Greenhouse technology

Transcription

1 Field: Agri-production Greenhouse technology Authors: Dr. Jose A. Acosta Dr. Silvia Martínez Martínez 2017 Boosting Adult System Education In Agriculture - AGRI BASE Erasmus+ K2 Action Strategic Partnership

2 Table of contents 1. Introduction Types of greenhouses Cooling systems Heating systems Carbon dioxide enrichment techniques Light management techniques Greenhouse cover materials AGRIBASE. Boosting Adult System Education in Agriculture 1

3 1. Introduction All greenhouse cultivation systems, regardless of geographic location, consist of fundamental climate control components, and depending on their design and complexity, they can provide a greater or lesser amount of climate control, and subsequent plant growth and productivity Today s greenhouse technologies mean it is possible to cultivate all horticultural species in any region of the world, provided that the greenhouse is properly designed and equipped to control the climatic parameters. However, for profitable and sustainable cultivation of the target crop, much stricter selection of the region is necessary, on the basis of climatic conditions and the requirements of the selected horticultural crop. In order to achieve optimal plant growth and maximize the yield, microclimate in greenhouses should be closely monitored by an advanced mechatronic systems. According to the intrinsic greenhouse features, the setting and tuning of greenhouse climate controllers is by no means an easy or standard procedure. The dynamic behavior of the greenhouse microclimate is a combination of physical processes involving energy transfer (radiation and heat) and mass balance (water vapor fluctuation and CO2 concentration). These processes depend on the environmental conditions, structure of the greenhouse, type and state of the crop, and on the effect of the control actuators. AGRIBASE. Boosting Adult System Education in Agriculture 1

4 2. Types of greenhouses In the present chapter, the most representative greenhouse types are presented. This systematic presentation allows the evaluation of the influence of various factors, such as the climate, the local building regulations, the cultivated crops and the availability of building materials, on the greenhouse design. Climate is a major factor influencing both the structural and the functional characteristics of greenhouses. The design of a greenhouse aims at exploiting the external climatic conditions for improving the indoor microclimate. For this reason, the overall greenhouse design is strongly influenced by the climate and the latitude of the location. Moreover, various load requirements for greenhouses depend on the climatic conditions. 1. Glass and rigid plastic greenhouses Glass greenhouses were extensively used before the appearance of plastic covering materials. It has important advantages, such as: 1. very low degradation due to environmental causes and agrochemicals. 2. low thermal radiation transmittance. 3. high visible radiation transmittance. Glass or rigid plastic covered greenhouses are characterized by long lifetime and are usually well-equipped systems. Attempts have been made to replace glass with rigid plastic sheets, such as polymethyl methacrylate (PMMA), polycarbonate (PC), and polyvinyl chloride (PVC), that exhibit higher insulating performance. AGRIBASE. Boosting Adult System Education in Agriculture 2

5 Photo I: Glass greenhouse 2. Low plastic tunnels Low tunnels and walk-in tunnels could be called miniature greenhouses. Many types of tunnels, consisting of a semi-cylindrical supporting framework covered with plastic film, have been developed. The supporting framework can be made of curved wood, bamboo or steel. The arches are secured into the ground at a spacing of 2-3 m. The plastic film is stretched over the arches and buried into the ground along the sides. In addition to the supporting arch, a string or wire retains the film in place. The film can be lifted between the supporting arch and the retaining wire on both sides for ventilation. The main advantages of low tunnels are their low cost and the simple construction. On the other hand, they provide small heating capability, allow poorly controlled ventilation and make plant husbandry difficult. AGRIBASE. Boosting Adult System Education in Agriculture 3

