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Cooling Systems for Greenhouses

Greenhouses capture the energy of sunlight. The radiation trapped in the greenhouse releases heat, as photons interact with matter and lose energy. While abundant sunshine promotes intense and rapid growth, excessive heat makes the use of cooling systems mandatory during the summer months, especially in Mediterranean climates such as Greece. Different methods of cooling have both advantages and disadvantages. The two main technologies used today for greenhouses are either cooling by ventilation or by cooler.


Evaporative Cooling


When the water evaporates, the air molecules are slowed down and cooled. The result is that the air temperature drops. Due to the simplicity and low energy consumption, evaporative cooling technologies are certainly the most energy-efficient and cost-effective solutions for cooling greenhouse spaces.


One of the most common systems that use exhaust cooling is the Wet Pad and Fan system. In this arrangement, large plates made of a porous material similar to cardboard, are placed on the walls of one side of the greenhouse, while on the other hand powerful ventilations are placed, which can replace the volume of air in the greenhouse 14 times per hour. Water is pumped through a system of pipes on top of the plates, where it is evenly distributed on the plates. The material of the plate is structured with thin wavy layers, maximizing the surface area and absorption of H2O. The absorbed moisture in the plates evaporates immediately, as the ventilations draw air through the porous material. The colder air that carries water, is evenly dispersed through the greenhouse, with the help of wobbly fans, gets heat from the environment and is evacuated from the ventilations as the freshly frozen air is attracted by the wet plates, continuing the process.


Another method of evaporative cooling is the high-pressure fog system. This arrangement bypasses the need for wet plates. Instead, a very fine water fog (H2O) is emitted into the greenhouse space at extremely high pressure. This fog evaporates easily, and cold air is distributed through the greenhouse through a combination of high-power ventilations and air circulation fans, similar to the Pad and Fan system. The placement and quantity of fog transmitters, ventilations and air circulation fans determines how effective the system will be in smoothly distributing cold air to the plant space within the greenhouse.


While it typically requires much lower energy consumption and easier and cheaper maintenance than chillers, evaporative cooling increases the amount of water vapor contained in the greenhouse air and thus increases humidity. Depending on the phase of plant growth, excess moisture can cause mold, fungi or insects that will attack the growing flowers. Therefore, monitoring humidity levels is important when using these techniques. Usually, in Greece, when it is hot there is also drought, so the subsequent increase in humidity is not so significant as to negatively affect the cannabis plants. In climates that are warm and also have high ambient humidity, wet plates are not effective because the warm and humid air flowing through the plates will already be close to saturation and will not be able to evaporate the cool water on the plates.


Cooling with Cooler


Coolers use a “liquid” or coolant, which undergoes closed-cycle expansion, evaporation, compression and condensation, to extract and evacuate heat from the greenhouse. Air or water cooling units differ because either only air or water are used to cool the hot refrigerant in the condensation phase.


A measuring valve acts as an expander, controlling the flow of the refrigerant into the exhaust chamber. As the coolant expands and evaporates due to the lower pressure, it extracts heat from the water or water/glycol solution through the heat exchanger, a device in which the two liquids do not come into contact but exchange thermal energy. The water or water/glycol mixture, initially gains this heat from the surrounding warm air in the greenhouse. After the heat is absorbed by the refrigerant, it must be emptied from the greenhouse environment, to keep the surrounding air temperature low.


The heated refrigerant is then further heated by the compressor, because the increasing pressure causes an increase in thermal energy. Finally, the compressed refrigerant enters the condenser, which is also a heat exchanger. At this stage, the units can use either water or air (water or water/glycol is used to provide heat to the refrigerant in the evaporation phase in both types of coolers), to absorb heat and cool the refrigerant, allowing it to thicken. The heat absorbed by the air or water is emptied from the unit outside the greenhouse area or transferred to another system for further use (such as dehumidification).


Units cooled with air usually use outside air to cool the coolant, however this is not practical when the temperatures of the outside air are high. Water cooling units are much more efficient in these cases. In water-based coolers, after the coolant releases heat into the water, the water is either emptied into a cooling tower, where it is channeled and dissipates the heat out or the heated water is used to provide thermal energy to supply other greenhouses or other systems of the installation. An example is hot water that can be used for hot air used to restart a dehumidifier wheel. Hot water can also be further heated and used as an importer for steam generator.


Although cooling systems with a cooler have the advantage of not increasing the humidity inside the greenhouse, high energy consumption, complex and costly maintenance and environmentally harmful waste (coolant and water/glycol) must be taken into account. In hot and dry areas, evaporative cooling is presented as the best option. However, for producers who wish to fully seal their medicinal crop from the external environment, water-based coolers are the only option that provides the type of cooling required to ensure a successful harvest.


By Angela Swift

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