1、1Renewable and Sustainable Energy ReviewsHigh-brightness LEDsEnergy efficient lighting sources and their potential in indoor plant cultivationABSTRACTThe rapid development of optoelectronic technology since mid-1980 has significantly enhanced the brightness and efficiency of light-emitting diodes (L
2、EDs). 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 perspectives for
3、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 such a
4、s 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.Contents1. Introduction 2. LED development.3. Color ratios and photosynthesis 4. LEDs and indoor plant cultivatio
5、n.4.1. Plant tissue culture and growth4.2. Space agriculture84.3. Algaculture4.4. Plant disease reduction5. Intermittent and photoperiod lighting and energy saving6. Conclusion1. IntroductionWith impacts of climate change, issues such as more frequent and serious droughts, floods, and storms as well
6、 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 failures due to droughts
7、 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 control of spectra
8、l 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 2and maintain useful light output for years. LEDs do not contain electrodes and thus do not burn out like
9、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 developmentLED is a uniq
10、ue type of semiconductor diode. It consists of a chip of semiconductor material doped with impurities to create a pn 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 from electrodes with diffe
11、rent 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 pn junction. The materials used for an LED have a direct band gap wit
12、h 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.13LED development began with infrar
13、ed and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors. J.Margolin reported that the first known light-emitting solid state diode was made in 1907 by H. J. Round.
14、No practical use of Rounds 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 phosphorus to provide a 655nm red ligh
15、t with brightness levels of approximately 110 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 quantum efficiencies at low curre
16、nts. 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 calculators, digital watches, a
17、nd 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 by a rapid growth in the use of
18、 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. Along this development period, LED
19、s 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. The basic structure of the mater
20、ial 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 gallium aluminum phosphide) visible
21、 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 improved.Shuji Nakamura at Nichia Chemi
22、cal 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 advanced than standard LED materials. B
23、ut 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 4addition, the brightness level is low and the o
24、verall 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 in an e
25、ntire generation of new applications that include telecommunications products, automotive applications, traffic control devices, and full-color message boards. Even LED TVs can soon become commercially available.Compare to incandescent lights 1000-h and fluorescent lights 8000-h life span, LEDs have
26、 a very significantly longer life of 100,000 h. In addition to their long life, LEDs have many advantages over conventional light source. These advantages include small size, specific wavelength, low thermal output, adjustable light intensity and quality, as well as high photoelectric conversion eff
27、iciency. Such advantages make LEDs perfect for supporting plant growth in controlled environment such as plant tissue culture room and growth chamber. Table 1 is a list of some common types of LEDs as compiled from .Table 1Some common types of LEDs.Peak wavelength(nm) Color Material and structure of
28、 LEDs Substrate730 Far-red GaAs GaP700 Red GaP:Zn-O GaP660 Red GaAl0.35As GaAs650 Red GaAs0.6P GaAs630 Orangered GaAs0.35P0.65:N GaP610 Orange GaAs0.25P0.75:N GaP590 Yellow GaAs0.15P0.85:N GaP585 Yellow GaAs0.14P0.86:N GaAs565 Green GaP:N GaP450 Blue GaN/SiC 3. Color ratios and photosynthesisThe chl
29、orophyll molecules in plants initiate photosynthesis bycapturing light energy and converting it into chemical energy to help transforming water and carbon dioxide into the primary nutrient for living beings. The generalized equation for the photosynthetic process is given as:CO2 + H2Olight（CH 2O） +
30、O2where (CH2O) is the chemical energy building block for thesynthesis of plant components.Chlorophyll molecules absorb blue and red wavelengths most efficiently. The green and yellow wavelengths are reflected or transmitted and thus are not as important in the photosyntheticprocess. That means limit
31、 the amount of color given to the plants and still have them grow as well as with white light. So, there is no need to devote energy to green light 5when energy costs are aconcern, which is usually the case in space travel.The LEDs enable researchers to eliminate other wavelengths found within norma
32、l white light, thus reducing the amount of energy required to power the plant growth lamps. The plants grow normally and taste the same as those raised in white light. Red and blue light best drive photosynthetic metabolism. These light qualities are particularly efficient in improving the developme