6 3. Plastic film greenhouses In many countries, especially in those with a warm climate, plastic film covered greenhouses are extensively used. The first plastic film covered greenhouses were developed as plastic covers installed over simple wooden frames, which also were used to support the culture. Even though plastic film greenhouses passed through several stages of development and important improvements have been introduced, they still remain cheaper than glasshouses. Moreover, if their design follows certain specifications, they provide important advantages with respect to their functionality. A large variety of plastic film greenhouses have been developed worldwide. They range from simple structures of wood with nailed-on plastic film and roundarched tunnels to complex constructions Tunnel greenhouses The simplest form of plastic film greenhouse is the single-span tunnel greenhouse. The most important advantage of the single-span tunnels is the relatively simple construction system and its good wind resistance. On the other hand, tunnel greenhouses have some functional disadvantages. The ventilation is insufficient when the sheet Parting mechanism is used. However, a rollup ventilation system in the sidewalls can improve the ventilation efficiency. AGRIBASE. Boosting Adult System Education in Agriculture 4

7 Photo II: Tunnel greenhouse 3.2. Plastic pitched roof greenhouses Different types of pitched roof greenhouses with plastic film cover are built around the world. The columns are made of wood, steel or concrete. In many cases, they are simple structures. The roof frame is made of wood. The plastic film often is nailed onto wooden laths, which makes the replacement of the film difficult and reduces the film durability Arch-shaped roof greenhouses The frame of arch-shaped roof greenhouses is made of steel tubes. The shape of the roof is either a round arch or a Gothic arch shape roof. Gothic arch structures have advantages over round arches because condensation water can flow off better on the inner side of the film. Therefore, fewer water drops fall onto the plants. Gothic arches also reduce the snow load because snow can slide easier down the roof to the ground or the gutter. However, the mechanical resistance of the Gothic arch is lower than that of the normal arch due to curvature discontinuity. AGRIBASE. Boosting Adult System Education in Agriculture 5

8 Roof and/or side openings provide ventilation. Plastic film structures have been developed, where one-half of the roof can open. Greenhouses with ventilators along the ridge can also be equipped with rolling ventilators at the sidewalls. Photo III: Gothic greenhouse 3.4. Inflated, double plastic film, greenhouses Recently, a new type of plastic film greenhouses with higher insulation performance has been developed for use under cold climatic conditions. The cover of these greenhouses consists of two layers of plastic film tightly fixed together along the frame elements of the walls and the roof. The space between the films is inflated at a pressure of Pa. In this way, the insulating performance and the resistance of the structure to wind or snow loads is enhanced. AGRIBASE. Boosting Adult System Education in Agriculture 6

9 3. Cooling systems Greenhouse technology is a breakthrough in the agriculture production technology, it integrates market driven quality parameters with production system profits. Although greenhouse protects crops from external bad weather, high temperature and humidity during summer months cause adverse effect on crop production. Thus, in such regions, reduction of air temperature inside the greenhouse or the regulation of temperature close to the ambient temperature during summer is necessary for successful crop production. Cooling is considered as the basic necessity for greenhouse crop production to overcome the problems of high temperature during summer months. Development of suitable cooling system that provides optimized micro climate for crop growth is a difficult task as the design is closely related to the local environmental condition. Besides, the selection of appropriate technology for cooling depends on the crop to be grown, maintenance, ease of operation and economic viability. Hence, understanding of greenhouse, its size, shape, covering material and external weather helps in the development of suitable cooling system. Evaluating micro climate in different designs of the greenhouse and establishing physical and physiological relationship of crops is necessary for greenhouse designers to improve cooling system that is suitable for crop growth. 1. Natural Ventilation One of the simplest and more effective ways to reduce the difference between inside and outside air temperature is to improve ventilation. And its driven force is the pressure differences, which is created and maintained by wind or temperature gradients. It depends on the crop. AGRIBASE. Boosting Adult System Education in Agriculture 7

