The moisture content of humid air is removed by condensing it to liquid water by exposing it to a cold surface with a temperature lower than the dew point of the greenhouse air. The active dehumidification in the tool creates the cold surface by using a heat pump.
The greenhouse air is passed by forced ventilation through the dehumidification units - here, it first passes through the cooling unit and condenses - the water is recovered. The latent heat of condensation is regained by a heat pump, and the colder, drier greenhouse air is then passed through a hot surface to heat it back to the initial greenhouse temperature. The warmed, dry air is channeled back to the greenhouse.
The heat recovered can be used to immediately heat the dehumidified air, or stored, for instance in an aquifer or other forms of heat storage.
The dehumidification capacity of the cold surface depends on:
the cold water temperature,
water flow over the heat exchanger
airflow of the greenhouse air over the cold surface
humidity levels inside the greenhouse.
The dehumidification capacity decreases rapidly at greenhouse air temperatures below 15°C and at low humidity levels. Dehumidification capacity can be increased by increasing the heat exchange surface area per m2 of the greenhouse, though it could be an expensive measure.
Contribution to energy-efficiency and resource-use:
Mechanical dehumidification removes moisture from the air, and the condensed water can be recovered.
As the greenhouse air is inevitably cooled down to condense the moisture, energy is required to heat it up back to the initial greenhouse temperature, especially in cold climates/periods. Recovering and using the latent heat of water condensation for re-heating can reduce the energy use, though it comes with a significant consumption of electrical power.
Mechanical dehumidification reduces the dependence on external climate conditions, hence the greenhouse can be closed, conserving CO2.
Steps towards sustainable greenhouses:
The high electricity demand for mechanical dehumidifiers can be sourced from renewable sources.
Mechanical dehumidification facilitates closing of the greenhouse - this means that further energy savings can be achieved in colder climates/during colder periods by increased greenhouse insulation and increased use of energy screens.
A study compared different types of dehumidification in greenhouses and found the annual energy savings in a typical Dutch tomato greenhouse to be 225 MJ/m2 per year by using a cooling system to condense moisture and recover excess sensible and latent heat, and 250 MJ/m2 per year - by using a hygroscopic dehumidification system.