In few cases, greenhouses may 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. An investment in a cooling system must therefore be a well considered decision. The preferred cooling method for a greenhouse is through evaporative cooling. Evaporative cooling is the process of having particles of water evaporate, which leads to a drop in temperature through increased humidity. There are two systems which use this principle in commercial greenhouses: high pressure fogging and the Pad & Fan cooling system. In the few years, semi-closed greenhouses have been developed based on the same principles as the Pad & Fan system.
Pad & Fan Cooling
The Pad & Fan cooling method is a widely applicable cooling system due to it’s 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 and 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.
Both Pad & Fan and High Pressure Fogging are used in developments of the semi-closed greenhouse. The main difference is the method in which the cooled air is introduced to 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.