Application of ultra-high brightness LED in solar city lighting system

1 Development History of LED

LED has a history of nearly 40 years. The first batch of commercialized LEDs appeared in 1968. Although LEDs have been limited by color and luminous efficiency for many years, GaP and GaAsP LEDs have many advantages such as long life, high reliability, low operating current, and compatibility with TTL and CMOS digital circuits. Therefore, they are favored by users and have begun to enter various application fields including aerospace, aircraft, automobiles, industrial applications, communications, etc. By 1996, LED sales in the world had reached several billion US dollars.

In the past decade, the frontier topics of LED material and device process technology research have been ultra-high brightness and full color. The so-called ultra-high brightness LED (UHB) refers to an LED with a luminous intensity of 100mcd or more, also known as a candela (cd) level LED. Due to the use of AlGaInP and InGaN as the light-emitting layer, the development of ultra-high brightness LEDs has progressed very rapidly and has reached a performance level that conventional materials GaA1As, GaAsP, and GaP cannot achieve. In 1991, Toshiba Corporation of Japan and HP Corporation of the United States developed InGaA1P (λ=620nm) orange ultra-high brightness LEDs, and in 1992, InGaA1p (λ=590nm) yellow ultra-high brightness LEDs were put into practical use. In the same year, Toshiba Corporation developed InGaA1P (λ=573nm) yellow-green ultra-high brightness LEDs with a normal light intensity of 2cd. In 1994, Nichia Corporation of Japan developed InGaN (λ=450nm) blue (green) ultra-high brightness LEDs.

At this point, the three primary colors of red, green, blue, as well as orange and yellow required for color display have all reached candela-level luminous intensity, achieving ultra-high brightness and full color, making outdoor full-color display of light-emitting tubes a reality.

Through technical transformation, technical research, introduction of foreign advanced equipment and some key technologies, my country's LED production technology has taken a step forward. Taiwan is the world's main production base for LEDs and tube cores, with an annual output of about 4 billion LEDs, complete product varieties and specifications, and performance reaching or approaching the world's advanced level. The tube cores produced are also exported in large quantities to Japan and other countries.

2 Basic performance of ultra-high brightness LED

2.1 Basic characteristics of various ultra-high brightness LEDs

Ultra-high brightness red A1GaAsLED has higher luminous efficiency. The lumen efficiency of transparent substrate (TS) A1GaAsLED (640nm) is close to 10lm/w, which is 10 times greater than that of red GaAsP-GaPLED. Ultra-high brightness InGaAlPLED provides the same colors as GaAsP-GaPLED, including: green-yellow (560nm), light green-yellow (570nm), yellow (585nm), light yellow (590nm), orange (605nm), light red (625nm), and dark red (640nm). The lumen efficiency of InGaAlPLED absorption substrate (AS) is 101m/w, and that of transparent substrate (TS) is 201m/w. In the wavelength range of 590-626nm, the lumen efficiency is 10-20 times higher than that of GaAsP-GaPLED; in the wavelength range of 560-570, it is 2-4 times higher than that of GaAsP-GaPLED. Ultra-high brightness InGaN LED provides blue and green light, with a wavelength range of 450-480nm for blue, 500nm for blue-green, and 520nm for green; its lumen efficiency is 3-151m/w.

2.2 Comparison between ultra-high brightness LEDs and incandescent lamps

At present, the lumen efficiency of ultra-high brightness LEDs has exceeded that of incandescent lamps with filters. The luminance of some varieties is higher than 1000mcd, which can replace incandescent lamps with a power of less than 1W. LED arrays can even replace incandescent lamps with a power of less than 150W. For many urban lighting system applications, incandescent lamps use filters to obtain red, orange, green and blue, while ultra-high brightness LEDs can directly obtain the same colors. In recent years, ultra-high brightness LEDs made of AlGaInP and InGaN materials combine multiple (red, blue, green) ultra-high brightness LED chips together to obtain seven colors including red, orange, yellow, green and blue without filters. Its luminous efficiency has now exceeded that of incandescent lamps and is close to that of forward fluorescent lamps. In addition, a large color screen composed of ultra-high brightness LEDs can show the sky and the ocean and realize three-dimensional animation.

