Active heat dissipation
Adding fans to force heat dissipation, water cooling technology, semiconductor cooling chips.

‧ Passive heat dissipation
Natural heat dissipation, heat pipes plus fins, temperature spreader plus fins, loop heat pipe technology, etc.

For street lamp applications, outdoor fan installation will have stability problems. Water cooling and semiconductor cooling chips require additional power for heat dissipation, and the heat dissipation end also requires additional structure for heat dissipation. In the application of LED street lamps, such designs should be avoided in harsh outdoor environments. The following will analyze the principles and relevant comparisons of passive heat dissipation forms.

‧ Natural heat dissipation
Natural heat dissipation uses heat conduction, heat convection and heat radiation as the basic heat dissipation principles. It mainly uses heat conduction to conduct heat to the outer wall of the lamp, and then dissipates heat through convection between the surface area of ​​the lamp or fins and the surrounding air and radiation heat dissipation to surrounding objects (Figure 2).

‧ Heat pipe with fins
The heat transfer phenomenon of heat pipes utilizes the phenomenon that when a substance changes phase, it can absorb or emit high heat energy, thus making heat pipes a device with extremely high heat transfer efficiency. Generally, heat pipes still need to add fins at the condensation end to dissipate heat, which is a very mature technology and is mostly used in computer type equipment.
‧ Heat spreader with fins
The principle and theoretical framework of the heat spreader and heat pipe are the same, only the direction of heat conduction is different. The heat conduction method of the heat pipe is one-dimensional, which is a linear heat conduction method, while the heat conduction method of the heat spreader is two-dimensional, which is a surface heat conduction method.
‧ Loop heat pipe heat dissipation
When heat is transferred from the evaporator to the working medium in the loop heat pipe, the working medium absorbs heat and evaporates and flows to the condenser. After releasing heat and condensing, it returns to the evaporator through the capillary force of the porous material in the evaporator and repeats the cycle. Among the passive

heat dissipation components, the loop heat pipe just makes up for the fact that heat pipes and heat spreaders cannot conduct heat over long distances, and can conduct heat to a farther or easier place to dissipate heat.

The loop heat pipe technology was originally used in space technology, artificial satellites, etc. It is a new attempt in LED lighting . The principle is shown in Figure 3 (A) and the application diagram is shown in Figure 3 (B). The heat pipe can use the lamp housing to dissipate heat, and there is no need to install additional fins. If aesthetics are considered, the heat pipe can be placed inside the lamp housing.

Figure 3 (A) Schematic diagram of loop heat pipe (B) Schematic diagram of loop heat pipe LED lamp application

The heat dissipation component is basically just a tool. There is no real distinction between good and bad. The important thing is to use these tools to achieve the purpose of optimizing the application of heat dissipation. The following will compare the above heat dissipation mechanisms in the application of LED street lights.
‧ Natural heat dissipation
The advantages are the lowest heat dissipation cost and the simplest structure. It is more inclined to small wattage heat dissipation, such as the application of MR16.
Most of the LED street light products on the market that use their own structural heat dissipation method are products below 100 watts. They use the low density of LED lamp beads to overcome the problem of excessive heat flux, but the loose arrangement may produce multiple images in optics.
In practical applications, if the lamp uses fins for heat dissipation (the fins are exposed to the atmosphere), special attention should be paid to dust or other foreign matter accumulating on the fins, which will reduce the heat dissipation effect and affect the life of the LED.
‧ Heat pipe plus fins
The more mature heat pipe technology in the current market, generally a single 6 mm diameter round tube heat pipe can solve about 40 to 50 watts of heat. Considering the loss of efficiency due to bending and flattening, a heat pipe can solve about 30 to 40 watts of heat.
For a 100-watt lamp, about three heat pipes can take the heat out of the LED module and then guide it to the fins for heat dissipation. For high-wattage LED street lamp products such as 100 watts or more, this is a more reasonable and low-cost solution. This heat dissipation method is currently the majority in the LED street lamp market.
‧ Heat spreader plus fins
The biggest advantage of the heat spreader is to solve the heat dissipation of high heat flux, or solve the sudden expansion thermal resistance, and expand the point heat source that is not easy to dissipate heat into a surface heat source that is easier to dissipate heat, as shown in Figure 4. The figure shows a simple aluminum plate plus fins on the left, three heat pipes plus fins in the middle, and a heat spreader plus fins on the right. The entire temperature distribution shows that when the heat source is on the right side of the entire heat dissipation module, three heat pipes plus fins are obviously better than a simple aluminum plate plus fins, but if a heat spreader is used, the temperature distribution is obviously better, and the thermal resistance is reduced from 0.61K/W to 0.54K/W. This emphasizes the uniformity of heat transfer to determine the utilization rate of the fins.

Figure 4 Comparison of heat dissipation simulation between aluminum plate, heat pipe and vapor chamber

The biggest limitation of the heat dissipation method of the vapor chamber plus fins is that the heat pipe can transfer heat horizontally or vertically, but the vapor chamber can only transfer heat vertically, which limits the design of the lamp (Figure 5). Currently, there are not many manufacturers using this heat dissipation method in the market. The application method is to use six to eight LEDs as a module, and combine multiple modules into a high-wattage LED street lamp.

Figure 5 Comparison of heat transfer between vapor chamber and heat pipe

‧ Heat dissipation by loop heat pipe
The maximum wattage of a single module of loop heat pipe on the market is 160 watts (Figure 6). In addition to solving the problem of high heat flux, it also has the advantages of long-distance heat transfer, anti-gravity and flexible heat pipe. If the heat pipe can be effectively combined with the lamp housing, the overall weight of the lamp can be greatly reduced. Through the loop heat pipe, the heat of the LED module located at the bottom of the lamp is moved up to the lamp housing, and the heat is dissipated through the large area of ​​the upper lamp housing. In this way, the fins on the top of the lamp housing will not be seen, making the lamp housing not only an appearance part but also a functional part (Figure 7).

