In a fogging system, very fine droplets of water are sprayed by high-pressure nozzles on a pipe network distributed across the greenhouse over the crop canopy. These droplets evaporate by using a large amount of heat from the greenhouse air - thereby making the air cooler and more humid. This way of cooling is called ‘evaporating cooling’ - the same principle is also used to cool in Pad and Fan greenhouses. Evaporative cooling works best in regions that have a low absolute humidity.
To avoid excessive humidity in the greenhouse air (which could lead to fungal diseases in the crops), the humid greenhouse air needs to be replaced with dry air, either by natural ventilation from outside the greenhouse, or by active dehumidification.
The main advantages of fogging over pad and fan cooling are: more uniform cooling can be achieved over the greenhouse area, water-use efficiency is higher, and the greenhouse design can be simpler without the need for cooling pads and fans. However, the main challenge is that if the quality of water used for fogging is not maintained, it could easily lead to clogging of the nozzles with particulate matter or calcium deposits.
Contribution to energy balance of greenhouse:
The evaporative cooling acting in fogging systems reduces the greenhouse temperature, and can be used to regulate humidity.
Impact on resource use of greenhouse:
The main resource consumption is that of water, which may pose a problem in water-scarce areas.
The compression pumps which generate the high-pressure droplet spray require electricity.
Possible steps towards sustainable, energy-efficient greenhouses
The energy supply for the compressors, and possible for the water purification, can be derived from renewable resources.