Recently, I heard many experts and scholars that the LED as a street lamp to replace the high-pressure sodium lamp 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. The long-term light decay of LED street lamps is serious and the life is not long.
4. Unstable LED luminescence (short-term light decay), which changes with time and season
5. LED street lights themselves are too heavy, which is not conducive to installation and wind.
6. LED street lamps are too expensive to be popularized.
Of course, all of this is based on the experience of many domestic test circuits. There is a certain basis.
However, the performance of the domestic test circuit can not represent the current status of the LED is worth exploring.
1. Luminous efficiency problem
At present, the luminous efficiency of LEDs is not as good as that of high-pressure sodium lamps. LEDs currently operate at 65-75 lumens per watt, while high pressure sodium lamps can reach 125 lumens per watt. However, the spectrum of high-pressure sodium lamps is more concentrated in yellow, and its color temperature is only 2000-2500 o K, while the color temperature of LEDs is higher, which can reach 3500-4500 or more. 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 20 to 40, which is dim; and the color rendering index of the LED is high, which can reach 75-80. So the road is bright and comfortable. Therefore, from the actual luminous effect, the LED can be much higher than the high-pressure sodium lamp. The 100W LED can replace the 250W high pressure sodium lamp or the 300W mercury lamp. The 100W LED has an output luminous flux of only about 6250 lumens (after secondary optics design, there will be a loss), the lumens when reaching the road surface is still 6000 lumens, and the average illumination of the road surface can reach 16Lux (12m 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 100W 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, 100W LED can replace 250W high-pressure sodium lamp, LED can save 2.5 times.
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 path must be designed to make the light intensity bat-shaped. So this problem is very easy to solve.
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, according to its light decay to 80% of the life of 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. However, many domestic manufacturers can only achieve a 10% drop in 10,000 hours.
Far from meeting the requirements for use.
In fact, LED life issues are mainly caused by long-term work at high temperatures. The life at different junction temperatures is shown in the following figure:
When the junction temperature is raised from 115 ° C to 135 ° C, the lifetime is reduced from 50,000 hours to 20,000 hours.
At present, the luminous efficiency of LEDs has not been raised to a very high degree, and it is only possible to improve the heat dissipation to extend the life of LEDs.
One of Taiwan's Light Torch Technology has solved this problem well. They used loop heat pipe (LHP) cooling technology to dissipate heat.
The loop heat pipe cooling technology can be used to reduce the junction temperature of the LED to 65 ° C, which greatly improves the service life of the LED lamp. After three years of actual testing, they showed that their light decay was less than 3% when they worked 12 hours a day. It is estimated that after ten years of work, its light decay is only 20%. This basically solves the problem of life.
4. The problem of the stability of LED illuminance
This problem is actually caused by the buck of the battery discharge and the temperature characteristics of the LED.
Usually, the discharge process of the battery has a pressure drop of more than 10%. For a 1W LED, if the forward voltage changes from 3.4 to 3.1V, its forward current will be reduced from 350mA to 100mA. That is to change 250mA. Its output luminous flux will be reduced by about 60%.
In addition, the volt-ampere characteristics of LEDs typically have a negative temperature coefficient of approximately -2 mV/°C.
If the ambient temperature changes by 50 ° C, then the forward voltage may change by 0.1V.
For a 1W LED, the forward current is likely to vary by 100mA, from 350mA to 250mA. The amount of luminescence will also be reduced by 20%. It is equivalent to the light decay with temperature.
In order to completely solve this problem, it is necessary to use a constant current chip to keep the forward current of the LED unchanged. The PAM2842 is a constant current control chip that can supply 30 1 watt LEDs. It can keep the LED forward current within 3% regardless of the voltage change caused by temperature change or battery discharge. That is to keep its short-term light decay within 3%.
5. The weight of LED lamps
Indeed, at present, the LED lamps produced by most manufacturers are extremely heavy, usually exceeding 10 kg. Such a large self-weight often makes the poles overload, reducing their wind resistance. This is mainly due to the use of a huge radiator. Similarly, if a heat pipe is used as the heat sink, the weight can be greatly reduced.
The 100W LED lamp (LT-100-W2) produced by Light Torch Technology Co., Ltd. has a weight of only 5.5 kg and can completely replace the 250 watt high pressure sodium lamp. Because of its light weight, it can be used with a 12m high pole, because the bat-shaped secondary light path can reach an illumination range of 66m.
6. Cost of LED lamps
At present, because the unit price of the LED itself is relatively high, the cost of the entire LED is relatively expensive. However, due to the rapid advancement of technology in LEDs. Its cost reduction is also very fast. Now the price of 1W LED is about 8 yuan, and 100 is only 800 yuan. The unit price of the entire lamp will be around 2,000 yuan. The unit price of solar lamps can also be controlled at around 4,500 yuan.
In fact, for LED lamps, you should not only consider the unit price of a single lamp, but should consider the engineering investment of the entire line. You should not only consider its initial investment, but should consider its operating investment. If solar street lights are used, their annual savings far exceed their initial investment. Here is an example to illustrate: assuming that the unit price of ordinary high-pressure sodium lamps is 1,500 yuan, and the price of solar 100-watt LED lamps is 13,500 yuan each. It is assumed that the illumination range can reach 66 meters. So for the three different distances of 3 km, 5 km and 10 km, let's compare their initial investment (lamps), erection costs (only considering cables, not considering transformers) and operating costs (electricity). The results are shown in the following table:
Therefore, in five years, the use of solar LED street lights can save a total of 17,800, 334,000, and 2.46 million. The savings are considerable.
Therefore, in summary, the key technical problem of LED street lamps is to use the loop heat pipe to dissipate heat and use a constant current chip to stabilize the flow. I hope that the country should make it a standard. Such LED street lights can be widely promoted.
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