10 Continuous vertical roof windows result in the decrease of air temperature and humidity ratio with time and approached to a steady state. Wind and buoyancy effect are the main controlling agents for getting a favorable micro climate and air exchange rate. Buoyancy effect cannot be neglected completely, though its effect on natural ventilation is not prominent when external wind speed exceeds m/s. Relative larger ventilation openings provided at the roof and ridge can realize acceptable natural ventilation cooling. If the greenhouse is equipped with ventilation openings, both near the ground and at the roof, then this type of ventilation replaces the internal hot air by external cooler one during the hot sunny days with weak wind. The external cool air enters the greenhouse through the lower side openings while the hot internal air exits through the roof openings due to density difference between air masses of different temperature causing the lowering of temperature in the greenhouse. Photo IV: Natural ventilation 2. Evaporation Cooling One of the most efficient solutions for alleviating the climatic conditions is to use evaporative cooling systems, based on the conversion of sensible heat into latent heat by means of evaporation of water supplied directly into the greenhouse atmosphere (mist or fog system, sprinklers) or through evaporative pads (wet pads). AGRIBASE. Boosting Adult System Education in Agriculture 8

11 2.1. Fan-Pad System The fan-and-pad cooling system is most commonly used in horticulture. Air from outside is blown through pads with as large a surface as possible. The pads are kept permanently wet by sprinkling. The water from the pads evaporates and cools the air. For this reason, the outside air humidity must be low. In order to achieve an optimal cooling the greenhouse should be shaded. The water flow rate, water distribution system, pump capacity, recirculation rate and output rate of the fan-and-pad cooling system must be carefully calculated and designed to provide a sufficient wetting of the pad and to avoid deposition of dissolved material in it. The pad material should have high surface, good wetting properties and high cooling efficiency. A suggested pad thickness is 200 mm. The pad area depends on the air flow rate necessary for the cooling system and the permissible surface velocity over the pad. Average face velocities are 0,75-1,5 m s -1. The pad area should be about 1 m 2 per m 2 greenhouse area. The maximum fan-to-pad distance should be 40 m. When starting the cooling system, the water flow through the pad should be turned on first to prevent the pads from clogging. A basic air flow rate of m 3 per m 2 greenhouse area per hour will permit satisfactory operation of an evaporative cooling system. AGRIBASE. Boosting Adult System Education in Agriculture 9

12 Photo V: Fan-pad system 2.2. Fog System Fog system is based on spraying the water as small droplets with high pressure into the air above the plants in order to increase the water surface in contact with the air. Free fall velocity of these droplets is slow and the air streams inside the greenhouse easily carry the drops. This can result in high efficiency of water evaporation combined with keeping the foliage dry. Fog systems can be high (40 bars) or low (5 bars) pressure systems. High pressure systems produce droplets of μm while low pressure systems produce droplets with diameter higher than 200 μm. High pressure systems are more effective than low pressure. The nozzles of the fog system should be located at the highest possible position inside the greenhouse to allow water evaporation before the water drops to crop or the ground. During the operation of the fog system a vent opening of 20 % of the maximum aperture should be maintained. AGRIBASE. Boosting Adult System Education in Agriculture 10

13 Photo VI: Fog system 3. Shading The entry of unwanted radiation can be controlled by the use of shading or reflection techniques. Shading can be obtained by various methods such as by the use of paints, external shade cloths, use of nets (of various colors), partially reflective shade screens and water film over the roof and liquid foams between the greenhouse walls. Shading is the ultimate solution to be used for cooling greenhouses, because it affects the productivity. However, in some cases, a better quality can be obtained from shading. One of the most used methods adopted by growers due its low cost is white painting, or whitening, the cover material. The use of screens has progressively been accepted by growers and has gained, through the last decade, a renewed interest as shown by the increasing area of field crops cultivated under screenhouses. AGRIBASE. Boosting Adult System Education in Agriculture 11

14 Photo VII: Shading system 4. Roof Evaporation Cooling Roof evaporation cooling is sprinkling of water onto a surface of the roof so as to form a thin layer of the free water surface causing increase in the evaporation rate, and to fall to the wet bulb temperature of the closely surrounded air. Photo VIII: Roof evaporation cooling system AGRIBASE. Boosting Adult System Education in Agriculture 12

