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Current status and trends of light-receiving methods in plant factories
As an important physical environmental factor, light plays a key role in regulating plant growth and material metabolism. One of the main features of plant factories is that the full artificial light source and the intelligent regulation of the light environment have become the common consensus in the industry. The plant factory combines modern industry, biotechnology, nutrient solution cultivation and information technology to implement high-precision control of environmental factors in the facility. It has full closure, low requirements for the surrounding environment, shortens plant harvest period, saves water and fertilizer, and has no The advantages of pesticide production and non-discharge of waste are 40~108 times of the production efficiency of open land. Intelligent artificial light source and its light environment control play a decisive role in its production efficiency. As an important physical environmental factor, light plays a key role in regulating plant growth and material metabolism. One of the main features of plant factories is that the full artificial light source and the intelligent regulation of the light environment have become the common consensus in the industry. The need for light in plants The life of plants is inseparable from light. Light is the only source of energy for plant photosynthesis. Light intensity, light quality (spectrum) and periodic changes in light have a profound impact on crop growth and development. Intensity has the greatest impact on plant photosynthesis. Light intensity The intensity of light can change the shape of the crop, such as flowering, internode length, stem thickness and leaf size and thickness. Plants' requirements for light intensity can be divided into hi-light, hi-light, and low-light plants. Most of the vegetables belong to hi-light plants, and their light compensation points and light saturation points are relatively high. In the artificial light plant factory, the relevant requirements of crop light intensity are an important basis for selecting artificial light sources. Understanding the illumination requirements of different plants is to design artificial light sources. Improving the production performance of the system is extremely necessary. Light quality (spectral) distribution also has an important impact on plant photosynthesis and morphogenesis (Fig. 1). Light is part of the radiation, and radiation is an electromagnetic wave. Electromagnetic waves have wave characteristics and quantum (particle) characteristics. The quantum of light is called photon, and it is also called photon in the field of horticulture. Radiation with a wavelength range of 300-800 nm is called physiologically active radiation of plants; and radiation with a wavelength range of 400-700 nm is called photosynthetically active radiation (PAR) of plants. The two most important pigments for plant photosynthesis are Chlorophyll and Carotenes. Figure 2 is a spectral absorption spectrum of each photosynthetic pigment, in which the chlorophyll absorption spectrum is concentrated in the red-blue band, and the illumination fill light system artificially fills light according to the spectral requirements of the crop to promote photosynthesis of the plant. Photoperiod The relationship between photosynthesis and photomorphogenesis of plants and the length of day (or light period) is called the photoperiod of plants. Periluminality is closely related to the number of hours of illumination, and the number of hours of illumination refers to the time when the crop is illuminated by light. Different crops require a certain amount of light hours to complete the photoperiod. According to the different photoperiod, it can be divided into long-day crops, such as cabbage, which require more than 12~14h of light hours at a certain stage of birth; short-day crops, such as onions and soybeans, need 12~14h or less. Light hours; medium-day crops, such as cucumbers, tomatoes, peppers, etc., can bloom and bear under long or short sunshine. Among the three elements of the environment, the light intensity is an important basis for selecting artificial light sources. At present, there are various expression methods for light intensity, including the following three types. (1) Illumination refers to the luminous flux surface density (luminous flux per unit area) accepted on the illuminated plane, in units of lux (lx). (2) Photosynthetically active irradiance PAR, unit: W/m2. (3) Photosynthetically active photon quantum density PPFD or PPF is the photon number of photosynthetically active radiation arriving or passing per unit time per unit area, in mol/(m2s). Mainly refers to the light intensity of 400~700nm directly related to photosynthesis. It is also the most commonly used indicator of light intensity in the field of plant production. Light source analysis of a typical fill light system Artificial fill light is achieved by installing a fill light system to increase the light intensity in the target area or to extend the illumination time to achieve the plant's light requirements. In general, fill light systems include fill light devices, circuits, and their control systems. The fill light source mainly includes several common types such as incandescent lamp, fluorescent