Greenhouse Cooling Systems

Horticooler Greenhouse: Enhancing Climate Control for Optimal Growth

Lowering the temperature inside the greenhouse

In some cases, greenhouses require cooling systems. This occurs when the daily average temperature is too high for the crops to function properly. In this case, cooling the greenhouse is the only way to keep the crops inside healthy and productive. Cooling a greenhouse is more expensive than heating a greenhouse, investments in cooling systems must therefore be a well considered decision.

Mechanical Cooling

Mechanical cooling lowers the temperature in a greenhouse by using refrigerant compressors or absorbers, desiccant dryers or other systems that require energy to directly condition the greenhouse. Cooling a greenhouse by only direct or indirect evaporation of water is not considered mechanical cooling, but as adiabatic cooling.

Chillers as cold-water source

In projects where mechanical cooling is applied, DutchGreenhouses® uses advanced screw-chillers with compressors to efficiently produce cold water at temperatures around 6° C. These chiller systems are located outside as a centralised, closed loop, source of cold water. In greenhouse cultivation, water should be the cold source rather than a synthetic refrigerant.

Harvesting condensation water

With cold water far below the dew point, condensation occurs, causing water vapor to liquefy at the heat exchangers where they are used. By changing the state of water (from gas to liquid) and collecting it, the absolute humidity automatically decreases in the greenhouse. This condensation water can be "harvested" and reused for cultivation processes. As a closed-loop system, mechanical cooling does not use any water, but produces it!

Centralized or de-centralized cooling

With cold water available, there are two different strategies to apply mechanical cooling in the greenhouses: centralized and de-centralized. Centralized mechanical cooling refers to climate-chambers used in some semi-closed greenhouses. Air is cooled and conditioned in a corridor, after which it is introduced into the greenhouse via channels under cultivation gutters or above the crops.

De-centralized cooling is cooling inside the greenhouse cultivation area. Examples for this are air handling and HortiCoolers, where the actual cooling and conditioning of the air occurs where the air is needed. If executed properly, de-centralized cooling creates a more homogenous climate throughout the crop.

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Evaporative Cooling in Greenhouses

Evaporative cooling is the process by which water particles evaporate, leading to a drop in temperature due to increased humidity. There are two systems that use this principle in commercial greenhouses: high-pressure misting and the Pad & Fan cooling system.

Pad & Fan Greenhouse

The Pad & Fan cooling method is a widely applicable cooling system due to its simplicity. As the name indicates, it uses a combination of wet-pads and ventilators. A set of large fans blow air out of the greenhouse, causing a suction-draught through the greenhouse. On the opposite sides of the fans, pad-walls are installed which are kept wet. The suction-movement caused by the fans, forces hot outside air to be pulled into the greenhouse through the wet pads. Water from the pads evaporates into the hot air, increasing its humidity whilst lowering the temperature.

High Pressure Fogging

Similar to the Pad & Fan system, High Pressure Fogging works on the principal of evaporative cooling. Under a high pressure, water is introduced to a nozzle with a miniscule opening. In essence, water is divided into tiny particles, creating fog. When the temperature and humidity-levels in the greenhouse allow it, these water particles are absorbed into the surrounding air before they hit the leave of the crop. Just like Pad & Fan, the humidity is increased and the greenhouse air is chilled. High Pressure Fogging is also installed as humidity control in mechanically cooled greenhouses.

Climate Chambers

Both Pad & Fan and High Pressure Fogging are used in developments of the semi-closed greenhouse. The main difference is the way in which the cooled air is introduced into the greenhouse. Semi-closed greenhouses make use of a so-called climate chamber, where the air is either cooled or heated. With large ventilators this treated air is blown into the greenhouse in polyethylene tubes underneath the hanging gutters.