15 4. Heating systems The heating requirements of a greenhouse depend on the optimal temperature required for plant development, the location and construction of the greenhouse, and the total outside exposed area of the structure. As much as 25 percent of the daily heat requirement may come from the sun, but a lightly insulated greenhouse structure will need a great deal of heat on a cold winter night. An adequate heating system is required to maintain the optimal day and night temperatures. Heating systems may be fueled by electricity, gas, or oil. Heat may be distributed by forced hot air, radiant heat, or hot water. The choice of a heating system and the type of fuel used depend on local availability, plant requirements, costs, and individual preferences 1. Heating systems by warm air The warm air generators can be of indirect or direct combustion. The heaters can be connected to some circulation system, which distributes the warm air through a perforated pipe. Those heaters work with gas oil or propane gas. Even the heaters have a mechanical system of turn on and off by means of thermostat. In hot air generators of indirect combustion, the fuel is combusted in a fuel box and the hot exhaust is passed through thin-walled, metal tubes (heat exchangers). Heat is transferred to the metal and the exhaust is removed from the greenhouse through an exhaust stack. The hot air may be blown directly into the greenhouse or forced through a polyethylene tube (jet tube) running the length of the greenhouse. These systems may use kerosene, liquid propane or natural gas as a fuel source. The system is designed to reach the temperature required by the crop. AGRIBASE. Boosting Adult System Education in Agriculture 13

16 Photo IX: Hot air generator 2. Heating systems by warm water The warm air distribution system is based on the circulation of warm water by the pipelines. The water is warmed in the boiler to approximately 60-80ºC and the pipelines are placed in the greenhouse. The pipelines are made of plastic or metal (the polyethylene is cheaper than the steel or the aluminium). Hot water is produced in a central boiler and is usually pumped through the greenhouse. An advantage of the hot water system is that, if the boiler fails, the hot water in the pipes acts as a heat reservoir for a short period of time. A standard system includes water delivery pipes, heat exchangers and circulation pumps for each cycle, electrical valves, railway radiation pipes and a full set of accessories. AGRIBASE. Boosting Adult System Education in Agriculture 14

17 Photo X: Hot water distribution system 5. Carbon dioxide enrichment techniques Carbon dioxide (CO2) enrichment enhances the growth and production of nearly every crop. Greenhouse operators all over the world are keen to inject extra CO2 into the growing environment, but it is expensive because a part of the valuable CO2 gas leaks to the outside. In hot weather when the greenhouse is vented, it's not feasible to maintain a high CO2 level. Carbon dioxide is often produced by burning fossil fuel. In that case, the heat generated during CO2 enrichment is either utilized or stored for later use. Hence, a heat storage tank has become a standard piece of equipment in countries with cooler night temperatures. CO 2 concentration and plants The concentration of CO2 in ambient outside air commonly varies from 300 to 500 ppm by volume depending on the season, time of day and the proximity of CO2 producers such as combustion or composting, or CO2 absorbers such as plants or bodies of water. Plants growing in greenhouses, particularly with double-layer structures and a reduced air infiltration rate, can reduce CO2 levels to well below ambient levels, greatly reducing the rate of photosynthesis. Conversely, enriching the concentration of CO2 above ambient levels will significantly increase the rate of photosynthesis. In general, a drop in CO2 levels below ambient has a stronger negative effect on plant growth than the positive effects of enriching CO2 levels above ambient. AGRIBASE. Boosting Adult System Education in Agriculture 15