lamp, metal halide lamp, high pressure sodium lamp and LED. Due to the low efficiency of incandescent lamps and low efficiency of photosynthetic energy, it has been eliminated by the market, so this article will not do detailed analysis. Fluorescent lamps are a type of low pressure gas discharge lamp. The glass tube is filled with mercury vapor or an inert gas, and the inner wall of the tube is coated with a phosphor, and the color of the light varies depending on the fluorescent material applied in the tube. The fluorescent lamp has good spectral performance, high luminous efficiency, low power, long life (12000h) compared with incandescent lamps, and relatively low cost. Because the fluorescent lamp itself generates less heat, it can be placed close to the plant for illumination. It is suitable for three-dimensional cultivation, but the spectral layout of the fluorescent lamp is unreasonable. The most common method in the world is to add a reflector to maximize the effective light source composition of the crop in the cultivation area. Japan's adv-agri company has also developed a new fill light source HEFL. HEFL is actually a category of fluorescent lamps. It is a general term for cold cathode fluorescent lamps (CCFL) and external electrode fluorescent lamps (EEFL). It is a hybrid electrode fluorescent lamp. The HEFL lamp is extremely thin, with a diameter of only about 4mm. The length can be adjusted from 450mm to 1200mm according to the cultivation needs. It is an improved version of the conventional fluorescent lamp. A metal halide lamp is a high-intensity discharge lamp formed by adding various metal halides (tin bromide, sodium iodide, etc.) to a discharge tube to generate different wavelengths on the basis of a high-pressure mercury lamp. The halogen lamp has high luminous efficiency, high power, good light color, high life and large spectrum. However, since the luminous efficiency is lower than that of the high-pressure sodium lamp, the life is shorter than that of the high-pressure sodium lamp, and it is currently used only in a few plant factories. High pressure sodium lamps are of the type of high pressure gas discharge lamps. The high-pressure sodium lamp is a high-efficiency lamp in which a high-pressure sodium vapor is charged in a discharge tube and a small amount of cesium (Xe) and a halide of a mercury lamp metal is added. Because high-pressure sodium lamps have high electro-optical conversion efficiency and low manufacturing cost, high-pressure sodium lamps are the most widely used in agricultural facilities for light-filling applications. However, due to the shortcomings of low photosynthetic efficiency, low-efficiency short-boards are caused. . On the other hand, the spectral components emitted by high-pressure sodium lamps are mainly concentrated in the yellow-orange light band, lacking the red and blue spectrum necessary for plant growth. As a new generation of light source, light-emitting diode (LED) has higher electro-optical conversion efficiency, adjustable spectrum and high photosynthetic efficiency. LED can emit monochromatic light required for plant growth. Compared with ordinary fluorescent lamps and other complementary light sources, LEDs have the advantages of energy saving, environmental protection, long life, monochromatic light, and cold light source. As the electro-optical efficiency of LEDs further increases, the cost of scale effects decreases, and LEDs will become the mainstream equipment for agricultural facilities. By comparison, the characteristics of different fill light sources can be clearly understood, as shown in Table 1. The light intensity of mobile light-filling device is closely related to the growth of crops. Three-dimensional cultivation is used in plant factories. However, due to the structure of the cultivation frame, the light and temperature distribution between the shelves is uneven, which will affect the yield of crops and harvest. The period is not synchronized. In 2010, Beijing No. 1 Company successfully developed the manual lifting and filling device. The principle is to rotate the small roller film to drive the transmission shaft and the winder fixed on it to realize the purpose of retracting the wire rope. The fill light wire rope is connected to the reel of the lifter through a plurality of sets of reversing wheels, thereby achieving the effect of adjusting the height of the fill light. In 2017, the company designed and developed a new mobile fill light device that automatically adjusts the fill level in real time based on crop growth requirements. The adjustment device is now installed on the 3-layer light source lifting type stereoscopic cultivation frame. The top layer of the device is the best lighting level, so the high-pressure sodium lamp is arranged; the middle layer and the bottom layer are equipped with a lifting and lowering adjustment system, and the LED lamp is installed, according to the light sensor. The detection signal automatically adjusts the height of the fill light to provide a suitable lighting environment for the crop. In contrast to the mobile fill light device tailored for three-dimensional cultivation, the Netherlands has developed a fillable light device that can be moved horizontally. In order to avoid the influence of the shadow of the fill light on the growth of the plant in the sun, the fill light can