33、ntal characteristics associated with autotrophic growth habits. Nevertheless, photosynthetically inefficient light qualities also convey important environmental information to a developing plant. For example, far-red light reverses the effect of phytochromes, leading to changes in gene expression, p
34、lant architecture, and reproductive responses. In addition, photoperiod (the adjustment of light and dark periods) and light quality (the adjustment of red, blue and far-red light ratio) also have decisive impacts on photomorphogenesis.The superimposed pattern of luminescence spectrum of blue LED (4
35、50470 nm) and that of red LED (650665 nm) corresponds well to light absorption spectrum of carotenoids and chlorophyll. Various plant cultivation experiments are possible when these twokinds of LED are used with the addition of far-red radiation (730735 nm) as the light source. Along the line of the
36、 LED technology advancement, LEDs become a prominent light source for intensive plant culture systems and photobiological researches. The cultivation experiments which use such light sources are becoming increasingly active. Plant physiology and plant cultivation researches using LEDs started to pea
37、k in 1990s and become inevitable in the new millennium. Those researches have confirmed that LEDs are suitable for cultivation of a variety of algae, crop, flower, fruit, and vegetable.Some of the pioneering researches are reviewed in the followings.Bula et al. have shown that growing lettuce with r
38、ed LEDs in combination with blue tubular fluorescent lamp (TFL) is possible. Hoenecke et al. have verified the necessity of blue photons for lettuce seedlings production by using red LEDs with blue TFL. As the price of both blue and red LEDs have dropped and the brightness increased significantly, t
39、he research findings have been able to be applied in commercial production. As reported by Agence France Press, Cosmo Plant Co., in Fukuroi, Japan has developed a red LED-based growth process that uses only 60% of electricity than a fluorescent lighting based one.Tennessen et al. have compared photo
40、synthesis from leaves of kudzu (Pueraria lobata) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. The responses of photosynthesis to CO2 are similar under the LED and xenon arc lamps at equal photosynthetic irradiance. There is no statistical significant difference between
41、the white light and red light measurements in high CO2. Some leaves exhibited feedback inhibition of photosynthesis which is equally evident under irradiation of either lamp type. The results suggest that photosynthesis research 6including electron transport, carbon metabolismand trace gas emission
42、studies should benefit greatly from the increased reliability, repeatability and portability of a photosynthesis lamp based on LEDs.Okamoto et al. have investigated the effects of different ratios of red and blue (red/blue) photosynthetic photon flux density (PPFD) levels on the growth and morphogen
43、esis of lettuce seedlings. They have found that the lettuce stem length decreases significantly with an increase in the blue PPFD. The research has also identified the respective PPFD ratio that (1) accelerates lettuce seedlings stem elongation, (2) maximizes the whole plant dry weight, (3) accelera
44、tes the growth of whole plants, and (4) maximizes the dry weights of roots and stems. Photosynthesis does not need to take place in continuous light. The solid state nature allows LEDs to produce sufficient photon fluxes and can be turned fully on and off rapidly (200 ns), which is not easily achiev
45、able with other light sources. This rapid onoff feature has made LEDs an excellent light source for photosynthesis research such as pulsed lighting for the study of photosynthetic electron transport details. The off/dark period means additional energy saving on top of the LEDs low power consumption.
46、4. LEDs and indoor plant cultivation4.1. Plant tissue culture and growthTissue culture (TC), used widely in plant science and a number of commercial applications, is the growth of plant tissues or cells within a controlled environment, an ideal growth environment that is free from the contamination
47、of microorganisms and other contaminants. A controlled environment for PTC usually means filtered air, steady temperature, stable light sources, and specially formulated growth media (such as broth or agar). Micropropagation, a form of plant tissue culture (PTC), is used widely in forestry and flori
48、culture. It is also used for conserving rare or endangered plant species. Other uses of PTC include:1short-term testing of genetic constructions or regeneration oftrans genic plants,2 cross breeding distantly related species and regeneration of the novel hybrid,3 screening cells for advantageous cha
49、racters (e.g. herbicidere sistance/tolerance),4embryo rescue (i.e. to cross-pollinate distantly related specie sand then tissue culture there sulting embryo which would normally die),5 large-scale growth of plant cells in liquid culture inside bioreactors as a source of secondary products (like recombinant proteins used as biopharmaceuticals).6production of doubled monoploid plants from haploid cultures to achieve homozygous lines more rapidly in breeding programs (usually by treatment with colchicine which causes doubling of the chromosome number).Tissue culture and growth