18 CO 2 enrichment requirements CO2 enrichment is not required as long as the crops are growing and developing to the complete satisfaction of the grower, or if high ventilation rates make CO2 enrichment uneconomical. CO2 enrichment should be considered, however, if crop production and quality are below required levels. In general, crop production times from late fall through early spring increases the potential need for CO2 enrichment as it coincides with reduced ventilation rates due to colder outdoor air temperatures. As ventilation rates are increased for cooling and dehumidification from late spring to early fall, the cost of CO2 enrichment escalates while the benefit to the crop may be minimal or reduced. As photosynthesis and CO2 consumption happens only during daylight hours, CO2 enrichment at night is not required. In general, CO2 enrichment systems should be turned on 1 or 2 hours after sunrise, and turned off several hours before sunset, however, additional CO2 enrichment may be needed if supplemental grow-lighting is used. CO 2 enrichment techniques Maximize natural (Free) CO2 supply: maximize ventilation rates whenever possible starting 1 or 2 hours after sunrise when the overnight build-up of CO2 has been depleted. Improve horizontal air flow to distribute available CO2 evenly throughout the crop and to reduce the leaf boundary layer, which will improve the diffusion of CO2 into the stomatal openings of each leaf. Keep plants healthy and well-watered so they are not forced to close their stomatal openings due to stress. Depending on the crop, consider using natural sources of CO2 such as decomposing straw bales and/or organic soil mixes in your production system. Liquid or Bottled CO2 Gas: When outside air conditions are too extreme for ventilation, additional CO2 is available in the form of liquid or bottled CO2 AGRIBASE. Boosting Adult System Education in Agriculture 16

19 gas. Specific processes are required for the safe and proper handling as well as the effective use of CO2 from these sources. Liquid CO2 must be fully vaporized before delivering into the greenhouse, and manufacturers instructions and local codes should be strictly adhered. CO2 from Carbon-Based Fuels: Gas-fired appliances generate CO2 and water vapor as primary byproducts of combustion. These appliances include equipment that is specifically designed and certified as CO2 generating appliances, un-vented forced-air primary space heaters, and hot water boiler heating systems with flue gas condensers specifically designed for CO2 enrichment. AGRIBASE. Boosting Adult System Education in Agriculture 17

20 6. Light management techniques To produce greenhouse crops during the winter, artificial lighting is sometimes essential. Supplemental lighting also improves plant growth during cloudy, low-light conditions and increases total daily light levels during short days of natural light. Another potential reason for supplementing light is to extend the day length or photoperiod to initiate or prevent flowering. The amount of light needed for extending the day length is usually considerably lower than the amount of light provided in support of overall plant growth. Incandescent light The incandescent light bulb may be used to extend day length, but it offers little help as a grow light. Most of the electrical energy going into incandescent bulbs is released as heat rather than light. For enhancing plant growth, incandescent bulbs are energy inefficient and rather ineffective light sources. Halogen Incandescent light The halogen incandescent bulb is a more energy-efficient modification of the traditional incandescent bulb. Halogen bulbs are filled with halogen gas, usually bromine or compounds of bromine, in contrast to the vacuum or a low-pressure argon/nitrogen mixture in conventional incandescent bulbs. The change in the gas mixture allows for a higher operating temperature, resulting in a bulb with higher efficacy, intensity and color temperature. AGRIBASE. Boosting Adult System Education in Agriculture 18

21 Depending on the wattage and type of gas mixture, the halogen version can be 30 to 60 percent more energy efficient (lumens per watt of electrical energy) than the traditional incandescent bulb. The lifetime of the bulb is often more than double the number of hours expected for a traditional incandescent bulb. Fluorescent light Fluorescent bulbs give off more uniform light than incandescent lamps. Fluorescent bulbs are available in 28- to 225-watt configurations and are often mounted in banks to cover larger growing areas. Fluorescent lighting is rarely used in commercial greenhouses for producing mature flowering or fruiting plants because of its low-intensity output and the considerable shading the fixtures and reflectors create. In commercial operations, the primary use of fluorescent tubes is to provide light over germination benches. Compact fluorescent lamps (CFLs) are replacing incandescent bulbs in some applications. CFLs are often used alone or mixed with incandescent or halogen incandescent bulbs to extend the day length in some photoperiodic crops. Since CFLs are more energy efficient, using about 75 percent less energy than incandescent bulbs, the shift to CFLs for day length control is easily justified. High-Intensity Discharge (HID) High-intensity discharge lamps are more costly to purchase and operate but offer the highest quality output of available grow-light options. High-pressure sodium (HPS) lamps are the most commonly used HID fixtures in commercial greenhouses. HPS lamps provide a large portion of yellow-orange light but are limited in the shorter, blue wavelengths. HPS lamps come in various suitable wattage levels, from 400 to 1,000 watts, for greenhouse applications. The choice of HPS lamp type and wattage is dependent on crop needs, available natural light and the greenhouse design. AGRIBASE. Boosting Adult System Education in Agriculture 19