Figure 6 General loop heat pipe standard module

Figure 7 Application of loop heat pipe in LED street light

The actual test results of the 150W LED street lamp on the market using loop heat pipe cooling are shown in Figure 8. After effective heat dissipation design, the weight is only 8.5 kg. In an environment with an ambient temperature of 30℃ and no wind, the measured chip temperature is 71℃ and the thermal resistance value is 0.27K/W. At present, there are still a few LED street lamps made with loop heat pipe cooling in the market, but because of its overall light weight, it has also attracted a lot of attention.

Figure 8 Measured results of loop heat pipe in LED street lamp

Due to recent industry investment, LED street lamps have gradually matured in solving heat problems. The above solutions will be considered differently due to the application method and the angle of entry. The above methods are evaluated from four angles: reliability, cost, design difficulty, and weight, and a simple comparison is made (Figure 9). If the main market is low-priced, cost is the first consideration, and fins or heat pipes plus fins are the better choice; if there are safety concerns and a lightweight street lamp is required, loop heat pipe cooling will be a good choice after effectively reducing costs.

Figure 9 Comparison of different heat dissipation methods

Light uniformity is another major technical challenge for LED street lights

. LED luminous efficiency/lifespan/light color/ color rendering Better than traditional light sources Comparing
traditional light sources with LED light sources Overall, the luminous efficiency and lifespan are shown in Figure 10, and the light color and color rendering are shown in Figure 11.

Figure 10 Comparison of luminous efficiency and lifespan of light sources

Figure 11 Comparison of color temperature and color rendering of light source

From Figure 10, we can see that the luminous efficiency of LED lamps is higher than that of mercury lamps, but overall, high pressure sodium lamps have the highest luminous efficiency. However, the service life of LED lamps is much longer than that of other lamp sources. If factors such as lamp efficiency and power efficiency are taken into consideration, the advantages of LED lamp sources can be more clearly seen.

When walking on the road at night, if a person whose face and clothes color are not clear can come from the opposite direction, it will make people feel uneasy and scary. The property of light source that can show the color of an object is called color rendering index (CRI). When lighting the road, choosing a light source with high color rendering index makes it easier to identify traffic conditions and effectively reduce the crime rate. In Figure 11, LED lights are better than high-pressure sodium lamps and mercury lamps in color rendering.

In terms of color temperature selection, due to different environments and design conditions in different places, for Orientals, mainly because of high temperature and high humidity, they will choose white or high color temperature lighting. If it is for atmosphere lighting, they will choose low color temperature. For Taiwan, which is located in the subtropics, mercury lamps or white light LEDs are better.

With luminous efficiency, life, light color and color rendering index as the four main evaluation points, mercury lamps, high-pressure sodium lamps and LED lamps, which currently have the largest market share of street light sources, are compared (Figure 12).

Figure 12 Comprehensive comparison of light sources

‧ LED light uniformity must meet IES specifications
LED has strong directivity, so it must be used in street lamps by using reflectors or optical lenses to meet the uniformity requirements of general road standards. The most commonly cited foreign standards for general road lighting are the standards of the North American Lighting Association (IES) and the International Commission on Illumination (CIE). Since the brightness of street lamps in various parts of China is generally higher than the national standard, twice as bright as that in the United States and 1.5 times brighter than that in Japan, resulting in a waste of power resources, this article will discuss the IES specifications.

Street lamps will be classified according to the type of road and area where they are installed. Table 2 is the recommended value of the IES brightness standard, and Table 3 is the recommended value of the IES illumination standard. The following uses a commercially available 150-watt LED street lamp with a luminous efficiency of 58 lumens per watt . The specifications and light shapes are shown in Figure 13. The simulated road lighting results are shown in Table 4 (the simulation software is Dialux version 4.4). The results obtained under the simulation conditions in Table 4 show an average illuminance of 11 lux. For asphalt roads, it can be installed on expressways and main roads in residential areas. If it is to be installed in mixed areas or commercial areas, the luminous efficiency or wattage must be increased. In terms of illuminance uniformity, the illuminance uniformity of the simulation results is 1.43, which is less than 3 and meets all road types and regional classifications.

Figure 13 Specifications and light patterns of commercially available 150W LED lamps

The average brightness of the simulation result is 0.55cd/㎡. For asphalt roads, it can only be installed on secondary roads in residential areas. If it is to be installed on expressways and main roads in residential areas, it must be increased to 0.6cd/㎡. In terms of brightness uniformity, the brightness uniformity of the simulation result is 1.33, which is less than 3, and meets all road types and regional classifications.

Therefore, taking the above-mentioned commercially available 150-watt simulation example, due to cost considerations, only 58 lumens per watt LED is used, so in terms of average illumination and average brightness, it can only be installed on secondary roads in residential areas, but in terms of illumination uniformity and brightness uniformity, it can meet all road types and regional classifications.

LED street light technology/specifications are expected to promote market growth

The above comparison of luminous efficiency, lifespan, color temperature and color rendering shows that with the current specifications of LED, it does have the potential to replace traditional light sources. The design should be based on the road type and installation area to select the appropriate LED. And because of the continuous investment of industry resources, the uniformity of LED street lights can meet the road lighting standards through the technology of reflective plates and optical lenses.

LED street light system technology has solutions in terms of heat, light, mechanics and electricity. With the release of relevant standards, the technology and quality of lamps can be effectively controlled. This will strengthen the confidence of consumers and the government. It saves electricity and money. I believe that the promotion of LED street lights can provide a good solution to the current energy shortage.