The new generation of red, green and blue ultra-high brightness LEDs have achieved unprecedented performance. Recently, Japan's Nichia Corporation has used its InGaN blue LED and fluorescent technology to launch a new white light solid-state light-emitting device product with a color temperature of 6500K and an efficiency of 7.5 lumens per watt. The advent and industrialization of ultra-high brightness LEDs have added new vitality to solar lighting systems.

3 Application of ultra-high brightness LED in solar city lighting system

3.1 Basic structure of ultra-high brightness LED bulb

When applied to solar city lighting systems, the light produced by a single ultra-high brightness LED tube is dim and too directional, resulting in poor overall visual effects. Therefore, when applied to solar city lighting systems, multiple LED tubes must first be concentrated together to increase their luminous brightness and area; secondly, according to the performance differences of LED tubes of different colors, the design is optimized through matching of relevant parameters to combine them into DC 12V, 6V, 4.8V and other pulse series products. The main technical parameters of typical DC LED energy-saving bulbs are shown in Table 1.

Finally, according to the requirements of the place of use, reflectors with various angles of 120°, 90°, and 70° were designed and placed in a plastic lampshade with high light transmittance (Figure 1) to improve the light spot formed by the excessive directionality of the LED. At the same time, a grid-type refractive surface was designed on the inner surface of the lampshade to make the light emitted by the LED reflect and refract multiple times, ultimately achieving uniform light emission in all directions for the LED high-efficiency energy-saving bulb and forming a special optical effect.

3.2 Application Examples

At present, the solar city lighting system using the LED high-efficiency energy-saving bulbs mentioned above has established demonstration projects in Wuhan, Shenzhen, Beijing and other places. It is currently further widely used in Wuhan, Ezhou, Hubei, Xinyang, Henan, Dalian, Liaoning, Pingxiang, Jiangxi and other places, and has been recognized by the society and consumers.

Taking the Wuhan Two Rivers and Four Banks Lighting Project as an example, the construction site is located along the Yangtze River and Han River in Wuhan. It is one of the key lighting areas in Wuhan. The lighting system is required to be clean and environmentally friendly, safe and energy-saving, durable, reliable in operation, flexible in use, and high in performance-price ratio.

Therefore, the system uses 1,500 LED energy-efficient bulbs to outline the building complex. Since the system controller has a rich control program for the changing of colored lights, it can automatically control the lighting to select single color, multi-color, flashing, chasing light, jumping, shining and other changes, fully showing the strong dynamic and smooth visual effects of the solar LED lighting system. At the same time, it is estimated that the system saves more than 85% of electricity compared with ordinary lighting systems, and the annual electricity consumption is only more than 500 yuan, which solves the problem that after many lighting projects in my country are completed, the lighting attractions cannot be fully lit or cannot be lit for a long time due to insufficient electricity fees, thus affecting the actual lighting effect of the city.

In addition, as a demonstration city for the application of renewable energy in my country, Shenzhen's municipal government departments have decided to apply or transform the lighting projects of 56 of the city's more than 70 parks into solar city lighting systems.

3.3 The significance of the practical application and commercialization of ultra-high brightness LEDs in solar lighting systems

The practical application and commercialization of ultra-high brightness LEDs in solar lighting systems have brought a new revolution to urban lighting technology. Although the cost of this new type of bulb made of multiple ultra-high brightness LEDs is still high, with each bulb costing about 20 yuan, LED energy-saving bulbs are not only colorful, but also green, environmentally friendly, energy-efficient and safe. For incandescent lamps of the same brightness, the power consumption of LEDs is only 10%-20% of the former; the life of incandescent lamps generally does not exceed 2,000 hours, while the life of LEDs is as long as 100,000 hours. This solid light source with small size, light weight, good directionality, energy saving, long life, and resistance to various harsh conditions is used in solar urban lighting systems and some special occasions such as decorative lighting for mining, diving, rescue, and military devices. It is generally cost-effective and will inevitably have an impact on the traditional light source market.

4 Conclusion

At present, although the cost of ultra-high brightness LED new energy-saving bulbs is relatively high, they have been successfully applied to solar city lighting systems and can be used for lighting in some other special occasions. As the production scale of ultra-high brightness LEDs expands and the cost is further reduced, its advantages in energy saving and long life are enough to make up for its current high price. Ultra-high brightness LEDs will become a very competitive new electric light source for city lighting systems.