22 HID light fixtures have a bulb, reflector, ballast and capacitor. The bulb functions at a high temperature and becomes hot during operation, so it is imperative to have good ventilation and place the bulbs at the proper distance from the plants. Photo XI: High-Intensity Discharge light Light-Emitting Diodes (LEDs) Light-emitting diodes are receiving a lot of interest for use in various applications, including greenhouse lighting. An LED is a solid-state, light-emitting device that in comparison to traditional lighting sources, such as gas-filled pressurized bulbs, filaments and electrodes, allows extended hours of operation and potentially more efficient conversion of electric energy to light. The LEDs are arranged in arrays or panels with options to customize the overall light quality. Panels specifically designed for plant growth usually consist of LEDs that emit red, 630 to 660 nanometers (nm), and blue, 450 nanometers (nm), light. The primary reason for choosing these types of LEDs is the observed high rate of photosynthesis. Although red and blue light support photosynthesis, most plants have developed in a full spectrum of natural light. Monochromatic LEDs used for supplemental lighting in a greenhouse can be expected to work well since even low levels of natural light are sufficient to complement the spectrum. AGRIBASE. Boosting Adult System Education in Agriculture 20

23 7. Greenhouse cover materials A covering material is chosen for its optical and mechanical properties and on the basis of climate and location. Good agricultural practices dictate that greenhouse plastic should have maximum solar transmission (so dust washes away easily and does not stick) and be opaque to long-wave radiation to reduce heat loss at night. Greenhouse films are composed of polymers and additives. Polymers are the basic component, while additives provide a variety of different properties including infrared absorption/reflection and light diffusion. Greenhouse cladding films range in thickness from 80 to 200 μm. Film width is up to 20 m. Single layer or multilayer (typically three-layer) films are widely used in commercial production, but multilayer films are preferred as they combine the positive properties of their individual components (e.g. good mechanical resistance and good light transmission). Weathering depends on the photo-additives incorporated in the film as well as on the geographic location and the exposure of the film to pesticide treatments. Polymers and additives Polymers are large molecules formed by the association of smaller units called monomers. The most common polymers used in horticulture are low density polyethylene (LDPE), ethylene vinyl acetate (EVA) and ethylene butyl AGRIBASE. Boosting Adult System Education in Agriculture 21

24 acrylate(eba). These three polymers cover more than 80 percent of the world market. Other materials are also popular, such as PVC in Japan or linear low density polyethylene (LLDP) in the rest of the world. In comparison with glass, a property common to all plastic materials is their low density and therefore low weight The low density and thickness of plastic materials is a great advantage in horticulture since it facilitates transportation, handling and installation. The light weight and flexibility of the covering material allows a significant reduction in the size and number of the supporting members, making the greenhouse frame lighter compared with the glasshouse frame, and thus much cheaper. Additives are an essential part of the covering materials. They are dispersed between the chains of polymer molecules without interacting chemically. Additives are used to facilitate the manufacturing of the film as well as to improve its performance under field conditions; the type and quantity of additive depends on which properties of the covering material need improving. The two most common additives in horticulture are UV (ultraviolet) stabilizer additives and IR (infrared) absorbing additives. UV stabilizers absorb UV radiation or protect the polymer molecules. As a consequence, the film ages more slowly. Good greenhouse film should block long-wave IR radiation (wavelength μm) so as to reduce heat loss. So-called thermal films are particularly effective for increasing leaf temperature in passive, unheated greenhouses during clear nights. Polyethylene films are very transparent to long-wave IR radiation, therefore IR-absorbing additives are commonly used to improve the thermal properties of the films. AGRIBASE. Boosting Adult System Education in Agriculture 22

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