Cooling system in greenhouse includes cooling fans and cooling pads.Its working principle is as follows,

The air inside the greenhouse is drawn out by the exhaust fan, resulting in negative pressure inside, so the dry and hot air will be pulled in the greenhouse by the pressure through the wet cooling pad, and then it becomes cool after passing the cooling pad. The cool air then absorbs the heat inside greenhouse and drawn out by the fan, the temperature is reduced at last.
The core part of the cooling system is the cooling pad. It works by the evaporation of water.