Aat Dijkshoorn, projectleader 'Het Nieuwe Telen'
Netherlands: Dehumidification of the greenhouse climate
The five priority areas are: uneven climate, dehumidification, energy loss, isolation, and crop activation. In this second article, in a series of five, Aat Dijkshoorn describes dehumidification in a greenhouse when the screens are closed.
At this time of the year, many Dutch greenhouse growers close the energy screens during the night. Will this lead to an over humid climate in the greenhouse? The level of humidity in the air is the result of supply and the discharge of moister. When the supplied moister is higher than the discharged moister, the absolute level of humidity in the greenhouse rises. The absolute level of humidity can be tracked via a climate computer. It is better to track the absolute humidity level than the relative humidity level. This because the relative humidity level strongly depends on the temperature.
During the night, the crop evaporates, which causes a higher level of humidity in the greenhouse. Evaporation is necessary to supply minerals to the growing parts of the plants. Especially calcium is important, because it can only reach the new cells through water. However, when the evaporation is much higher than requested by the plant, the energy consumption will be unnecessarily high. An evaporator of 10 grams per hour costs 6 watts of heating.
Thus, when the climate is too humid, it is important to take a closer look at the supply of moister. Is it possible to lower the minimum pipe or even remove it? It is often possible to realize enough evaporation with minimal air movements. However, this highly relies on the permeability of the crop and the current air movement. Heavy ventilation also leads to high evaporation, which consequently leads to the consumption of more energy.
Cultivation in the ground also leads to higher levels of humidity. Then more moister needs to be discharged.
Moister under closed screens can be discharged in several ways. It is possible to discharge the redundant moister via condensation at the (cold) outside facade. However, this method is not profitable for large greenhouses; they have relatively less facade. Then, the moister will be discharged by the screens, if they contain moisture-permeable fibres or openings. The discharge of redundant moister is a two-staged process; first through the screen and afterwards discharged by condensation or ventilation.
When the greenhouse climate becomes too humid, it is important to reduce the supply. Then, it is advisable to increase the amount discharged above the screens. This means that you have to avoid working with screen gaps. It is better to ventilate above the screens. Therefore, some windows need to be changed considerably, depending on the temperature outside.
The level of humidity can be tracked by measuring the moister under and above the screens. A climate measuring box above the screens in combination with a measurement of the humidity outside the greenhouse is very useful.
When regulating the climate in the greenhouse it is useful to take into account the above mentioned aspects. This will enable growers to find a balance between the supply and discharge of the moister in their greenhouse. It is also advisable to contact your suppliers for more information about your screens for example.
Click here to read the the first article about uneven climate.
More information:
www.energiek2020.nu
Email: adijkshoorn@ltoglaskracht.nl