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Moisture: friend or foe?

Energy conservation is an urgent issue in greenhouse horticulture, and within cultivation always has a direct link with moisture management. During an interactive meeting organized by Ridder on 19 June in the World Horti Center, Frank Kempkes, DLO researcher at Wageningen UR, highlighted relevant insights in the field of greenhouse climate optimization within the themes of energy conservation, moisture management, light optimisation (including LED light), screen techniques and heat management.

So is moisture a friend or a foe? Most of the growers present at the event chose 'friend'.

How much moisture should plants evaporate to maintain a good greenhouse climate? And when evaporation is controlled, how much energy should it take? To keep the moisture percentage in balance, there are some disruptive elements, but also controlling ones, thankfully.

Click here for a photo impression of the afternoon (in Dutch)

Frank Kempkes, researcher at Wageningen UR gave a masterclass on this subject at the World Horti Centre in Honselersdijk, the Netherlands.

Insulating screens
Insulating screens conflict with moisture management. The benefits of screens include reducing heat demand (energy saving), and ensuring a homogeneous build-up of vertical (greenhouse climate).

To remove moisture, screens tend to be opened. Yet, open screens create a climate imbalance. That's why growers prefer to keep their screens closed. This is why the screens and moisture management conflicts.

Frank is not in favor of opening screens. It contributes to dehumidification, however it also disrupts the greenhouse climate. "You'd rather create barriers that limit air movement," he said.

Using two to three screens in a greenhouse seems to become standard. Frank explains that 3 screens on 2 wire beds is the compromise. Otherwise, the Law of diminishing returns comes into play.

Moreover, he touches on the importance and effect of the various types of screen properties. The optical, water transport, radiation properties and air permeability of the screen affect light transmission, (convective) fluxes, sensible, latent and radiant heat.

In the example calculation shown by Frank, you could see that screen material affects energy consumption and the amount of light. "However, don't underestimate the screen's mode of use either. By mode of use, I mainly mean screen time and the opening of the screens."

Michel Grootscholten (MG Grand bv), and Johan van Erven (Ridder).
Click here for a photo impression of the afternoon (in Dutch)

What's the relationship between exposure and evaporation? Evaporation costs energy. The heat output of Son-t is different than that of LED. It also has a different effect on evaporation differently and therefore moisture management. When LED radiation is used, a large part is absorbed by leaves.

Eric van Nieuwkoop, Amos Westerveld (Agro Care), and Matthijs van Dentop (Beja Fleur). Click here for a photo impression of the afternoon (in Dutch)

Active or non-active dehumidification
There are several ways to dehumidify. Frank mentions some of them: opening windows, screen kiosks, forced exchange through screens (such as the Airmix), blowing in outside air (by using a heat exchanger) or by active dehumidification (by using a heat pump).

Side note: the more active dehumidification, the greater the investment. There needs to be a balance between the supply and the removal of moisture. Climate control tools help to control the climate and therefore moisture. When these tools are correctly used, it is possible to get good results.

Panel discussion
It is difficult to give an unequivocal answer to the questions raised in the first paragraph as there is no one-size-fits-all approach as each greenhouse, strategy, and crop is different.

However, what is clear is that opening screens is not the best solution for dehumidification, because of the disruptive influences. One should also not underestimate the heat that comes from lamps.

The greenhouse deck is often wet on the inside resulting in condensation which affects light transmission. Paul van Gils (Lumiforte) says there are possibilities to dehumidify through the windows. "The droplets in the greenhouse are unnecessary. There are simple solutions such as anti-condensation coating. This is applied to dry glass." Antireflect coating, which is now mainly applied on the outside, can also be applied on the inside. It antireflects, thus allows for more light in the greenhouse and removes excess moisture."

Peter Geelen, and Jan Voogt of Plant Empowerment were also present during the masterclass. They asked who actually carries out moisture percentage measurements. Only a few participants did. Indeed, Joost Veenman from Ridder agreed on the importance of measurements. Measurement are the starting point for accurate moisture management.

No one-size-fits-all approach
A one-size-fits-all approach doesn't work," emphasizes Johan van Erven (Ridder). We should aim for tailor-made solutions. "As it is a complex issue, there is no conclusive advice for everyone here."

The event made clear that energy conservation and moisture management are pressing issues in greenhouse horticulture.

Click here for a photo impression of the afternoon (in Dutch)

For more information:
Paul van Gils
[email protected]
tel 06 460 57 223

Rob van Hulzen
Van der Ende Group
[email protected]

Johan van Erven
[email protected]
+31 6 53352477

Erik Stappers
Philips Horticulture
[email protected]
+31 6 1583 3825

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