Dehumidification: Forced Ventilation (with heat recovery)

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Sleeves for distribution of the dehumidifed air mounted below the hanging gutters

Humidity can be controlled in the greenhouse by forced ventilation i.e. exchanging humid air inside the greenhouse with dry air from outside the greenhouse withe the use of fans, suction ventilators, and a distribution system. Outside air almost always contains less moisture content than the greenhouse air. The dry air blown in displaces the humid greenhouse air outside, lowering the humidity. The exchanged dry air can be distributed across the greenhouse by sleeves below the (hanging) gutters or above the crop. The dehumidification capacity of the system is the maximum amount of air blown into the greenhouse by the forced ventilation, in m³ per m² per hour.

In periods when the outside air is much colder than the greenhouse air (and therefore much drier - the warmer the air, the more moisture it can hold), forced ventilation causes a loss of warmth (i.e. sensible heat) to the outside. Using an air-to-air heat exchanger, the sensible heat from the outgoing humid air can be recovered to heat the incoming cold dry air. A suction ventilator sucks in the warm, humid greenhouse air and passes it through tubing that brings it into contact with the cold, dry air being brought in from outside, and transferring heat to the outside air. This way, even for low outside temperatures, the incoming dry air is heated to a temperature close to the greenhouse temperature.

The recovery percentage of sensible heat from the outgoing humid greenhouse air is determined by the efficiency of the air-to-air heat exchanger. A 100% recovery percentage would indicate that all the sensible heat has been transferred from the inside air to the outside air – this theoretical value is only possible if the contact surface between the two bodies of air is infinite, and if exactly the same amount of air is pushed out as is pulled in. More realistic values for the recovery percentage range from 60 - 75%.

More information on forced ventilation can be found on the KasKieswijzer.

Contribution to energy-efficiency and resource-use:

Dehumidification via forced ventilation removes moisture from the greenhouse and loses it to the outside air.

Based on whether the sensible heat from the outgoing greenhouse is recovered or not, additional energy may be required to heat the incoming air to the greenhouse temperature. However, the energy loss can be better controlled compared to natural ventilation.

The fans and heat exchangers will consume electrical power.

The reduced need for natural ventilation conserves CO₂.

Steps towards sustainable greenhouses:

The reduced need for natural ventilation leads to a possibility to keep the greenhouse more closed - thus there is a potential for energy saving in cold climates/periods, facilitated by improved insulation, increased screening, and usage of multiple screens, provided that sufficient air movement is ensured at the same time.