Management of high heat under shelter

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serre horticulture afrique

ventilation aeration serreAeration: it is the renewal of warm indoor air by cooler outdoor air. Ventilation can be static or dynamic. This is the simplest and most economical way, but the indoor temperature will not drop below the outdoor temperature.
Cooling, misting or nebulization: this is the exploitation of the thermodynamic properties of water during its transition from liquid to gaseous state: this transformation reduces the internal temperature below the external temperature.
Films, screens and bleaching: it is the reduction of the heating of the air under the shelter by limiting the penetration of light radiation. This does not allow the indoor temperature to be lower than the outdoor temperature.
ventilation aeration laterale serreStatic ventilation ventilation: this is the use of natural air movements by convection. The efficiency of static aeration depends on:
– of the greenhouse height; the higher the greenhouse, the more efficient the ventilation will be due to the chimney effect,
– the number and size of openings. These can be lateral or located at the ridge. The ridge opening, along the entire length of the greenhouse, is the most efficient system. When there are only openings on the side walls, they should be as open as possible while keeping a tarpaulin up to one metre above the ground to limit the entry of rain under the greenhouse. The side walls can also be made of a movable tarpaulin that is rolled up in dry weather and unrolled in wet weather.
Dynamic ventilation: dynamic ventilation only applies to closed greenhouses. The air is renewed by fans placed on one of the greenhouse facades and by openings on the opposite facade. These fans work like indoor air extractors: they create a vacuum inside the greenhouse that causes cooler outdoor air to enter through the opposite openings. A thermostat in the centre of the greenhouse activates the fans at a temperature threshold set by the farmer. The temperature obtained can only be lowered to the outside temperature if the following conditions are met:
– helical fans with large blades that circulate a large volume of air without causing sudden movements of the air;
– a number of fans to obtain an air change equal to 40 to 80 times the volume of the shelter per hour. The main disadvantage is the risk of failure: the temperature can rise very quickly in the enclosed shelter. It must be possible to intervene quickly with a generator to compensate for power outages or with a spare motor in the event of a mechanical failure.
Cooling system: It is a dynamic ventilation system coupled with an air cooling system by a cardboard facade supplied with cold water. In the shelter, the temperature becomes cooler than the outside temperature. The coupling of ventilation to the increase in humidity makes it the most efficient system for reducing the temperature in greenhouses with a length of less than 30 m. However, this method is only effective if the atmosphere under the greenhouse is sufficiently dry – relative humidity below 60% – to allow water to evaporate. Its most efficient use is obtained in desert regions. Provision must be made for rapid intervention in the event of a power failure or interruption. In addition, cooling is a costly technique in terms of investment and energy.
Misting: A network of sprinklers is installed above the plants; low-flow nozzles, with a pressure of 3 to 4 bars, produce fine droplets (diameter 20 at 100μm) whose evaporation reduces the temperature of the greenhouse. The disadvantage of misting is to create a fine rain that falls back on the plant; this wetting of the foliage creates conditions conducive to the development of certain diseases favoured by humid environments (botrytis, oidium, bacterial mange).
Fine fogging: Fine fogging does not have the disadvantages of misting. Droplets are produced by nozzles with lower flow rates (less than 7 l/h with a pressure of 30 to 70 bar). The droplets are so fine (diameter less than 10 μm) that they evaporate before reaching the plants, without wetting their foliage.
Cover made of a thermal film: polyethylene vinyl acetate (EVA) for an antiheat cover, this cover film reflects the wavelengths of the short infrared, which corresponds to the part of the solar spectrum not used by the plant and producing only heat. At the hottest time of the day, this film reduces the greenhouse temperature by 5 to 6°C compared to a simple polyethylene cover.
Thermal screens: placed inside the greenhouse, above the plants, in order to reflect some of the sun’s rays. They are made of strips of cloth, woven or non-woven, or plastic, the outer side of which is covered with aluminium (face upwards). These bands have low emissivity, high reflection and zero transmission. The daytime temperature of the greenhouse is regulated but the night-time temperature of the greenhouse is increased. It is then necessary to ensure that a sufficient thermal amplitude is maintained for the development of the plant, ideally 5 to 10 °C between the average day and night temperatures, by modulating the opening of the shelters.
Bleaching: In hot weather, it is advisable to bleach the outside of the plastic sheeting as this limits the penetration of light energy under the greenhouse. Lime or specific bleaching products with good rain resistance can be used. This technique has its limits: on rainy days, the product should be removed and the greenhouse should be brushed again on sunny days, which is something we never do! To avoid this inconvenience, there are commercially available products that become translucent when wet; but specific thinners are required to remove them, which increases the cost of installation and environmental pollution. On the other hand, bleaching the reinforcement with paint is strongly recommended because it reduces the heating of the metal and, as a result, the degradation of the plastic sheeting cover, thus extending the life of the greenhouse cover.

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