be pushed horizontally through the telescopic slide to both sides of the bracket, so that the sunlight can be fully irradiated onto the plant; in the rainy days without sunlight, Pushing the fill light to the middle of the bracket, so that the light of the fill light evenly fills the plant; moving the fill light horizontally through the slide on the bracket avoids frequent disassembly and removal of the fill light, and reduces The labor intensity of employees has effectively improved work efficiency. The design idea of ​​the typical crop fill light system is not difficult to see from the design of the mobile fill light device. The design of the plant factory fill light system is usually designed with the core of the light intensity, light quality and photoperiod parameters of different crop growth stages and special means of end regulation. Content, relying on intelligent control systems to implement the implementation, to achieve the ultimate goal of energy saving and high yield. At present, the design of the fill light design for leafy vegetables has gradually matured. For example, leafy vegetables can be divided into four stages: seedling stage, middle growth stage, late growth stage and terminal treatment; fruit vegetables can be divided into seedling stage, vegetative growth stage, flowering stage and harvesting stage. From the light intensity of the fill light, the light intensity at the seedling stage should be slightly lower, at 60~200 mol/(m2s), and then gradually increase. Leafy vegetables can reach up to 100~200 mol/(m2s), fruit vegetables can reach 300~500 mol/(m2s), to ensure the photosynthesis requirements of photosynthesis in different growth stages, and to achieve high yield; for light quality The ratio of red to blue is crucial. In order to increase the quality of the seedlings and prevent the length of the seedlings, the proportion of red and blue is generally set at a lower level, and then gradually reduced to meet the needs of plant light form formation, and the red and blue ratio of leafy vegetables can be set (3~6):1. For the photoperiod, similar to the light intensity, it should show an increasing trend with the extension of the growth period, so that the leafy vegetables have more photosynthesis time for photosynthesis. The light-filling design of fruit and vegetables will be more complicated. In addition to the above basic rules, we should focus on the photoperiod setting during flowering, and we must promote the flowering results of vegetables so as not to be counterproductive. It is worth mentioning that the light formula should include the end-disposing light environment setting content, for example, the continuous fill light can greatly improve the yield and quality of the hydroponic vegetable seedlings, or the UV treatment can significantly improve the sprouts and leafy vegetables (especially purple The nutritional quality of leaves and red leaf lettuce. In addition to optimizing the fill light for selected crops, the light source control systems of some artificial light plant factories have also developed rapidly in recent years. This control system is generally based on the B/S architecture, and remotely controls the temperature of the crop growth process through WIFI. Environmental factors such as humidity, light, CO2 concentration, etc. are automatically controlled and are not subject to external conditions. This intelligent fill light system uses LVD induction lamp as the fill light source, combined with remote intelligent control system, can meet the needs of plant wavelength illumination, especially suitable for light-controlled plant facility cultivation environment, which can meet the market demand well. Conclusion Plant factories are considered to be an important way to solve the world's resources, population and environmental problems in the 21st century, and are an important way to achieve food self-sufficiency in high-tech projects in the future. As a new type of agricultural production, plant factories are still in the stage of learning and growth, and need to invest more attention and research. This paper describes the characteristics and advantages of the common fill light illumination method in plant factories, and introduces the design ideas of the typical crop fill light system. It is not difficult to find that it is necessary to deal with the weak light and ultraviolet light caused by bad weather such as cloudy weather and haze. Adversity, while ensuring high-yield and stable production of crops, LED light source equipment is most in line with current development trends. In the future, the development direction of plant factories should be re-typed with high-precision, low-cost sensors and remotely controllable, tunable spectral fill light system and expert control system. At the same time, the future plant factory will continue to develop with low cost, intelligence and adaptability. The use and popularization of LED light source provides guarantee for high-precision environmental control of plant factories. LED light environment regulation is a complex process involving light quality, light intensity, photoperiod and other comprehensive control, pending the relevant experts and scholars to supplement the artificial light plant factory LED Light lighting is thoroughly researched and used for promotion. (Author: Zhao Jing, Zhou yield, Bo Yunlong, etc.)