1、 1 Renewable and Sustainable Energy Reviews High-brightness LEDs Energy efficient lighting sources and their potential in indoor plant cultivation ABSTRACT The rapid development of optoelectronic technology since mid-1980 has significantly enhanced the brightness and efficiency of light-emitting dio
2、des (LEDs). LEDs have long been proposed as a primary light source for space-based plant research chamber or bioregenerative life support systems. The raising cost of energy also makes the use of LEDs in commercial crop culture imminent. With their energy efficiency, LEDs have opened new perspective
3、s for optimizing the energy conversion and the nutrient supply both on and off Earth. The potentials of LED as an effective light source for indoor agriculturalproduction have been explored to a great extent. There are many researches that use LEDs to support plant growth in controlled environments
4、such as plant tissue culture room and growth chamber. This paper provides a brief development history of LEDs and a broad base review on LED applications in indoor plant cultivation since 1990. Contents 1. Introduction 2. LED development. 3. Color ratios and photosynthesis 4. LEDs and indoor plant c
5、ultivation. 4.1. Plant tissue culture and growth 4.2. Space agriculture8 4.3. Algaculture 4.4. Plant disease reduction 5. Intermittent and photoperiod lighting and energy saving 6. Conclusion 1. Introduction With impacts of climate change, issues such as more frequent and serious droughts, floods, a
6、nd storms as well as pest and diseases are becoming more serious threats to agriculture. These threats along with shortage of food supply make people turn to indoor and urban farming (such as vertical farming) for help. With proper lighting, indoor agriculture eliminates weather-related crop failure
7、s due to droughts and floods to provide year-round crop production, which assist in supplying food in cities with surging populations and in areas of severe environmental conditions. The use of light-emitting diodes marks great advancements over existing indoor agricultural lighting. LEDs allow the
8、control of spectral composition and the adjustment of light intensity to simulate the changes of sunlight intensity during the day. They have the ability to produce high light levels with low radiant heat output 2 and maintain useful light output for years. LEDs do not contain electrodes and thus do
9、 not burn out like incandescent or fluorescent bulbs that must be periodically replaced. Not to mention that incandescent and fluorescent lamps consume a lot of electrical power while generating heat, which must be dispelled from closed environments such as spaceships and space stations. 2. LED deve
10、lopment LED is a unique type of semiconductor diode. It consists of a chip of semiconductor material doped with impurities to create a p n junction. Current flows easily from the p-side (anode), to the n-side (cathode), but not in the reverse direction. Electrons and holes flow into the junction fro
11、m electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The color (wavelength) of the light emitted depends on the band gap energy of the materials forming the p n junction. The materials used for an LED ha
12、ve a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light. The key structure of an LED consists of the die (or light-emitting semiconductor material), a lead frame where the die is placed, and the encapsulation which protects the die (Fig. 1). Fig.1 LED dev
13、elopment began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices 3 with ever-shorter wavelengths, producing light in a variety of colors. J.Margolin reported that the first known light-emitting solid state diode was m
14、ade in 1907 by H. J. Round. No practical use of Round s diode was made for several decades until the invention of the first practical LED by Nick Holonyak, Jr in 1962. His LEDs became commercially available inlate 1960s. These GaAsP LEDs combine three primary elements: gallium, arsenic and phosphoru
15、s to provide a 655nm red light with brightness levels of approximately 1 10 mcd at 20mA. As the luminous intensity was low, these LEDs were only used in a few applications, primarily as indicators. Following GaAsP, GaP (gallium phosphide) red LEDs were developed. These device sex hibit very high qua
16、ntum efficiencies at low currents. As LED technology progressed through the 1970s, additional colors and wavelengths became available. The most common materials were GaP green and red, GaAsP orange, and high efficiency red and GaAsP yellow. The trend towards more practical applications (such as in c
17、alculators, digital watches, and test equipment) also began to develop. As the LED materials technology became more advanced, the light output was increased, and LEDs became bright enough to be used for illumination. In 1980s a new material, GaAlAs (gallium aluminum arsenide) was developed followed
18、by a rapid growth in the use of LEDs. GaAlAs technology provides superior performance over previously available LEDs. The voltage requirement is lower, which results in a total power savings. LEDs could be easily pulsed or multiplexed and thus are suitable for variable message and outdoor signs. Alo
19、ng this development period, LEDs were also designed into bar code scanners, fiber optic data transmission systems, and medicalequipment. During this time, the improvements in crystal growth and optics design allow yellow, green and orange LEDs only a minor improvement in brightness and efficiency. T
20、he basic structure of the material remained relatively unchanged. As laser diodes with output in the visible spectrum started to commercialize in late 1980s, LED designers used similar techniques to produce high-brightness and high reliability LEDs. This led to the development of InGaAlP (indium gal
21、lium aluminum phosphide) visible light LEDs. Via adjusting the energy band gap InGaAlP material can have different color output. Thus, green, yellow, orange and red LEDs could all be produced using the same basic technology. Also, light output degradation of InGaAlP material is significantly improve
22、d. Shuji Nakamura at Nichia Chemical Industries of Japan introduced blue LEDs in 1993. Blue LEDs have always been difficult to manufacture because of their high photon energies (2.5 eV) and relatively low eye sensitivity. Also, the technology to fabricate these LEDs is very different and less advanc
23、ed than standard LED materials. But blue is one of the primary colors (the other two being red and green). Properly combining the red, green, and blue light is essential to produce white and full-color. This process requires sophisticated software and hardware design to implement. In addition, the b
24、rightness level is low and the overall light output of each RGB die being used degrades at a different rate resulting in an eventual color unbalance. The blue LEDs available today consist of GaN (gallium nitride) and SiC (silicon carbide) construction. The blue LED that becomes available in production quantities has result
